Arrange the following compounds in increasing order of their reactivity towards HCN.
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → (C), (D), (B), (A)
Addition of HCN to a carbonyl compound is a nucleophilic addition: the cyanide ion attacks the carbonyl carbon, and the product is a cyanohydrin:
\( \displaystyle \ce{CH3CHO + HCN} \xrightarrow{\ce{OH^-}} \ce{CH3CH(OH)CN} \)
The reactivity depends on two factors. Steric: bulky groups around the carbonyl carbon hinder the approach of the nucleophile. Electronic: alkyl groups push electrons towards the carbonyl carbon and make it less positive. Both factors make aldehydes more reactive than ketones, and ketones with bulky groups the least reactive.
(A) Acetaldehyde, \( \ce{CH3CHO} \): one small methyl group and a hydrogen; the most reactive.
(B) Acetone, \( \ce{CH3COCH3} \): two methyl groups.
(D) Methyl tert-butyl ketone, \( \ce{CH3COC(CH3)3} \): one methyl and one bulky tert-butyl group.
(C) Di-tert-butyl ketone, \( \ce{(CH3)3CCOC(CH3)3} \): two bulky tert-butyl groups; the carbonyl carbon is the most crowded, so the least reactive.
Shortcut: The order starts with (C) and ends with (A): Options (2) and (4). Methyl tert-butyl ketone is more hindered than acetone, so (D) comes before (B), which leaves Option (2).
NCERT (2026-27 edition):
Reactivity
