Question

While this may or may not be true of your specific sample, why is it that the benzoin sample often shows traces of benzoic acid as an impurity whereas the benzoic acid doesn’tgenerally show traces of the benzoin as an impurity?

Answer 1

Benzoin i.e. PhCOCH(OH)Ph is generally formed from benzaldehyde i.e. PhCHO through the benzoin condensation reaction, where an alcoholic solution of the aldehyde is boiled in presence of a small amount of NaCN. However, there is always a possibility that some of the benzaldehyde is oxidized to benzoic acid. Thus, the sample of benzoin often contains trace of benzoic acid. But benzoic acid is prepared by either partial oxidation of toluene or by base induced cannizzaro reaction of benzaldehyde. So, there is no chance of any contamination from benzoin.