Why might the presence of benzoic acid in the starting material benzaldehyde be deleterious to the benzoin condensation catalyzed by thiamine?
In this experiment, thiamine is the catalyst which is produced by mixing hydrochloride salt (thiamine HCl) with NaOH. In the presence of benzoic acid, NaOH will react with benzoic acid to form a salt so that there is not enough NaOH to turn thiamine HCl into thiamine, greatly slowing down the real benzoin condensation reaction.
Benzoic acid will condense into something else if it is present.
Benzoic acid will oxidize benzaldehyde to benzoic acid.
Benzoic acid will reduce benzaldehyde to benzal alcohol.