Racemates from Nucleophilic Addition

Aldehydes and ketones react with hydrogen cyanide by nucleophilic addition to form hydroxynitriles. The reaction uses HCN in the presence of KCN, which provides the CN⁻ nucleophile. In practice KCN is used with a little dilute acid, because HCN itself is a very toxic gas.Mechanism (ethanal + HCN → 2-hydroxypropanenitrile):
1. The C=O bond is polar, so the carbonyl carbon is δ+. The CN⁻ ion uses the lone pair on its carbon to attack the δ+ carbon, and the π bond breaks, moving a pair of electrons onto the oxygen.
2. The negatively charged intermediate takes an H⁺ ion from HCN (or from the acid), forming the OH group.CH₃CHO + HCN → CH₃CH(OH)CNWhy a racemate forms: the groups around the carbonyl carbon are trigonal planar. The CN⁻ ion is equally likely to attack from above or below the plane. Attack from one side gives one enantiomer, and attack from the other side gives the other. Since both happen equally often, the product is a racemic mixture and is optically inactive.A chiral product only forms if the carbonyl carbon ends up with four different groups. So a racemate is formed from any aldehyde except methanal, and from an unsymmetrical ketone such as butanone. Propanone gives 2-hydroxy-2-methylpropanenitrile, in which the central carbon has two CH₃ (methyl) groups, so it is not chiral.The optical activity of a product can be used as evidence for a mechanism: an optically inactive (racemic) product fits attack on a planar group from both sides. The same reasoning applies to S_N1 reactions, where a planar carbocation is attacked from either side.