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15.5: Regioselective Formation of Enolates

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Organic Chemistry

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Regioselective Formation of Enolates

15.5: Regioselective Formation of Enolates

As depicted in the figure below, the unsymmetrical ketones can form two possible enolates:  less substituted or more substituted enolates. Usually, the thermodynamic enolates are formed from the more substituted α-carbon atom, while the kinetic enolates are formed faster by deprotonation from the less substituted position. The thermodynamic enolates have lower energy, so they are  more stable. But the energy required to form kinetic enolates is less.


This regioselectivity in enolate formation is essentially an acid-base reaction and is controlled by various factors such as solvent, base, cation, and temperature. Protic solvents and weaker bases favor the formation of thermodynamic enolates, while aprotic solvents and stronger bases favor the kinetic enolates. Thermodynamic enolates are formed at higher temperatures and have longer reaction times because of the higher energy barrier. On the other hand, kinetic enolates are formed at lower temperatures with short reaction times. The conditions favoring thermodynamic enolates encourage a reversible reaction which is not the case with conditions involved for kinetic enolates intermediate.


Regioselective Formation Enolates Unsymmetrical Ketones Less Substituted Enolates More Substituted Enolates Thermodynamic Enolates Kinetic Enolates Deprotonation Acid-base Reaction Solvent Base Cation Temperature Protic Solvents Weaker Bases Aprotic Solvents Stronger Bases Energy Barrier Reversible Reaction

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