Question

You are planning to purify an impure sample of benzoic acid,known to be contaminated with naphthalene. Propose a strategy for purifying this sample,making use of acid-base principles,using equations to further clarify your answer.

Answer 1

Benzoic acid reacts with aqueous NaOH to form a water soluble salt, while naphthalene, which is not soluble in water to any appreciable extent, does not react with NaOH.So,upon shaking the impure sample with NaOH (aq), the naphthalene remains as a solid, while the benzoic acid dissolves in the aqueous medium as sodium benzoate. 

After filtration,HCl is added to the filtrate to regenerate benzoic acid, which precipitates as almost pure crystals. Cooling maximizes the yield of the purified product,after which pure crystals can be obtained by filtration. Alternatively, before beginning the reaction with aqueous NaOH,the impure sample is first dissolved in diethyl ether (benzoic acid and naphthalene are both soluble in ether) and placed in a separatory funnel. After shaking thoroughly with aqueous NaOH, the aqueous layer (which now contains sodium benzoate) is collected. The rest of the procedure is as described earlier.