Work-up

In chemistry, work-up refers to the series of manipulations required to isolate and purify the product(s) of a chemical reaction. The term is used colloquially to refer to these manipulations, which may include: deactivating any unreacted reagents by quenching a reaction. cooling the reaction mixture or adding an antisolvent to induce precipitation, and collecting or removing the solids by filtration, decantation, or centrifugation. changing the protonation state of the products or impurities by adding an acid or base. separating the reaction mixture into organic and aqueous layers by liquid-liquid extraction. removal of solvents by evaporation. purification by chromatography, distillation or recrystallization. The work-up steps required for a given chemical reaction may require one or more of these manipulations. Workup steps are not always explicitly shown in reaction schemes. Written experimental procedures will describe work-up steps but will usually not formally refer to them as a work-up. The Grignard reaction between phenylmagnesium bromide (1) and carbon dioxide in the form of dry ice gives the conjugate base of benzoic acid (2). The desired product, benzoic acid (3), is obtained by the following work-up: The reaction mixture containing the Grignard reagent is allowed to warm to room temperature in a water bath to allow excess dry ice to evaporate. Any remaining Grignard reagent is quenched by the addition of water. Dilute hydrochloric acid is added to the reaction mixture to protonate the benzoate salts, as well as to dissolve the magnesium salts. White solids of impure benzoic acid are obtained. The benzoic acid is decanted to remove the aqueous solution of impurities, more water is added, and the mixture is brought to a boil with more water added to give a homogeneous solution. The solution is allowed to cool slowly to room temperature, then in an ice bath to recrystallize benzoic acid. The recrystallized benzoic acid crystals are collected on a Buchner funnel and are allowed to air-dry to give pure benzoic acid.