The aldehydes and ketones undergo many nucleophilic reactions. Aldehydes are generally more reactive than Ketones in nucleophilic addition reactions due to steric and electronic reasons. The aldehydes and ketones undergo a number of reactions due to acidic nature of α-Hydrogen also. Aldol and cross aldol reactions occur in carbonyl compounds which contain α-Hydrogen. Aldehydes and ketones having at least one α-Hydrogen undergo reaction in dil. alkali to form β-hydroxy aldehyde or β-hydroxy ketone. This is aldol reaction. Aldol or ketol lose water to form α,β unsaturated carbonyl compounds which is called aldol condensation. Carbonyl compounds react with ammonia derivatives such as NH2OH, NH2NHCONH2, 2,4 DNP to give products useful in their identification.
Ethanal undergoes nucleophilic addition reaction at faster speed than acetone, this is because of
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Electrophilicity of carbon of carbonyl group is high in ethanal
In nucleophilic addition, the nucleophile attacks the electrophilic carbonyl carbon. Aldehydes are generally more reactive than ketones for two reasons:
Electronic: alkyl groups release electrons (+I effect) and reduce the positive charge on the carbonyl carbon. Ethanal has one such group, propanone (acetone) two, so the carbonyl carbon of ethanal is more electrophilic.
Steric: the single alkyl group of ethanal hinders the approach of the nucleophile less than the two methyl groups of acetone.
Why the other options are incorrect
Option (1) Acetone has 6 α-hydrogen atoms — Incorrect. True as a fact, but α-hydrogens decide aldol reactions, not the rate of nucleophilic addition.
Option (3) Electrophilicity of carbon of carbonyl group is less in ethanal — Incorrect. It is the other way round; lower electrophilicity would make ethanal slower.
Option (4) Nucleophilicity of carbon of carbonyl group in ethanal is high — Incorrect. The carbonyl carbon is an electrophilic centre, not a nucleophilic one.
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.”