Keto-enol Tautomerism - Mechanism

Mechanism

The conversion of an acid catalyzed enol to the keto form proceeds by a two step mechanism in an aqueous acidic solution.For this it is necessary that the alpha carbon(carbon closest to functional group) contains at least one hydrogen atom known as alpha hydrogen.This atom is removed from the alpha carbon and bonds to the oxygen of the carbonyl carbon to form the enol tautomer. The existence of hydrogen atom at alpha carbon is necessary but not sufficient condition for enolization to occur. In order to be acidic, the alpha hydrogen should be positioned such that may line up parallel with antibonding pi-orbital of the carbonyl group. The hyperconjugation of this bond with C-H bond at alpha carbon reduces the electron density out of C-H bond and weakens it. Thus the alpha hydrogen becomes acidic. When this requirement is not enforced, for example in the adamantanone or other polycyclic ketones, the enolization is impossible or very slow. (J. E. Ordlander et al., Resistance of Adamantanone to Homoenolization, 1969), (J.B. Stothers and C.T. Tan,Adamantanone: Stereochemistry of its Homoenolization as shown by 2H Nuclear Magnetic Resonance, 1974)

First, the exposed electrons of the C=C double bond of the enol are donated to a hydronium ion (H₃O+). This addition follows Markovnikov's rule, thus the proton is added to the carbon with more hydrogens. This is a concerted step with the oxygen in the hydroxyl group donating electrons to produce the eventual carbonyl group.