(A), (B), and (C) are three non-cyclic functional isomers of a carbonyl compound with molecular formula C4H8O. Isomers (A) and (C) give a positive Tollens' test, whereas isomer (B) does not give a Tollens' test but gives a positive iodoform test. Isomers (A) and (B) on reduction with Zn(Hg)/conc.HCl give the same product (D). Out of (A), (B), and (C) isomers, which one is least reactive towards the addition of HCN?

Isomer (B), 2-butanone, is least reactive towards the addition of HCN. Towards addition reactions, aldehydes are more reactive than ketones due to steric and electronic reasons. A nucleophile can approach more easily to the aldehyde carbonyl carbon than that of a ketone as in aldehydes, only one alkyl group is present, and in ketones, two bulky alkyl groups are present. Due to the +I effect of alkyl groups, the carbonyl carbon in aldehydes is more electrophilic than that in ketones. In ketones, the +I effect of two alkyl groups reduces the electrophilicity of the carbonyl carbon. In aldehydes, the +I effect of only one alkyl group reduces the electrophilicity of the carbonyl carbon.