Aldehydes are more reactive than ketones towards nucleophilic addition because in a ketone

  • A. the carbonyl group is absent
  • B. two alkyl groups release electron density towards the carbonyl carbon and also hinder attack
  • C. the carbonyl carbon carries a full positive charge
  • D. the carbon to oxygen bond is much longer

Explanation

Alkyl groups push electron density towards the carbonyl carbon, reducing its positive character, and their bulk physically blocks the approaching nucleophile, so two of them make a ketone markedly less reactive than an aldehyde with only one. Methanal, having none at all, is the most reactive carbonyl compound of the series. Both electronic and steric factors point the same way here.

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