Purification by Sub-Zero Temperature Washing After the synthesis of an ester from the corresponding earboxylic acid, the product is usually contaminated with the unreacted acid. Purification by crystallization and recrystallization is sometimes quite tedious. It is well known that the rate of hydrolysis of esters is greatly reduced a t sub-zero temperatures, whereas the diffusion-controlled rate of acid and base is still relatively fast. Thus, most of the free acid (>95%) can be removed efficiently by washing (50ml X 5) the pre-coaled ester solution dissolved in suitable organic solvent with a low concentration of aqueous 0 to -5°C. When thus done in a cold room, the ester product will he left essentially untouched. base (e.g., 0.01 M )a t about ' For instance, most of the unreacted starting material, p-nitrophenol, is successfully removed by this simple method during the synthesis of bis-p-nitrophenol sulfite ester, despite the fact that the spontaneous hydrolysis of this ester is rather fast1 in aqueous solution a t room temperature. I acknowledge the financial support of this work by grant #NSF PCM 76-19561 awarded to Professor E. T. Kaiser.
' King, L.-H., and Kaiser, E. T., J. Amer. Chem. Soc., 96,1410,1974. Searle Chemistry Laboratory The University of Chicago Chicago, Ill. 60637
384 1 Journal of Chemical Education
Samuel S.-T. C h u
