Question

(a) Give a plausible explanation for each one of the following :

(i) There are two –NH₂ groups in semicarbazide. However, only one such group is involved in the formation of semicarbazones.

(ii) Cyclohexanone forms cyanohydrins in good yield but 2,4,6-trimethylcyclohexanone does not.

(b) An organic compound with molecular formula C₉H₁₀O forms 2,4-DNP derivative, reduces Tollen’s reagent and undergoes Cannizzaro’s reaction. On vigorous oxidation it gives 1,2-benzene-di-carboxylic acid. Identify the compound.

Answer 1

(a) (i) 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.

(ii) Formation of cyanohydrin involves the nucleophilic attack of cyanide ions (CN^–) at the carbonyl carbon. In cyclohexanone, reaction proceeds but in 2,4,6-trimethylcyclohexanone, the methyl groups cause steric hindrance and yields are poor.

(b)  The compound forms 2,4-DNP derivative. It shows that it is a carbonyl compound. Further it reduces Tollens’ reagent which shows that it contains aldehydic group. It undergoes Cannizzaro reaction indicating that aldehyde group is without any α-hydrogen. On vigorous oxidation, it gives 1,2-benzenedicarboxylic acid which shows that there are two carbon residues on benzene ring. Since the molecular formula is C₉H₁₀O, it fits into the structure, 2-ethylbenzaldehyde.