Why are aldehydes generally more reactive than ketones towards nucleophiles?
The carbonyl carbon is the reactive spot, and two effects make aldehydes more open to attack.
The carbonyl carbon is the reactive spot, and two effects make aldehydes more open to attack. Sterically, an aldehyde has only one bulky group plus a small H, so the nucleophile reaches the carbon easily; a ketone has two bulky groups blocking it. Electronically, the two alkyl groups of a ketone push electron density onto the carbonyl carbon, making it less electrophilic. Both effects favour faster addition to aldehydes.
Key point
Aldehydes are less hindered and more electrophilic, so nucleophilic addition is faster than for ketones.
Common mistake
thinking more alkyl groups make a ketone more reactive.
Memory tip
fewer/ smaller groups on the C=O → easier nucleophile attack (aldehyde wins).
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