Which of the following electrophilic aromatic sustitution is reversible?

Options

A. Halogentation
B. Nitration
C. Sulphonation
D. Alkylation

Answer

3

Explanation

In halogenations, nitration and other irreversible electrophilic substitution the activation energy for the formation of the sigma complex is considerably greater than for the expulsion of `H^(+),K_(2)gt gt K_(-1)` In sulphonation the activation energy for the loss of `SO_(3)` is only slightly greater than for the loss of `H^(+)` and `K_(2)` slightly greater than `K_(-1)` As a result the sigma complex can react about equally well in either direction and hence sulphonation is reversible. The equilibrium may be shifted toward desulphonation (reverse of sulphonation) by using hot dilute aq. `H_(2)SO_(4)`.