Aromatic hydrocarbons like benzene does electrophilic aromatic substitution reactions. We are very well aware of electrophile (E+) and nucleophile (Nu-). In this type of reactions we will make the use of electrophiles and will substitute another electrophile from the aromatic ring, that is usually a proton (H+). The driving force for this reaction is the electron richness of the aromatic ring which makes it a better nucleophile. That is also a reason for the benzene having more tendency to do electrophilic addition and substitution reactions rather than nucleophilic addition and substitution. Electrophiles are electron deficient and they can get easily attracted towards the electron rich benzene. Benzene also prefer electrophilic substitution over electrophilic addition reactions because of the regaining of aromaticity after the substitution reaction. Aromatic conjugation is the most stable conjugation and which will make the system more stabilized.

Electrophilic aromatic substitution reactions occurs in three steps. They are

1. Generation of electrophile

2. Attack of electrophile on the aromatic ring

3. Substitution of old electrophile with new electrophile

The potential energy curve for the electrophilic aromatic substitution reaction is given as

The important electrophilic aromatic substitution reactions of benzene are

• Friedel Craft's alkylation

• Friedel Craft's acylation

• Nitration

• Sulphonation

• Halogenation

• Gattermann Formylation

• Gattermann-Koch Formylation

Friedel Craft's alkylation

This reaction involves the substitution of an alkyl group on the benzene ring through EAS mechanism. The reagents used for this reaction are an alkyl halide (Generally Alkyl chloride) with a Lewis acid like AlCl3. The reaction proceeds through three steps as follows

Step-1 : Formation of electrophile

Here, the Lewis acid AlCl3 takes the Cl- ion from methyl chloride and which results in the formation of a carbocation. This carbocation will act as electrophile in this reaction. Here, AlCl3 acts as a catalyst.

Step-2 : Attack of electrophile and forming sigma complex

The attacking of electrophile with the pi-electrons of benzene result in the formation of a pi-complex. Upon bonding with the electrophile the pi-bond will break and the benzene ring will loose its aromaticity. Because of the loosing of aromaticity, this step will be the slow step and rate determining step. The formed positive charge on the benzene ring will be delocalized and the resonance stabilized unit is called sigma complex or arenium ion.

Step-3 : Substitution of old electrophile with new electrophile to regain aromaticity

In this step the sigma complex will remove the old electrophile, that is H+, to regain its aromaticity. The H+ ion make bond with the extra Cl- ion present with AlCl4- to form HCl and the catalyst AlCl3 will be free at the end of the reaction.

Friedel Craft's Acylation

Friedel Craft's acylation is very similar to the alkylation. Here, instead of an alkyl halide we may use acid halides or anhydrides. Acylium cation is the electrophile in this reaction and the reaction proceeds in three steps.

Mechanism using acid halide :

Mechanism using anhydride :

Nitration

The substitution of NO2+ electrophile on the benzene ring is known as nitration of aromatic compounds. The nitronium ion is prepared by the reaction between sulphuric acid nitric acid below 50 degrees celsius to prevent the decomposition of nitric acid and to control the exothermic nature of the reaction. The mixture of concentrated H2SO4 and HNO3 is known as nitrating mixture.

Mechanism :

Sulphonation

The substitution of -SO3H group on the benzene ring is known as sulphonation. In this reaction we will use a mixture of sulphuric acid (H2SO4) and sulphur trioxide (SO3) to produce bezene sulphonic acid. The reaction goes through the attack of pi-electrons of benzene on the partially positively charged sulphur in SO3. Sulphonation is a reversible process.

Mechanism :

Halogenation

The substitution of halogen on benzene is known halogenation. In order to perform halogenation we may react benzene with halogen in the presence of a Lewis acid catalyst, such as AlCl3 or FeCl3.

Mechanism :

Gattermann Formylation

The formation of benzaldehyde from benzene by reacting with HCN, HCl and H2O is known as Gattermann formylation. In this reaction aldenium ion acts as the electrophile.

Mechanism :

Gattermann-Koch Formylation

In this reaction also the benzene will be converted to benzaldehyde using CO, HCl and AlCl3. Here, the formyl cation will be acting as an electrophile and reaction proceeds through EAS mechanism.

Mechanism :