Aldehydes are generally more reactive than ketones in nucleophilic addition reactions. Which of the following is one of the reasons for this?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → The two alkyl groups of ketones reduce the electrophilicity of the carbonyl carbon more effectively than the single alkyl group of aldehydes.
A nucleophile attacks the electrophilic carbonyl carbon. Aldehydes react faster for two reasons.
Electronically, alkyl groups push electrons towards the carbonyl carbon; a ketone has two of them and an aldehyde only one, so the ketone’s carbonyl carbon is less positive.
Sterically, the two larger groups of a ketone crowd the carbonyl carbon and hinder the approach of the nucleophile.
Why the other options are incorrect
Option (1) Aldehydes are easily oxidised even by mild oxidising agents, whereas ketones are generally oxidised under vigorous conditions. — Incorrect. The statement itself is true, but it compares the oxidation of aldehydes and ketones. It says nothing about how readily a nucleophile adds to the carbonyl carbon.
Option (3) The carbon–oxygen double bond is polarised in aldehydes because of the hydrogen atom on the carbonyl carbon, but it is not polarised in ketones. — Incorrect. The C=O bond is polarised in all carbonyl compounds, ketones included, because oxygen is more electronegative than carbon. The carbonyl carbon of a ketone is also an electrophilic centre.
Option (4) The single alkyl group of aldehydes increases the electrophilicity of the carbonyl carbon, whereas the two alkyl groups of ketones reduce it. — Incorrect. An alkyl group reduces the electrophilicity of the carbonyl carbon. The single alkyl group of an aldehyde does the same, only less than the two alkyl groups of a ketone.
NCERT (2026-27 edition):
8.4 Chemical Reactions – “(ii) 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.”