C11 INTRODUCTION TO ORGANIC CHEMISTRY

Stability of carbanion

o Alkyl group and other electron-releasing
groups decreases the stability (destabilised) of
carbanions.

o Electron withdrawing group (e.g: halogen)
stabilised carbanions through the inductive
withdrawal of electron density

101

H HR R

H C- H > H C- R > H C- R > R C- R

Methyl 1o 2o 3o
anion
carbanion carbanion carbanion

The descending order of stability of carbanion

102

Reagents of Organic reactions

Reagents in organic reaction can be considered as
an electrophile or a nucleophile.

i. Electrophile

Electrophiles are ions or molecules that are
electron-deficient and can accept electron-pairs.

examples :
• cations such as H+, H3O+, NO2+, Br+ etc.
• carbocations

• Lewis acids (defined as a substance that can

accept a pair of electrons to form a covalent

bond) such as AlCl3, FeCl3, BF3 etc. 103
• oxidizing agents such as Cl2, Br2 and etc.

• electrophilic sites are molecules with low
electron density around a polar bond

examples:

i) + - ii) + -
-C = O (carbonyl) - C – X (haloalkanes)

+ -
iii) -C – OH (hydroxy compounds)

104

ii) nucleophile

Nucleophiles are ions or molecules that are rich
in electrons and can donate electron-pairs.

examples :
• anions such as OH-, RO-, Cl-, CN- etc.
• carbanions
• Lewis bases (defined as a substance that can
donate a pair of electrons to form a covalent
bond) such as NH3, H2O, H2S etc.

•nucleophilic sites are electron rich sites (C-C
multiple bonds) in organic compounds, e.g : C=C,
C≡C and

105

Types of Organic Reactions :

• The four main types of organic reactions are:
i. Addition
ii. Substitution
iii. Elimination
iv. Rearrangement

106

i) Addition Reaction
• A reaction in which atoms or group of atoms are

added to adjacent atoms of a multiple bond.

• Involves compounds with multiple bonds
(unsaturated) such as alkenes and carbonyl
compounds.

• Two types of addition :-

a) Electrophilic Addition
b) Nucleophilic Addition

107

a) Electrophilic Addition
• Initiated by an electrophile accepting electron

from a nucleophile.
• Typical reaction of unsaturated compounds

such as alkenes and alkynes.

Example :

CH3CH=CH2 + Br2 CCl4 CH3CHBrCH2Br
room

nucleophilic site temperature

electrophile

108

b) Nucleophilic Addition

• Initiated by a nucleophile, which attacks an
electrophilic site of a molecule.

• Typical reaction of carbonyl compounds.

 CN
H3C C CH3
O
OH
H3C C CH3 + HCN
 109
nucleophile

electrophilic site

ii) Substitution Reaction

• A reaction in which an atom or group in a
molecule is replaced by another atom or group.

• A typical reactions of saturated compounds
such as alkanes, alkyl halides and alcohols;
and of aromatic compunds

• Three types of substitution reactions :-
a) free radical substitution.
b) electrophilic substitution.
c) nuclephilic substitution.

110

a) Free-radical Substitution

• Substitution reaction which involves
halogen atom that form free radicals as
the intermediate species.

Example :

CH3CH3 + Cl2 uv CH3CH2Cl + HCl

alkane halogen One H atom is
substituted by Cl

111

b) Electrophilic Substitution
• Typical reaction of aromatic compounds.

• The aromatic nucleus has high electron density,
thus it is a nucleophile and is exposed to
electrophilic attack.

Example:

+ Br2 FeBr3 Br

+ HBr

electrophile
nucleophilic site

112

c) Nucleophilic Substitution

• Typical reaction of saturated organic compounds
bearing polar bond as functional group, such as
haloalkane with alcohol.

Example :

 

H3CH2C Br + OH-(aq) Δ CH3CH2OH + Br-(aq)

electrophilic nucleophile

site

113

iii) Elimination Reaction

• An atoms or groups are removed from
adjacent carbon atoms of a molecule to form
a multiple bond (double or triple bond).

• Results in the formation of unsaturated
molecules.

Example : H H

HH KOH C C + HBr
HC C
H H
H Br ethanol, reflux
114
H

iv) Rearrangement Reaction

• A reaction that results in a product with a
different carbon skeleton from the reactant.

• In a rearrangement reaction, a molecule
undergoes a reorganization of its constituent
parts

Example: H CR
O
H CCR tautomerisme
H OH
HC

H

115

Exercise

Identify the type of chemical reactions below:

CH3 CH3

i) CH3 CH CH CH3 concentrated H2SO4 CH3 C CH CH3 + H2O
OH


ii) H3C CH3 H3C CH3
C Br + NaOH(aq) C OH + NaBr(aq)
CH3 CH3

116

H CH3

iii) + CH3Cl AlCl3 + HCl

iv)CH3 CH2 OH + HCl CH3 CH2 Cl + H2O

v) CH3 CH3 + Cl2 uv CH3 CH2 Cl + HCl

117

O H + NaCN OH
vi) CH3 C H2SO4 CH3 C H

CN

vii) CH3CH CH2 + Br2 CH3CH CH2
Br Br

118

Answer 119
i) Elimination reaction

ii) Nucleophilic substitution reaction
iii) Electrophilic aromatic substitution reaction
iv) Nucleophilic substitution reaction
v) Free radical substitution reaction
vi) Nucleophilic addition reaction
vii) Electrophilic addition reaction