Ch18: Mechanism ofTautomerisation

Chapter 18: Enols and Enolates

Mechanism of Tautomerisation

Tautomerisation is both acid and base catalysed.
The mechanisms of these process are shown below for propanone.
In each case note that it is catalytic since the acid or the base is regenerated.

MECHANISM OF ACID CATALYSED TAUTOMERISATION
Step 1: First, an acid-base reaction. The Lewis basic O atom of the carbonyl is protonated by the acid catalyst giving an oxonium ion.
Step 2: Another acid-base reaction. Removal of an α-hydrogen by a water molecule functioning as a base allows formation of the C=C and neutralises the positive charge on the O giving the enol and regenerates the catalyst.

MECHANISM OF BASE CATALYSED TAUTOMERISATION
Step 1: First, an acid-base reaction. Hydroxide functions as a base and removes the acidic α-hydrogen giving the reactive enolate
Step 2: The negative charge is resonance stabilised to the more electronegative O atom.
Step 3: An acid-base reaction. The alkoxide deprotonates a water molecule reforming the catalyst hydroxide and the enol

© Dr. Ian Hunt, Department of Chemistry