Hydrolysis of the Pyrethroid Insecticide Cypermethrin in Aqueous Media

Abstract

Hydrolysis of (1R, cis, αRS)- and (1R, trans, αRS)-isomers of cypermethrin [(RS)-α-cyano-3-phenoxybenzyl (1 RS)-cis, trans-3- (2,2-dichlorovinyl) -2,2-dimethylcyclopropanecar-boxylate] was studied in buffer solutions at pH 3.0, 7.0 and 11.0, and in natural river and sea water at 15°C, 25°C, 40°C and 55°C under laboratory conditions. For making suitable approximations, the expressions for kA, kNand kBof Eq. (1) were calculated, using values of kH at three pH’s. kh = ka[h+] + kn + kb[oh-] (1)- The pH-rate profile thus obtained showed that hydrolysis of both isomers proceeded predominantly through neutral (pH independent) and base-catalyzed processes in the regions below pH 5.0 and above pH 7.0, respectively, whereas both reactions occurred between pH 5.0 and 7.0. The trans-isomer was hydrolyzed approximately 1.2-1.7 times faster than the cis-isomer at any pH tested between 25°C and 50°C. The rates of hydrolysis of both isomers in buffer solutions were similar to those in one sample of river and one sample of sea water. The cleavage of the ester linkage proceeded more rapidly than the hydration of the CN group at any pH and temperature tested. © 1985, Pesticide Science Society of Japan. All rights reserved.