Electrophilic substitution of reactions

 Electrophilic substitution of benzene

Nitration, Acylation, Sulphonation

Nitration :-

Benzene reacts with concentrated nitric acid in the presence of concentrated sulphuric acid at 60° C to form nitro benzene. 

Mechanism :-

Step1 : Formation of the electrophile (NO₂⁺)

Step2 : The electrophile attacks the benzene ring to form a carbonium ion.

Step3 : Loss of proton yields nitro benzene.

Sulphonation :-

Benzene reacts with concentrated sulphuric acid at 120° C or fuming sulphuric acid at room temperature to give benzene sulphonic acid.

Note:- Fuming sulphuric acid is concentrated sulphuric acid that contains added sulpher trioxide.

Mechanism :-

Step1 : Electrophile is formed in this reaction the electrophile is sulphur trioxide (SO₃). In concentrated sulphuric acid, SO₃ is produced as follows. in fuming sulphuric acid, this step is not important because the dissolved SO₃ reacts directly.

Step2 : The electrophile attacks the benzene ring to form a carbonium ion.

 Step3 : Loss of proton.

Step4 : Addition of proton gives benzene sulphonic acid.

Acylation :-

Benzene reacts with acid chlorides or anhydrides in the presence of aluminium chloride to give aromatic ketones.

Mechanism :-

Step1 : Formation of the electrophile (CH₃ – C⁺ = O)

Step2 : The electrophile attacks the benzene ring to form a carbonium ion.

Step3 : Loss of proton gives an aromatic ketone.

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