What is the correct order of reactivity of the following towards nucl…

The correct answer is Option (1) → Methanal > Ethanal > Acetone

In nucleophilic addition, a nucleophile attacks the carbonyl carbon, which carries a partial positive charge, and adds across the \( \ce{C=O} \) bond. Two factors decide how fast this happens:

• Electronic factor: alkyl groups push electrons towards the carbonyl carbon (+I effect). Each alkyl group makes the carbon less positive (less electrophilic), so it attracts the nucleophile less.

• Steric factor: alkyl groups are larger than H atoms. Each alkyl group crowds the carbonyl carbon and makes it harder for the nucleophile to reach it.

So the fewer alkyl groups on the carbonyl carbon, the more reactive the compound.

• Methanal, \( \ce{HCHO} \) — first (most reactive). The carbonyl carbon carries two H atoms and no alkyl group. Nothing reduces its positive charge and nothing blocks the nucleophile.

• Ethanal, \( \ce{CH3CHO} \) — second. It has one methyl group and one H atom. The methyl group makes the carbon a little less positive and a little more crowded than in methanal.

• Acetone (propanone), \( \ce{CH3COCH3} \) — third (least reactive). It has two methyl groups. They reduce the positive charge on the carbonyl carbon the most and hinder the approach of the nucleophile the most.

NCERT (2026-27 edition):
Reactivity – “Aldehydes are generally more reactive than ketones in nucleophilic addition reactions due to steric and electronic reasons. Sterically, the presence of two relatively large substituents in ketones hinders the approach of nucleophile to carbonyl carbon than in aldehydes having only one such substituent. Electronically, aldehydes are more reactive than ketones because two alkyl groups reduce the electrophilicity of the carbonyl carbon more effectively than in former.”