Acid-base extraction is frequently used as the first step in a work-up procedure following a chemical synthesis to remove acidic and basic starting materials or impurities. Acid-base extraction is typically a precursor to more complicated purification techniques, such as recrystallization, if the product synthesized is still not completely pure.
Organic synthesis often uses acid-base extractions during work-up procedures. For example, consider a Fischer esterification –– the condensation of a carboxylic acid with an alcohol to form an ester. The post-reaction mixture often consists of small amounts of leftover acid and alcohol, in addition to the desired ester. Acid-base extraction can be used to easily separate out the acidic starting materials from the ester. By rinsing the crude product mixture with a weak base (e.g. sodium bicarbonate), the carboxylic acid and alcohol will be washed away with the aqueous layer, leaving purified ester in the organic layer. The choice of base used for extraction is critical, as a strong base (e.g. sodium hydroxide) will hydrolyze the ester.
Another common example of acid-base extraction occurs following peptide coupling, where the amide product must be separated from leftover carboxylic acid and amine. The carboxylic acid can be removed by rinsing the organic layer with weak base (sodium bicarbonate), while the amine can be removed by rinsing with a weak acid (10% hydrochloric acid). Following these two extractions, the amide will remain in the organic layer and has been significantly purified.