C11 INTRODUCTION TO ORGANIC CHEMISTRY

Example 1
Draw all the chain isomers of C5H12

CH3 CH2 CH2 CH2 CH3 CH3
pentane
CH3 CH CH2 CH3
CH3 2-methylbutane

H3C C CH3

CH3

2,2-dimethylpropane

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b) Positional isomers

• These isomers have a substituent group in different
positions in the same carbon skeleton.

Example 2 :
Draw all the positional isomers of C5H11Cl

54321

CH3 CH2 CH2 CH2 CH2 Cl

1-chloropentane

5 4 3 21 5 4 32 1

CH3 CH2 CH2 CH CH3 CH3 CH2 CH CH2 CH3

Cl Cl 52
2-chloropentane 3-chloropentane

Example 3
Draw all the positional isomers of C4H8 and C8H10

432 1 43 2 1

CH3 CH2 CH CH2 CH3 CH CH CH3
1-butene 2-butene

CH3 CH3 6 CH3
1 CH3 5
1 1
6 62 2
2
53 3
53 4
4 4 CH3
CH3
1,2-dimethylbenzene 1,3-dimethylbenzene

1,4-dimethylbenzene

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Exercise:
Draw all the chain & positional isomers of C4H9Cl

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Answer CH3
CH3 CH CH2 Cl
chain isomer
CH3 CH2 CH2 CH2 and

Cl

CH3 and CH3CH2 CH CH3
CH3 C CH3 Cl

Cl

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Answer

positional isomer and CH3 CH2 CH CH3
CH3 CH2 CH2 CH2 Cl

Cl

CH3 CH3
CH3 C CH3 and CH3 CH CH2 Cl

Cl

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c) Functional group isomers

 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 are :

Classes of compounds General Formula
Alcohol & Ether
Aldehyde & Ketone CnH2n + 2O
Carboxylic acid & Ester CnH2nO
Alkene & Cycloalkane CnH2nO2
CnH2n

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Classes of General Example 4:
compounds Formula Functional Group Isomers

Alcohol & Ether CnH2n + 2O Molecular formula C2H6O
CH3 CH2 OH
Aldehyde & CnH2nO
Ketone Alcohol (ethanol)
H3C O CH3
Ether (methoxymethane)
Molecular formula C3H6O
CH3 CH2 C H

O
Aldehyde (propanal)
H3C C CH3

O
A ketone (propanone)58

Classes of General Example 4:
compounds Formula Functional Group Isomers
Molecular formula C3H6O2
Carboxylic acid CnH2nO2
& Ester CH3 CH2 C OH

O
Carboxylic acid
(propanoic acid)

H3C O C CH3

O
Ester (methyl ethanoate)

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Alkene & CnH2n Molecular formula C6H12
Cycloalkane

H2C CHCH2CH2CH2 CH3
Alkene (1-hexene)

HH

HC H

HC CH

HC CH

H CH

HH

Cycloalkane (cyclohexane)

or

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2. Stereoisomers

Isomers with the same structure but that differ in
respect to the arrangement of their atoms in
space
Types of stereoisomers are:
a) Diastreomer
b) Enantiomer

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a) Diastereomers

 Diastereomers are stereoisomers which are
not mirror images of each other.

 Class of diastereomers:
i. cis - trans isomers
ii. other diastreomers
(molecule with two or more chiral
carbons)

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i. cis - trans isomers

 also known as geometric isomers.

 occurs only in two classes of compounds;
alkenes and cyclic compound

 are stereoisomer that differ by groups being on
the same sides (cis-isomer) or opposite sides
(trans-isomer) of a site of rigidity ( C=C) in a
molecule

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The requirements for the existence of cis - trans
isomerism are:

i. a restricted rotation of a C – C double bond
in an alkenes or a C-C single bond in a cyclic
compound.

ii. each carbon atom of a site of restricted rotation
has two different groups attached to it.

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i. cis – trans isomers of C – C double bond in
alkene

Example

a. 2-butene

cis - groups CH3 same on the trans - groups CH3 same on the
same side opposite side

CH3 CH3 CH3 H

CC CC

HH H CH3
cis-2-butene
trans-2-butene

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Example

b. Identify the cis – trans isomers of 3-methyl-2-pentene

CH3 CH3 CH3 CH2 CH3
C C C C

H CH2 CH3 H CH3

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Example (answer)

CH3 CH3 CH3 CH2 CH3
C C C C

H CH2 CH3 H CH3

cis isomers trans isomers

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ii. cis – trans isomers of cyclic compound
Example

a. 1,2-dimethylcyclohexane

HH H CH3

CH3 CH3 CH3 H
cis-1,2-dimethylcyclohexane trans-1,2-dimethylcyclohexane

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Example

b. Identify the cis – trans isomers of 1,3-dichlorocyclopentane

H Cl
H Cl HH
Cl Cl

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Example (answer) Cl
HH
H Cl
H Cl
Cl trans isomers
cis isomers

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• Geometric isomerism (cis-trans isomerism) is not
exhibited if one of the doubly bonded carbons
attached to 2 identical atoms or groups of atoms.

Example

a) H CH3 b) H Cl
C CH3 Cl
C CH3
H3C CH3 C

c) Cl
C

H CH3

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b) Enantiomers

• Every object has a mirror image. However the mirror
image and the object itself may or may not be
superimposable.

• Enantiomers are molecules like left and right hands
in which are mirror images, but they are not
identical, or superimposable.

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• Enantiomers are stereoisomers which are non-
superimposable with its mirror image molecule
due to the existence of chirality center in the
molecule.

• Chirality center is a carbon atom in a molecule
which is bonded to four different atoms /
groups of atoms.

• It also known as asymetric carbon atom or
stereocenter and is often marked by an
asterisk ()

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3D structrure of chiral molecule
Example : CHClFBr molecule

Molecule and
its image are
mirror image

Molecule and its
image are not
superimposable

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3D structrure of achiral molecule
Example : CH2ClBr molecule

Molecule and its
image are mirror
image

• Achiral Molecule and its
compounds /non image are
enantiomers are superimposable
molecules and
its mirror image 75
molecule
superimposable

Example
Draw a pair of enantiomers of 2-bromobutane

2-bromobutane, CH3 CH2 CH CH3

Br

CH3 CH3

C H C
Br H
CH3CH2 CH2CH3
Br
mirror

a pair of enantiomers 76

Example 13
Clasiffy each of the following pairs as enantiomers.

a) CH3 CH3 Non enantiomers
A pair of enantiomers
CH3 H3C

Cl Br Br Cl

b) CH3 CH3

Br Cl Br
H Cl H

c) H Br Br H

F A pair of enantiomers

F

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• Enantiomer compounds
i. are chiral molecules
ii. have identical physical properties
iii. have the ability to rotate the plane-polarized
light through/ in a polarimeter and said to be
optically active.

• However, enantiomer molecules are differ in
their behavior toward plane-polarised light.

* A polarimeter is an instrument that allows polarized light to
travel through a sample tube containing an organic
compound. It permits the measurement of the degree to
which an organic compound rotates plane-polarized light.

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a polarimeter

A schematic diagram of a polarimeter

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• Plane-polarized (polarized) light is the light that
has an electric vector that oscillates in a single
plane. Plane-polarized light arises from passing
ordinary light through a polarizer.

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• A pair of enantiomers rotate the plane polarised
light in opposite direction.

• If one of enantiomer rotates the light in the
clockwise direction, the rotation, α, is said to be
positive(+) and the substance is said to be
dextrorotatory.

• If the rotation is counterclockwise, the rotation is
said to be negative(-) and the substance is said
to be levorotatory

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11.4 REACTIONS OF ORGANIC
COMPOUNDS

86

11.4 REACTIONS OF ORGANIC COMPOUNDS

At the end of the lesson, student should be able to:
a) explain covalent bond cleavage

i. homolytic cleavage
ii. heterolytic cleavage
b) differentiate homolytic cleavage and heterolytic
cleavage.
c) state the relative stabilities of primary, secondary
and tertiary free radicals, carbocations and
carbanions

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11.4 REACTIONS OF ORGANIC COMPOUNDS

At the end of the lesson, student should be able to:
d) compare the stabilities of carbocation and

carbanions by using the inductive effect of alkyl
group

e) define
i. electrophile and nucleophile
ii. Lewis acid and Lewis base

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11.4 REACTIONS OF ORGANIC COMPOUNDS
At the end of the lesson, student should be able to:
e) state the main types of organic reactions:

i. addition (nucleophilic and electrophilic)
ii. substitution (nucleophilic and electrophilic)
iii. elimination
iv. rearrangement
g) identify the main types of organic reaction in
given a reaction equation

89

11.4 REACTIONS OF ORGANIC COMPOUNDS

 Reactions of organic compounds involve the
forming and breaking of covalent bonds.

 A covalent bond may break in two fundamentally
different ways:
a) Homolytic cleavage
b) Heterolytic cleavage

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a) Homolytic Cleavage

• Occurs in a non-polar bond of two atoms of

similar electronegativity. unpaired
electron

XX X +X

free radicals

• A single bond breaks symmetrically into two
equal parts; leaving each atom with one
unpaired electron, known as a free radical as
the product.

• Shown by a single headed arrow, 91

free radical : an uncharged chemical species that
contains an unpaired electron

Example

Homolytic cleavage in some molecules

i) Cl Cl uv Cl + Cl

Free radicals

HH H H
HC
ii) H C C H HC+

HH H H
H
H
H C+H
iii)

HCH

H H 92

b) Heterolytic Cleavage two atoms with

• Occurs in a polar bond of
different electronegativities.

• A single bond breaks unsymmetrically in which
both bonding pair electrons are transferred to the
more electronegative atom thus form a cation and
anion.

bonding pair e

δ - δ+ A- + B+

AB

anion cation

• Shown by a double headed arrow,

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A heterolytic cleavage that forms a carbocation

CH3 CH3

H3C δ+ δ - H3C + + -

C Cl C Cl

CH3 CH3

carbocation anion

• chlorine is more electronegative than carbon thus the
C—Cl bond is polar.

• both bonding pair electrons are transferred to Cl atom.

• a carbocation (CH3)3C+ and a chloride, Cl- ion is formed.

• If the heterolytic cleavage involves carbon atom, either a

carbocation (+ve charge on the carbon atom) or a

carbanion (–ve charged carbon atom) is formed. 94

A heterolytic cleavage that forms a carbanion

H3C CH3 H3C CH3 +
 - 
C- + Li
C Li

CH3 CH3 cation

carbanion

• carbon is more electronegative than Li thus the
C—Li bond is polar.

• both bonding pair electrons are transferred to
carbon atom.

• a carbanion (CH3)3C- and a lithium, Li+ ion is
formed.

95

The differences between homolytic cleavage
and heterolytic cleavage

homolytic cleavage heterolytic cleavage

Occurs in a non-polar bond Occurs in a polar bond of
of two atoms of similar two atoms with different
electronegativity. electronegativities.

A single bond breaks A single bond breaks

symmetrically into two equal unsymmetrically in which

parts; leaving each atom with both bonding pair electrons

one unpaired electron, known are transferred to the more

as a free radical as the electronegative atom thus

product. form a cation and anion.

Stability of free radical

• The stability of free radical increases as more
alkyl groups (electron releasing group) are
attached to the carbon atom with the unpaired
electron.

• The alkyl group is an electron-releasing group
which will increase the electron density at C•
inductively, thus help to stabilize the unpaired
electron of carbon atom.

HH CH3 CH3

H C < H3C C < H3C C < H3C C

H H H CH3
2º
Methyl 1º 3º
free radical free
free radical free
radical radical

Ascending order of stability of carbon free radical

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Stability of carbocation

• The stability of carbocation increases with the
number of alkyl groups present.

• The alkyl group is an electron-releasing group
which will increase the electron density at C+
inductively, thus help to stabilize the positive
charge on the carbocation.

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HH CH3 CH3

H + < H3C + < H3C C+ < H3C C+

C C

HH H CH3

methyl 1º 2º 3º
carbocation carbocation
carbocation carbocation

The ascending order of stability of carbocation

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