2. COURSE OUTCOME
Ability to explain the relationship between the structure,
physical and chemical properties of the different bonds and
functional groups in organic compounds (CO2)
Course Learning Outcome
The student should be able to: -
Name alkanes.
Explain aliphatic properties.
Predict, draw and name the products of functional group reactions.
Draw the mechanistic pathway.
3. Fossil Fuels:
Many alkanes occur in
nature, primarily in
natural gas and
petroleum.
Natural gas is com-posed
largely of methane, with
lesser amounts of ethane,
propane and butane.
Petroleum is a complex mixture of compounds, most of which are hydrocarbons
containing one to forty carbon atoms. Distilling crude petroleum (called refining),
separates it into usable fractions that differ in boiling point.
gasoline: C5H12—C12H26
kerosene: C12H26—C16H34
diesel fuel: C15H32—C18H38
4. Sources of Alkanes: Petroleum and Natural Gas
.
Petroleum is the source of alkanes. It is a complex mixture of
mostly alkanes and aromatic hydrocarbons with smaller amounts
of oxygen-, nitrogen-, and sulfur-containing compounds
Natural gas is a gaseous mixture of hydrocarbons recovered from
natural sources. It is mostly methane (CH4, BP -162oC) with small
amounts of ethane (C2H6, BP -88oC) and propane (C3H8, BP -42o).
Petroleum Refining
Liquid petroleum and natural gas are usually separated at the
wellhead and shipped independently to processing (refining) plants.
The liquid petroleum (crude) is separated by distillation according to
the volatility (BPs) of the different size hydrocarbons. The fractions
collected by refining are still mixtures of hydrocarbons that have
commercial value.
5. Refining crude petroleum into usable fuel and other petroleum products.
(a) An oil refinery. At an oil refinery, crude petroleum is separated into
fractions of similar boiling point by the process of distillation.
(b) Schematic of a refinery tower. As crude petroleum is heated, the lower-
boiling, more volatile components distill first, followed by fractions of
progressively higher boiling point.
6. Hydrocarbon Fractions from Petroleum
boiling range
of fraction (oC)
size range name and use
below 20 C1 to C4
natural gas, bottled
gas, petrochemicals
20 to 60 C5 to C6 petroleum "ether".
solvents
60 to 100 C6 to C7 ligroin, solvents
40 to 200 C5 to C10 straight-run gasoline
175 to 325 C12 to C18 kerosene and jet fuel
250 to 400 C12 and higher
gas oil, fuel oil
and diesel oil
nonvolatile
liquids C20 and higher
mineral oil,
lubricating oil
nonvolatile
solids
C20 and higher paraffin wax,
asphalt, tar
7. Petroleum Technologies
.
Technologies exist to interconvert the various hydrocarbons using
catalysts
Cracking is a process for breaking down larger alkanes into
smaller alkanes by heating. The mixture of larger alkanes is
heated in the absence of oxygen at high temperatures (~500oC) for
only a few minutes in the presence of catalysts. By this method,
alkanes of size C12 and larger can be turned into gasoline-size
alkanes (C5 to C10).
Isomerization
Since the 1920s, it has been known that highly branched alkanes
perform better in the internal combustion engine than straight-
chain alkanes. Catalytic isomerization changes straight-chain
alkanes into the more useful branched alkanes.
hexane
acid catalyst
+
branched alkanes
CH3CH2CH2CH2CH2CH3
CH3CH2CH2CHCH3
CH3
CH3CH2CHCH2CH3
CH3
8. Catalytic Reforming
Alkanes are transformed into cycloalkanes and aromatic
hydrocarbons by catalytic reforming.
heptane
silica-alumina
catalyst, 500oC
20 atm H2
+ 4H2
CH3CH2CH2CH2CH2CH2CH3
CH3
The aromatic hydrocarbons produced by catalytic reforming
are used as additives in gasoline and as starting materials for
the petrochemical industry. Production of these aromatics is in
the billions of pounds per year in the United States.
9. Crude
Petroleum
Refining
straight-
chain
alkanes
of
different
sizes
Cracking
Isomerization
Reforming
smaller alkanes
branched alkanes aromatics
Petroleum Products
Daily consumption of petroleum in the United States is over 17
million barrels which amounts to close to one billion tons per year.
Of this total, approximately 43% goes into gasoline, another 25%
into fuel oil, and approximately 7.5% into jet fuel. Thus, about
75% of all the petroleum consumed is burned as a source of
energy. The remainder is used as "feedstock" for polymers (~4%)
and the chemical industry (~3%), and the many other products
used in our society such as oils, lubricants and asphalt.
An Overview of Petroleum Refining
10. Combustion
All hydrocarbons undergo combustion, the reaction with oxygen
that liberates energy. Thus, all hydrocarbons are potential "fuels",
materials that burn in oxygen releasing energy.
Heat of Combustion
The heat of combustion (Hcomb) is the amount of heat liberated
when one mole of material undergoes combustion at 1 atm pressure
to produce gaseous CO2 and liquid water.
CH4 + 2O2 CO2(g) + 2H2O(l)
methane
Hcomb = -882 kJ/mol (or -55.1 kJ/g)
For a gasoline-size hydrocarbon::
2C8H18 + 25O2 16CO2(g) + 18H2O(l)
Hcomb = -5452 kJ/mol (or -47.8 kJ/g)
Note, the total amount of heat liberated increases with the size of
the hydrocarbon, but that doesn't make it a better fuel. On a per
weight basis, methane is a better fuel than the octane.
11. Gasoline Performance: The Octane Rating
The combustion of alkanes is a complicated reaction probably
involving free radicals. Much attention has been directed towards
the combustion of hydrocarbons in the internal combustion engine.
Since the 1920s, it has been known that some hydrocarbons tend to
give better performance during combustion. Some fuels cause
"knocking", the premature ignition of the fuel before the piston is
in the firing position for a power stroke. Knocking causes loss of
power.
Branched hydrocarbons were found to perform better than straight-
chain alkanes in the internal combustion engine. In 1927, an
arbitrary engineering performance standard was developed called
"the octane rating." The performance of the branched alkane
"isooctane" (actually 2,2,4-trimethylpentane) in a specific internal
combustion engine was given a rating of 100. Heptane, which
causes severe knocking, was given a rating of 0.
A fuel with a
performance equivalent
to a mixture of 75%
isooctane and 25%
heptane is given an
octane rating of 75.
"isooctane"
100
heptane
0
CH3CCH2CHCH3
CH3
CH3 CH3
CH3CH2CH2CH2CH2CH2CH3
12. Octane Boosters
.
It is common practice to add octane boosters to gasoline to
improve the performance of the fuel. Many years ago,
tetraethyllead, (C2H5)4Pb, was an important additive for this
purpose. It is now illegal to use "leaded" gasoline in an
automobile in the United States. Aromatics and "oxygenated
fuels" are blended into gasoline to raise the octane rating
Some Octane Ratings of
Hydrocarbons and Additives
Octane Rating
heptane 0
1-pentene 91
2,2,4-trimethylpentane 100
benzene 106
methanol 107
ethanol 108
methyl t-butyl ether 116
toluene 118
Methyl t-butyl ether (MTBE) is an
oxygenated fuel blended into
gasoline to improve performance
and reduce air pollution.
Production of MTBE increased over
the past 10 years to many billions of
pounds per year in the United
States. However, MTBE is being
phased out because of
environmental and health concerns.
15. Alkanes: Compounds with C-C single bonds and C-H bonds only (no
functional groups)
Connecting carbons can lead to large or small molecules
The formula for an alkane with no rings in it must be CnH2n+2 where the
number of C’s is n
Alkanes are saturated with hydrogen (no more can be added)
They are also called aliphatic compounds
ALKANES AND ALKANE
ISOMERS
16. CH4 = methane, C2H6 = ethane, C3H8= propane
The molecular formula of an alkane with more than three
carbons can give more than one structure
C4 (butane) = butane and isobutane
C5 (pentane) = pentane, 2-methylbutane, and 2,2-
dimethylpropane
Alkanes with C’s connected to no more than 2 other C’s are
straight-chain or normal alkanes
Alkanes with one or more C’s connected to 3 or 4 C’s are
branched-chain alkanes
ALKANE ISOMERS
17. Isomers that differ in how their atoms are arranged in chains are called
constitutional isomers
Compounds other than alkanes can be constitutional isomers of one
another
They must have the same molecular formula to be isomers
CONSTITUTIONAL ISOMERS
18. We can represent an alkane in a brief form or in many types of extended
form
A condensed structure does not show bonds but lists atoms, such as
CH3CH2CH2CH3 (butane)
CH3(CH2)2CH3 (butane)
Structural formulas
CONDENSED STRUCTURES OF
ALKANES
20. Alkyl group – remove one H from an alkane (a part of a structure)
General abbreviation “R” (for Radical, an incomplete species or the “rest”
of the molecule)
Name: replace -ane ending of alkane with –yl ending
-CH3 is “methyl” (from methane)
-CH2CH3 is “ethyl” from ethane
ALKYL GROUPS
23. * There is no 4˚ hydrogen…Why or why not? Let’s talk about
this…
ALKYL GROUPS
(CONTINUED)
24. Hydrogen atoms are classified as primary (10), secondary (20), or
tertiary (30) depending on the type of carbon atom to which they are
bonded
25. CYCLOALKANES
Cycloalkanes have molecular formula CnH2n and contain carbon atoms
arranged in a ring. Simple cycloalkanes are named by adding the prefix
cyclo- to the name of the acyclic alkane having the same number of
carbons.
26. Compounds are given systematic names by a process that uses
Follows specific rules
Find parent hydrocarbon chain
NAMING ALKANES
27. 1. Find the parent carbon chain and add the suffix.
Note that it does not matter if the chain is straight or it bends.
IUPAC systematic Nomenclature -
Alkanes
28. Also note that if there are two chains of equal length, pick the
chain with more substituents. In the following example, two
different chains in the same alkane have seven C atoms. We
circle the longest continuous chain as shown in the diagram
on the left, since this results in the greater number of
substituents.
29. 2. Number the atoms in the carbon chain to give the first substituent the
lowest number.
NAMING ALKANES
(CONT..)
30. If the first substituent is the same distance from both ends, number the
chain to give the second substituent the lower number.
NAMING ALKANES
(CONTINUED)
31. When numbering a carbon chain results in the same numbers from either
end of the chain, assign the lower number alphabetically to the first
substituent.
NAMING ALKANES
(CONTINUED)
32. 3. Name and number the substituents.
• Name the substituents as alkyl groups.
• Every carbon belongs to either the longest chain or a substituent, not
both.
• Each substituent needs its own number
• If two or more identical substituents are bonded to the longest chain, use
prefixes to indicate how many: di- for two groups, tri- for three groups,
tetra- for four groups, and so forth.
NAMING ALKANES
(CONTINUED)
33. 4. Combine substituent names and numbers + parent and
suffix.
• Precede the name of the parent by the names of the substituents.
• Alphabetize the names of the substituents, ignoring all prefixes except
iso, as in isopropyl and isobutyl.
• Precede the name of each substituent by the number that indicates its
location.
• Separate numbers by commas and separate numbers from letters by
hyphens. The name of an alkane is a single word, with no spaces after
hyphens and commas.
NAMING ALKANES
(CONTINUED)
37. 2. Name and number the substituents. No number is needed to
indicate the location of a single substituent.
For rings with more than one substituent, begin numbering at one
substituent and proceed around the ring to give the second substituent the
lowest number.
CYCLOALKANES NAMING
(CONT..)
38. With two different substituents, number the ring to assign the
lower number to the substituents alphabetically.
Note the special case of an alkane composed of both a ring and a long
chain. If the number of carbons in the ring is greater than or equal to the
number of carbons in the longest chain, the compound is named as a
cycloalkane.
CYCLOALKANES NAMING
(CONT..)
41. Some organic compounds are identified using common names that do not
follow the IUPAC system of nomenclature. Many of these names were
given long ago before the IUPAC system was adopted, and are still widely
used. Additionally, some names are descriptive of shape and structure, like
those below:
Nomenclature—Common Names
CYCLOALKANES NAMING
(CONT..)
42. REACTION OF ALKANES
Combustion of Alkanes
• Alkanes undergo combustion—that is, they burn in the presence of
oxygen to form carbon dioxide and water.
• This is an example of oxidation. Every C—H and C—C bond in the
starting material is converted to a C—O bond in the product.
43. Halogenation of Alkanes
C H + X2
250-400o
or h
C X + HX
Reactivity:- X2 : F2 > Cl2 > Br2 (> I2)
H : 3o
> 2o
> 1o
> H3C-H
Chlorination - a substitution reaction
CH4 + Cl2
h
or
CH3Cl + HCl
REACTION OF ALKANES
(CONT..)
44. Polychlorination
CH3Cl + Cl2 CH2Cl2 + HCl
CH2Cl2 + Cl2 CHCl3 + HCl
CHCl3 + Cl2 CCl4 + HCl
dichloromethane
methylene chloride
trichloromethane
chloroform
tetrachloromethane
carbon tetrachloride
Iodination and fluorination
• iodine does not react while, fluorine reacts very readily
order of halogen reactivity: F2 > Cl2 > Br2 (> I2)
REACTION OF ALKANES
(CONT..)
45. E.G: CHLORINATION OF METHANE
Requires heat or light for initiation.
The most effective wavelength is blue, which is absorbed
by chlorine gas.
Many molecules of product are formed from absorption
of only one photon of light (chain reaction).
CHAPTER 4 45
46. THE FREE-RADICAL CHAIN REACTION
Initiation: Generates a radical intermediate.
Propagation: The intermediate reacts with a stable molecule to
produce another reactive intermediate (and a product molecule).
Termination: Side reactions that destroy the reactive intermediate.
47. INITIATION STEP: FORMATION OF
CHLORINE ATOM
A chlorine molecule splits homolytically into
chlorine atoms (free radicals).
48. LEWIS STRUCTURES OF FREE RADICALS
Free radicals are reactive species with odd numbers of
electrons.
49. PROPAGATION STEP: CARBON RADICAL
The chlorine atom collides with a methane molecule
and abstracts (removes) an H, forming another free
radical and one of the products (HCl).
50. PROPAGATION STEP: PRODUCT
FORMATION
The methyl free radical collides with another
chlorine molecule, producing the organic product
(methyl chloride) and regenerating the chlorine
radical.
54. 54
The intermediate alkyl radical
The nature of the intermediate free radical determines the product:
CH3CH2CH3
X
propane
CH3CH2CH2
n-propyl radical
CH3CHCH3
isopropyl radical
CH3CH3
X
CH3CH2
ethane ethyl radical
X2
CH3CH2X
haloethane
X2
CH3CH2CH2X
1-halopropane
X2
CH3CHXCH3
2-halopropane
X2
CH3X
halomethane
CH4
X CH3
methane methyl radical
55. SYNTHESIS OF ALKANES AND
CYCLOALKANES
Hydrogenation of alkenes and alkynes
CnH2n CnH2n+2
H2
Pt, Pd or Ni
alkene alkane
H2/Ni
C2H5OH
25o
, 50 atm
(CH3)3CH
Pt
+ 2 H2
+ H2
Pd
56. Reduction of alkyl halides
i. Hydrolysis with Grignard reagent
R-X + Mg R – Mg –X
RMgX + HOH R-H + Mg(OH)X
CH3CH2CH2MgBr + H2O CH3CH2CH3 + Mg(OH)Br
ii. Reduction of alkyl halide with metal and acid
(Zn in CH3COOH or HBr)
R-X R-H
CH3CHBrCH2CH3 CH3CH2CH2CH3
iii. Reaction with LiAlH4
C9H19CH2-Br C9H19CH3
57. Alkylation of terminal alkynes
An acetylenic hydrogen is weakly acidic:
C C HR
Na
NH3
C CR
-
Na+
+ 1/2H2
a sodium
acetylide
(CH3)2CHC C H
NaNH2
ether
(CH3)2CHC C
-
Na+
+ NH3
58. Alkylation of terminal alkynes (cont..)
The anion formed will react with a primary halide:
C C-
Na+
R + CH3X C CCH3 + NaXR
1. NaNH2
2. CH3Br
H2/Pt
59. Corey – Posner – Whitesides - House Synthesis
R-X + 2Li
diethyl ether
RLi + LiX
alkyllithium1o
, 2o
,
or 3o
2RLi + CuI R2CuLi + LiI
lithium
dialkylcuprate
a Gilman reagent
R2CuLi + R'X R-R' + RCu +LiX
1o
alkyl or 2o
cycloalkyl halide
