Basic conditions keto-enol tautomerism

Under basic conditions, the keto-enol tautomerism operates by a different mechanism than it does in acid. In base, the proton is first removed from its old position in the OH group, and then replaced on carbon. In acid, the proton was first added on carbon, and then removed from the hydroxyl group. 

We will now explore a mechanism for keto-enol tautomerism. We said that compounds will tau-tomerize in the presence of either add or base, so we will need to explore two mechanisms one under acidic conditions, and one under basic conditions. 

Decarboxylation, or loss of CO2, is not a typical reaction of carboxylic acids under ordinary conditions. However, j8-ketoacids are unusually prone to decarboxylation for two reasons. First, the Lewis basic oxygen of the 3-keto function is ideally positioned to bond with the carboxy hydrogen by means of a cyclic six-atom transition state. Second, this transition state has aromatic character (Section 15-3), because three electron pairs shift around the cyclic six-atom array. The species formed in decarboxylation are CO2 and an enol, which tautomerizes rapidly to the final ketone product. 

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