Online Chemistry notes

Electrophilic substitution reactions of benzene with mechanism.

Chemist
electrophilic substitution reactions

The most important/common reactions of benzene are electrophilic substitution reactions. In electrophilic substitution reactions an electrophile attacks the benzene and substitutes one of the hydrogen atoms of benzene ring to give substituted product.

Electrophiles : (Electron lovers) . Electrophiles are electron deficient species which attack on electron rich centre during chemical reactions. These are either a positively charged species or the neutral species having electron deficient centre. Eg. Cl+, CH3+, SO3, AlCl3, BF3, etc.

General mechanism of electrophilic substitution reactions

Four types of electrophilic substitution reactions of benzene are described below with their mechanism.

Halogenation of benzene

Benzene reacts with halogen in presence of a Lewis acid such as FeCl3 to give halobenzene. For example, chlorine reacts with benzene in presence of ferric chloride or AlCl3 as catalyst to give chlorobenzene.

Similarly, benzene reacts with bromine in presence of ferric bromide as catalyst to give bromobenzene.

Mechanism of halogenation of benzene

This reaction involves the following steps:

Nitration of benzene

When benzene is heated with conc. HNO3 in the presence of conc. H2SO4 at about 600C gives nitrobenzene.

Mechanism of nitration of benzene

This reaction involves the following steps:

Sulphonation of benzene

When benzene is heated with conc. H2SO4 , benzene sulphonic acid is formed.

Mechanism of sulphonation of benzene

This reaction involves the following steps:

Friedel – Craft’s reaction of benzene

Friedel- Craft’s reaction is of two types- alkylation and acylation.

Friedel Craft’s alkylation reaction

Introduction of an alkyl group ( – R ) in the benzene ring by treating benzene with an alkyl halide (R-Cl or R-Br) in the presence of anhydrous AlCl3 is known as Friedel- Craft’s alkylation reaction. Eg.

Mechanism of Friedel-Craft’s alkylation reaction

This reaction involves the following steps:

Friedel craft’s acylation reaction

Introduction of an acyl group (i.e.keto group) ( ) in the benzene ring by treating benzene with an acylating agent like acid chloride (RCOCl) or acid anhydride in the presence of anhydrous AlCl3 is known as Friedel- Craft’s acylation reaction. Eg.

Similarly, benzene when treated with benzoyl chloride in the presence of anhydrous AlCl3 gives benzophenone. This reaction is also called benzoylation reaction.

Mechanism of Friedel-Craft’s acylation reaction:

This reaction involves the following steps:

References