Description: Ketones and other carbonyl compounds containing a hydrogen adjacent to the carbonyl are in equilibrium with a constitutional isomer called an enol.
Notes: Generally speaking the “keto” form is more stable due to the strong C–O π bond. The rate of interconversion between keto and enol is accelerated by acid.
Several situations lead to the enol form being more stabilized than normal:
- aromaticity (example 3)
- conjugation (example 2)
- hydrogen bonding (example 3)
Mechanism: Protonation of the ketone oxygen by an acid (Step 1, arrows A and B) leads to formation of an oxonium ion, which is then deprotonated at carbon to give the enol (Step 2, arrows C, D, and E).
- H–A here could be water (H2O) or an acid of some kind that is added such as H2SO4 or HCl.
- The rate of tautomerism is greatly increased by the addition of strong acid, since formation of the oxonium ino makes the adjacent C–H bond considerably more acidic (and easier to remove).
- It’s incorrect to draw the mechanism for tautomerism like this:
If you make a model you will see why this is impossible. If you compare the positions of Ha and Hb in the starting material and the product you should see why this does not work.