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#1 2011-07-12 19:56:53


Direction of ketone enolization in acid or base

A poster has asked in which direction enolization would occur for methylcyclohexanone and 3-methyl-2-butanone. In the first reaction, it is an acid catalyzed bromination in acetic acid. In the second reaction, it is a haloform reaction (chlorination to chloroform) with chlorine and NaOH.

The regiochemistry of these two reactions will be different. In the haloform reaction, the least substituted carbon is the most acidic (the least number of electron donating carbon atoms). After the initial formation of an enolate, each subsequent enolation is enhanced by the electron withdrawing properties of the chlorine atom. You will need to exchange the bromine atoms with chlorine.
This example is excerpted from A Handbook of Organic Chemistry Mechanisms, available at or

In the second reaction, the acid catalyzed enolation favors formation of the most substituted alkene, the most stable. Bromination of the enol will give a ketone that is less basic and will resist formation of an enol, therefore monobromination reactions are possible.

A Minisampler of A Guide to Organic Chemistry Mechanisms.


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