When propanal is reacted with potassium cyanide under weakly acidic conditions the resulting mixture does not rotate plane polarised light. Explain this observation.

A common mistake is to answer this question with the explanation that the light is not rotated because no optical isomers are present. However, this is incorrect. The only other time where plane polarised light is not rotated is when a racemic mixture is present.This reaction involves a nucleophilic attack at a carbon oxygen double bond. The nucleophile can attack above or below the plane. This means that there are two conformations that the product can take. These two conformations are two different optical isomers. Both attacks are equally as likely, so an equal mixture of the two products is formed.A good answer would explain the racemic mixture, and draw out the nucleophilic attack for explanation.