Benzoin 2-hydroxy-1,2-diphenylethanone

Aromatic aldehydes generally do not produce cyanohydrins on reaction with hydrogen cyanide, but undergo the benzoin condensation (Scheme 6.12). The initial product from nucleophilic attack by cyanide ion is depro-tonated to form a resonance-stabilized carbanion, which attacks a second molecule of the aldehyde. Elimination of HCN leads to an a-hydroxy ketone, benzoin (2-hydroxy-1,2-diphenylethanone). The benzoin condensation is catalysed specifically by cyanide ion, which assists in both the formation and stabilization of the carbanion. The reaction is limited to aromatic aldehydes, since the aryl ring also stabilizes the anion. 

Racemic 2-hydroxy-1,2-diphenylethanone (benzoin) is deprotonated by potassium hydride to give the enediolate, which can be rapidly reprotonated to the enediol by (27 ,3R)-dipivaloyltartaric acid. (S)-Benzoin (2) of 80% ee is formed very slowly from the enediol, whereas the latter is rapidly oxidized to diphenylethanedione (1) in the presence of air148. 

Hydroxy-1,2-diphenylethanone. See Benzoin Hydroxydiphenyl methane. See Benzhydrol 2-Hydroxydiphenyl sodium salt. See Sodium o-... 

The Benzoin Condensation. Treatment of benzenecarboxaldehyde (benzalde-hyde, CeHsCHO) and similar aldehydes with sodium cyanide (Na " CN") in ethanol (ethyl alcohol, CH3CH2OH) as solvent at reflux for brief periods results in the formation of a dimer, 2-hydroxy-l,2-diphenylethanone (benzoin) (Equation 9.49). A mechanism has been proposed (Scheme 9.60) that accounts for the following observations ... 

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