The carbonyl group of the aldehyde has two faces, and the cyanide ion can attack from either the front or the back face. In each case, a distinct product is formed. The two products are non-superimposable and are therefore non-identical. They are actually mirror images of each other called enantiomers. Structures that exist as two enantiomers are called chiral, and in this case attack at either face of the aldehyde is just as likely, so a 50:50 mixture of the two enantiomers is formed, known as a racemic mixture.
The carbonyl group of acetone also has two faces which the cyanide ion can attack. However, in this case, attack at either side forms the same product, by rotating one to match the other. They are therefore superimposable with rotation about an axis.
M. North, Synlett, 1993, 807–820.
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