A chemist is performing a liquid-liquid extraction to separate benzoic acid from a nonpolar organic impurity. To selectively move the benzoic acid into the aqueous layer, which of the following reagents would be most effective?
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The answer
Correct answer: C. An aqueous solution of sodium bicarbonate, which deprotonates the carboxylic acid.
Tested concept: Liquid-liquid extraction
Explanation
Benzoic acid is a carboxylic acid, which is acidic. To move it from an organic layer into an aqueous layer during extraction, it needs to be converted into its conjugate base, which is ionic and thus water-soluble. Sodium bicarbonate (NaHCO₃) is a weak base that is strong enough to deprotonate a carboxylic acid, forming a carboxylate salt that dissolves in the aqueous layer. Pure water would not effectively deprotonate the acid for efficient separation. Hydrochloric acid (HCl) is a strong acid and would keep the benzoic acid in its protonated, organic-soluble form. Diethyl ether is an organic solvent and would keep the benzoic acid in the organic layer, not move it to the aqueous layer.
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