Chapter 16 hydrocarbons

Chapter Overview - Sections
• Introduction and background
• Types of Hydrocarbons
• Alkanes
• Cycloalkanes
• Radical substitution reactions
• Oxidation and reduction of organic
compounds
• Alkenes
• Isomerism
• Alkynes
• Benzenes and substituted benzenes

• Classify hydrocarbons as aliphatic and aromatic.
• Describe nomenclature of alkanes and cycloalkanes.
• Describe the mechanism of free radical substitution in alkanes exemplified by
methane and ethane.
• Describe the structure and reactivity of alkanes as exemplified by ethane.
• Describe the chemistry of alkenes by following the reaction of ethene.
• Describe what is meant by the term delocalized electrons in the context of the
benzene ring.
• Describe addition reactions of the benzene and methyl benzene.
• Describe the mechanism of electrophilic substitution in benzene.
• Describe the preparation of alkynes using elimination reactions.
• Describe acidity of alkynes.
• Describe and differentiate between substitution and addition reactions.
After completing this lesson, you will be able to

• Explain the shapes of alkanes and cycloalkanes exemplified by ethane and
cyclopropane.
• Explain unreactive nature of alkanes towards polar reagents.
• Explain what is meant by a chiral center and show that such a center gives
rise to optical isomerism.
• Explain the nomenclature of alkenes.
• Explain shape of ethane molecule in terms of sigma and pi C-C bonds.
• Explain dehydration of alcohols and dehydrohalogenation of RX for the
preparation of ethane.
• Explain the shape of benzene molecule (molecular orbital aspect).
• Explain isomerism in alkanes, alkenes, alkynes and substituted benzene.
• Define homoloytic and heterolytic fission, free radical initiation, propagation
and termination.

• Identify organic redox reaction.
• Define and explain with suitable examples the terms isomerism, stereoisomerism and
structural isomerism.
• Define resonance, resonance energy and relative stability.
• Define the terms hydrogenation, halogenation, hydrohalogenation, hydration,
halohydration, epoxidation, ozonolysis and polymerization.
• Compare the reactivity of benzene with alkanes and alkenes.
• Compare the reactivity of alkynes with alkanes, alkenes and arenes.
• Discuss chemistry of benzene and ethyl benzene by nitration, sulphonation, halogenation,
Friedal Craft’s alkylation and acylation.
• Discuss the shape of alkynes in terms of sigma and pi C-C bonds.
• Discuss chemistry of alkynes by hydrogenation, hydrohalogenation, hydration, bromination,
ozonolysis and reaction with metals.
• Apply the knowledge of position of substituents in the electrophilic substitution of benzene.
• Use the IUPAC naming system for alkynes.

16.0 – Introduction and background
Dr. Hashim Ali
Post-Doc Uppsala University, Sweden.
PhD Computational Biology, KTH, Stockholm, Sweden.
Federal Board of Intermediate and Secondary
Education (FBISE)
Chemistry F.Sc
II

ethene
methane
C
H
HH
H
16.0.1 – Introduction- Hydrocarbons
• Organic compounds which contain
carbon and hydrogen only are
called hydrocarbons.
• The ability of carbon atoms to
attach with each other to form a
chain or ring is called catenation.

16.0.2 – Introduction - Hydrocarbons in daily life
• Motor oil consists of several hydrocarbons.
It lubricates the moving parts of car
engines.
• Asphalt pavement on highways is made of
hydrocarbons found in petroleum.
• Many candles are made of paraffin wax, a
solid mixture of hydrocarbons.
• These forms of transportation are fueled by
different mixtures of hydrocarbons.
• These rain boots are made of a mixture of
several hydrocarbons.
• This lighter burns the hydrocarbon names
butane.

16.0.2.1 – Introduction - Hydrocarbons in daily life - Methane
• Methane is extremely flammable and used as fuel (Sui gas?) to produce
light and warmth.
• It is also used as fuel and in chemical reactions.

16.0.2.2 – Introduction - Hydrocarbons in daily life - Ethane
• Ethane is used in the production of ethylene for making plastics, and anti-
freeze and detergents.
• It is a ripening agent for foods, a refrigerant, a substance in producing
welding gas and a primary ingredient in mustard gas.

16.0.2.3 – Introduction - Hydrocarbons in daily life - Propane
• Propane is also called “liquid petroleum gas”, commonly used to heat
homes and hot water, cooking food, power your bbq (tikka) sessions, and
fuel cars.
• It is also used by industries and agriculture for all kinds of applications.

16.0.2.4 – Introduction - Hydrocarbons in daily life - Butane
• Butane is also called “liquid petroleum gas”, commonly used as a fragrance
extraction solvent, in fire lighters, cooking food, power your bbq (tikka)
sessions, and fuel cars.
• It is also used by industries to produce ethylene and butadiene.

16.0.2.5 – Introduction - Hydrocarbons in daily life - Ethene
• Ethene is often used in the manufacture of many important polymers like
polyethene and polyvinyl chloride (PVC).
• It is used to prepare other important chemicals like ethyl alcohol and
acetaldehyde etc.
• It is used as a general anesthetic.
• It is used to prepare epoxyethane, used in detergents.

16.0.2.6 – Introduction - Hydrocarbons in daily life - 1,3-
butadiene
• 1,3-butadiene is often used in the manufacture of styrene-butadiene
rubber, polybutadiene rubber and adiponitrile, which is further hydrated to
produce nylon.

16.0.2.7 – Introduction - Hydrocarbons in daily life - Ethyne
• Ethyne (acetylene) is often used as a fuel in welding. An acetylene flame
can reach temperatures of 6000°F.
• Acetylene is also used to make carbon volatile to make it more efficient in
carbon dating.
• It is also used for lighting, especially for portable lamps used by miners and
in the headlights of early automobiles.
• It is also used to harden steel that is too big to be hardened in a furnace.

16.0.2.8 - Introduction - Hydrocarbons in daily life - Propyne
• Propyne is commonly used as a substitute for acetylene as fuel for welding
torches.
• It is also being investigated as possible fuel for rockets in space craft.

16.0.2.9 - Introduction - Hydrocarbons in daily life - Benzene
• Benzene is one of the most
commonly used hydrocarbon, to
which we are most exposed to in
our daily lives.

16.0 - Background to types of hydrocarbons

16.0 – Background to hydrocarbons
Dr. Hashim Ali
Education (FBISE)
Chemistry F.Sc
II

• I am assuming that one is absolutely clear about valence electrons, covalent bonds and
maximum number of bonds an element can form. Just a reminder!
• Also the focus will be on exactly one carbon atom at a time. The orbitals and other numbers are
for that atom only and nothing to do with the other participating atoms for concept!
• Understanding the concept of hybridization is extremely important for organic chemistry in
particular
o Why and how double and triple bonds are formed between C-C atoms? Describe theory and experimental
setting for energy spectra required and molecule shape in each type of bond.
o What is a hybrid orbital? How is it formed? How many orbitals are formed? What is sp, sp2 and sp3 hybrid
orbitals?
o What are sigma and pi bonds?
o How does sp3 orbital lead to formation of single bonds only, sp2 to double bonds and sp to triple bond?
How many sigma and pi bonds can be formed in each of these?
• Finally, I will give the explanation that you can write in exams in case of question!
• I will tackle each question one by one but make sure not to move ahead until you have
understood the concept in each question clearly!
16.0.3 – Background - Orbital hybridization
Education (FBISE)
Chemistry F.Sc
II

Why and how double and triple bonds are formed between C-C atoms? Describe theory and
experimental setting for energy spectra required and geometry of molecule in each type of bond.
• A carbon atom contains four valence electrons and can form four covalent bonds with other atoms.
• The static configuration of Carbon atom is (1s2, 2s2, 2p2) or precisely (1s2, 2s2, 2px
1,2py
1,2pz
0).
• On excitement, with a small amount of heat, the C atom can be excited to form (1s2, 2s1, 2px
1,2py
1,2pz
1).
• A single bond means sharing one electron with another atom, a double bond means sharing two electrons with
another atom, a triple bond means sharing three electrons with another atom and a tetra-bond means sharing
four electrons with another atom.
• The valence shell of carbon is completed, if it shares 4 electrons with 1 or more atoms.
• In theory, under pure orbitals and only for single bonds, forming the first bond requires least energy, the second
bond requiring more energy than the first, the third bond requiring more energy than the second and the fourth
bond requiring the most energy, even if they are all single bonds or double bonds or triple bonds due to
repulsive forces between electrons.
• All single bonds are spherical theoretically.
16.0.3.1 – Background - Orbital hybridization - Bonds

Why and how double and triple bonds are formed between C-C atoms? Describe theory and
experimental setting for energy spectra required and geometry of molecule in each type of bond.
• However, in practical experiments, one observes that all C–H single bonds formed require the
same energy (413 kJ per mole).
• All single bonds are short and elliptical in shape, which is theoretically not explained under pure
orbitals.
• Similarly for an atom with one double bond, the formation of third and fourth bonds required the
same energy, and the double bond required slightly more energy than the single bonds.
• For an atom with a triple bond, the energy required for formation of the fourth bond was equal to
the energy required for formation of single bonds. The triple bond required [single bond energy +
(twice the energy difference between single bonds and double bonds)].
• How then do you explain this anomaly between experimental data and theoretical concept?
16.0.3.1 – Background - Orbital hybridization - Bonds
Diff = 256
Diff = 503

What is a hybrid orbital? How is it formed? How many orbitals are formed? What is sp, sp2 and
sp3 hybrid orbitals?
• Developed in the 1930s, chemists explained the difference in energy
spectrum by a phenomenon called hybridization.
• The basic claim is that some (one, two or all three) 2p orbitals in an
excited C atom mix together with 2s orbital to form the same number of
hybrid orbitals - the word hybrid means mixing of two unique units to
form a third unique unit that has some properties from both the original
units.
• A hybrid orbital has the same character as the number of s and p orbitals
composing the hybrid orbital - an sp bond has 50% s and 50% p
characteristics while an sp3 bond has 25% s and 75% p characteristic.
• If one 2p orbital (assume 2px) hybridizes with 2s orbital, then two
hybrid sp orbitals are formed.
• If two 2p orbital (assume 2px and 2py) hybridizes with 2s orbital, then
three hybrid sp2 orbitals are formed.
• If all three 2p orbital (assume 2px,2py and 2pz) hybridizes with 2s orbital,
then four hybrid sp3 orbitals are formed.
• NOTE: After hybridization, the total number of orbitals remains the same
4 as before hybridization regardless of the type of hybridization.
• Remember sp hybridization means two sp hybridized orbitals and two
remaining p orbitals.
• Remember sp2 hybridization means three sp2 hybridized orbitals and one
• Remember sp3 hybridization means four sp3 hybridized orbitals and zero
• NOTE: Overlap is only possible between same energy orbitals. A p orbital
can NEVER overlap with a sp, sp2 or sp3 orbital and vice versa.
16.0.3.2 – Background - Orbital hybridization - Hybrid orbitals

What are sigma (represented by σ)and pi (represented by π) bonds?
• Sigma bonds are the bonds formed by
overlapping one hybridized orbital with
another hybridized orbital from C atom or
with orbitals from other (than C) elements.
• If there are any pure p orbitals after
hybridization (i.e., two p orbitals in sp and
one p orbital in sp2), they overlap with other
pure p orbitals to form additional bonds to
sigma bond with the same atom. These
additional bonds are called pi bonds and can
only be formed between pure p-orbitals.
16.0.3.3 – Background - Orbital hybridization - Sigma and pi
bonds

How does sp3 orbital lead to formation of single bonds only, sp2 to double bonds and sp to triple
bond? How many sigma and pi bonds can be formed in each of these?
• NOTE: Only (and not more than) one hybridized orbital from
one carbon atom can overlap with a hybridized orbital from
another carbon atoms. This means that exactly one sigma bond
can be formed between a C-atom and any other atom and the
remaining orbitals overlap with other atoms.
• In sp3 hybridization, four sp3 hybridized orbitals form four
sigma bonds between four different atoms and no pi bonds
with any other atom - meaning there are four equal energy
single bonds formed as in the case of CH4 or H3C–CH3 as seen
experimentally.
• In sp2 hybridization, three sp2 hybridized orbitals form three
sigma bonds with three atoms and 1 pi bond with one of the
three atoms - there are two single bonds (equal energy) and
one double bond (sigma + pi bond) with slightly more energy
than the single bonds.
• In sp hybridization, two sp hybridized orbitals form two sigma
bonds with two atoms and 2 pi bonds with one of the two
atoms - there is one single bond and one triple bond (sigma +
2 pi bonds) with exactly the same energy description as
observed experimentally.
16.0.3.4 – Background - Orbital hybridization - Bond formation

16.0.3 - Background - Orbital hybridization
• Hybridization is the concept of mixing atomic
orbitals into new hybrid orbitals (with different
energies, shapes, etc., than the component atomic
orbitals) suitable for the pairing of electrons to form
chemical bonds in valence bond theory.
• Hybrid orbitals are very useful in the explanation of
molecular geometry and atomic bonding properties.
• Hybrid orbitals are assumed to be mixtures of atomic
orbitals, superimposed on each other in various
proportions.
• For example, in methane, the C hybrid orbital which
forms each carbon–hydrogen bond consists of 25% s
character and 75% p character and is thus described
as sp3 (read as s-p-three) hybridized.

16.0 - Background to hydrocarbons

Dr. Hashim Ali
16.1 – Types of hydrocarbon
Education (FBISE)
Chemistry F.Sc
II

Hydrocarbons
Open chain
acyclic hydrocarbons
Closed chain or
cyclic hydrocarbons
Saturated
Hydrocarbons
(Alkanes/paraffin)
Unsaturated
Hydrocarbons
Alkenes/Olefins Alkynes/Acetylenes
Alicyclic
Hydrocarbons
(Cycloalkanes)
Aromatic
Hydrocarbons
Ethane
Ethene
Ethyne cyclohexane
benzene
C
H
H
C
H
H
C
H
H
H
C
H
H
H
CH C H
16.1.0 – Types of hydrocarbon - Overview
• Hydrocarbons have been divided into various classes on the basis of their structure.

Hydrocarbons
Open chain
Closed chain or
cyclic hydrocarbons
Saturated
Hydrocarbons
(Alkanes/paraffin)
Unsaturated
Hydrocarbons
Alicyclic
Hydrocarbons
(Cycloalkanes)
Aromatic
Hydrocarbons
C
H
H
H
C
H
H
H
C
H
H
C
H
H
CH C H
16.1.1 – Types of hydrocarbons - Open chain acyclic
hydrocarbons
• The hydrocarbons in which carbon atoms are
attached with each other to form open chains are
called open chain hydrocarbons.
• Open chain  no cycle between molecules.
• Depending on presence/absence of double and
triple bonds between carbon atoms, they can
further be classified into:
o Saturated hydrocarbons.
o Unsaturated hydrocarbons.

Hydrocarbons
Open chain
Closed chain or
cyclic hydrocarbons
Saturated
Hydrocarbons
(Alkanes/paraffin)
Unsaturated
Hydrocarbons
Alicyclic
Hydrocarbons
(Cycloalkanes)
Aromatic
Hydrocarbons
Pentane (straight chain)
Methyl butane (branched chain)
• The hydrocarbons in which carbon atoms are
attached with each other through single bonds.
• They are called ‘saturated’ because no further
atoms or groups can be attached to the carbon
atoms of such hydrocarbons.
• Each carbon atom is sp3 hybridized.
• For example Alkanes.
• Saturated hydrocarbons may have straight
chain (where carbon atoms in a molecule form
a chain that runs from one end to the other) or
branched chain (where alkyl groups are bonded
with a central carbon atom of main chain).
• Called paraffins because historically found in
paraffin wax in petroleum products.
16.1.1.1 – Types of hydrocarbons - Open chain acyclic
hydrocarbons - Saturated hydrocarbons

Hydrocarbons
Open chain
Closed chain or
cyclic hydrocarbons
Saturated
Hydrocarbons
(Alkanes/paraffin)
Unsaturated
Hydrocarbons
Alicyclic
Hydrocarbons
(Cycloalkanes)
Aromatic
Hydrocarbons
• The hydrocarbons in which at least two
carbon atoms are attached through double
or triple bonds, and are sp2 or sp hybridized.
• For example alkenes and alkynes.
• They can further be classified into alkenes
(double bond) or alkynes (triple bond).
16.1.1.2 – Types of hydrocarbons - Open chain acyclic
hydrocarbons - Unsaturated hydrocarbons

Hydrocarbons
Open chain
Closed chain or
cyclic hydrocarbons
Saturated
Hydrocarbons
(Alkanes/paraffin)
Unsaturated
Hydrocarbons
Alicyclic
Hydrocarbons
(Cycloalkanes)
Aromatic
Hydrocarbons
1-Pentene (straight chain)
2-Methyl 2-butene (branched chain)
• The unsaturated hydrocarbons in which at
least two carbon atoms are sp2 hybridized.
• The sp2 hybridization causes the formation
of a double bond between these carbon
atoms.
• Alkenes may have a straight chain or
branched chain.
• Olefin is a word originating from French
meaning oil forming with reference to oily
ethylene dichloride.
16.1.1.2.1 – Types of hydrocarbons - Open chain acyclic
hydrocarbons - Unsaturated hydrocarbons - Alkenes or olefins

Hydrocarbons
Open chain
Closed chain or
cyclic hydrocarbons
Saturated
Hydrocarbons
(Alkanes/paraffin)
Unsaturated
Hydrocarbons
Alicyclic
Hydrocarbons
(Cycloalkanes)
Aromatic
Hydrocarbons
2-Butyne (straight chain)
5-Methyl 2-hexyne (branched chain)
16.1.1.2.2 – Types of hydrocarbons - Open chain acyclic
hydrocarbons - Unsaturated hydrocarbons - Alkynes or acetylene
• The unsaturated hydrocarbons in which at
least two carbon atoms are sp hybridized.
• The sp hybridization causes the formation
of a triple bond between these carbon
atoms.
• Alkenes may have a straight chain or
branched chain.

Hydrocarbons
Open chain
Closed chain or
cyclic hydrocarbons
Saturated
Hydrocarbons
(Alkanes/paraffin)
Unsaturated
Hydrocarbons
Alicyclic
Hydrocarbons
(Cycloalkanes)
Aromatic
Hydrocarbons
Cyclopropane
benzene
16.1.2 – Types of hydrocarbons - Closed chain hydrocarbons
• The hydrocarbons in which carbon atoms attach
with each other to form rings or cycles.
• They can be classified into two types.
• Alicyclic hydrocarbons.
• Aromatic hydrocarbons.

Hydrocarbons
Open chain
Closed chain or
cyclic hydrocarbons
Saturated
Hydrocarbons
(Alkanes/paraffin)
Unsaturated
Hydrocarbons
Alicyclic
Hydrocarbons
(Cycloalkanes)
Aromatic
Hydrocarbons
cyclopentane
cyclopentene
CHHC
H2C CH2
CH2
cyclohexane
16.1.2.1 – Types of hydrocarbons - Closed chain hydrocarbons -
Alicyclic hydrocarbons
• The cyclic hydrocarbons, which do not
contain benzenoid ring are called alicyclic
hydrocarbons.
• Alicyclic hydrocarbons possess two
hydrogen atoms less than their
corresponding open chain hydrocarbons.

Hydrocarbons
Open chain
Closed chain or
cyclic hydrocarbons
Saturated
Hydrocarbons
(Alkanes/paraffin)
Unsaturated
Hydrocarbons
Alicyclic
Hydrocarbons
(Cycloalkanes)
Aromatic
Hydrocarbons
toluene
benzene
16.1.2.2 – Types of hydrocarbons - Closed chain hydrocarbons -
Aromatic hydrocarbons
• The cyclic hydrocarbons, which contain benzenoid ring are
called alicyclic hydrocarbons.
• In these compounds, all the carbon atoms present in the
ring are sp2 hybridized.
• They are called aromatic because most of these compounds
have a sweet aroma (smell).
• Benzene is the simplest aromatic hydrocarbon.
• Benzene has a regular hexagonal structure with alternate
single or double bonds between carbon atoms.

• What is catenation?
• What are hydrocarbons?
• Why saturated hydrocarbons are called paraffins?
• Why unsaturated hydrocarbons are called olefins?
• What is hybridization?
• Why benzene is called aromatic hydrocarbon?
• What is hexagonal structure?
16.1.3 - Quick quiz

Chapter 16 Section 1 - Types of hydrocarbons

Dr. Hashim Ali
16.2 – Alkanes
Education (FBISE)
Chemistry F.Sc
II

Pentane (straight chain)
Methyl butane (branched chain)
16.2.0 – Alkanes - Introduction
• Simplest organic molecules with only
Carbon and Hydrogen atoms.
• They contain only single bonds between all
C-C atoms.
• They do not contain any ring/cycle.
• Commercially important as found in
natural gas and oils

• In order to clearly identify compound, a systematic
method of naming has been developed.
• It is known as the IUPAC (International Union of Pure
and Applied Chemistry) system of nomenclature.
• The names are correlated with structure such that the
reader or listener can deduce the structure from the
name.
• Remember: The correct name will have only one
structure
o i.e., given a IUPAC name, you should be able to draw the
structure.
o AND given the structure, you should be able to get the exact
name back!
• If either of the above is not true, then your drawn
structure/deduced name from structure is wrong!
16.2.1.0 – Alkanes - Nomenclature - Background to IUPAC

1. Naming hydrocarbons starts with selecting/identifying the longest chain (even around
corners) (optionally followed by straightening to ease reading).
C C C C C
C
C
C
CH H
H
H H
H H
H
H
H
HH
H
H
H
H H
H
HH
Which is the longest chain (in number of C
atoms) here? Is it with 5C?
Not 6C either!
Not 7C either!
C C C C
CH H
H
H
H
HH
H
H
H
C
H
H
H C C
H
HH
H
H
H
C H
8C is correct!
Name starts with
“oct” (8C).
All single bonds in
main chain 
Name ends with
“ane”.
Main chain name
 Octane
16.2.1.1 – Alkanes - Nomenclature - IUPAC Rules

2. If there are two or more chains of
equal lengths, the chain with larger
number of branches is selected as
main chain.
Longest chain is 7C but there are several
of them!
Some are equivalent!
Count number of branches
for all non equivalent ones
4 branches are the
most!
Name starts with
“hept” (7C).
All single bonds
in main chain 
Name ends with
“ane”.
Main chain name
 heptane
C C C C C
C
C
C
CH H
H
H H
H H
H
H
HHH
H H
H
HH
CH
H
H
C
CH H
HH
H
CH H
HH
H
C
C C C C C
C
C
C
CH H
H
H H
H H
H
H
HHH
H H
H
HH
CH
H
H
C
CH H
HH
H
CH H
H
C
H
H

3. Number the main chain starting from the end nearest to the substituent.
The main chain can be numbered from left
to right or right to left! Which way to
number?
Starting from left to right,
the first substitute is at C2.
Starting from right to left,
the first substitute is at C3.
So the chain is numbered
from the end with C2.
The main
heptane chain
should be
numbered from
left to right in
this case!
C C C C C
C
C
C
CH H
H
H H
H H
H
H
HHH
H H
H
HH
CH
H
H
C
CH H
HH
H
CH H
H
C
H
H

4. When two identical substituents are present at equal distance from either end,
number the chain starting with the end, which gives their minimum sum.
Both the left to right and right to left order
for main chain give position 2 for the first
substituent. Which to choose?
Starting from right to left
end, the sum of all
substitutes is 2+4+5+6 = 17.
Starting from left to right
end, the sum of all
substitutes is 2+3+4+6 = 15.
So the chain is numbered
from the left to right.
The main
heptane chain
should be
numbered from
left to right in
this case!
C C C C C
C
C
C
CH H
H
H H
H H
H
H
HHH
H H
H
HH
CH
H
H
C
CH H
HH
H
C
H
H
CH H
H

5. The position of substituent is indicated by the number of C-atom to which it is
attached. The number is prefixed to the name of group separated by hyphen.
The selected chain is highlighted and carbon atoms are
numbered from left to right as C1, C2, C3, C4, C5, C6 and
C7.
Methyl(CH3), Ethyl(C2H5) and
Propyl(C3H7) are attached to the
main chain.
Name each substituent (side
chain) and determine its position
on the main chain by the carbon
to which it is attached.
The alkane is named
heptane and the side
chains are 2-Methyl, 3-
Ethyl and 4-Propyl.
The name of the alkane
is “2-Methyl-3-Ethyl-4-
Propylheptane”.
C2 C3 C4 C5 C7
C
C
C
CH H
H
H H
H H
H
H
HHH
H H
H
HH
C1
H
H
H
C
CH H
HH
H
C6
H
H H

6. Names of alkyl groups are written before the name of parent hydrocarbon in
alphabetical order or in order of increasing size, separated by a hyphen.
The selected chain is highlighted and carbon atoms are
numbered from left to right as C1, C2, C3, C4, C5, C6 and
C7.
Methyl(CH3), Ethyl(C2H5) and
Propyl(C3H7) are attached to the
main chain.
Name each substituent (side
chain) and determine its position
on the main chain by the carbon
to which it is attached.
Increasing size order
would be Methyl < Ethyl <
Propyl. The name of the
alkane is “2-Methyl-3-
Ethyl-4-Propylheptane”.
In alphabetic order, it will
be “3-Ethyl-2-Methyl-4-
Propylheptane”.
C2 C3 C4 C5 C7
C
C
C
CH H
H
H H
H H
H
H
HHH
H H
H
HH
C1
H
H
H
C
CH H
HH
H
C6
H
H H

7. When two or more like groups are present, their numbers are indicated by prefixes
di-, tri-, tetra-, etc. Their numbers are grouped together and are separated by
commas.
Two Methyl groups are present. How to
represent them?
Use di-methyl as name (di = 2).
The name of the alkane is “2,6-di-Methyl-3-Ethyl-4-Propylheptane”.
C C C C C
C
C
C
CH H
H
H H
H H
H
H
HHH
H H
H
HH
CH
H
H
C
CH H
HH
H
C
H
H
CH H
H

8. If two identical groups appear at the same C-atom, the number is separated twice
separated by commas.
Two Methyl groups are present on the
same atom. How to represent them?
The name of the alkane is “2,2-di-Methyl-3-Ethyl-4-Propylheptane”.
Use di-methyl as name (di = 2) and
write the position twice.
C C C C C
C
C
C
CH H
H
H H
H H
H
H
HH
H H
H
HH
CH
H
H
C
CH H
HH
H
C
H
H H
CH H
H

9. The longest chain of the substituent is numbered starting with the carbon attached
directly to the main chain. Parentheses are used to separate the numbering of the
substituent and the main chain.
The side chain also contains substitutes.
How to represent them?
The name of the alkane is “2-Methyl-3-Ethyl-4-(1-MethylPropyl)heptane”.
Number the side chain starting from the carbon
attached to the main chain
C C C C C
C
C
C
CH H
H
H H
H H
H
H
HH
H H
H
H
CH
H
H
C
CH H
HH
H
C
H
H H
C H
H
H
H
Determine the name and position of the substituent
on side chain and name according to the rule.

1. The prefix iso-, which stands for isomer, is commonly given to 2-methyl alkanes.
The prefix will be placed in front of the alkane name that indicates the total number
of carbons.
The name of the alkane is “iso-octane”.
16.2.1.2 – Alkanes - Nomenclature – Common System rules
C C C C C
C
H H
H
H
HH
H H
H
CH
H
H
C
H
H H
H
HH

2. The prefix neo- refers to a substituent whose second-to-last carbon of the chain is
tri-substituted (has three methyl groups attached to it). The prefix will be placed in
front of the alkane name that indicates the total number of carbons.
The name of the alkane is “neo-octane”.
16.2.1.2 – Alkanes - Nomenclature – Common System rules
C C C C
C
H HH
H H
H
CH
H
H
C
H
H HHH
CH H
H
H

• The structural formula and names for the simple alkanes are shown in the following table.
Number of C atoms Formula Alkane name Condensed Structure
1 CH4 methane CH4
2 C2H6 ethane CH3CH3
3 C3H8 propane CH3CH2CH3
4 C4H10 butane CH3(CH2)2CH3
5 C5H12 pentane CH3(CH2)3CH3
6 C6H14 hexane CH3(CH2)4CH3
7 C7H16 heptane CH3(CH2)5CH3
8 C8H18 octane CH3(CH2)6CH3
9 C9H20 nonane CH3(CH2)7CH3
10 C10H22 decane CH3(CH2)8CH3
11 C11H24 undecane CH3(CH2)9CH3
12 C12H26 dodecane CH3(CH2)10CH3
13 C13H28 tridecane CH3(CH2)11CH3
… … … …
20 C20H42 eicosane CH3(CH2)18CH3
21 C21H44 heneicosane CH3(CH2)19CH3
… … … …
30 C30H62 tricontane CH3(CH2)28CH3
16.2.1.2 – Alkanes - Nomenclature - Examples

• Name each of the following compounds according to IUPAC System.
CH3
CH3 CH3
CH3CHCH2CH CH2
CH3
CH3 CH3
CHCH2C
CH3 CH3
16.2.1.3 – Alkanes - Nomenclature - Activity
1,5-dimethyl hexane
2,2,4-trimethyl pentane

• Name each of the following compounds according to IUPAC System.
CH3 CH2
CH
CH3
CH3
CHCH2
CH3
CH3 CH2
CH CH3
CH3 CH2
2,4-dimethyl hexane
3-methyl pentane

• Indicate what is wrong with each of the following names. Give the correct
IUPAC name if possible.
a) 2-Dimethyl Propane
b) 2,2,3-Methyl Butane
c) 3,3-Dimethyl-5,5-Dimethyl Heptane.
d) 2,2-Diethyl-4,4Dimethyl Pentane.
e) 2,4-Diethyl Pentane.
f) 3-Ethyl-4-,Methyl Pentane.

• Write the structures of the following compounds.
a) Neo heptane.
b) Iso Heptane.
c) Tri-methyl Ethyl Methane.
d) Dimethyl Ethyl Isopropyl Methane.
e) Dimethyl Propyl Ethyl Methane.
f) 3-ethyl Hexane.

• Methane to Butane (C1 to C4) are colorless and odorless
gases.
• Pentane to heptadecane (C5 to C16) are colorless and
odorless liquids.
• The higher members from C18 onwards are waxy solids,
which are also colorless and odorless.
• Alkanes are non-polar or very weakly polar compounds.
• Alkanes are insoluble solvents like water but soluble in
non-polar solvents like benzene, ether, carbon
tetrachloride, etc.
• Their melting points, boiling points, density etc. increase
with the increase in number of carbon atoms.
o The boiling point increases by 20 to 30 °C for addition of each
CH2 group to the molecule.
o However, the boiling points of alkanes, having branched chain
structures are lower than their isomeric normal chain alkanes.
E.g., n-butane has a higher boiling point (55 °C) than iso-butane
(-10.2 °C)
• On the other hand, solubility decreases with increase in
mass.
Formul
a
Alkane
name
Boiling
point (°C)
Melting
point
(°C)
Density
(g/mL at
20°C)
CH4 methane -160 -182.5
C2H6 ethane -89 -183.3
C3H8 propane -42 -187.7
C4H10 butane -0.4 -138.3
C5H12 pentane 36.1 -129.8 0.5572
C6H14 hexane 68.7 -95.3 0.6603
C7H16 heptane 98.4 -90.6 0.6837
C8H18 octane 127.7 -56.8 0.7026
C9H20 nonane 150.8 -53.5 0.7177
C10H22 decane 174.0 -29.7 0.7299
C11H24 undecane 195.8 -25.6 0.7402
C12H26 dodecane 216.3 -9.6 0.7487
C13H28 tridecane 235.4 -5.5 0.7546
… … … … …
C20H42 eicosane 343.0 36.8 0.7886
C21H44 heneicosan
e
356.5 40.5 0.7917
… … … … …
C30H62 tricontane 449.7 65.8 0.8097
16.2.2 – Alkanes - Physical
Properties

• Alkanes are the simplest organic
compounds, comprising of only sp3
hybridized C and H atoms.
• The generic formula of alkanes is CnH2n+2.
• The maximum number of Hydrogen atoms
that can be present for a given number of C
atoms is also CnH2n+2.
• Remind: Isomers are structurally/chemically
distinct compounds with same chemical
formula.
• Simple C1 to C3 alkanes have single, unique
structures (no isomers) while C4 alkane has
two isomers, C5 alkane has 3 isomers and
higher alkanes have more possibilities for
isomeric structures.
n-butane
iso-butane
16.2.3 – Alkanes - Structure

n-pentane isopentane
neopentane
16.2.3 – Alkanes - Structure

• Branched alkanes are more stable than linear alkanes. E.g., 2-
methylpropane is more stable than n-butane.
Property N-butane isobutane
Melting point -139°C -161°C
Boiling point -0.4°C -10.2°C
Heat of formation ΔHf -125.6 kJ/mol (-30.0 kcal/mol) -135.6 kJ/mol (-32.4 kcal/mol)
Heat of combustion ΔHc -2877 kJ/mol (-687 kcal/mol) -2868 kJ/mol (-685 kcal/mol)
Property n-pentane isopentane neopentane
Boiling point 36.1°C 30°C 9.5°C
Heat of formation ΔHf -147 kJ/mol (-35.1 kcal/mol) -154.1 kJ/mol (-36.8 kcal/mol) -168.0 kJ/mol (-40.1 kcal/mol)
Heat of combustion
ΔHc
-3509 kJ/mol (-839 kcal/mol) -3502 kJ/mol (-837 kcal/mol) -3493 kJ/mol (-835 kcal/mol)
16.2.4 – Alkanes - Relative stability

• The alkanes or Paraffins are inert towards acids,
alkalies, oxidizing and reducing agents under
normal conditions.
• The un-reactivity of alkanes can be explained on the
basis of inertness of a sigma bond and non-polar C-
H/C-C bonds.
o In a sigma bond, the electrons are tightly held between
the nuclei. A lot of energy is required to break it.
Moreover, the electrons present in a sigma bond can
neither attack on any electrophile nor a nucleophile can
attack on them. Hence alkanes are less reactive.
o The electronegativity of carbon (2.5) and hydrogen (2.1)
do not differ appreciably and thus bonding electrons
between C-H and C-C are equally shared making them
almost nonpolar. In view of this, the ionic reagents such
as acids, alkalies, oxidizing agents, etc., find no reaction
site in the alkane molecules to which they could attach.
16.2.5 – Alkanes - Reactivity

• However, under suitable condition,
Alkanes give two types of reactions.
o Thermal and catalytic reactions.
o Substituted reactions.
• These reactions take place at higher
temperature or on absorption of
light energy through the formation
of highly reactive free radicals.
16.2.6 – Alkanes - Reactions

Chapter 16 Section 2 - Alkanes

Dr. Hashim Ali
16.2 – Cycloalkanes
Education (FBISE)
Chemistry F.Sc
II

• Another type of molecule containing only sp3 hybridized C and H atoms
connected by single bonds is possible with a ring of 3 or more C atoms.
• These are the cycloalkanes which are fairly common in the world of organic
chemistry, both man-made and natural.
cyclohexane cyclobutane
cyclopropane
16.2.7 – Cycloalkanes - Introduction

cyclohexane cyclobutane cyclopropane
1. According to IUPAC system, cyclo alkanes with one ring are named by prefixing
cyclo to the name of the corresponding alkane having the same number of carbon
atoms as the ring.
16.2.8.1 – Cycloalkanes - Nomenclature - IUPAC rules

Methyl cyclo propane 1,2-dimethyl cyclo butane 1,2-dimethyl cyclo hexane
2. The substituents are numbered in such a way that the sum of numbers is kept
minimum.

cyclo pentene cyclo 1,3-pentadiene
3. If the alicyclic hydrocarbon is unsaturated , the rules applied to alkenes (for double
bond) or alkynes (for triple bond) are used.
4. Multiple bonds are given the lowest possible number.
cyclo 1,4-hexadiene

• Like alkanes, the low polarity of all
the bonds in cycloalkanes means that
the intermolecular forces between
molecules of cycloalkanes are the
very weak induced dipole-dipole
forces, also known as London forces,
which can be easily overcome.
• Like alkanes, cycloalkanes also have
low melting and boiling points.
• But cycloalkanes are slightly more
rigid and stable than alkanes.
16.2.9 – Cycloalkanes - Physical properties

• Cycloalkanes have a generic formula of CnH2n.
• Note that there are two less Hydrogen atoms compared to the analogous alkane.
16.2.10 – Cycloalkanes - Structure

• Very similar reactivity to the closely
related alkanes, which have the same
types of bonds.
• Since C and H atoms have very similar
electronegativities, both the C-H and C-
C bonds are non-polar.
• As a result, cycloalkanes like alkanes
are not very reactive functional group.
• One notable exception are very small
cycloalkanes especially cyclopropane
due to extremely condense molecule
and large bond angle in the ring 
larger repulsive forces between atoms.
60°
cyclopropane
90°
cyclobutane
cyclopentane
108°
cyclohexane
120°
16.2.11 – Cycloalkanes -Reactivity

• What are polar, non-polar and weakly polar compounds?
• What are isomers?
• What are inert compounds?
• What is sigma bond?
• What are intramolecular and intermolecular forces?
16.2.12 – Quick quiz

Chapter 16 Section 2 - Cycloalkanes

Dr. Hashim Ali
16.3 – Radical Substitution Reactions
Education (FBISE)
Chemistry F.Sc
II

• Primary carbons, are carbons attached to one other carbon. (Hydrogens – although
usually 3 in number in this case – are ignored in this terminology, as we shall see).
• Secondary carbons are attached to two other carbons.
• Tertiary carbons are attached to three other carbons.
• Finally, quaternary carbons are attached to four other carbons.
• Radical, also called Free Radical, in chemistry, is a molecule that contains at least one
unpaired electron.
C
H
HH
H
C
H
H
H
C
H
H
H
C
H
H
C
H
H
H
C
H
H
H
C
H
H
C
H
H
H
C
H
H
H
C
H
C HH
H
Methane
(unique)
0 carbons
attached
1 carbon directly
attached
Primary carbon (1°) Secondary carbon (2°)
2 carbon directly
attached
3 carbon directly
attached
Tertiary carbon (3°)
4 carbon directly
attached
Quaternary carbon (4°)
C
H
H
H
C
H
H
H
C
H
C HH
H
C HH
16.3.1 – Radical substitution reactions - Classification of carbons

• When treated with Br2 or Cl2, radical substitution of
R-H generates the alkyl halide R-X and HX.
• Alkane R-H relative reactivity order : tertiary(3°) >
secondary(2°) > primary(1°) methyl.
• Halogen reactivity F2 > Cl2 > Br2 > I2
• Only chlorination and bromination are useful in the
laboratory.
• Bromination is selective for the R-H that gives the
most stable radical.
• Chlorination is less selective.
• Reaction proceeds via a radical chain mechanism
which involves radical intermediates.
• The termination steps are of low probability due to
the low concentration of the radical species meaning
that the chances of them colliding is very low.
The bond dissociation energy, that is the energy
required to break the bond in a homolytic fashion,
generating R. and H.
16.3.2 – Radical substitution reactions - overview

• When reaction mechanisms are being
described, a curly arrow is sometimes used
to show the movement of a pair of electrons.
• The beginning of the arrow shows where the
electron pair starts from and the arrow head
shows where the pair ends up.
• A half-arrow is used to show the movement
of a single electron in reactions involving
free radicals.
• The beginning of the arrow shows where the
single electron starts from and the half
arrow head shows where it ends up.
16.3.3 – Radical substitution reactions - Interesting information

• The mechanism for the bromination of methane is shown below, but the
mechanism for chlorination or with higher alkanes is the same.
• Note that it contains three distinct types of steps, depending on the net
change in the number of radicals that are present.
o Initiation
o Propagation
o Termination
16.3.4 – Radical substitution reactions - Mechanism: Reaction of
methane with bromine

• Heat or UV light cause the weak halogen bond to undergo homolytic cleavage.
• Homolytic cleavage is the breaking of a covalent bond in such a way that each
fragment gets one of the shared electrons.
o The word homolytic comes from the Greek homoios, "equal", and lysis, "loosening".
• Homolytic cleavage produces free radicals — atoms with unpaired valence electrons.
• Heterolytic or ionic fission is the breaking of a covalent bond in such a way that one
atom gets both of the shared electrons.
o The word heterolytic comes from the Greek heteros, "different", and lysis, "loosening".
• Heterolytic cleavage is most likely to occur in polar bonds. And the electrons will
move toward the more electronegative atom.
• This generates two bromine radicals and initiates the chain process.
16.3.4.1 – Radical substitution reactions - Mechanism: Reaction
of methane with bromine - Step 1: initiation

• A bromine radical abstracts a hydrogen to form HBr and a methyl radical.
• The methyl radical abstracts a bromine from another molecule of Br2 to form
the methyl bromide product and another bromine radical.
• The bromine radical produced can itself undergo the first reaction of this
step, creating a repeatable cycle.
16.3.4.2 –Radical substitution reactions - Mechanism: Reaction
of methane with bromine - Step 2 propagation

• Various reactions between the possible pair of radicals allow for the
formation of ethane, Br2, or the product, methyl bromide.
• These reactions remove the radicals and do not perpetuate the cycle.
16.3.4.3 – Radical substitution reactions - Mechanism: Reaction
of methane with bromine - Step 3 termination

Chapter 16 Section 3 - Radical substitution reactions

Dr. Hashim Ali
16.4 – Oxidation and reduction in organic compounds
Education (FBISE)
Chemistry F.Sc
II

• Oxidation  loss of electrons
• Reduction  gain of electrons
• Species that loses electron is said to be oxidized and is
known as reducing agent.
• Species that gains electrons is said to be reduced and is
known as oxidizing agent.
• Hence the redox reactions are the transfer of electrons
between two species (atoms, ions or molecules).
• Oxidation, [O], and reduction, [R], are opposites and
both must occur simultaneously , hence redox
reactions.
• Organic chemists will normally describe a reaction as
either oxidation or reduction, depending on the rate of
the major organic component.
16.4.1 – Oxidation and reduction in organic compounds -
Introduction

• We consider that an atom is considered to be reduced
if it gains electrons and to be oxidized if it loses
electrons. (Remember: Oil vs Rig)
• Oxidation is any reaction that leads to an increased
oxidation state, e.g., +1 to +3, while reduction is any
reaction that leads to a decreased oxidation state, e.g.,
+1 to -1.
• In practice, oxidation occurs when a bond between a
carbon and an atom that is less electronegative than
carbon is replaced by one that is more electronegative
than carbon (More C-O bonds and less C-H bonds).
• Reduction occurs when a bond between a carbon and
an atom that is more electronegative than carbon is
replaced by one that is less electronegative than
carbon (more C-H bonds and less C-O bonds as
example).
16.4.2 – Oxidation and reduction in organic compounds - Redox
reactions in organic chemistry

Chapter 16 Section 4 - Redox in organic compounds

Dr. Hashim Ali
16.5 – Alkenes
Education (FBISE)
Chemistry F.Sc
II

1. The longest continuous chain containing double bond is selected as parent chain.
The ending ‘ane’ is replaced by ‘ene’.
Which is the longest chain including
double bonded C here? Is it with 8C?
6C is correct!
Name starts with
“hex” (6C).
Double bond in
main chain 
Name ends with
“ene”.
Main chain name
 hexene
H
C C C C C
C
C
C
CH H
H H
H
HH
HH
H
H
H
H H
H
HH
C C C C
CH
H
H
HH
H
H
H
C
H
H
H C
C
H
H
H
H
H
H
C H
16.5.1.1 – Alkenes - Nomenclature (IUPAC system) - Rules

2. The chain is numbered in such a manner as to give minimum to the doubly bonded
C-atoms.
Which is the first carbon in the main chain? The
one on the left or the one on the right?
Starting from left the
double bonded carbon
is at C5
In this case, the
chain is numbered
to select double
bond at C1, i.e.,
from right to left!
Starting from right the
double bonded carbon
is at C1
C C C C
CH
H
HH
H
H
H
C
H
H
H C
H
H
C
H
HH
C HH
H

3. The position of double bond is indicated by the lower number of C-atom.
In this case, the
double bond is
positioned at 1.
C C C C
CH
H
HH
H
H
H
C
H
H
H C
H
H
C
H
HH
C HH
H
Which is the position of double bond?
The two carbons
connected by double
bond are C1 and C2.

4. The lower number of C-atom is placed before the name of parent alkene. The
position of the double bond is mentioned with the ene.
The name of the
parent chain, with
position of double
bond, is hex-1-ene.
C C C C
CH
H
HH
H
H
H
C
H
H
H C
H
H
C
H
HH
C HH
H
Given that the double bond is at C1, what is the
position before main alkene chain?

5. The presence of more than one double bond is indicated by the suffix ‘-di ene’ for
two double bonds, ‘-tri ene’ for three double bonds and so on.
The name of the
parent chain, with
two double bonds,
is hex-1,3-diene.
C C C C
CH
HH
H
H
C
H
H
H C
H
H
C
H
HH
C HH
H
If two double bonds are present in the alkene
chain, what is the name of alkene chain?
There are two double
bonds present at C1
and C3.

6. Alkyl groups are indicated by the methods mentioned in alkanes.
The name of the
complete alkene is
2-propyl-1,3-
hexadiene.
C C C C
CH
HH
H
H
C
H
H
H C
H
H
C
H
HH
C HH
H
Side chains are named as?
A propyl side chain is
present in this alkene
at C2.

• Name each of the following Olefins according to IUPAC System.
C
H
H
C
H
H
C
H
H
C
H
C
H
H
H
C
H
H
C
H
C
H
H
H C
H
C
H
H
C
H
H
H C
H
H
CC
H
H
H
H
C
H
HH
16.5.1.2 – Alkenes - Nomenclature - Activity
ethene propene
3-hexene 2-methyl-propene

• Name each of the following Olefins according to IUPAC System.
C
H
C
H
C
H
H
C
H
C
H
H
H CCC
H
H
C
H
C
H
H
C
H
H
H
CCC
H
H
H
C
H
HH
C HH
C
H
H
H
H
CCC
H
H
H
C
H
HH
C HH
C
H
H
H
C
H
H
HC
C
H
HH
C HH
H
1,3-penta-diene 1,2,3-hexa-triene
2,3-dimethyl-2-butene 2,2,5,5-tetramethyl-3-hexene

• Name the compounds (a) and (b) by IUPAC nomenclature, and
compounds (c) and (d) by their trivial/common names, and (e) by its
derived name.
3-methyl-1,2-butadiene 2-ethyl-1,3-butadiene
β-ethane-⍺,β-butadiene
1-propene
ƛ-heptene

• Write structural formulas for the following compounds and discuss the
geometric isomerism in each case.
a) 1,3-buta diene.
b) 1,2-penta diene.
c) 2,4-hexa diene.
d) 2-methyl-1,3-buta diene.
e) 3-methyl-1,3 pentadiene.

• 3 factors influence the stability of alkenes.
o Degree of substitution: More highly
alkylated alkenes are more stable, so stability
of alkene decreases in the following order.
tetra > tri > di > mono-substituted
o Steriochemistry: Trans alkenes are more
stable than cis-alkenes due to reduced stearic
interaction when R groups are on opposite
sides of the double bond.
o Conjugation: Conjugated alkenes are
alkenes with adjacent double bonds. Isolated
alkenes are alkenes without adjacent double
bonds. Conjugated alkenes are more stable
than isolated alkenes.
• 1,3-pentadiene is more stable than 1,4-
pentadiene.
16.5.2 – Alkenes - Relative stability

• The carbon atom linked through π-bond are
sp2 hybridized.
• Therefore, each atom carries three sp2-
hybrids and one p-orbital.
• The p-orbital overlap to form π-bond.
• The hybrid orbitals form σ-bond due to
linear overlap.
• The carbon-carbon distance in ethene is
shorter (1.34Å) than the C-C bond distance
of ethane (1.54Å) due to increased electron
density between carbon atoms.
• Carbon atoms are coplanar.
• The rotation of one C-atom with respect to
other is restricted, which results in cis-trans
isomerism in alkene.
16.5.3 – Alkenes - Structure

• Alkenes can be prepared through the following methods.
o Dehydration of alcohols.
o Dehydrohalogenation of alkyl halides.
o Reduction of carboxylates through Kolbe’s electrolytic method.
o Dehalogenation of vicinal dihalides.
o Hydration of alkynes.
16.5.4 – Alkenes - Preparation

• Dehydration is the process of removal
of water molecule from a compound.
• Dehydration is the opposite of
hydration (adding a water molecule).
• When vapors of alcohol are passed over
heated alumina, dehydration takes
place with the formation of alkene.
• Phosphorus pentoxide (P4O10),
concentrated sulphuric acid (H2SO4) or
phosphoric acid (H3PO4) can also be
used for dehydration.
• The ease of dehydration of various
alcohols is in the order
o Tertiary alcohol > secondary alcohol >
primary alcohol
H
C
H
R C
OH
H
H
H
CR C
H
H
+ H2O
Al2O3
340°-450°
Primary alcohol alkene
75% H2SO4
140°-170°
R CH
OH
+ H2O
CH3CH2 R CH CH2CH2
Secondary alcohol alkene
Tertiary alcohol alkene
20% H2SO4
140°-170°
R C
OH
+ H2OCH3 R C CH2
CH3 CH3
16.5.4.1 – Alkenes - Preparation - By dehydration of alcohol

• Dehydrohalogenation is the process of removal of hydrogen halide (HX) from alkyl
halides.
• Alkyl halides on heating with alcohol potassium hydroxide undergo dehydrohalogenation
and form alkenes.
+ KOH
H
C
H
R C
X
H
H
H
CR C
H
H
+ KX + H2O
alkeneAlkyl halide
H
C
H
C
Br
H
H + KOHH
H
C C
H
H
+ KBr + H2O
H
etheneethyl bromide
+ KOH
H
C
H
CH3 C
Br
H
H
H
CCH3 C
H
H
+ KBr + H2O
propenepropyl bromide
16.5.4.2 – Alkenes - Preparation - By dehydrohalogenation of
alkyl halides

• Sodium or potassium salt of a dicarboxylic acid on electrolysis gives an alkene.
• When an aqueous solution of sodium or potassium salt of a dibasic acid is electrolyzed, an
alkene is produced.
• Such an electrolysis, where a compound containing caroxylate is electrolyzed, is known as
Kolbe’s electrolysis.
• For example, electrolysis of sodium succinate (a sodium salt of butanedioic acid) gives ethene.
16.5.4.3 – Alkenes - Preparation - By reduction of carboxylates

• Vicinal dihaloalkanes are those dihalogen derivatives of alkanes in which two halogen
atoms are on the adjacent carbon atoms.
• Alkenes can be obtained from vicinal dihaloalkanes by dehalogenation.
• When such a dihaloalkane is heated with zinc in methanol, an alkene is formed.
+ Zn(methanol)
H
C
B
r
CH3 C
Br
H
H + ZnBr2
H
CCH3 C
H
H
propenevicinal propyl
dibromide
16.5.4.4 – Alkenes - Preparation - By dehalogenation of vicinal
dihalides
+ Zn(methanol)
H
C
X
R C
X
H
H + ZnBr2
H
CR C
H
H
alkenevicinal alkyl
dihalide

• Alkynes react with hydrogen gas in the presence of suitable catalysts like finely divided Nickel (Ni), Platinum (Pt) or
Palladium (Pd).
• In the first step, called partial hydrogenation, alkenes are formed, which then take up another molecule of hydrogen to
form an alkane.
• On the other hand, if you utilize Lindlar’s catalyst (a mixture of Pd, CaCO3, Pb salts and quinoline) or nickel boride in
hydrogenation, you can partially hydrogenate alkyne to cis-alkene only, i.e., a stereospecific reaction.
16.5.4.5 – Alkenes - Preparation - By hydrogenation of alkynes
Ethyne Ethene Ethane
H C C H + H2
H
C C
H H
H
200°CNi, 200°CNi,
+ H2
C
H
H
H
C
H
H
H
Propyne Propene
C C H + H2
H
C C
H
H
200°CNi,
Propane
200°CNi,
+ H2
C
H
H
H
CH
H
H
C
H
H
C
H
H
H
C
H
H
H

• There is a relatively diffuse region of high electron density in alkenes as
compared to alkanes.
• This is due to the π-bonds in alkenes.
• Since an ethene π-bond is weaker than σ-bond, it requires less energy to
break a π-bond.
• Hence the reactions of alkenes involve weaker π-bond and electrophilic
addition occurs.
• It involves the change of a π-bond to σ-bond through addition reactions.
16.5.5 – Alkenes - Reactivity

16.5.6.0 – Alkenes - Reactions - Overview
• Alkenes can undergo the following reactions.
o Hydrogenation.
o Hydrohalogenation.
o Hydration
o Halogenation
o Halohydration
o Epoxidation
o Ozonolysis followed by reduction of ozonide
o Polymerization

• A process in which a molecule of
hydrogen is added to an alkene in the
presence of a catalyst and at moderate
pressure (1-5 atm) to give a saturated
compound is known as catalytic
hydrogenation.
• It is a highly exothermic process.
• The amount of heat evolved when one
mole of an alkene is hydrogenated is
called Heat of Hydrogenation.
• The heat of hydrogenation of most
alkenes is about 120 kJ mole-1 for each
double bond present in a molecule.
• The catalysts employed are platinum(Pt),
palladium(Pd) and Raney Nickel.
16.5.6.1 – Alkenes - Reactions - Hydrogenation

• Raney Nickel is prepared by
treating a Ni-Al alloy with caustic
soda.
• Catalytic hydrogenation of alkenes
is used in the laboratory as well as
in industry.
• In industry, it is used for the
manufacture of vegetable ghee
from vegetable oils.
• In industry, it is used as a synthetic
as well as an analytical tool.
Ni-Al + NaOH + H2O Ni + NaAlO2 + 3/2H2
+ H2
Ni
iso-pentane3-methyl-1-butene
Ni
cyclohexanebenzene
16.5.6.1 – Alkenes - Reactions - Hydrogenation

• Alkenes react with aqueous solution
of halogen acid to form alkyl halides.
• The order of reactivity of halogen
acids is HI > HBr > HCl.
• The addition of a hydrogen halide to
an alkene takes place in two steps.
o Alkenes accepts the proton of
hydrogen to form a carbocation.
o The carbocation then reacts with the
halide ion.
• Notice that ethylene is a symmetrical
alkene: it has the same substituents
(two hydrogen atoms) on either end
of the C=C double bond.
ethene Ethyl chloride
carbocation Ethyl bromide
16.5.6.2 – Alkenes - Reactions - Hydrohalogenation

• As such, it doesn't matter which carbon the
"H" ends up attached to and which carbon
the "Cl" ends attached to in this addition
reaction -- the product is still the same,
ethyl chloride.
• But what about cases where the alkene is
not symmetrical? In those cases, two
different "regioisomers" could be formed.
For example, see the addition of HCl to
propene.
• In fact, the only product that is formed in
the reaction of propene with HCl is
isopropyl chloride.
• None of the regioisomer (n-propyl chloride)
is formed.
• As such, we call this a "regiospecific"
reaction, since only one of several possible
regioisomers was formed.
+
+
ethene Ethyl chloride
propene 1-propyl chloride
2-propyl chloride
+
+
16.5.6.2 – Alkenes - Reactions - Hydrohalogenation (ctd)

• In these sorts of cases, when the degree of substitution on
either end of the double is not identical, we can
use "Markovnikov's rule" to predict which
regioisomer will form predominantly, if not exclusively,
in the addition of HX to an alkene.
• In simplest terms, Markovnikov's rule states that:
o "In the addition of HX to an alkene, the acid hydrogen
(H) becomes attached to the carbon with fewer alkyl
substituents, and the halide (X) group becomes
attached to the carbon with more alkyl substituents."
• Another way of saying this is that "the hydrogen-rich
atom becomes hydrogen-richer," i.e., the hydrogen of HX
gets attached to the carbon that had more hydrogens in
the first place.
• Because there are only three possible degrees of alkyl
substitution on either end of a C=C double bond -- no
alkyl groups (and two hydrogens), one alkyl group (and
one hydrogen), or two alkyl groups (and no hydrogens) --
it should be a simple matter to apply Markovnikov's rule.
2-methyl-propene 2-chloro-2-methylpropane
1-chloro-2-methylpropane
2-methyl-2-pentene 2-chloro-2-methylpentane
3-chloro-2-methylpentane
16.5.6.2 – Alkenes - Reactions - Hydrohalogenation (ctd)

• Addition of water is called hydration.
• Some reactive alkenes react with water in the presence of
suitable substances such as acids etc. to form alcohol.
• It is possible as alkenes are soluble in cold concentrated
sulfuric acid.
• They react by addition to form alkyl hydrogen sulphate.
• These alkyl hydrogen sulphates on boiling with water
decompose to give corresponding alcohols.
• The carbon-carbon double bond in alkenes such as
ethene react with concentrated sulfuric acid.
• It includes the conversion of the product into an alcohol.
• Alkenes react with concentrated sulfuric acid in the cold
to produce alkyl hydrogensulfates.
• Ethene reacts to give ethyl hydrogensulfate.
• The structure of the product molecule is sometimes
written as CH3CH2HSO4, but the version in the equation
is better because it shows how all the atoms are linked up.
You may also find it written as CH3CH2OSO3H.
• Reaction of propene with H2SO4 is typical of the reaction
with unsymmetrical alkenes.
• An unsymmetrical alkene has different groups at either
end of the carbon-carbon double bond.
• If sulfuric acid adds to an unsymmetrical alkene like
propene, there are two possible ways it could add.
• You could end up with one of two products depending on
which carbon atom the hydrogen attaches itself to.
• However, in practice, there is only one major product, due
to Markownikov’s Rule.
ethene ethyl hydrogensulfate
100°C
ethanol
propan-2-ol
propene propyl hydrogensulfate
16.5.6.3 – Alkenes - Reactions - Addition of sulphuric acid +
hydration

• The alkenes react with halogen in an inert solvent
like carbon tetrachloride at room temperature to
give vicinaldihalides or 1-2 dihalogenated products.
• Br2 and Cl2 are effective electrophilic reagents.
• Flourine is too reactive to control the reaction.
• Iodine does not react.
• Mechanism:
o A bromine molecule becomes polarized as it
approaches the alkene.
o This polarized bromine molecule transfers a positive
bromine atom to the alkene resulting in formation of
a bromonium ion.
o The nucleophilic bromide ion then attacks on the
carbon of the bromonium ions to form
vicinaldihalides and the reddish brown color of
bromine is discharged.
• This test is applied for the presence of a double bond
in a molecule.
16.5.6.4 – Alkenes - Reactions - Halogenation
Vicinal dihalideethene
Vicinal dibromideethene

• Addition of hypohalous acid (HOX)
is called halohydration.
• Alkenes react with hypohalous acid
to give halohydrin.
• In this reaction, molecules of the
solvent become reactants too.
Bromohydrin/
1-bromo-ethan-2-ol
ethene
CH2=CH2 + Br2 + H2O CH2–CH2–OH + HBr
Br
16.5.6.5 – Alkenes - Reactions - Halohydration

• Epoxidation is the formation of epoxides, which is a cyclic ether with a three-
atom ring. .
• Peracids such as per oxyacetic acid or phenol benzoic acid react with alkenes
to form epoxides.
A generic epoxide
CH3–CH=CH2 + C6H6C–O–O–H CH3–CH–CH2 + C6H6C–O–O–H
O
O
Epoxy propane Benzoic acidpropene Phenol benzoic
acid
CHCl3
16.5.6.6 – Alkenes - Reactions - Epoxidation

• Ozone (O3) reacts vigorously with alkenes to form unstable molozonide.
• The unstable molozonide rearranges spontaneously to form an ozonide.
Molozonide
(unstable)
ethene
CH2=CH2 + O3
H–C–C–H
O
H H
O
O
C C
H
O
O O H
H H
Ozonide
rearrangement
16.5.6.7 – Alkenes - Reactions - Ozonolysis

• Ozonides are unstable compounds and are reduced directly on treatment
with zinc and H2O.
• The reduction produces carbonyl compounds (aldehydes or ketones).
• Ozonolysis is used to locate the position of a double bond in an alkene.
• The C-atom’s double bond is changed to carbonyl group.
16.5.6.7 – Alkenes - Reactions - Reduction of ozonide

• Polymerization is a process in which
small organic molecules (which are
monomers) combine together to
form a large molecule.
• The substances so produced are
called polymers.
• Ethene polymerizes to polythene at
400°C at a pressure of 100 atm.
• A good quality polythene is obtained
when ethane is polymerized in the
presence of aluminium triethyl
Al(C2H5)3 and titanium tetrachloride
TiCl4.
Polyethene
(polyethylene)
Ethene
monomer
nCH2=CH2 –––(CH2–CH2)n–––
Pressure = 100 atm
Traces of O2 (0.1%)
16.5.6.8 – Alkenes - Reactions - Polymerization

• Examples of natural and synthetic polymers
Polymer monomer Where you find it
Natural Protein Amino acid Wool, silk, muscle etc.
Starch Glucose Potato, wheat etc.
Cellulose Glucose Paper, wood, dietary fibre,
chromosomes.
DNA Nucleotide genes
Synthetic Poly (ethane) Ethane Bags, washing-up bowls etc.
Poly (chloroethane)
(PVC)
Chloroethane Fabric coatings, electrical
insulation, toys.
Poly (phenylethene) or
polystyrene
Phenylethene/ethenyl
benzene
Expanded polystyrene
polyester Ethane-,2-diol and
benzene- 1,2-
dicarboxylic acid
Skirts, shirts and trousers.
16.5.6.9 – Alkenes - Reactions - Interesting information

• The word “conjugation” is derived from a Latin word that means “to link together”.
• In organic chemistry, conjugation describes the situation when p- systems are linked together.
• There are two types of p- systems.
o An isolated p- system exists only between a single pair of adjacent atoms (e.g., C=C).
o An extended p- system exists over a longer series of atoms (e.g., C=C–C=C or C=C–C=O etc.). An extended
p-system results in an extension of the chemical reactivity.
• The fundamental requirement for the existence of a conjugated system is revealed if one
considers the orbital involved in the bonding within the p- system.
• A conjugated system requires that there is continuous array of “p” orbitals that can align to
produce a bonding overlap along the whole system.
• If a position in the chain does not provide a “p” orbital OR if the geometry prevents the correct
alignment, then the conjugation is broken at that point.
• You can investigate these differences by studying the following examples. Pay particular
attention to the “p” orbitals.
• The result of conjugation is that there are extra p bonding interactions between the adjacent p
systems that results in an overall stabilization of the system.
16.5.7 – Alkenes - Conjugation

System P system type
ethene isolated
propene isolated
1,2-
propadiene
(allene)
cumulated
1,3-
butadiene
conjugated
1,3-
pentadiene
conjugated
System P system type
1,4-
pentadiene
isolated
1,3-
cyclopenta
diene
conjugated
1,3-
cyclohexadi
ene
conjugated
1,4-
cyclohexadi
ene
isolated
benzene conjugated
16.5.7 – Alkenes - Conjugation

• What is conjugation?
• What are conjugated alkenes?
• What is pi bond?
• What are s and p orbitals?
• What is dehydration?
• What is dehydrohalogenation?
• What is hydrogenation?
• What is ozonolysis?
• What is Markownikov’s Rule?
• What is electrophilic reagent?
• What is nucleophiles reagent?
• What is halohydration?
• What is polymerization?
16.5.8 – Alkenes - Quick quiz

• When 2-Methyl propene reacts with HCl:
o What are the structures of the two possible intermediate carbocations?
o Which of the two ions is the most stable?
o What will be the major product of the reaction between 2-Methyl propene and
HCl?
16.5.8 – Alkenes - Quick quiz

Chapter 16 Section 5 - Alkenes

Dr. Hashim Ali
16.6 – Isomers and isomerism
Education (FBISE)
Chemistry F.Sc
II

• Compounds that have the same
molecular formula but different
chemical structures are called isomers
and the phenomenon is called
isomerism.
• Since isomers have the same molecular
formula, the difference in their
properties must be due to different
modes of combination or arrangement
of atoms within the molecule.
• There are two main types of isomerism:
o Structural isomerism.
o Stereoisomerism
16.6.1 – Isomers and isomerism - Types of isomerism
Isomerism
Structural isomerism Stereo-isomerism
Chain isomerism
Position isomerism
Functional group
isomerism
Metamerism
Tautomerism
Optical isomers
Geometrical isomers

• When the isomerism is due to
difference in the arrangement of
atoms within the molecule, without
any reference to space, the
phenomenon is called structural
isomerism.
• Structural isomers are compounds
that have the same molecular formula
but different structural formulas.
• Structural isomerism is of five types.
o Chain isomerism.
o Position isomerism.
o Functional group isomerism.
o Metamerism.
o Tautomerism.
16.6.1.1 – Isomers and isomerism - Types of isomerism -
Structural isomerism
Isomerism
Structural isomerism Stereo-isomerism

• Chain isomers have the same
molecular formula but differ in
order in which the carbon atoms
are bonded to each other.
• For example
o N-Butane and iso-butane C4H10.
o 2-Methylbutane and 2,2-
Dimethylpropane C5H12.
n-butane iso-butane
2-methylbutane 2,2-dimethylpropane
16.6.1.1.1 – Isomers and isomerism - Types of isomerism -
Structural isomerism - Chain isomerism

• Position isomers have the same
molecular formula but differ in the
position of a functional group on the
carbon chain.
• For example
o Bromobutane and 2-Bromobutane
C3H7Br.
o Propyl alcohol and Isopropyl alcohol.
bromobutane 2-bromobutane
Propyl alcohol iso-propyl alcohol
Structural isomerism - Position isomerism

• Functional isomers have the
same molecular formula but
different functional groups.
• For example
o Acetone and Propionaldehyde.
o Acetic acid and Methyl formate.
acetone Propionaldehyde
Acetic acid Methyl formate
Structural isomerism - Functional group isomerism

• This type of isomerism is due to the
unequal distribution of carbon atoms
on either side of the functional group.
• Members belong to the same
homologous series.
• For example:
o 2-pentanone and 3-pentanone.
o Di-ethyl ether and methyl propyl ether.
2-pentanone
3-pentanone
diethyl ether
methyl propyl ether
Structural isomerism - Metamerism

• This type of isomerism is due to the
simultaneous existence of a compound in
two forms in equilibrium with one
another.
• It involves the shifting of position of
proton.
• Such a hydrogen atom is known as
“mobile” hydrogen.
• For example:
o Keto-Form and Enol-Form of acetone.
Keto-form of acetone
Enol-form of acetone
Structural isomerism - Tautomerism

• When isomerism is caused by the different arrangements/orientation of
atoms or groups in space, the phenomenon is called Stereoisomerism.
• The stereoisomers have the same structural formulas but differ in
arrangement of atoms in space or in configuration (which refers to the three-
dimensional arrangement of atoms that characterize a particular
compound).
• Stereoisomerism is of two types:
o Optical isomerism.
o Geometrical or Cis-Trans isomerism.
16.6.1.2 – Isomers and isomerism - Types of isomerism -
Stereoisomerism

• A carbon atom which is bonded to four
different groups is called an asymmetric
carbon atom.
• The term asymmetric carbon atom is
rather misleading.
• It only means that a carbon atom is
bonded to four different groups and that
a molecule of this type lacks plane of
symmetry. Such a molecule is called
asymmetric (Latin a = without), that is
without symmetry.
• Presently the term Dissymmetric or
Chiral is often used for asymmetric
molecules.
16.6.1.2.0.1 – Isomers and isomerism - Types of isomerism -
Stereoisomerism - Background - Chiral molecule

• A plane which divides an object into two symmetrical
halves, is said to be the plane of symmetry.
• For example a person or a hat has a plane of
symmetry.
• A person’s hand or gloves lack plane of symmetry.
• An object lacking a plane of symmetry is called
dissymmetric or chiral.
• A symmetric object is referred to as achiral.
• A dissymmetric object can not be superimposed on
its mirror image.
• A left hand for example does not possess a plane of
symmetry, and its mirror image is not another left
hand but a right hand.
• The two are not identical because they can not be
superimposed.
• If we were to lay one hand on top of the other, the
fingers and the thumb would clash.
• Chiral molecules has at least one asymmetric center
and does not have a plane of symmetry.
• Achiral molecule has a plane of symmetry.
Stereoisomerism - Background - plane of symmetry

• Light from ordinary electric lamp is
composed of waves vibrating in many
different planes.
• When it is passed through Nicole prism
(made of calcite, CaCO3) or polaroid lens,
light is found to vibrate in only one plane,
and is said to be plane-polarized or simply
polarized.
• The diagrams illustrate the vibrations of
ordinary and polarized light from a beam
propagated perpendicularly to the plane or
paper.
• Solutions of some organic compounds have
the ability to rotate the plane of polarized
light.
• These compounds are said to be optically
active compounds.
• This property of a compound is called
optical activity.
Stereoisomerism - Background - Optical activity
Sample polarimeter

• Optical activity in a compound is detected and measured by means of a polarimeter.
• When a solution of a known concentration of an optically active material is placed inside polarimeter, the beam of
polarized light is rotated through a certain number of degrees, either to the right (clockwise) or to the left (anti-
clockwise).
• The compound, which rotates the plane of polarized light to the right (clockwise), is said to be Dextrorotatory. It is
indicated by the sign (+).
• The compound, which rotates the plane of polarized light to the left (anticlockwise), is said to be Laevorotatory. It is
indicated by the sign (-).
• The magnitude of rotation in degrees is referred as observed rotation, alpha.
Stereoisomerism - Background - Optical activity

• An optically active compound can exist in two isomeric forms, which rotate the plane of polarized light in
opposite directions.
• These are called optical isomers and the phenomenon is called optical isomerism.
• The isomer, which rotates the plane of polarized light to the right (clockwise direction), is known as
Dextrorotatory isomer or (+) isomer.
• The isomer, which rotates the plane of polarized light to the left (anticlockwise direction), is known as
Laevorotatory isomer or (-) isomer.
Stereoisomerism - Optical isomerism

Stereoisomerism - Optical isomerism

• Lactic acid (2-Hydroxypropanoic acid) is an optical
isomer.
• It contains one asymmetric carbon atom.
• Two non-identical three-dimensional structures are
possible for Lactic acid, which can not be superimposed
on each other.
• On the mirror image of the other, such non-
superimposable mirror image forms are optical
isomers and are called enantiomers.
• Thus, three forms of Lactic acid are known - two are
optically active and one is optically inactive.
o The (+) Lactic acid or Dextrorotatory Lactic Acid isomer
rotates the plane of polarized light to the right (clockwise
direction).
o The (-) Lactic acid or Laevorotatory Lactic acid isomer rotates
the plane of polarized light to the left (anti-clockwise
direction).
o The (±) Lactic acid isomer, which is an equimolar mixture of
(+) and (-) forms (racemic mixture), does not rotate the plane
of polarized light and is optically inactive.
L (-) lactic acid
C
O
O H
C
H
H
C HOH
H
D (+) lactic acid
C
O
O H
C
H
H
CH O H
H
Stereoisomerism - Optical isomerism - Lactic acid

• Tartaric acid (2,3-Dihydrobutanedioic acid) is an optical
isomer.
• It contains two asymmetric carbon atoms.
• Four forms of tartaric acid are known, two of which are
optically active and two are optically inactive.
• The two optically active forms are related to each other as
an object to its mirror image and are thus enantiomers.
• Thus, the tartaric acid molecule has four possible
stereoisomers: two are optically active and two are
optically inactive.
o The (+) Tartaric acid or Dextrorotatory Tartaric Acid isomer
rotates the plane of polarized light to the right (clockwise
direction).
o The (-) Tartaric acid or Laevorotatory Tartaric acid isomer
rotates the plane of polarized light to the left (anti-clockwise
direction).
o One (±) Tartaric acid isomer is called meso form and
superimposable with its mirror image. Thus the chirality of the
two asymmetric carbon atoms cancels. Both isomer and its
mirror image do not rotate the plane of polarized light and is
optically inactive.
o Another (±) Tartaric acid isomer, which is an equimolar mixture
of (+) and (-) forms (racemic mixture), does not rotate the plane
of polarized light and is optically inactive.
C
O
O H
C
O
O H
C
C H
O HH
OH
D (+) tartaric acid
C
O
O H
C
O
O H
C
C
H
O HH
OH
L (-) tartaric acid
C
O
O H
C
O
O H
C
CH
O HH
O H
H
H
C
O
O
C
O
O
C
C H
OH H
OH
=
DL (±) tartaric acid
Stereoisomerism - Optical isomers - Tartaric acid

• Geometrical isomerism (also
called cis-trans isomerism)
results from a restriction in
rotation about double bonds, or
about single bonds in cyclic
compounds.
Stereoisomerism - Geometrical isomers

• The carbon atoms of the carbon-carbon double bond are sp2
hybridized.
• The carbon-carbon double bond consists of a sigma bond and a
pi bond.
• The sigma bond is formed by the overlap of sp2 hybrid orbitals.
• The pi bond is formed by the overlap of p orbitals.
• The presence of the pi bond locks each molecule in one position.
• The two carbon atoms of the C=C bond and the four atoms that
are attached to them lie in one plane and their position in space
are fixed.
• Rotation around the C=C bond is not possible because rotation
would break the pi bond.
o This restriction of rotation about the carbon-carbon double bond is
responsible for the geometrical isomerism in alkenes.
o A popular analogy is based upon two boards and two nail. Driving
one nail through two boards will not prevent free rotation of the
two boards but once a second nail is used, the boards can not be
freely rotated.
Stereoisomerism - Geometrical isomers - alkenes

• Consider the case of 2-butene, which exists in two special
arrangements.
• These two compounds are referred to as geometrical
isomers and are distinguished from each other by the
terms cis and trans.
o The cis isomer is one in which two similar groups are on the
same side of the double bond.
o The trans isomer is that in which two similar groups are on
opposite sides of the double bond.
• Consequently, this type of isomerism is called cis-trans
isomers.
o Cis isomers are very common in nature while trans isomers are
rare and usually artificially produced.
o Hydrogen atoms are on the same side of cis isomers but on
different sides in trans isomers.
o Cis isomers are loosely packed while trans isomers can be tightly
packed (higher density and lesser space). This leads to higher
melting and boiling points for trans isomers than cis isomers.
• Geometrical isomers are stereoisomers because they have
the same structural formula but different spatial
arrangement of atoms.

• The conversion of cis-isomer into trans-isomer or vice
versa is possible only if either isomer is heated to a high
temperature or absorbs light.
• The heat supplies the energy (about 62 Kcol/mole) to
break the pi bond so that rotation about sigma bond
becomes possible.
• Upon cooling, the reformation of the pi bond can take
place in two way giving mixture of trans-2-butene plus
cis-2-butene.
• The trans isomers are more stable than the
corresponding cis isomers.
o This is because in the cis isomer, the bulky groups are on the
same side of the double bond.
o The stearic repulsion of groups makes the cis isomers less
stable than the trans isomer in which the bulky groups are far
(they are on the opposite sides of the double bond).
• The geometrical isomers have different physical and
chemical properties.
• They can be separated by conventional physical
techniques like fractional distillation and gas
chromatography, etc.
cis-2-butene trans-2-butene
Less polar More polar

• All alkenes do not show geometrical isomerism.
• Geometrical isomerism is possible only when each
double bonded carbon atoms is attached to
different atoms of groups.
• The following examples illustrate this condition for
the existence of geometrical isomers.
o Example 1: propene (CH2=CHCH3).
• No geometric isomers are possible for
propene.
• This is because one of double bonded
carbons has two identical groups (H
atoms) attached to it.
o Example 2: 3-Hexene (CH3CH2CH=CHCH2CH3)
• This is because each double bonded
carbon atom is attached to two different
groups (CH3CH2 and H).
o Example 3: Butenedoic acid (HOOC-CH-CH-
COOH)
• Geometrical isomers are possible
because each double bonded carbon
atoms has two different groups attach to
it (H and COOH).
propene
C H
C
H
H
C H
H
H
3-hexene
C H
C
H
H
C H
H
H
C
C
C
H
H
H
H
H
H
C H
C
H
HC H
H
H
C
C
C
H
H
H
H
H
H
cis-3-hexene
C H
C
H
H C H
H
H
C
C
C
H
H
H
H
H
H
trans-3-
hexene
or
C
O
C
OO
H
C
C
H
H
O
H
Butanedoic acid
C
O
C
OO
H
C
C
H
H
O
H
cis-butanedoic
acid
or
C
O
C
C
H
H
C
OO
H
O
H
trans-butanedoic
acid

• Geometrical isomerism is also possible in
cyclic compounds.
• There can be no rotation about carbon-
carbon single bonds forming a ring
because rotation would break the bonds
and break ring.
• For example, 1,2-dimethylcyclopropane
exists in two isomeric forms.
o In cis-1,2-dimethylcyclopropane, the two
methyl groups are on the same side of ring.
o In trans-1,2-dimethylcyclopropane, the two
methyl groups are on the opposite side.
o As mentioned earlier, a requirement for
geometrical isomerism in cyclic compounds
is that there must be at least two other
groups besides hydrogen on the ring and
both should be on different ring carbon
atoms. For example, no geometric
isomerism is possible for 1,1-
dimethylcyclopropane.
cis-1,2-dimethylcyclopropane
trans-1,2-dimethylcyclopropane
1,1-dimethylcyclopropane
Stereoisomerism - Geometrical isomers - cyclic compounds

• Define or explain the following terms.
o Structural isomerism.
o Stereoisomerism
o Geometrical isomerism
o Optical isomerism
o Asymmetric carbon.
o Chiral molecule.
• State the necessary condition for a compound to show geometrical isomerism.
Illustrate your answer with examples.
• Which of the following compounds show geometrical isomerism?
o 2- Butene.
o 2-Methyl-Butene.
o 2-Pentene.
o 1,2-Dichloropropane.
16.6.2 – Isomers and isomerism - Quick quiz

• Which of the following compounds show isomerism.
o CH3-CH=CH2.
o CH3CH2CH2CH=CHCH3.
• What is optical activity? How is it measured?
• Write a note on optical isomerism of Lactic acid.
• Write a note on optical isomerism of Tartaric acid.
• An acid of formula C5H10O2 is optically active. What is its structure?
• How does cis-isomer convert into trans-siomer?
• The trans-isomer is more stable. Why?
16.6.2 – Isomers and isomerism - Quick quiz

Chapter 16 Section 6 - Isomerism

Dr. Hashim Ali
16.7 – Alkynes
Education (FBISE)
Chemistry F.Sc
II

• The parent hydrocarbon is the continuous
chain containing triple bond.
• The ending ‘ane’ of the alkane is changed
to ‘yne’.
• The main chain is numbered starting
from the terminal carbon nearer to the
triple bond.
• Triple bond is given the number of the
lower carbon atom attached to the triple
bond separated by hyphen.
• If two or more triple bonds are there in
the molecule, use the prefixes di-, tri- etc.
• Alkyl groups are indicated by the
methods described while naming alkanes.
C C C C
H
C
C
C
C
C
HHH
H
H
HH
H
H
H
H
H
H
H
H
C C C C
H
C
C
C
C
C
HHH
H
H
HH
H
H
H
H
H
H
H
H
C C C C
H
C
C
C
C
C
HHH
H
H
HH
H
H
H
H
H
H
H
H
123456
C C C C
H
C
C
C
C
C
HHH
H
H
HH
H
H
H
H
H
H
H
H
1 2 3 4 5 6
What is the main side chain and what is
the position of each carbon in that chain?
Longest chain but without
triple bond - Not main chain
Longest chain with triple
bond - main chain
Correct Incorrect3-methyl-hex-1-yne
16.7.1.1 – Alkynes -
Nomenclature - Rules

1. The suffix ene denotes the presence
of a double bond.
2. The suffix yne denotes the presence
of a triple bond.
3. The suffix ene always precedes yne in
the name of compound, even when
the double bond is assigned the large
number.
4. The position number for the double
bond is placed before the name of
parent hydrocarbon.
5. The position number for the triple
bond is placed between ene and yne.
6. If same number would result from
each terminal, then the double bond
is given the lower possible number.
C C C C
H
C
C
C
C
C
HHH
H
H
HH
H
H
H
H
H
H
H
H
123456
What is the main side chain and what is
the position of each carbon in that chain?
Longest chain but without
triple bond - Not main chain
Longest chain with
double and triple bond -
main chain
Incorrect
Correct
4-propyl-hex-1-ene-5-yne
C C C C
H
C
C
C
C
C
HHH
H
H
HH
H
H
H
H
H
H
H
H
C C C C
H
C
C
C
C
C
HHH
H
H
HH
H
H
H
H
H
H
H
H
C C C C
H
C
C
C
C
C
HHH
H
H
HH
H
H
H
H
H
H
H
H
1 2 3 4 5 6
16.7.1.1 – Alkynes -
Nomenclature - Rules -
Contain double + triple bonds

• Give the IUPAC name of the following.
o CH3CH2C=CH.
o HC=C–C=CH.
o CH2=CH–C=CH.
o CH=C–C=C–CH=CH–CH3.
o CH3–CH2–C=C–CH2–CH3.
−
−−
−
−
−−
16.7.1.2 – Alkynes - Nomenclature - Activity
but-1-yne
but-1,3-diyne
but-1-ene-3-yne
hept-5-ene-1,3-diyne
hex-3-yne

• Two pi-bonds are formed by parallel overlapping of
p orbitals.
• Sigma bond is formed by linear overlapping of p
orbitals.
• The two carbons of alkyne are sp-hybridized and
are linked by a sigma bond due to sp-sp (linear)
orbital overlap.
• The unhybridized two p-orbitals on one carbon
overlap with two p-orbitals on the other carbon
atom to form two pi-bonds.
• The cloud of pi-electrons is present cylindrically
symmetrical about the carbon-carbon sigma bond.
• Rotation about carbon-carbon sigma bond does not
cause any change in energy and electron density.
• It is a linear molecule and hence geometrical
isomerism is not observed in it.
16.7.2 – Alkynes - Structure

• Alkynes are more stable as compared to
alkenes due to the presence of extra pi-
bond.
• The presence of pi electrons in a
cylindrically symmetrical manner around
the carbon-carbon sigma bond causes it
to attract both nuclei more firmly, which
in turn means more energy is required to
break the pi-electron system in alkynes as
compared to alkenes.
• This is supported by the thermodynamic
data of the alkynes and alkenes.
o ΔHd of 1-hexyne = 290 kJ/mole
o ΔHd of 1-hexene = 126 kJ/mole.
16.7.3 – Alkynes - Relative stability

• In general alkynes are non-polar and insoluble in water but soluble in non-polar organic
solvents.
• They are colorless and odorless except acetylene, which has a garlic-like odor.
• The melting points, boiling points and densities increase gradually with the increase in
molecular masses.
• The first three members of alkynes (C2-C4) are gases, the next eight (C5-C12) are liquids,
and the higher members are solids at room temperature.
16.7.4 – Alkynes - Physical properties

• Alkynes can be prepared by the following methods.
o Elimination reactions.
• Elimination of Hydrogen halide
– Dehydrohalogenation of vicinal dihalide.
– Dehydrohalogenation of geminal dihalide.
o Alkylation of sodium acetylide (NaC2H).
16.7.5 – Alkynes - Preparation

• Alkynes can be prepared by dehydrohalogenation
of vicinal and germinal dihalides in the presence of
some alkaline reagents.
• A vicinal dihalide contains two halogens atoms on
adjacent carbon atoms.
• Elimination of two molecules of halogens from the
two adjacent carbon atoms produces an alkalyne.
• Higher alkalynes are also formed in the presence of
alcoholic KOH, e.g.,
o In the presence of a strong base such as KOH and at
high temperature, triple bond at terminal C atom
migrates to give more disubtitutued alkyne.
o Therefore alcoholic KOH is useful when
rearrangement is not possible.
o 1-alkynes can be prepared from vic-dihalides with
sodium amide in liquid ammonia.
ethyne1,2-dibromoethane
CH2CH2
Br
HC CH
Br
Base
-HBr
CH CH2
Br
Base
-HBr
bromoethane
dibenzylethyne1,2-dibenzyl-1,2-
dibromoethane
C6H5–CH–CH–C6H5
Br Br
KOH
Alc. 100-200°C
C6H5 C C C6H5
1-benzyl-2-
sodiumethyne
benzyl-1,2-
dibromoethane
C6H5–CH–CH2
Br Br
liq. NH3
-33°C
C6H5 C C- Na+
+ 3NaNH2
+ 2NaBr + 2NH3
C6H5 C C- Na+
H2O
C6H5 C CH
benzylethyne
16.7.5.1.1 – Alkynes - Preparation - Elimination of hydrogen
halide - Vicinal dihalide

• Alkynes can be prepared by
dehydrohalogenation of vicinal and
germinal dihalides in the presence of
some alkaline reagents.
o A geminal dihalide are those dihalides
that contain two halogen atoms on the
same carbon atom.
o Geminal dihalide on treatment with
strong base gives alkyne.
propyne1,1-dibromopropane
CH3CH2CH
Br
Br H3C C CH +2NaBr+ 2NH3+ 2NaNH2
NH3
Sodium amide
Sodium
bromide
Ammonia
16.7.5.1.2 – Alkynes - Preparation - Elimination of hydrogen
halide - Geminal dihalide

• Alkyne is an unsaturated hydrocarbon and shows addition reactions.
• It also undergoes substitution reactions due to easy cleavage of C-H bonds.
• The pi-electrons are present cylindrically symmetrical about the carbon-
carbon sigma bond and the removal of terminal hydrogen is possible without
disturbing carbon-carbon bonding. Therefore electrophile substitution
reactions are possible in 1-alkynes.
16.7.6 – Alkynes - Reactivity

• In ethyne and other terminal alkynes
like propyne, the hydrogen atom is
bonded to the carbon atoms with sp-s
overlap.
• As sp hybrid orbital has 50% S-
character in it, it renders the carbon
atoms to be more electronegative.
• As a result, the sp-hybridized carbon
atom of a terminal alkyne pulls the
electrons more strongly making the
attached hydrogen atom slightly acidic.
• This Hδ+ can be substituted with metal.
Thus substitution reactions occur due
to Hδ+.
16.7.7.1 – Alkynes - Reactions - Acidity of terminal alkynes

• When 1-alkyne or ethyne is treated with sodamide in liquid ammonia or passed over molten sodium,
alkynides or acetylides are obtained.
o Sodium acetylide is a very valuable reagent for chemical synthesis and is essentially ionic in nature.
• Acetylides of copper and silver are obtained by passing acetylene in the ammoniacal solution of cuprous
chloride and silver nitrate respectively.
o Silver and copper acetylides react with acids to regenerate alkynes.
o These alkynides are used for the preparation, purification, separation, and identification of alkynes.
Terminal alkyne
R C C H +NaNH2
liq NH3
R C C Na+NH3
Disodium acetylideethyne
H C C H + 2Na Na C C- Na+
+ NH3
Disilver acetylide (white ppt).ethyne
H C C H + 2AgNO3 Ag C C- Ag + 2NH4NO3 + 2H2O+ 2NH4OH
Dicopper acetylide (reddish brown ppt).ethyne
H C C H + Cu2Cl2 Cu C C- Cu + 2NH4Cl + 2H2O+ 2NH4OH
Ag C C Ag+ dil. H2SO4 H C C H + 2Ag2SO4
Ag C C Ag+ dil. HNO3 H C C H + 2AgNO3
ethyneDisilver acetylide
16.7.7.1.1 – Alkynes - Reactions - Acidity of terminal alkynes -
Examples

• Alkynes undergo addition reactions in an analogous fashion to those of
alkenes.
• The high electron density of the pi-bonds makes them nucleophilic.
• Two factors influence the relative reactivity of alkynes compared to alkenes:
o Increased nucleophilicity of the starting pi-system.
o Stability of any intermediates (for example carbocations).
16.7.7.2 – Alkynes - Reactions - Addition

• Alkynes react with hydrogen gas in the presence of suitable catalysts like finely divided Nickel (Ni), Platinum (Pt) or
Palladium (Pd).
• In the first step, called partial hydrogenation, alkenes are formed, which then take up another molecule of hydrogen to
form an alkane.
• On the other hand, if you utilize Lindlar’s catalyst (a mixture of Pd, CaCO3, Pb salts and quinoline) or nickel boride in
hydrogenation, you can partially hydrogenate alkyne to cis-alkene only, i.e., a stereospecific reaction.
Ethyne Ethene Ethane
H C C H + H2
H
C C
H H
H
200°CNi, 200°CNi,
+ H2
C
H
H
H
C
H
H
H
Propyne Propene
C C H + H2
H
C C
H
H
200°CNi,
Propane
200°CNi,
+ H2
C
H
H
H
CH
H
H
C
H
H
C
H
H
H
C
H
H
H
16.7.7.2.1 – Alkynes - Reactions - Addition - Hydrogenation

• Alkynes can be reduced to trans-alkenes
using Na in NH3 (liq.)
• This reaction is also stereospecific giving
only the trans-alkene via an anti-addition.
• Note that the stereochemistry of this
reaction complements that of catalytic
hydrogenation.
• The reaction proceeds via single electron
transfer from the Na with H coming from
the NH3.
• These reaction conditions do not reduce
alkenes, hence the product is the alkene.
16.7.7.2.2 – Alkynes - Reactions - Addition - Dissolving metal
reduction

• Alkynes react with hydrogen
chloride and hydrogen bromide
to form dihaloalkenes.
• The reaction occurs in
accordance with Markownikov’s
Rule.
H C C H + H–Br
alkyne Vinyl bromide
H
C C
H H
Br
Vinyl bromide
H
C C
H H
Br
+ H–Br
1,1-dibromoethane
H
C C
H
H
Br
Br
H
Markownikov’s
Addition
16.7.7.2.3 – Alkynes - Reactions - Addition - Hydrohalogenation

• Water adds to acetylene in the presence of mercuric sulphate (HgSO4) dissolved in
sulphuric acid (H2SO4) at 75°C.
• Ethyl alcohol is an unstable compound with hydroxyl group attached to a doubly bonded
carbon atom and isomerizes to acetaldehyde.
• Except acetylene, all other alkynes give ketones.
• This reaction is industrially important because aldehydes can be prepared by this method.
alkyne vinyl alcohol
H C C H + H–OH
H
C C
H H
OH
HgSO4
H2SO475°C,
16.7.7.2.4 – Alkynes - Reactions - Addition – Hydration and
rearrangement of alcohol
C
propyne water acetone (ketone)
acetaldehyde

• Chlorine and bromine add to
the acytylenic triple bond in
the presence of Lewis acid as
catalyst.
• The halogenation may be
stopped at the dihaloalkene
stage because the double bond
of dihaloalkene is less
nucleophilic than even the
triple bond itself.
CCl4
-34°C
C C C H + Br2
H
H
H C
C
H
H
H
Br
C
Br
H
1,2-dibromopropenepropyne
C
C
H
H
H
Br
C
Br
H
+ Br2
CCl4
-34°C
C
H
H
H C C
Br
Br
Br
Br
H
1,1,2,2-tetrabromopropane1,2-dibromopropene
16.7.7.2.5 – Alkynes - Reactions - Addition - Bromination

• When ozone reacts with alkyne
followed by aqueous work up,
we get 2RCO2H.R C C R + O3
R C
O
RC
O
O
alkyne Acid anhydride
+H2O R C
OH
O
+ RC
OH
O
R C
O
RC
O
O
Carboxylic acidAcid anhydride
16.7.7.2.6 – Alkynes - Reactions - Addition - Ozonolysis

16.7.7 – Alkynes - Reactions - Summary

• What reducing agent would you use to convert an alkalyne to a:
o Cis-alkene.
o Trans-alkene.
16.7.8 – Quick quiz

Chapter 16 Section 7 - Alkynes

Dr. Hashim Ali
16.8 – Benzene and aromatic hydrocarbons
Education (FBISE)
Chemistry F.Sc
II

• Michael Faraday discovered Benzene in 1825
during destructive distillation of vegetable
oil.
• Hoffman isolated it from coal tar.
• The molecular formula of benzene is C6H6.
• The molecular weight of benzene is 6*12 +
6*1 = 78.
• The special features unique to benzene are
Resonance and electrophylic substitution
reactions.
• As a functional group, benzene and
substituted benzene are called arenes.
• Benzene is a clear yellow highly flammable
liquid with a sweet odor and is commonly
found in cigarette smoke, crude oil and
gasoline.
16.8.0 – Benzene - Introduction

• The following procedures are adopted
for naming mono-substituted benzenes.
1. Parent name is benzene and the
substitution is indicated by a prefix, e.g.,
methyl, ethyl, chloro, nitro etc.
2. The substituent and the benzene ring
taken together may form a new parent
name. The largest parent name is
preferred, e.g., C6H5CH3 may be named
as i) Methyl benzene ii) Phenyl methane.
According to the largest rule, methyl
benzene is preferred.
3. Mono substituted derivatives of
benzenes are named by prefixing the
name of substitute to the word ‘benzene’.
Cl
chlorobenzene
NO2
nitrobenzene
Br
bromobenzene
CH3
methylbenzene
16.8.1.1 – Benzene - Nomenclature - Mono-substituted benzenes

• Many aromatic compounds have been known by their common or trivial
names, which have been retained by IUPAC system.
NO2
nitrobenzene
Br
bromobenzene
CH3
toluene
OCH3
anisole
OH
phenol
NH2
aniline
CH=CH2
styrene
COOH
benzoic acid
CH
cumene
CH3CH3

• In fact, MOST aromatic compounds are known by their common or trivial
names, which have been retained by IUPAC system.

• When there are two substituents on benzene ring, their relative positions are
indicated by prefixes ortho(o), meta(m) and para(p) in common system of
naming and by numerals while naming according to IUPAC system.
CH3
o-dimethyl benzene
CH3
1,2-dimethyl benzene
CH3
m-dimethyl benzene
CH3
CH3
p-dimethyl benzene
CH3
16.8.1.2 – Benzene - Nomenclature - Di-substituted benzenes

1. If the substituents are different and one of them is an alkyl group, the numbering is started
from the ring carbon, which is linked to the alkyl group and the second substituent gets the
lowest possible number.
2. When a common name is used, the substituent which is responsible for name, e.g., CH3 in
toluene and -OH in phenol, is considered to be on carbon -1, i.e., numbering is started from
the carbon of ring bearing that group such a di-substituted compound.
CH3
Cl
The name of the
parent chain, with
position of
substituents, is o-
chlorotoluene or 2-
chlorotoluene.
Given that the there are two substituents with benzene, what is the name of
main chain? What is the position of substitutes on the main benzene chain?
Can it be 5-methyl chlorobenzene?
Or is it 5-chlorotoluene?
Or is it 2-methyl chlorobenzene?
Or is it 2-chlorotoluene?
According to the rule, it has to be
toluene.
According to the rule, 2-
chlorotoluene is the smallest.
16.8.1.2 – Benzene - Nomenclature - Di-substituted benzenes

Chapter 16 hydrocarbons

Recommended

Recommended

More Related Content

What's hot

What's hot (20)

Similar to Chapter 16 hydrocarbons

Similar to Chapter 16 hydrocarbons (20)

More from Hashim Ali

More from Hashim Ali (11)

Recently uploaded

Recently uploaded (20)

Chapter 16 hydrocarbons

Editor's Notes