The enol of acetoacetic acid: A computational study of the relative stabilities of the ketone and carboxylic acid isomers

Ab initio molecular orbital calculations at the MP2/6-311+G**//MP2/6-311+G** level show that the enol tautomer of acetoacetic acid in which the ketone group is enolized, 2, is more stable than its isomer in which the carboxylic acid group is enolized, 3, by 11-3 kcal mol^-1 . This difference may be attributed to a difference in the strength of resonance interactions between the carbonyl and hydroxyl groups of the enols: the carbonyl group in both isomers is conjugated with two hydroxyl groups, but in the acid enol, 3, both interactions are vinylogous, whereas in the ketone enol, 2, one of the vinylogous effects is replaced by a stronger direct interaction.