Question

What is keto-enol tautomerism?

Answer 1

`H_3C-C(=O)-CH_3 rightleftharpoons H_2C=C(-OH)-CH_3`

Explanation:

The equation above describes keto-enol tautomerism for acetone. As written, the equilibrium lies strongly to the left. It's worth mentioning that this is NOT resonance.

This tautomerism offers a potential for `C-C` bond formation, especially for 1,3-dicarbonyl species, where the equilibrium may lay towards the enol side.

The alcoholic (i.e. enolic) hydrogen is potentially acidic, and may be removed by a strong base, to give the enolate, which itself may give equilibrium quantities of its tautomer, the carbanion, as shown:

`H_2C=C(-O^_)-CH_3 rightleftharpoons H_2^(-)C-C(=O)-CH_3`

For `1,3-"dicarbonyls"` such as `"acetylacetone"`, the enolic form is strongly favoured in that it forms a 6-membered ring stabilized by hydrogen bonding, `H_3C-C(=O)CH=C(-OH)CH_3`.