Return to Contents  Chapter 18: Enols and Enolates  Ch 18 contents
Mechanism of Tautomerisation
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.
mechanism of acid catalysed tautomerisation
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
mechanism of base catalysed tautomerisation
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
 


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© Dr. Ian Hunt, Department of Chemistry, University of Calgary