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Why do carboxylic acids not undergo the typical addition–elimination reactions shown by aldehydes and ketones?

1. Because the O–H bond is more polar than the >C=O group
2. Because the carboxylate ion becomes ionised
3. Because the carboxylate ion is stabilised by resonance
4. Because it is present as –COOH and therefore has no carbonyl group

Correct answer: Because the carboxylate ion is stabilised by resonance

Solution

In carboxylic acids the carbonyl carbon is deactivated toward nucleophilic addition because the -COOH group is resonance stabilised (the lone pair on the -OH oxygen delocalises onto C=O), reducing the electrophilicity of the carbonyl carbon. Hence they resist the addition reactions typical of aldehydes/ketones.

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