JEE Main 2025 Jan 24 Shift 1, Chemistry Q15: Properties and Important Reactions of Aldehydes & Ketones

Two effects govern the rate of nucleophilic addition to a carbonyl:

1. Steric crowding around the C=O carbon hinders the incoming nucleophile and slows the reaction.

2. Electron-withdrawing substituents make the carbonyl carbon more electrophilic and speed up the reaction; electron-donating groups slow it.

Compare the four carbonyls:

- Acetophenone (Ph-CO-CH${}_3$) has a methyl and a phenyl on the carbonyl carbon: most steric hindrance plus mild electron donation. Slowest.

- p-Tolualdehyde (p-CH${}_3$-C${}_6$H${}_4$-CHO): aldehyde (low steric), but the para-methyl donates electrons by $+I$/hyperconjugation, slightly deactivating.

- Benzaldehyde (Ph-CHO): aldehyde with no activating or deactivating ring substituent.

- p-Nitrobenzaldehyde: aldehyde with a strong $-M/-I$ NO${}_2$ group para to CHO, sharply increasing the electrophilicity of the carbonyl. Fastest.

Order of reactivity: acetophenone $<$ p-tolualdehyde $<$ benzaldehyde $<$ p-nitrobenzaldehyde.

$\therefore$ Option (3).