LECTURE NOTE SK025 (ORGANIC) KMKt

6.3 (c) The influence of ortho-para and meta directing
substituents towards electrophilic aromatic substitution
reaction.

i. Effect of Substituents on Electrophilic Aromatic Substitution

A substituent on the benzene ring affects 2 aspects of electrophilic
aromatic substitution :
a) Reaction rate
b) Orientation

Effect of substituent

Reaction rate Orientation

Activating Deactivating Ortho-para Meta

i. Reaction Rate

• A substituted benzene reacts faster or slower
towards further substitution.

• Example: more reactive or less reactive

i. Activating Groups (activators)

• Activate the benzene ring towards electrophilic attack,
making it more reactive than benzene via Inductive Effect or
Resonance Effect.

• Electron-donating groups / electron-releasing groups are
activating groups

Example

Benzene rings that contain an electron-donating group (activating
group):

CH3

Alkyl group activates the benzene ring
via Inductive Effect

OH NH2 Hydroxy and amino groups
activate the benzene ring via
Resonance Effect

ii. Deactivating Groups (deactivators)

• Deactivate a benzene ring towards electrophilic attack,
making it less reactive than benzene.

• Electron-withdrawing groups are deactivating groups.

Example
Benzene rings that contain an electron-withdrawing group
(deactiving group):

Cl CN COOH

Example

H HNO3, H2SO4 NO2

50-55 °C Nitro compound

Deactivating group H Activating group

Cl OH

Relative rate 0.033 1 1000
of nitration reactivity

ii. Orientation

The existing substituent on the benzene ring
determines the position of the second
substituent.

A

ortho ortho

meta

meta

para A = substituent

A A

A = ortho-para director E

+

A 1,2- (ortho) E

+ E+ 1,4- (para)

A

A = meta director 1,3- (meta)

E

a) Ortho-para director

Example Ortho and para isomers

CH2CH3 CH 2CBHr CH2CH3

Br2, FeBr3 3

+

ethylbenzene Br

ii. Meta director

NO2 NO2
NO2
conc. HNO3, conc. H2SO4

100 °C

nitrobenzene

Thus,

1. All ortho-para directors (except halogens) ⇒
activating groups.

2. All meta directors ⇒ deactivating groups.
3. The halogens are ortho-para directors but

deactivating groups.

Ortho-para directors which are activating groups

Increasing ring activation General structure

●● ●● ●● —R or —Z ●●
—NH2 —NHR —NR2
Alkyl have lone
●● (Inductive effect) pair electron
(Resonance effect)
—●O●H

●● Ortho-para directors which are deactivating groups

—●O●R

●●

—NHCOR

—R —●●F●●●● —●●C●●l●● General structure :

—●●B●●r●● ●● ●●—X●● (halogens)

—●●●●I ●●

Meta director which are deactivating groups

—CHO

—COR General structure:
—COOR —Y (δ+ or + )
—COOH

—CN partial full
—SO3H charge charge

—NO2
+

—NR3

Increasing ring deactivation

6.3 (d) Predict the product of electrophilic aromatic
substitution of monosubstituted benzene.

EXERCISE 1 OH

Cl2
FeCl3

6.3 (e) Reactions of Alkylbenzene

(i) OXIDATION WITH HOT ACIDIFIED KMnO4 or K2Cr2O7

Benzylic H

CH3

CH3 CH(CH3)2

Benzylic hydrogen
Hydrogen atom bonded to a sp3 hybridized
carbon atom that bonded to a benzene ring.

73

Reactions of Alkylbenzene

Example…

The alkylbenzenes with alkyl groups other than
methyl will produce benzoic acid, carbon dioxide
and water.

+ CO2 + H2O

74

Example… Reactions of Alkylbenzene

+ CO2 + H2O

Compounds without a benzylic H are inert to
oxidation.

75

Reactions of Alkylbenzene

(ii) Halogenation ( free radical substitution)
• Take place at high temperature or in the

presence of uv light.
• Mechanism: free–radical substitution.

• Cl or Br replaces H atom of alkyl group.

Example…

CH3 CH2Cl

Cl2
heat or light

toluene benzyl chloride

76

Benzene vs Alkylbenzene

KMnO4/H+ @K2Cr2O7/H+ no reaction occur
benzene cannot undergo
∆
oxidation

X2 no reaction occur
CH2Cl2 benzene cannot undergo
halogenation in inert
Where X: Cl2 @ Br2 solvent

X2 no reaction occur
uv benzene cannot undergo
halogenation under
Where X: Cl2 @ Br2 sunlight

77

Benzene vs Alkylbenzene

COOH

KMnO4/H+ @K2Cr2O7/H+
∆

R
no reaction occur
can’t undergo
X2 halogenation in inert
CH2Cl2 solvent

Where X: Cl2 @ Br2

R-X

X2
uv

Where X: Cl2 @ Br2

78

Benzene vs Alkylbenzene

Examples ... COOH

KMnO4/H+ @K2Cr2O7/H+

∆

CH3CHCH3 Purple colour of KMnO4
decolourised @ orange
Cl2 colour of K2Cr2O7 turns green
CH2Cl2
no rxn. occur

Br

CH3CCH3

Br2
uv

reddish brown colour of 79
bromine decolourised

Exercise 1

Suggest the reagents and products formed for the
following reaction: A

O Cl2 ,AlCl3 NO2
C-CH3 C

B

CH3CH2Cl , FeCl3

E Br2 D K2Cr2O7/H+ F
uv
∆

80

Exercise 2

Compound A and B are hydrocarbon with the

structural formula: CH3

AB

Name the compound A and B

Write the equation for the reaction between B and bromine
under sunlight

The product of bromination of B depends on the reaction
conditions. State the conditions and the product formed8.1

THE END

Edited By: Revised By:
AO KAD

Approved By:
ZA

82



INTRODUCTION Name of haloalkanes according to IUPAC
TO nomenclature

HALOALKANES Give the structural formulae of
haloalkanes

Classify 1˚, 2˚, 3 ˚ haloalkanes

Describe haloalkanes as compounds that
contain polar bond & carbon bearing the

halogen is susceptible to nucleophilic
attack

HALOALKANES

❑ Contain halogen atom X bonded to
sp3 hybridized C atom

C+δ ●●X●●●●–δ

❑ General formula: CnH2n+1X (acyclic)
CnH2n-1X (cyclic)
R—X ; X = F, Cl, Br, I

❑ Also known as alkyl halides

CLASSIFICATION

❑ Depends on the classification of C that

bonds to the halogen

CLASS EXAMPLE

Primary (1o)
Halogen is bonded to 1o C

Secondary (2o)
Halogen is bonded to 2o C

Tertiary (3o)
Halogen is bonded to 3o C

Note: CH3–Cl is methyl haloalkane

EXERCISE

Classify each alkyl halide as 1o, 2o and 3o.

(a) CH3CH2CH2CH2CH2—Br (b) F

(c) CH3 (d) I
CH3—C—CHCH3
CH3Cl

IUPAC NAMES

❑ Find longest C chain

parent name = heptane

parent name = heptane

IUPAC nomenclature for alkyl
halides follows the basic
rules as described in alkanes

—F ☞ fluoro

—Cl ☞ chloro

—Br ☞ bromo

—I ☞ iodo

❑ Number parent chain beginning at the
end nearer to the first substituent,
regardless of whether it is alkyl or halo

5–bromo–2,4–dimethylheptane

Substituent 5-bromo
Parent 2,4-dimethyl

heptane

❑ Treat halogens exactly like alkyl for

numbering and alphabetizing purpose

(because the reactivity of halogen and alkyl group are almost similar)

2–bromo–5–methylhexane

Substituent 2–bromo
5–methyl

Parent hexane

F

2 1 CH2CH3
3

45

1–ethyl–2–fluorocyclopentane

Substituent 1–ethyl
Parent 2–fluoro

cyclopentane

3 2
4

5 1
6

1-chloro-3-ethylbenzene

Substituent 1–chloro
Parent 3-ethyl

benzene

EXERCISE

Give the IUPAC name for each compound.

No Structure No Structure
1 4

25

36

POLAR C—X BOND

❑ The electronegative halogen X creates a
polar C–X bond, making C atom electron
deficient

Attack!
(by nucleophile)

δ+ δ–
Cx

❑ The electrophilic C of alkyl halide is
susceptible to nucleophilic attack

HALOALKANES
(ALKYL HALIDES)

7.2 Chemical Properties
of Haloalkanes

Chemical Reactions of Haloalkanes
Properties of
Haloalkanes Explain Nucleophilic Substitution reaction
(Alkyl Halides)
Explain SN1 and SN2 mechanisms

Illustrate SN1 and SN2 mechanism

Compare relative reactivities of 1˚, 2˚, 3 ˚
haloalkanes towards hydrolysis or alcoholysis
Explain elimination reaction

Grignard Reagents

Explain the use of haloalkanes in the synthesis
of Grignard Reagents, RMgX / ArMgX
Deduce the structural formulas of alcohols and
carboxylic acids prepared using Grignard
Reagents

NUCLEOPHILIC SUBSTITUTION
REACTION

❑ Haloalkanes undergo substitution
reactions with nucleophiles

R—X + ●●Nu– R—Nu + ●●X–
haloalkane nucleophile leaving grou

A nucleophile replaces a leaving
group on an sp3 hybridized carbon

NUCLEOPHILES

Strong Weak
nucleophile nucleophile

HO‒ H2O
ROH ; e.g: CH3OH
–OR ; e.g: CH3O‒
CN– NH3
CH3COO–

1. Reaction with NaOH

alkyl group nucleophile leaving group
CH3—OH + Cl–
CH3—Cl + –OH
hydroxide

2. Reaction with Alkoxide Ion, RO-

alkyl group nucleophile leaving group
CH3CH2—OCH3 + Br–
CH3CH2—Br + –OCH3
methoxide

3. Reaction with excess Ammonia

alkyl group nucleophile leaving group

CH3CH2CH2—Br + NH3 CH3CH2CH2—NH2
(excess)
bromopropane propanamine

+ NH4+ Br–

React with 1o amine to form 2o amine
Example :

NOTE :
* Similar reaction can be used to produce tertiary amine.

4. Reaction with KCN or NaCN or CN-

alcohol
reflux

alkyl group nucleophile leaving group
CH3CH2—Cl
+ –CN alcohol CH3CH2—CN + Cl–
cyanide reflux

5. Reaction with Alcohol, RO-

alkyl group nucleophile

CH3 CH3 + CH3OH
Cl methanol

1–chloro–1,3–dimethylcyclopentane

leaving group

CH3 CH3 HCl
+

OCH3

6. Reaction with acetate ion

OOCCH3

alkyl group nucleophile leaving group
O O

CH3—Br + –O—C—CH3 CH3—O—C—CH3
bromomethane acetate methyl acetate
+ Br –

7. Reaction with water, H2O

R—X + H2O R—OH + HX

alcohol

alkyl group nucleophile leaving group

CH3CH2—Br + H2O CH3CH2—OH + Br –
bromoethane ethanol

SN1 Mechanisms SN2
Unimolecular of Bimolecular

Nucleophilic Nucleophilic Nucleophilic
Substitution
Substitution Substitution

Reaction Reaction

SN2 MECHANISM

❶ Bimolecular Nucleophilic Substitution

The reaction involve two molecule
(Haloalkane & Nucleophile)