Arrange the following compounds in decreasing order of the reactivity towards nucleophilic addition reaction.
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
In a nucleophilic addition, the nucleophile attacks the carbonyl carbon. The reactivity depends on two factors:
Steric factor: bulky groups around the carbonyl carbon hinder the attack. H is the smallest group, so aldehydes are more reactive than ketones, and HCHO is the most reactive.
Electronic factor: groups that release electrons reduce the positive charge on the carbonyl carbon. Alkyl groups do this by the +I effect. A benzene ring does it more strongly, by resonance, so aromatic aldehydes and ketones are less reactive than aliphatic ones.
Matching the given compounds:
C: HCHO, formaldehyde (two H atoms): most reactive
D: \( \ce{CH3CH2CHO} \), propanal (aliphatic aldehyde)
B: \( \ce{C6H5CHO} \), benzaldehyde (aromatic aldehyde)
A: \( \ce{C6H5COCH3} \), acetophenone (aromatic ketone): least reactive
Shortcut: HCHO (C) must come first, which leaves Options (1) and (2). The aliphatic aldehyde (D) is more reactive than the aromatic aldehyde (B), so the answer is Option (2).
NCERT (2026-27 edition):
Reactivity
