Decarboxylation [of β-keto acids]

Decarboxylation [of β-keto acids] Definition:


When carboxylic acids containing a carbonyl group two bonds away (on the β carbon) are heated, carbon dioxide is lost.



Decarboxylation [of β-keto acids] Explained:


β-Keto (or 3-keto) acids have another carbonyl group in the β position (or on the third carbon atom), from where their names are derived. Other carboxylic acids which do not have a carbonyl function in this place do not decarboxylate, regardless of the presence of carbonyl groups elsewhere in the molecule. β-Keto acids are very unstable and readily undergo the elimination of carbon dioxide or simply decarboxylation under mild conditions. Ketones are obtained in this reactions.

Decarboxylation occurs readily whenever a carboxyl group is bonded to the α carbon of another carbonyl group for two reasons. First, the oxygen of the β-keto function is ideally positioned to bond with the carboxy hydrogen forming a cyclic six-atom transition state. And this transition state has aromatic character because three electron pairs shift around the cyclic six-atom array which is a second good reason. Thus enol is formed which tautomerizes rapidly to the final ketone product.