Question

why is it that benzoin sample often show traces of benzoic acid as an impurity whereas the benzoic acid doesn’t generally show traces of benzoin as an impurity?

Answer 1

First fact is clear that in benzoin condensation, benzaldehyde is key raw material   KCN and NaOH used as transforming reagent .While carrying out benzoin condensation , benzaldehyde might be oxidized into benzoic acid, so this benzoic acid percolate into desired benzoin product.

But benzoic acid does not shows benzoin , because there is lot of difference in polarity of both compound , we know that if polarity difference and solubility of compound becomes  same , then that impurity remain imparted with each other .

Here route of designing involves conversion of benzaldehyde into benzoin , once reaction gets completed , benzoin can be extracted into suitable solvent while if there is any acid impurity is formed , it gets knocked into water, because acids are highly water soluble .