intro organic

CHAPTER 11 : INTRODUCTION TO ORGANIC CHEMISTRY

CHAPTER 11 : INTRODUCTION TO ORGANIC CHEMISTRY 11.1 Introduction 11.2 Empirical molecular and structural formulas 11.3 Functional groups and homologous series 11.4 Classification of carbon atoms in organic molecules 11.5 Isomerism 11.6 Reactions in organic compound

Organic and Inorganic Compound Organic compound Inorganic compound
were defined as
compounds that
could be obtained
from living
organisms
were those that
came from
nonliving sources

Some examples of carbon compounds in our daily lives :- methane (a component of natural gas) Methyl salicylic acid (aspirin-a drug) alanine (amino acid-a protein component) cocaine (a pain killer)

11.2 EMPIRICAL, MOLECULAR AND
STRUCTURAL FORMULAE
Empirical formula is the simplest formula that
shows the relative numbers of the different kinds
of atoms in a molecule.
Molecular formula is a formula that states the
actual number of each kind of atom found in the
molecule. Example : C 2 H 4 , C 2 H 4 O 2

Quantitative example :- A sample of hydrocarbon contains 85.7 % carbon and 14.3 % hydrogen by mass. Its molar mass is 56. Determine the empirical formula and molecular formula of the compound.

Solution :- = 7.142 = 14.3  Empirical formula = CH 2 n (empirical molar mass)= molar mass n ( 12 x 1 + 1 x 2 ) = 56 n = 4 Molecular formula = C 4 H 8 14.43 7.142 = 2 7.142 7.142 = 1 Smallest ratio 14.3 1.0 85.7 12.0 Moles (n) 14.3 85.7 Mass (g) H C Element

Practice Exercise:
1) A complete combustion of 10.0 g of compound X,
C x H y O z forms 12.0 g H 2 O and 22.0 g CO 2 . Its
molar mass is 60.
a) Determine the percentage composition of
C, H and O.
b) Determine the molecular formula of
compound X.
A complete combustion of 0.6 g of hydrocarbon,
C x H 12 forms 1.98 g CO 2 . Determine the percentage
of C and H in the hydrocarbon and write the
molecular formula.

Structural formula shows the order in which
atoms are bonded together
Representation of structural formula :- a) Condensed Structure b) Expanded Structure c) Skeletal Structure d) 3-Dimensional formula e) Ficher Projection

a) Condensed Structure
In condensed formulae all the hydrogen atoms
that are attached to a particular carbon are
usually written immediately after that carbon

Example : C 4 H 9 Cl CH 3 CHCH 2 CH 3 or CH 3 CH(Cl)CH 2 CH 3 Condensed structure Cl

b) Expanded Structure
Expanded structures indicate the way in which
the atoms are attached to each other and are not
representations of the actual shapes of the
molecules.
Example : C 4 H 9 Cl Expanded structure

c) Skeletal Structure
This structure shows only the carbon skeleton
The hydrogen atoms that are assumed to be
present, are not written.
Other atoms such as O, Cl, N and etc. are shown Example : CH 3 CH(Cl)CH 2 CH 3 Cl = 1.

2. = CH 2 =CHCH 2 OH 3. = OH

Practice Exercise : Rewrite each of the following structures using skeletal formula :- (CH 3 ) 2 CHCH 2 CH 2 CH(CH 3 )CH 2 CH 3 CH 2 = CHCH 2 CH 2 CH = CHCH 3 1. 2. 3. 4.

d) 3 - Dimensional formula (wedge - dashed wedge - wedge) Describes how the atoms of a molecule are arranged in space Example : (Bromoethane) Indication :- bonds that lie in the plane of the page bonds that lie behind the plane bonds that project out of the plane of the paper OR OR

e) Fischer Projection
Vertical lines represent bonds that project behind
the plane of paper
Horizontal lines represent bonds that project out of
the plane of paper
The intersection of vertical and horizontal lines
represent a carbon atom, that is stereocentre

Example : 2 – butanol , CH 3 CH(OH)CH 2 CH 3 OR

11.3 FUNCTIONAL GROUPS AND HOMOLOGOUS SERIES
A functional group is an atom or group of
atoms in an organic molecule which characterized
the molecule and enables the molecule to react
in specific ways (determines its chemical properties)

Some important functional groups in organic compounds :- CH  CH ethyne -yne C n H 2n-2 C  C (triple bond) alkynes CH 2 =CH 2 ethene -ene C n H 2n C = C (double bond) alkene CH 4 methane -ane C n H 2n+2 none alkane Example IUPAC nomenclature Prefix- -suffix General Formula Functional Group Homologous Series

aromatic ring methylbenzene CH 3 CH 2 Cl chloroethane haloalkane C n H 2n+1 X – X (halogen) haloalkane CH 3 OCH 3 methoxymethane alkoxyalkane C n H 2n+2 O – OR (alkoxy) ether CH 3 CH 2 OH ethanol alkanol C n H 2n+1 OH – OH (hydroxyl) alcohol -benzene C n H 2n-6 arene Example IUPAC nomenclature Prefix- -suffix General Formula Functional Group Homologous Series

carbonyl H ethanal carbonyl CH 3 propanone C OH O carboxyl OH ethanoic acid CH 3 C=O alkanoic acid CnH 2n O 2 carboxylic acid CH 3 C=O alkanone C n H 2n O ketone CH 3 C=O alkanal C n H 2n O aldehyde Example IUPAC nomenclature Prefix- -suffix General Formula Functional Group Homologous Series

acyl Cl ethanoyl chloride ester ethyl ethanoate amide ethanamide amino methanamine CH 3 NH 2 -amine C n H 2n+1 NH 2 -NH 2 amine CH 3 CONH 2 -amide C n H 2n+1 CONH 2 amide CH 3 COOCH 3 alkyl alkanoate C n H 2n O 2 ester CH 3 C=O alkanoyl chloride C n H 2n+1 COCl acyl chloride Example IUPAC nomenclature Prefix- -suffix General Formula Functional Group Homologous Series

11.4 CLASSIFICATION OF CARBON AND HYDROGEN ATOMS IN ORGANIC MOLECULES
Carbon atom classified
primary (1 o ) secondary (2 o ) tertiary (3 o ) quarternary (4 o ) depending on the number of carbon atoms bonded to it

A primary carbon – directly bonded to one other
carbon atom
(has 1 adjacent carbon atom)
Example : 1 o carbon 1 o H

A secondary carbon – directly bonded to two other
carbon atoms
(has 2 adjacent carbon atoms)

A tertiary carbon – directly bonded to three other
carbon atoms

A quarternary carbon – directly bonded to four other
carbon atoms
Example : 4 o carbon

Similarly, a hydrogen atom is also classified as
primary, secondary or tertiary depending on the
type of carbon to which it is bonded.
1° hydrogen atom bonded to a 1° C atom 2° hydrogen atom bonded to a 2° C atom 3° hydrogen atom bonded to a 3° C atom

Classification of haloalkanes (alkyl halides)
Alkyl halides are classified based on the carbon atom
to which the halogen is directly attached.
1 ° alkyl halide – the halogen atom is bonded to a primary carbon atom 2 ° alkyl halide – the halogen atom is bonded to a secondary carbon atom 3 ° alkyl halide – the halogen atom is bonded to a tertiary carbon atom

1 ° alkyl chloride 1 ° C 2 ° alkyl chloride 2 ° C 3 ° alkyl chloride 3 ° C

Classification of alcohols
Alcohols are classified based on the carbon atom
to which the hydroxyl group is directly attached.
1 ° alcohol – the hydroxyl group is attached to a 1 ° carbon atom 2 ° alcohol – the hydroxyl group is attached to a 2 ° carbon atom 3 ° alcohol – the hydroxyl group is attached to a 3 ° carbon atom

1 ° alcohol 1 ° C 2 ° C 2 ° alcohol 3 ° alcohol 3 ° C

Classification of amines
Amines are classified based on the number of alkyl
groups or carbon atoms that are directly attached
to the nitrogen atom
1 ° amine – N is bonded to one alkyl group 2 ° amine – N is bonded to two alkyl groups 3 ° amine – N is bonded to three alkyl groups

N bonded to one alkyl group A primary (1°) amine N bonded to two alkyl group A secondary (2°) amine N bonded to three alkyl group A tertiary (3°) amine

ISOMERISM Structural/Constitutional Isomerism Stereoisomerism Isomerism Chain isomerism Positional isomerism Functional group isomerism Geometric isomerism Optical isomerism

Isomerism is the existence of different compounds
with the same molecular formula but different
structural formulae
Isomers – different compounds that have same
molecular formula
Two types of isomerism structural isomerism stereoisomerism different order of attachment of atoms different spatial arrangement of atoms in molecules

Structural isomerism Chain/skeletal isomerism
Structural isomers are different compounds with
the same molecular formula but differ in the order
of attachment of atoms
Positional isomerism Functional group isomerism

The isomers differ in the carbon skeleton
( different carbon chain )
a) Chain/skeletal isomerism
They possess the same functional group and
belong to the same homologous series
Example : C 5 H 12 : CH 3 CH 2 CH 2 CH 2 CH 3 CH 3 CHCH 2 CH 3 CH 3 CH 3 -C-CH 3 CH 3 CH 3

b) Positional isomerism
These isomers have a substituent group in
different positions in the same carbon skeleton
Example : C 3 H 7 Cl i) CH 3 CH 2 CH 2 Cl 1-chloropropane CH 3 CHCH 3 Cl 2-chloropropane C 4 H 8 ii) CH 2 =CHCH 2 CH 3 CH 3 CH=CHCH 3 1-butene 2-butene

1,2-dimethylbenzene iii) C 8 H 10 CH 3 CH 3 CH 3 CH 3 1,3-dimethylbenzene CH 3 CH 3 1,4-dimethylbenzene

c) Functional group isomerism
These isomers have different functional groups
and belong to different homologous series with
the same general formula
Different classes of compounds that exhibit
functional group isomerism :-
carboxylic acid and ester C n H 2n O 2 alkene and cycloalkane C n H 2n aldehyde and ketone C n H 2n O alcohol and ether C n H 2n+2 O Classes of compounds General formula

Example : i) C 2 H 6 O CH 3 CH 2 OH ethanol CH 3 OCH 3 dimethyl ether ii) C 3 H 6 O CH 3 CH 2 C-H O propanal CH 3 C-CH 3 O propanone iii) C 3 H 6 O 2 CH 3 CH 2 C-OH O propanoic acid CH 3 C-O-CH 3 O methyl ethanoate

Stereoisomerism Geometric Isomerism Optical Isomerism
Geometric isomerism
occurs only in two classes of compounds :
Alkenes & cyclic compound (because of rigidity in molecules)

Geometric isomers (also called cis-trans isomers)
are stereoisomers that differ by groups being
on the same side (cis-isomer) or opposite
sides (trans-isomer) of a site of rigidity in a molecule
The requirements for geometric isomerism :
restricted rotation about a C=C,double bond, in
alkenes or a C-C single bond in cyclic compounds
each carbon atom of a site of restricted rotation has
two different groups attached to it

Examples : H 3 C CH 3 H H C C = i) cis -2-butene H 3 C C = C CH 3 H H trans -2-butene ii) H 3 C CH 2 CH 3 C = C H CH 3 trans -3-methyl-2-pentene H 3 C CH 3 C = C H CH 2 CH 3 cis -3-methyl-2-pentene

iii) H H CH 3 CH 3 cis -1,2-dimethylcyclohexane H H CH 3 CH 3 trans -1,2-dimethylcyclohexane Cl Cl H H iv) Cl Cl H H cis -1,3-dichlorocyclopentane trans -1,3-dichlorocyclopentane

If one of the doubly bonded carbons has 2 identical
groups, geometric isomerism is not possible.
Examples : CH 3 CH 2 i) C = C H H H 3 C 2-methyl-2-butene H ii) C = C CH 3 CH 3 Cl 1-chloro-2-methylpropene

cis-trans isomers have similar chemical properties
but different physical properties
They differ in melting and boiling points and
solubility due to different polarity of the molecules
cis -isomers polar molecules trans -isomers non-polar
Melting point: trans - isomer > cis -isomer
Boiling point: cis -isomer > trans - isomer
Stability: trans -isomer > cis -isomer

b) Optical isomerism
If a beam of light is passed through a piece of
polarizer prism, the emergent light vibrates in a
single plane, hence it is called a plane-polarized
light
Optically active compounds have the ability to
rotate plane-polarized light
The angle of rotation can be measured with an
instrument called polarimeter

Schematic representation of a polarimeter containing an optically active sample : clockwise rotation – plus sign (+) / dextrorotarory anticlockwise rotation – minus sign (-) / levorotorary

The requirements for optical isomerism :-
molecule contains a chiral carbon or chiral centre
(carbon atom with 4 different groups attached to it)
ii) molecule is not superimposable with its mirror image
A representation of a chiral molecule with
3-dimensional formula :-
P  Q  R  S *designates chiral centre

Enantiomers are a pair of mirror-image molecules
that are not superimposable (must have one or more
chiral carbons)
Examples : i) 2-butanol, CH 3 CHCH 2 CH 3 OH enantiomers :-

ii) 2-hydroxypropanoic acid, CH 3 CHCOOH OH :- COOH COOH OH HO H H CH 3 CH 3 enantiomers

A racemic mixture or racemate is an equimolar
mixture of enantiomers which is optically inactive
because the two components rotate plane-polarized
light equally (same degree of rotation but in opposite
direction– so they can cancel each other’s rotation)
A pair of enantiomers have identical chemical and
physical properties but differ in the direction of
rotation of plane-polarized light

A compound with n chiral centers can have a
maximum of 2 n stereoisomers
If a molecule contains two or more chiral centers,
diastereomers may exist
Diastereomers are stereoisomers that are not
mirror images of each other
All physical properties of diastereomers are usually
different from one another

Example : The 4 stereoisomers of 2-amino-3-hydroxybutanoic acid CH3CH-CHCOOH are shown below using OH NH 2 Fischer projection formula:- COOH COOH CH 3 CH 3 H H H H NH 2 H 2 N OH HO enantiomers COOH NH 2 H H HO CH 3 COOH H H OH H 2 N enantiomers CH 3 A B C D Four pairs of diastereomers are identified : A and C ; A and D ; B and C ; B and D

Meso compound is a stereoisomer that has more
than one chiral centres and that is superimposable
on its mirror image because of the presence of an
internal plane of symmetry , hence it is optically
inactive (does not cause a rotation of plane-polarized
light)

Example : Tartaric acid , HOOCCH(OH)CH(OH)COOH COOH OH OH H H COOH COOH COOH HO HO H H COOH COOH H H OH OH plane of symmetry plane of symmetry rotate 180 o P Q identical

At first glance, P and Q are assumed to be enantiomers
But if compound Q is rotated 180 o in the plane of
the paper, it is actually identical to compound P ,
therefore P and Q are superimposable mirror images
P and Q are the same compound
It is a meso compound

COOH OH OH H H COOH COOH COOH R S COOH COOH H HO H OH * not a plane of symmetry * not a plane of symmetry rotate 180 o different H OH H HO

R and S are related as mirror images and are not
superimposable even if rotated 180 o
Thus R and S constitute an enantiomeric pair
There are 2 pairs of diastereomers :
P and R & P and S

Further examples of meso compounds:
CH 3 CH 3 Cl Cl H H CHO CHO HO HO H H H OH plane of symmetry

11.6 REACTIONS OF ORGANIC COMPOUNDS
11.6.1 Types of Covalent Bond Cleavage/Fission
All chemical reactions involved bond breaking and
bond making
Two types of covalent bond cleavage :-
Homolytic cleavage
Heterolytic cleavage

a) Homolytic Cleavage
Occurs in a non-polar bond involving two atoms of
similar electronegativity
A single bond breaks symmetrically into two equal
parts, leaving each atom with one unpaired electron
Free radicals are formed in homolytic cleavage
X X X + X ≡ 2X • • • • • free radicals

b) Heterolytic Cleavage
Occurs in a polar bond involving unequal sharing
of electron pair between two atoms of different
electronegativities
A single bond breaks unsymmetrically and both the
bonding electrons are transferred to the more
electronegative atom
Cation and anion are formed in heterolytic cleavage
A B • • A • • - + B + A is more electronegative A + + B • • - B is more electronegative cation anion anion cation

Carbocations and free radicals are intermediates in
organic reactions.
They are unstable and highly reactive

11.6.2 Reaction Intermediates
a) Carbocation
Also called carbonium ion
A very reactive species with a positive charge
on a carbon atom
Carbocation is formed in heterolytic cleavage

Example : (CH 3 ) 3 C — Cl   (CH 3 ) 3 C + carbocation + Cl - anion
Chlorine is more electronegative than carbon and
the C—Cl bond is polar
The C—Cl bond breaks heterolitically and both the
bonding electrons are transferred to chlorine atom
to form anion and carbocation

b) Free Radical
A very reactive species with an unpaired electron
Formed in homolytic cleavage
Cl – Cl Example : uv Cl • free radicals + Cl • C C C • + C • H 3 C H CH 3 • + H •

11.6.3 Relative Stabilities of
Carbocations and Free Radicals
Carbocation and free radical
primary secondary tertiary depending on the number of carbon atoms directly bonded to the :-
positively charged carbon atom (for carbocation)
carbon atom with unpaired electron (for free radical)

The stability of carbocation increases with the
number of alkyl groups present
The alkyl groups are electron-releasing relative to hydrogen, thus help to stabilize the positive charge
on the carbocation

Carbocation Stability:
methyl cation primary (1°) secondary (2°) tertiary (3°) Increasing stability
As the number of alkyl groups attached to the
positively charged carbon atom increases, the
stability of carbocation increases

Likewise, the stability of free radical increases as
more alkyl groups are attached to the carbon atom
with unpaired electron
Free Radical Stability :
methyl radical primary (1°) secondary (2°) tertiary (3°) Increasing stability

11.6.4 Reagents and Sites of Organic Reactions a) Electrophile (E + )
Means ‘ electron loving ’
An electron-deficient species and electron-pair
acceptor that attacks a part of a molecule where
the electron density is high
An electrophile can be either neutral or positively
charged

Examples of electrophiles :-
cations such as H + , H 3 O + , NO 2+ , Br + etc.
carbocations.
Lewis acids such as AlCl 3 , BF 3 etc.
oxidizing agents such as Cl 2 , Br 2 and etc
Examples of electrophilic sites in organic molecules :-
molecules with low electron density around a
polar bond such as :-
 +  -  +  -  +  - C = O C – X C – OH carbonyl haloalkanes hydroxyl compound

b) Nucleophile (Nu - )
Means ‘ nucleus loving ’
An electron-rich species and electron-pair donor
that attacks a part of a molecule where the electron
density is low
A nucleophile can be either neutral or negatively
charged

Examples of nucleophiles :-
anions such as OH - , RO - , Cl - , Cn - etc.
carbanions. (species with –ve charge on C atoms)
Lewis bases which can donate lone pair electrons
such as NH 3 , H 2 O etc.
Examples of nucleophilic sites in organic molecules :-
molecules with high electron density around the carbon-carbon multiple bond such as :- -C=C- (alkenes) , -C  C-(alkynes), (benzene ring) and etc.

11.6.5 Types of Organic Reactions
The four main types of organic reactions are:
Addition
Substitution
Elimination
Rearrangement

Addition reaction
a) Electrophilic addition b) Nucleophilic addition A reaction in which atoms or groups added to a multiple bond

a) Electrophilic Addition
Initiated by an electrophile , which attacks a
nucleophilic site of a molecule
Typical reaction of unsaturated compounds such
as alkenes and alkynes
Example : CH 3 CH=CH 2 + Br 2 CH 3 CHBrCH 2 Br room temperature electrophile

b) Nucleophilic Addition
Initiated by a nucleophile , which attacks an
electrophilic site of a molecule
Typical reaction of carbonyl compounds
Example :   + H + CN -

2. Substitution Reaction A reaction in which an atom or group in a molecule is replaced by another atom or group a) Free-radical Substitution b) Electrophilic Substitution c) Nucleophilic Substitution

a) Free-radical Substitution
Substitution which involves free radicals as
intermediate species
Example : CH 3 CH 3 + Cl 2 CH 3 CH 2 Cl + H Cl uv light

b) Electrophilic Substitution
Typical reaction of aromatic compounds
The aromatic nucleus has high electron density,
thus it is nucleophilic and is tend to electrophilic
attack
Example : + Br 2 Fe catalyst Br + H Br electrophile Br  Br 

c) Nucleophilic Substitution
Typical reaction of saturated organic compounds
bearing polar bond as functional group, such as
haloalkane and alchohol
Example : CH 3 CH 2 Br + OH - (aq) CH 3 CH 2 OH + Br - (aq) nucleophile  

3. Elimination Reaction
A reaction in which atoms or groups are removed
from adjacent carbon atoms of a molecule to form
a multiple bond (double or triple bond)
Elimination reaction results in the formation of
unsaturated molecules
CH 3 C H 2 OH CH 2 = CH 2 + H 2 O Conc. H 2 SO 4  Example :

4. Rearrangement Reaction
A reaction in which atoms or groups in a molecule
change position
Occurs when a single reactant reorganizes the
bonds and atoms
Example : tautomerisation enol keto (more stable) H C C H OH R H C C R H H O

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