Decarboxylation of beta-keto carboxylic acids
Description: When carboxylic acids containing a carbonyl group two bonds away (“on the β carbon”) are heated, carbon dioxide is lost.
Notes: Although often not given, a typical temperature for this reaction is around 180° C. This is the final step in the malonic ester synthesis (and the related acetoacetic acid synthesis).
- The delta sign in example 1 is equivalent for “heat”.
- Note how the “beta-keto” (” β-keto”) group can be a ketone, carboxylic acid, or even an aldehyde. Essentially the reaction is effective as long as there is a carbonyl group in this position. Note that if the “keto” is three carbons away the process is not very effective (see mechanism below for explanation).
Mechanism: Decarboxylation proceeds in a one-step, 6-electron “pericyclic” mechanism to give an enol (Step 1, arrows A, B, C). The enol can subsequently tautomerize to give the ketone (or ester, or other corresponding carbonyl compound).
- The activation energy for decarboxylation is lowered in β-carbonyl compounds due to the fact that the carbonyl oxygen can participate in hydrogen bonding with the neighboring carboxylic acid.
- Breakage of C-C is stabilized by the fact that there is resonance in the enol form. This is why the reaction works well for β-carbonyl species but not for “alpha” or “gamma” carbonyls – they can’t form the enol.