Aldehydes And Ketones - Nucleophilic Addition Reactivity

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Ranking carbonyl compounds by their tendency to undergo nucleophilic addition reveals an order governed by sterics and electronics, so which species is most reactive?

RankGuru · Accepted Answer

Nucleophilic addition to a carbonyl is favoured when the carbonyl carbon is highly electrophilic and sterically open to attack. Two factors control this: alkyl or aryl groups donate electron density that reduces the partial positive charge on carbon, and bulky groups hinder the approach of the nucleophile. Methanal has only hydrogen atoms on the carbonyl carbon, so it experiences neither electron donation nor steric blocking and is the most reactive toward nucleophilic addition. Ethanal has one electron-donating, mildly hindering methyl group, so it is less reactive than methanal. Propanone has two methyl groups, increasing both electron donation and crowding, making it slower still. Benzophenone, with two bulky phenyl groups that also delocalise the carbonyl, is the least reactive of the set. The resulting order methanal > ethanal > propanone > benzophenone follows the NCERT discussion of carbonyl reactivity and is a frequent JEE ranking question. As a check, reactivity decreases steadily as hydrogen atoms are replaced by larger electron-rich groups, confirming methanal as most reactive.

Aldehydes And Ketones - Nucleophilic Addition Reactivity

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