Basic Alpha-Halogenation of Ketones and Aldehydes

• Make a tautomer of the carbonyl structure to make it an enol tautomer. This puts the hydrogen atom on top of the oxygen and an alkene out of the carbonyl carbon and the alpha carbon.

• Then pull the halogen apart, attaching it to the nucleophilic alpha carbon atom.

This leaves behind a halogen anion that “needs” a hydrogen ion to make it a complete acid.

• Then pull the hydrogen atom off the OH molecule on the original enol, making the molecule now a halogenated ketone plus the acidic hydrogen halide molecule.

BASIC ALPHA-HALOGENATION OF KETONES AND ALDEHYDES

We have just discussed the acidic alpha halogenation of ketones and aldehydes. As it turns out, the same thing can be done under basic circumstances. The biggest problem with this reaction is that it is not limited to monohalogenation and frequently polyhalogenation occurs. This is because the addition of an electronegative halogen atom makes the enolate more stable and more halogenation occurs. The rates of halogenation are the same for bromine, iodine, and chlorine. The reaction goes as is seen in figure 83:

This reaction proceeds as follows:

• An acid-base reaction occurs, removing the acidic hydrogen atom by virtue of its attachment to a basic hydroxyl group. This leaves the enolate behind plus water, which is not seen in the acidic version of this reaction.

Figure 83.