JEE Main · 2025 · 24 Jan 2025 · Shift 1 · Aldehydes Ketones and Carboxylic Acids

Nucleophilic addition at a carbonyl carbon is favoured by an electrophilic (electron-poor) carbonyl carbon and by low steric hindrance around it.

Ketones react slower than aldehydes because the second alkyl/aryl group adds both steric bulk and a +I electron-donating effect that reduces electrophilicity — acetophenone, a methyl ketone, is therefore the least reactive of the four.

Among the aldehydes, ring substituents change the electrophilicity of the carbonyl carbon: the $-C{H}_3$ group in p-tolualdehyde donates electron density into the ring, making its carbonyl carbon less electrophilic than in unsubstituted benzaldehyde. The $-N{O}_2$ group in p-nitrobenzaldehyde withdraws electron density strongly by resonance and induction, making its carbonyl carbon the most electrophilic and hence this aldehyde the fastest to react.

The order of increasing reactivity is
$\text{acetophenone}<\text{p-tolualdehyde}<\text{benzaldehyde}<\text{p-nitrobenzaldehyde}$