Question

Why does nucleophilic substitution reaction of chlorobenzene take place through benzyne mechanism and that of p-nitrochlorobenzene proceeds via addition-elimination mechanism ? Explain.

Answer 1

In p-nitrochlorobenzene, the carbanion (I) resulting through initial attack of the nucleophile is stabilized by electron-withdrawing $-N{O}_2$ group as well as by the benzene ring as shown in Fig.1

In the second step, the carbanion (I) loses a chloride ion to complete the substitution. (Refer Fig.2)

This two step bimolecular aromatic nucleophilic substitution mechanism is also called addition-elimination mechanism.

In contrast, the carbanion (II) resulting through nucleophilic attack on chlorobenzene is stabilized only by the benzene ring. (Refer Fig.3)

Since stabilization by $N{O}_2$ group is much greater than that by benzene ring, therefore, carbanion (I) is easily formed and hence p-nitrochlorobenzene undergoes substitution via addition-elimination mechanism. In contract, the carbanion (II) is only slightly stabilized and here is not easily formed.

Therefore, chlorobenzene undergoes nucleophilic substitution by an alternate mechanism involving benzyne intermediate or elimination-addition mechanism as shown in Fig. 4.