Question

In organic chemistry lab, we separated a sample of benzion and benzoic acid. We tested the purity of the samples we obtained using a TLC plate. All of our benzoin spots also showed an impurity of benzoic acid. However, our benzoic acid spot did not show an impurity of benzoin. Why is this?

Answer 1

In your question you have not mentioned the method used for the separation, Extraction or column chromatographic purification.

I persume that you have used column chromatographic purification, which separating both the spot, you must have used a higher percent of solvent mixture to run the column that is why you have observed the tailing of benzoic acid in benzoin.

Whereas, benzoic acid is polar and requires higher percent of solvent mixture. So by the time, benzoic acid eluded from the column, all the benzoin was already separated.

Suggestion to completely separated each other

Take a mixture of  benzion and benzoic acid in organic solvent (ethyl acetate) in a separating funnel. To this add saturated NaHCO₃ and shake, all the benzoic acid will be converted into sodium benzoate and go into water layer and benzoin will stay in the organic layer. Separate both layers

Concentrate the organic layer to obtain pure benzoin

To the aqueous layer add 5 N HCl until the pH of solution becomes acidic and the solid benzoic acid precipitates out. Filter the solid was with water and dry.