IMIDAZOLE CATALYSIS OF ORGANIC HYDROXYL GROUP DETERMINATION BY REACTION WITH PYROMELLITIC DIANHYDRIDE IN DIMETHYLFORMAMIDE

Imidazole is superior to pyridine, other substituted imidazoles and pyridines, and several tertiary amines in the catalysis of the reaction between pyromellitic dianhydride and organic hydroxyl groups. In dimethylformamide solution, quantitative reaction occurs within 5 minutes at room temperature or at 70-80 °C depending on reagent concentration. Hydrolysis of the excess anhydride is instantaneous and gives a colorless solution. Visual titration is carried out with either thymol blue or phenolphthalein. The overall precision for hydroxyl number determination is ±0.3%. Imidazole does not catalyze the reaction of aldehydes and phenols with pyromellitic dianhydride; however, it does cause partial reaction of tertiary alcohols and alkoxysilanes. Infrared and NMR evidence indicates the formation of an intermediate which reacts with the alcohol to give the ester product. Two possible mechanisms for the catalysis are proposed. © 1969, American Chemical Society. All rights reserved.