Formation of racemic mixture in Nucleophilic Addition.

A nucleophilic addition reaction involves a carbonyl and HCN in the presence of NaCN as the catalyst. Details of the mechanism can be found here.

However, this reaction is capable of forming a racemic mixture. This is possible when the following rules are fulfilled:

1. The carbonyl is asymmetrical.
2. The product formed causes the C on C=O to change from sp² to sp³.
3. The product is an optically active product.

Isn't it slightly ironic that an optically active product is formed and yet we actually obtain a racemic mixture? Before my explanation, it is important to note that a racemic mixture does not mean that an optically active compound is absent. In fact, a racemic mixture implies that the pair of enantiomers exist in the same mixture.

In the reaction between an asymmetrical carbonyl and HCN results in the cyanide ion able to attack the carbonyl above or below the plane of the carbonyl.

If the cyanide ion attacks above the plane, an optically active addition product is formed. If the cyanide attacks below the plane of the carbonyl, the mirror image is formed.

As it is equally probable for the nucleophile to attack above or below the plane of the carbonyl, both enantiomers are formed in equal proportions and hence a racemic mixture is obtained.

This manner of forming racemic mixture, is also applicable the reaction between an alkyl halide and nucleophile, in which, this reaction must occur in the S_N1 mechanism. The carbocation being planar is equally probable for it to be attacked above and below its plane.

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^{Article written by Kwok YL 2010.}

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