Question

Account for the following :

(i) CH₃CHO is more reactive than CH₃COCH₃ towards reaction with HCN.

(ii) There are two –NH₂ groups in semicarbazide (H₂NNHCONH₂). However, only one is involved in the formation of semicarbazone.

Answer 1

(i) It is a nucleophilic addition reaction, in which CN^– acts as a nucleophile. CH₃CHO undergoes nucleophilic addition reactions faster than CH₃COCH₃ as in CH₃COCH₃ there are two electron releasing methyl groups attached to the carbonyl carbon that hinders the approach of nucleophile to carbonyl carbon and reduce the electrophilicity of the carbonyl group while in CH₃CHO, there is only one methyl group attached to carbonyl carbon.

(ii) Semicarbazide has the following resonance structures arising due to the electron withdrawing nature of the O atom.

Lone pairs of N-1 and N-2 are involved in conjugation with  group while that of N-3 is not involved in resonance thus, it is involved in the formation of semicarbazone.