Question

$\left(A\right),\left(B\right)$ and $\left(C\right)$ are three non-cyclic functional isomers of a carbonyl compound with molecular formula $C_4{H}_{8}O$. Isomers $\left(A\right)$ and $\left(C\right)$ give positive Tollens' test whereas isomer$\left(B\right)$ does not give Tollens' test but gives protection Iodoform test. Isomers $\left(A\right)$ and $\left(B\right)$ on reduction with $Zn\left(Hg\right)/conc$. $HCl$ give the same product $\left(D\right)$.

Out of $\left(A\right),\left(B\right)$ and $\left(C\right)$ isomers, which one is least reactive towards addition of $HCN$?

Answer 1

Isomer (B) 2-butanone is least reactive towards addition of HCN. Towards addition reaction, aldehydes are more reactive than ketones due to steric and electronic reasons.

A nucleophile can approach more easily to aldehyde carbonyl carbon than that of ketone as in aldehydes, only one alkyl group is present and in ketones two bulky alkyl groups are present. Due to +I effect of alkyl groups, the carbonyl carbon in aldehydes is more electrophilic than that in ketones.

In ketones +I effect of two alkyl groups reduces the electrophilicity of carbonyl carbon. In aldehyde, +I effect of only alkyl group reduces the electrophilicity of carbonyl carbon.