Electrophilic Aromatic Substitution Reactions: A Comprehensive Guide, Study notes of Organic Chemistry

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ELECTROPHILIC SUBSTITUTION

REACTIONS OF SIMPLE AROMATIC

HYDROCARBONS AND THEIR

COMPARISION WITHCOMPARISION WITH

POLYNUCLEAR AROMATIC

HYDROCARBONS

CONTENTS

I.

INTRODUCTIONT TO SUBSTITUTION

a) TYPES OF SUBSTITUTION REACTIONb) ELECTROPHILE AND THEIR TYPES

II.

ELECTROPHILIC AROMATIC SUBSTITUTION

II.

GENERAL MECHANISM OF ELECTROPHILIC SUBSTITUTION

II.

ELECTROPHILIC SUBSTITUTION REACTIONS OF ARENE

II.

ELECTROPHILIC SUBSTITUTION IN POLYNUCLEAR AROMATICHYDROCARBONS

II.

REACTIVITY COMPARISION OF SIMPLE ARENE WITH POLYNUCLEARAROMATIC COMPOUNDS

II.

COMPARISION OF ELECTROPHILIC SUBSTITUTION IN AIPHATIC,AROMATIC SYSTEM AND NUCLEOPHILIC AROMATIC SUBSTITUTIONREACTION

II.

REFERENCES

TYPES OF SUBSTITUTION REACTIONS

Depending

up

on

the

nature

of

the

attacking

reagents

substitution reactions can be classified as below;

SUBSTITUTION REACTIONS

IONIC SUBSTITUTION

FREE RADICAL SUBSTITUTION

ELECTROPHILICSUBSTITUTION REACTION

NUCLEOPHILICSUBSTITUTION REACTION

ELECTROPHILIC SUBSTITUTION

These are the reactions in which an atom or group in amolecule is replaced by an electrophile.

Such reactions are shown by aromatic compounds and are very

rare

in

aliphatic

system

very

rare

in

aliphatic

system

TYPES OF ELECTROPHILIC REAGENTS Electrophilic reagents are characterized by an atom withan incomplete outer electron shell and therefore are veryreactive. The Electrophilic reagents are of two types;

a) Positive charged electrophileb) Neutral electrophile.

Positive electrophile

Neutral electrophile

Proton (H

)

Hydronium ion (H

3

0

)

Bromonium ion (Br

)

Nitronium ion (N

2

)

Carbonium ion Nitrosonium (N

)

Lewis acids such as,Boron trifluoride (BF

3

)

Aluminium trichloride (AlCl

3

)

Ferric chloride (FeCl

3

)

CarbenesNitrenes

Nitrosonium (N

)

Sulphonium (S

3

H

)

They both have 6 valance electrons (except H

) and can

accept an e

pair from a nucleophile (or) substrate to

acquire the stable octate.

C

C

E

:

E :

:

.. ..

Substrate

Electrophile

Product

The

common

well

known

electrophilic

substitution

reactions of aromatic compounds are,

Nitration

Halogenation

Sulphonation

Friedel

crafts

alkylation

Friedel

crafts

alkylation

Friedel crafts acylation

The various electrophilic substitution reactions

proceed by a common mechanistic pathway.

GENERAL MECHANISM OF ELECTROPHILIC SUBSTITUTION REACTION:

It occurs in the following steps Step-I Generation of electrophiles, either by

a) Spontaneous dissociation of the reagent. b) By acid catalysed dissociation.

Eu-Nu

E

Nu

( Spontaneous dissociation)

Eu-Nu

-A

Eu-Nu

• Nu--A

(Acid catalysed dissociation)

E

Step-III •

Elimination of a proton from the

σ

complex, by a base

(:B) to yield the final substitution product.

H

E

E

:

B

H

:

B

Now

let

us

discuss

some

common

electrophilic

substitution reactions of benzene.

:

B

H

:

B

Substitution

product

fast

ELECTROPHILIC SUBSTITUTION REACTIONS OF ARENES

1. Nitration:

Benzene reacts with a mixture of conc. nitric acid andsulphuric

acid

(nitrating

mixture)

to

form

nitrobenzene.

H

HO

NO

2

NO

2

Conc. .H

2

S

4

Nitrobenzene

2. HALOGENATION: Benzene reacts with chlorine (or) bromine in the presence of alewis

acid

catalyst

such

as

AlCl

3

,

FeCl

3

(or)

FeBr

3

to

form

corresponding substituted product.

H

Cl

2

Cl

HCl

Fecl

3

H

Br

2

Br

HBr

FeBr

3

MECHANISM

Step 1:

Generation of electrophile (Bromonium ion)

Br

Br

• FeBr

3

Br

• FeBr

4

Electrophile

Step 2:

Formation of

σ

complex (or) stable resonance hybrid

H

H

Br

H

Br

H

Br

Br

• Br

Step 3:

Elimination of a proton from a

σ

complex

H

Br

• Br

slow

H

Br

Br

FeBr

4

FeBr

3

Bromobenzene

fast

• HBr

MECHANISM

Step 1:

Generation of electrophile (Sulphur trioxide)

Step 2:

Formation of

σ

complex (or) stable resonance hybrid

HO

SO

3

H

H

SO

3

OH

H

3

O

HSO

• 4

SO

3

H

H

SO

3

H

SO

3

H

SO

3

• SO

3

slow

Step 3:

Elimination of a proton from a

σ

complex

H

SO

3

H

SO

3

SO

3

SO

3

H

HSO

4

fast

H

3

0

4. FRIEDEL CRAFTS REACTIONS: This reactions, which now occupied prestigious positionin Organic synthesis, was discovered in 1 by CharlesFriedel and M. Crafts. This involves the reaction of analkyl halide (or) acylchloride with

benzene in the

presence of lewis acids, generally AlCl

3

is known as

friedel

craft’s

reaction

3

friedel

craft’s

reaction

It can be divided into two types:a)

Friedel craft’s alkylation

b)

Friedel craft’s acylation