Racemates from Nucleophilic Addition

The carbonyl group, C=O, is planar. The carbonyl carbon and the three atoms bonded to it lie in one plane, in a trigonal planar arrangement with bond angles of about 120°.In nucleophilic addition, a nucleophile such as CN⁻ attacks the δ+ carbonyl carbon. Because the group is flat, the nucleophile can attack from either side of the plane (from above or below), and each side is equally likely.If the two groups already on the carbonyl carbon are different (any aldehyde except methanal, or an unsymmetrical ketone), the product carbon now has four different groups, so it is a chiral centre. Attack from one side gives one enantiomer and attack from the other side gives the other enantiomer. Since both are equally likely, the product is a racemic mixture, which is optically inactive.Example: ethanal, CH₃CHO, reacts with KCN followed by dilute acid to give 2-hydroxypropanenitrile, CH₃CH(OH)CN. Carbon 2 carries H, OH, CH₃ and CN, and the product is a racemic mixture of its two enantiomers.If the carbonyl carbon carries two identical groups, no chiral centre forms. Propanone gives 2-hydroxy-2-methylpropanenitrile, whose central carbon has two CH₃ groups, so it is not chiral.The same idea applies to reduction with NaBH₄, where the nucleophile is H⁻: butanone is reduced to a racemic mixture of butan-2-ol.