Mechanism of Aromatic Electrophilic Substitution Reaction

1
B.Sc. First year
Students
Semester – I
Unit-IV
Mechanism of Electrophilic
Aromatic Substitution
by
Dr Pramod R Padole

Unit IV- Aromatic Hydrocarbons
Orientation: Effect of substituent groups. Activating and
deactivating groups.
Theory of reactivity and orientation on the basis of inductive
and resonance effects (-CH3, -OH, -NO2 and –Cl groups).
D
Nomenclature and Isomerism of Aromatic Compounds:
Structure of Benzene: Kekule structure and Molecular orbital structure.
A
Aromaticity and Huckel’s rule Aromatic, antiaromatic and
non-aromatic systems
B
Mechanism of Electrophilic Aromatic Substitution: Nitration,
Friedal Craft Alkylation and Acylation.Nuclear and Side Chain
Halogination, Birch Reduction
C

LOGO
C) Mechanism of
Aromatic Electrohilic
Substitution:
Q.1) Discuss why benzene undergoes electrophilic substitution reaction
rather than addition reaction. (W-06, 2 Mark)

Aromatic Electrophilic Substitution
Reaction:
 The π-electrons in benzene are loosely held (e-s rich
centre) and are easily available to electrophilic
reagents.
 Thus, benzene acts as a source of electrons
(nucleophile) and will be attacked by electrophiles
only.
 Hence, it will undergo electrophilic substitution
reactions, such as Halogenation, Nitration,
Sulphonation, etc.
 The replacement of hydrogen from the benzene ring
by the electrophile such as Cl+, NO2
+, SO3, etc. is
known as electrophilic substitution reaction.

Reaction:

Reaction:
After the addition reaction, compound lost their aromatic character, so benzene
undergoes electrophilic substitution reaction rather than addition reaction.

Aromatic Electrophilic Substitution Reaction
Aromatic
Electrophilic
Substitution
Reaction
Aromatic Electrophilic Substitution Reaction:

LOGO
General Mechanism of
Aromatic Electrophilic
Substitution
Important three steps

 The substitution reactions of aromatic ring caused by the
presence of an active species, the electrophile (E+), are
commonly called as electrophilic substitution reaction.
 Reaction:
General Mechanism of Aromatic Electrophilic Substitution:
Mechanism:
General mechanism proceeds through the following steps.
Step-1) Formation or Generation of electrophile:
The active electrophile, E+, is formed from E-Nu reagent as follows.

Step-2) Formation of σ- complex
Or Attack of Electrophile on Benzene ring:
The electrophile so formed then attacks the benzene ring to
forms resonance structure of carbonium ion known as benzonium
cation (σ-complex).
This is a slow step hence it is a rate determining step.
Step-3) Formation of product
or Elimination of Proton(H+) form σ-complex:
Elimination (abstraction) of proton from σ-complex, by base; to form
the electrophilic substitution product.

Mechanism of Nitration of
Benzene:
Q.1) Explain / Discuss the mechanism of nitration of benzene.
(S-04, S-05, W-07,S-08, W-15, S-16 & W-17, 4 Mark)
Q.2) How does benzene react with Nitrating Mixture (Con. HNO3 + Con. H2SO4 ).
(W-05, & W-06, 2 Mark)
Q.3) Give / Explain the mechanism of following reaction:-
(S-06, S-10, S-11 & S-14, 3-4 Mark)
Q.4) How can you synthesize Nitrobenzene from benzene. (S-07, 1 Mark)
Q.5) How will you convert Benzene to Nitrobenzene? (W-08, S-09, W-11 & S-18, 2 Mark)
Q.6) How does benzene react with Con. HNO3 in presence of Con. H2SO4 .
(W-09, W-10 & W-15, 2 Mark)
Q.7) How will you obtain the nitrobenzene from benzene? (S-10 & S-11, 2 Mark)
Q.8) The mixture of conc. H2SO4 and conc. HNO3 is called_________. (S-12, ½ Mark)
Q.9) Complete the following reaction: (S-15, 2 Mark)
Q.10) Give the mechanism of Nitration of benzene. (W-16, 4 Mark)
Q.11) How does Benzene react with conc. HNO3 and conc. H2SO4? (W-18, 2 Mark)
Q.12) The nitrating mixture is ________________. (W-19, ½ Mark)
H
HNO3
con. H2SO4
+
Heat at 60o
C
or at 333K
? + ?

Benzene:
H
HO-NO2
con. HNO3
con. H2SO4
NO2
H2O
Benzene
+
or
Heat at 60o
C
or at 333K
+
Nitrobenzene
Mixture of (con.HNO3 + Con.H2SO4) is known as nitrating mixture.
Mechanism of Nitration of benzene proceeds through the three steps.
Step-1) Formation of electrophile, i.e., NO2
+ ion:
The active electrophile, nitronium ion, NO2
+ is formed from
nitrating mixture as follows.
HNO3 + 2 H2SO4 NO2 + H3O + 2 HSO4
nitronium ion
(electrophile)
Hydronium ion Base
OR
HNO3 + H2SO4 NO2 + H2O + 2 HSO4
nitronium ion
(electrophile)
Base

Benzene:
Step-2) Formation of resonance stabilized σ- complex:
The nitronium ion, NO2
+, attacks to the benzene ring; to form
an intermediate carbonium ion known as σ-complex which is stabilized
by resonance. This is a slow step hence it is a rate determining step.
(electrophile)
H
+ NO2
slow
RDS
H NO2 H NO2 H NO2
I II III
H NO2
Benzene
Resonance structures
Resonance hybrid
(-complex)
nitronium ion



Step-3) Formation of product or elimination of Proton from σ-complex :
or
Abstraction of proton by Base from σ-complex ; to form final product:
Elimination of proton from σ-complex, by base; to form nitrobenzene.
H NO2
Resonance hybrid
(-complex)


NO2
nitrobenzene
+ HSO4
Fast
+ H2SO4
Base
(Nucleophile)

Halogenation of Benzene
(Nuclear Halogenation):
Explain / Discuss the mechanism of chlorination of benzene?

Q.1) How will you obtain the Chloro-Benzene from Benzene?
(W-04, S-05, W-17 & W-18, 2 Mark)
Q.2) What happens when benzene reacts with Cl2 in presence of halogen Carrier
(AlCl3)? (S-06 & W-07,1Mark)
Q.3) What happens when benzene is treated with Cl2 in presence of FeCl3?
(W-11,1Mark)
Q.4) How does benzene reacts with Cl2 in presence of FeCl3?
(S-07, S-10, W-10 & S-11, 1 Mark)
Q.5) How will you convert benzene to Chloro-Benzene? (S-08 & S-19, 2 Mark)
Q.6) How can you synthesise Chlorobenzene from Benzene? (W-09, 1 Mark)
Q.7) Explain / Discuss the mechanism of chlorination of benzene?
(W-09, W-10, S-11, W-15 & S-19, 4 Mark)
Q.8) Complete the following reaction: (W-13 & S-15, 2 Mark)
Q.9) The molecular formula of chlorobenzene is ____________. (S-18, ½ Mark)
H
Cl-Cl
Fe / FeCl3 / FeBr3 / AlCl3 / AlBr3 / I2
Catalyst
i.e., Halogen carrier
or in absence of sunlight
+ ? + ?

Halogenation of Benzene (Nuclear halogenation):
 The replacement of hydrogen atom of benzene ring (nucleus)
by halogen is called nuclear halogenation.
 When benzene is reacted with Chlorine in presence of a
catalyst, such as, iron(Fe)/ iron chloride(FeCl3)/ FeBr3 / AlCl3 /
AlBr3 or iodine (is called as halogen carrier) or in absence of
sunlight; to form chlorobenzene.
H
Cl-Cl
Fe / FeCl3 / FeBr3 / AlCl3 / AlBr3 / I2
Cl
H-Cl
Benzene
Catalyst
i.e., Halogen carrier
or in absence of sunlight
+
chlorobenzene
Reaction with Cl2 :
+

Halogenation of Benzene (Nuclear halogenation):
Electrophilic or Ionic Mechanism of Chlorination of Benzene:
Chlorination of benzene proceeds through the three steps.
Step-1) Formation of electrophile, chloronium ion, Cl + :
The catalyst, AlCl3, polarises the chlorine-chlorine bond;
to form chloronium ion, the active electrophile,Cl+, is used for
substitution.
Cl Cl AlCl3
Cl AlCl4
+ +
chlorine
molecule
Catalyst
Chloronium ion
electrophile
Base
Lewis base Lewis acid

Step-2) Formation of resonance stabilized σ-complex:
The chloronium ion attacks the benzene ring; to form
σ-complex which is stabilized by resonance.
Step-3) Elimination of Proton or Formation of product, Chlorobenzene:
When σ-complex loses proton by base; to form final
product, chlorobenzene.
(electrophile)
H
+ Cl
slow
RDS
H Cl H Cl H Cl
I II III
H Cl
Benzene
Resonance structures
Resonance hybrid
(-complex)
chloronium ion



H Cl
Resonance hybrid
(-complex)


Cl
chlorobenzene
+ AlCl4
Fast
+ AlCl3 + HCl
Base
(Nucleophile)

LOGO
Side Chain Halogenation:
Discuss the mechanism of side chain chlorination of Toluene.

LOGO
Side Chain halogenations of Benzene:
Q.1) Discuss the mechanism of side chain chlorination of Toluene. (S-05, 4 Mark)
Q.2) How does toluene react with Chlorine in presence of light? (S-06, 2 Mark)
Q.3) What is side-chain halogenations? Explain with example. (S-08, 2 Mark)
Q.4) How will you obtain Benzyl chloride from Toluene? (S-11, 2 Mark)
Q.5) Explain the mechanism of following reaction. (S-13, 4 Mark)
Q.6) Complete the following reaction: (S-14 & W-16, 2 Mark)
Q.7) What happen when: Toluene reacts with Chlorine in presence of U.V. light?
(W-14 & S-17, 2 Mark)
Q.8) How will you convert Toluene to Benzyl chloride? (S-18, 2 Mark)
CH2-H
Cl2
CH2Cl
+
in presence of
u.v. / sun light
at Boiling temp.
+ HCl
or in absence of Halogen carrier
CH2-H
Cl2
+
in presence of
u.v. / sun light
at Boiling temp.
or in absence of Halogen carrier
? + ?

LOGO
Side Chain halogenations of Benzene:
 The replacement of hydrogen atom (one by one) of side chain (i.e. CH3-
group in Toluene) by halogen in presence of U.V. light (or sunlight) at
boiling temperature is called side chain halogenation.
 When Toluene is treated or reacted with chlorine in presence of U.V. /
sunlight and in absence of halogen carrier, at boiling point undergoes
side chain halogenation, where all the three hydrogen atoms of methyl
group (side chain) get successively replaced by chlorine; to form benzyl
chloride, benzal dichloride and finally benzo trichloride, respectively.

LOGO
Mechanism of side chain halogenation:
 The side chain halogenation proceeds by free radical mechanism.
 It involves the following steps:
Step-1) Chain Initiation:
Chlorine molecule undergoes homolytic fission; to form chlorine
free radicals
Step-2) Chain Propagation:
(a) Chlorine free radical attacks side chain –CH3 group in Toluene;
to form benzyl free radical

LOGO
Step-2) Chain Propagation:
(b) Benzyl free radical further attacks to chlorine molecule; to form
benzyl chloride and chlorine free radical.
 In step-2): Reaction (a) and (b) are repeated over and over again,
called as propagation step.
Step-3) Chain-Termination:
Chain reaction comes to stops (ends) by the combination of any
two radicals.
Similarly, benzal dichloride and benzo trichloride are formed by further
chain reaction.

Company
LOGO
Friedel-Craft’s Alkylation:
Preparation of Toluene:

Q.1) How will you obtain the Toluene from Benzene? (S-04, W-04, W-05, S-10 & W-17, 2 Mark)
Q.2) How does benzene react with CH3Cl in presence of anhydrous AlCl3?
(S-06, W-06, W-09, W-10 & S-19, 2 Mark)
Q.3) Discuss / Explain the mechanism of Friedel-Crafts Alkylation of benzene.
(W-08, S-12 & W-14, 4 Mark)
Q.4) Explain the mechanism of following reaction: (S-09, S-15 & S-17, 4 Mark)
Q.5) How will you convert benzene to toluene? (W-12, 2 Mark)
Q.6) In Friedel-Crafts Alkylation reaction, the electrophile is a ____________ ion. (S-13, ½ Mark)
Q.7) What happens when, Benzene is treated with methyl chloride in presence of anhydrous AlCl3.
(S-13, 2 Mark)
Preparation of Ethyl Benzene
Q.8) How will you convert benzene to Ethyl Benzene? (S-08, S-09 & S-11, 1 Mark)
Q.9) Explain / Discuss the mechanism of alkylation of Benzene. (W-17, W-18, S-19 &W-19, 4 Mark)
Q.10) Replacement of hydrogen atom of benzene by alkyl group is called _______ of benzene.
(W-18, ½ Mark)
+ CH3Cl Anhydr. AlCl3

By Dr Pramod R Padole
CH3Cl in presence of anhydrous AlCl3
Friedel-Crafts Alkylation:
 The replacement of hydrogen atom of benzene ring
(nucleus) by alkyl group is known as alkylation of benzene.
 When benzene is treated or reacted with alkyl halide in
presence of Lewis acid, anhydrous AlCl3 / FeCl3; to form
alkyl benzene.
 Preparation of Toluene or Reaction of CH3Cl:
methyl chloride
H
+ CH3 Cl
anhydrous
AlCl3
CH3
+ H Cl
+ Cl CH3
Benzene
Toluene
(methyl benzene)

 The replacement of hydrogen atom of benzene ring
(nucleus) by alkyl group is known as alkylation of benzene.
 When benzene is treated or reacted with alkyl halide in
presence of Lewis acid, anhydrous AlCl3 / FeCl3; to form
alkyl benzene.
 Preparation of Ethyl Benzene or Reaction of C2H5Cl:
H
+ C2H5 Cl
anhydrous
AlCl3
C2H5
+ H Cl
+ Cl C2H5
Benzene
ethyl benzene
ethyl chloride

Mechanism of alkylation of Benzene:
e.g.:The methylation of benzene by methyl chloride in
presence of AlCl3; to form toluene.
It involves the three steps-
Step-1) Formation of electrophile, CH3
+ :
The catalyst, Lewis acid, AlCl3, forms electrophile, a polar
addition compound with methyl chloride according to the
following equation.
CH3 Cl AlCl3 CH3 AlCl4
+ +
Catalyst methyl cation
electrophile
Base
Lewis base Lewis acid
methyl chloride

Electrophile,CH3
+, attacks to the benzene ring ; to form
σ-complex which is stabilized by resonance.

Step-3) Elimination of Proton or Formation of product,
Toluene:
 Elimination of proton from σ-complex by base; to form final
product, Toluene.

LOGO
Friedel-Crafts
Acylation
Reaction
Replacement of hydrogen atom of benzene ring (nucleus) by acyl group, (RCO- group),
is known as Acylation or Acetylation.

Friedel-Craft’s Acylation Reaction:
Q.1) How will you obtain the Acetophenone from Benzene? (S-04, W-05&S-06, 2 Mark)
Q.2) How does benzene react with CH3COCl (Acetyl Chloride) in presence of anhydrous AlCl3?
(W-05, 2 Mark)
Q.3) Discuss the mechanism of Friedel Crafts Acylation of Benzene. (W-06, 4 Mark)
Q.4) How can you synthesize Acetophenone from benzene. (S-07&W-09, 2 Mark)
Q.5) What happens when benzene reacts with Acetyl Chloride in presence of halogen Carrier
(AlCl3)? (W-07 & W-10, 2 Mark)
Q.6) How will you convert Benzene to Acetophenone? (S-08, W-08 & S-12, 2 Mark)
Q.7) How will you obtain Acetophenone from Benzene? (S-11& W-18, 2 Mark)
Q.8) Complete the following reaction. (S-14 & W-16, 2 Mark)
Q.9) Explain Friedel Crafts Acylation reaction. (S-18, 4 Mark)
+ CH3CO Cl
AlCl3 ? + ?

Acetophenone from Benzene:
Friedel-Crafts Acylation:
 Replacement of hydrogen atom of benzene ring (nucleus)
by acyl group, (RCO- group), is known as Acylation or
Acetylation.
 When benzene is heated or reacted with acetyl chloride in
presence of catalyst, Lewis acid, AlCl3/ FeCl3; to form
acetophenone (acetyl benzene or methyl phenyl ketone)

Mechanism:
The reaction proceeds through the following steps.
Step-1) Formation of electrophile, acetyl cation, +COCH3:
Polarization of acetyl chloride by catalyst, AlCl3,
generates acetyl cation (acyliumion) in the following
manner.

Mechanism:
The electrophile, acetyl cation, attacks the benzene ring;
to form an intermediate carbonium ion, σ-complex which is
stabilized by resonance.

Mechanism:
Step-3) Elimination of Proton or Formation of product,
Acetophenone:
Elimination of proton from σ-complex by base; to form
the product, acetophenone.

LOGO
“ Add your company slogan ”
Birch Reduction
Hydrogenation of Aromatic Rings
Or Preparation of 1,4-CycloHexadiene:
Or Preparation of 1,4-Hexadiene:
Aromatic rings can be reduced by catalytic
hydrogenation, but higher temperatures
(373-473 K) are required than for ordinary double bonds.

Birch Reduction - Hydrogenation of aromatic
(Benzene) ring:
Q.1) How will you obtain the 1,4-cyclo Hexadiene from Benzene? (S-06& W-07, 1-2 Mark)
Q.2) What is Birch reduction? Explain its mechanism. (S-07, 3 Mark)
Q.3) How does benzene react with Liquid ammonia and sodium in the presence of alcohol?
(W-09 & S-10, 1-2 Mark)
Q.4) How will you convert: Benzene to 1,4-cyclo Hexadiene? (W-11& S-19, 2 Mark)
Q.5) Complete the following reaction: (W-13, 2 Mark)
Q.6) What happen when: Benzene reacts with sodium in liquid ammonia in presence of
alcohol? (W-14 & S-17, 2 Mark)
Q.7) Discuss Birch reduction of benzene with mechanism. (W-15 & S-18, 4 Mark)

(Benzene) ring:
 Aromatic rings can be reduced by catalytic hydrogenation, but higher
temperatures (373-473 K) are required than for ordinary double bonds.
 When Benzene (aromatic) ring is hydrogenated (or reduced) by sodium
(or lithium or potassium) in liquid ammonia solvent (such reactions are
known as dissolving metal reductions) in the presence of an alcohol,
1,4- addition of hydrogen takes place; to form non-conjugated
cyclo-hexadiene.
 This reaction is called the Birch reduction.
 Thus, Benzene on Birch Reduction; to form 1,4-Hexadiene
(or 1,4-cyclohexadiene)

(Benzene) ring:
Mechanism of Birch Reduction:
 It involves solvated electrons, obtained from metal, like Li / Na / K in the
liq. NH3 & then transfer to Benzene ring.
Step-1) Loss of electron by sodium:
 The sodium becomes oxidised to Na+ in liq. NH3 solvent by the loss of
solvated e-.
Step-2) Attacks of solvated electron to Benzene ring:
Transfer of solvated electron to the benzene ring; to form ion radical (I).

(Benzene) ring:
Step-3) Protonation by Alcohol:
The radical ion (I) accepts a proton from the alcohol; to form a radical (II).
Step-4) Again loss of electron by sodium:
Radical (II) reacts with another sodium atom; to form a carbanion-(III).

(Benzene) ring:
Step-5) Formation of product,
1,4-cyclohexadiene:
Carbanion –(III) accepts another proton from alcohol;
to form the final product, 1,4-cyclohexadiene -(IV)
-----*****-----

LOGO
www.themegallery.com
Mechanism of Electrophilic Aromatic Substitution
by Dr Pramod R. Padole
Stay Home. Take Care

Mechanism of Aromatic Electrophilic Substitution Reaction

Recommended

Recommended

More Related Content

What's hot

What's hot (20)

Similar to Mechanism of Aromatic Electrophilic Substitution Reaction

Similar to Mechanism of Aromatic Electrophilic Substitution Reaction (20)

More from pramod padole

More from pramod padole (20)

Recently uploaded

Recently uploaded (20)

Mechanism of Aromatic Electrophilic Substitution Reaction