Nucleophilic Addition Mechanism

The reaction of aldehydes and ketones with HCN in the presence of KCN is a nucleophilic addition. The C=O bond is polar: oxygen is more electronegative than carbon, so the carbon is δ+ and the oxygen is δ−. The electron-deficient carbon is attacked by a nucleophile, an electron-pair donor.Step 1: the cyanide ion, :CN⁻, uses the lone pair on its carbon atom to attack the δ+ carbonyl carbon. At the same time the π bond of C=O breaks and its pair of electrons moves onto the oxygen. This forms a negatively charged intermediate with an O⁻ group.Step 2: the O⁻ uses a lone pair to accept a proton, H⁺, from an HCN molecule (or from H⁺ in solution), forming the OH group. Taking H⁺ from HCN regenerates a CN⁻ ion.Stereochemistry: the carbonyl group is planar (the carbon is trigonal planar), so the nucleophile can attack from above or below the plane with equal probability. For an aldehyde such as ethanal, the new carbon in the product has four different groups (H, CH₃, OH and CN), so it is chiral. Equal amounts of the two enantiomers form: a racemic mixture, which is not optically active.Worked example: propanone reacts by the same mechanism, but its product, (CH₃)₂C(OH)CN, has two identical methyl groups on the central carbon. That carbon is not chiral, so no enantiomers form. Butanone, CH₃COCH₂CH₃, has two different groups on its carbonyl carbon, so it gives a racemic mixture.