Kinetics and mechanisms of mercuration of aliphatic carboxylic acids

Abstract

The kinetics of the mercuration of acetic acid by mercuric acetate under anhydrous and aqueous conditions have been studied. The reaction is first order in mercuric acetate and possibly first order in acetic acid. Radicals are not involved. The reaction is insensitive to added salts and is not catalysed by acetate. Enol or carbanion intermediates are excluded by the absence of H-D exchange. A four-centre (S_F2) mechanism is suggested. The reaction is accelerated by perchloric acid and this is attributed to mercuration by acetoxymercuric perchlorate probably in the form of an ion pair. Mercuric perchlorate and mercuric nitrate are more effective mercurating agents than mercuric acetate. Although the perchloric acid catalysed reaction may follow the S_E2 mechanism an S_F2 pathway is preferred. Propionic acid is mercurated exclusively at the α-carbon atom and trimethylacetic acid is unreactive. Acetic acid is 2.7 times as reactive as propionic acid. Optically active methylethylacetic acid appears to be mercurated stereospecifically.