Question

Naphthalene is mostly insoluble in water. How would you recrystalize benzoic acid contaminated with naphthalene?

Answer 1

During the recrystallising, add benzoic acid contaminated with naphthalene to hot water then benzoic acid will be dissolved in water whereas naphthalene does not. Then filter the solution and cool the filtrate, benzoic acid will be crystallised out.

(OR)

we can separate them by the following steps.

1. Dissolve the mixture of benzoic acid and naphthalene in an organic solvent, such as diethyl ether.

2. Add 1 M NaOH to the mixture. This will react with the benzoic acid to form sodium benzoate. The naphthalene will remain unchanged.

3. Separate the aqueous and organic layers. You now have two solutions - an aqueous solution of sodium benzoate and an ether solution of naphthalene.

4. Evaporate the ether solution to yield pure naphthalene.

5. Add concentrated HCl to the sodium benzoate solution. This will reform benzoic acid. Since the acid is no longer a salt, it will precipitate from the aqueous solution.

6. Filter off the crystals of benzoic acid to yield the pure compound.