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Keto Enol Tautomerism - Notes and Mechanism 

Ketones and Enols exist in equilibrium when in acidic or basic solution. 

The ketone and enol are tautomers, structural isomers that convert between each other rapidly. This is different from a resonance structure, in which both exist at the same time as a hybrid. 

 

The ketone is usually favored over the enol, except in 2 cases. 

1 - Hydrogen bonding stabilization that takes place on 1,3-diones. 

2 - Aromatic stabilization of the enol. 

 

Tautomerization Mechanism

Base Catalyzed Mechanism: 

Tautomerization occurs in base via deprotonation of the alpha carbon. This generates a resonance stabilized anion known as an enolate, and the electrons resonate up to the electronegative oxygen, forming a double bond at the base of the carbonyl. The negative oxygen then gets protonated generating the enol. 

 

Acid Catalyzed Mechanism: 

Tautomerization occurs in acid via protonation of the carbonyl, followed by deprotonation of the alpha carbon, which forms a double bond and kicks the carbonyl’s pi bond up onto the oxygen, resulting in the formation of the enol. 

 

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