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A question in a practice paper I did recently showed the aldol condensation of benzaldehyde with ethanoic acid. I think I am right in saying that carboxylic acids do not form enolates because the carboxyl proton is much more acidic and so this reaction could not take place under basic conditions but it is possible to protonate a carboxylic acid and then have it take part in an aldol reaction?

Additionally, is this also possible for acyl chlorides, and if not, why?

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Carboxylic acids form enolates

I think I am right in saying that carboxylic acids do not form enolates because the carboxyl proton is much more acidic and so this reaction could not take place under basic conditions...

They do, if you beat them to it:

  1. Dissolve the carboxylic acid in a polar, aprotic solvent, such as 1,2-dimethoxyethane, 1,4-dioxane, tetrahydrofuran, etc.

  2. Add one quivalent of sodium hydride to deprotonate and form the carboxylate.

  3. Add one equivalent of a strong base, such as LDA (lithium diisopropylamide) to deprotonate the carboxylate at the $\alpha$-position.

  4. Add your aldehyde

What about acyl chlorides?

Adding a base, such as Hünig's base (plus catalytic amounts of a Lewis acid) to an enolizable acyl chloride under anhydrous conditions gives rise to a different pathway: a ketene can be formed!

How do ketenes react with aldehydes?

In the presence of a Lewis acid, they form $\beta$-lactones!

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  • $\begingroup$ What about reactions under acidic conditions? Or acyl chlorides? $\endgroup$ – bon Apr 14 '15 at 19:04

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