Question

Devise a procedure for the separation and purification of the mixture containing 0.25g benzoic acid, 0.20g benzil, 0.05 p-dichlorobenzene.

Available reagents: 1M NaOH, 1M HCI, 1M NaHCO3, 6 M NaOH, 6 M HCI, saturated solution of NaCl, diethyl ether, 95% ethanol, methanol, isopropyl alcohol, acetone, hexane, toluene, ethyl acetate, anhydrous sodium sulfate.

Answer 1

Both benzoic acid and p-dichlorobenzene are soluble in water and benzil is insoluble in water.

Benzil separation: Therefore, to separate benzil from this mixture, add 1L water to the mixture to completely dissolve benzoic acid (solublity 3.44 g/L) and p-dichlorobenzene (solubility 0.10 g/L). Benzil separates out from the aqueous phase, which can be filtered. The filtered benzil is dried and purified by recrystallization from ethanol.

The filtrate (aqueous solution) has benzoic acid (~0.25g) and p-dichlorobenzene (~0.05g). Since benzoic acid is a weak acid and dissociates into H+ and benzoate ions. Therefore, acidifying the aqueous solution results in precipitating benzoic acid. To ensure maximum amount of precipitation of benzoic acid add 200 mL of 6M HCl to the aqueous solution and filter it. Now the filtrate contains p-dichlorobenzene and the precipitate can be dried and purified by recrystallization method by dissolving it in acetone and slowly evaporating acetone.

The acidic aqueous solution should first be neutralized by adding NaOH (200mL of 6M) and evaporating the water off. This results in p-dichlorobenzene and NaCl in solid phase. To this add diethyl ether to dissolve p-dichlorobenzene, filter it, and recrystalize it to recover pure solid p-dichlorobenzene.