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Among the following carbonyl compounds, which one undergoes nucleophilic addition most readily?

1. CH3—C(=O)—H
2. CH3—C(=O)—CH3
3. benzene—C(=O)—H
4. benzene—C(=O)—CH3

Correct answer: CH3—C(=O)—H

Solution

Nucleophilic addition is fastest for the least hindered, least stabilised carbonyl. Acetaldehyde (CH3CHO) has only one +I alkyl group and no conjugation, so it reacts fastest; acetone has two alkyl groups, and the benzaldehyde/acetophenone carbonyls are deactivated by resonance with the ring.

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