Submitted to: International Congress of Pesticide Chemistry Abstracts
Publication Type: Abstract Only
Publication Acceptance Date: 4/30/2002
Publication Date: 8/4/2002
Technical Abstract: Endosulfan, an insecticide used on vegetables, fruits, cereals and cotton, exists as 7:3 isomeric mixture of alpha and beta when applied. However, this isomeric ratio is not conserved throughout the environmental compartments and the ratio is dependent on the compartment. Studies examining endosulfan emission from soil have shown very little loss of beta and concomitantly a large release of alpha-endosulfan. The alpha isomer has been shown to be less persistent than beta on vegetables and on foliage. An exception to the compartmental influence on the isomeric ratio was indicated in tree bark samples taken from over thirty countries around the world where no trend was observed. Several studies have noted isomerization of beta-endosulfan to alpha which led to the reexamination of the structure of the isomers. The alpha isomer was established unequivocally by X-ray crystallography as asymmetric (1). In addition, a eutectic concentration of ca. 63% alpha and 37% beta was observed (2). A mechanism was proposed for the isomerization of beta to alpha. Computational chemistry demonstrated that the isomerization mechanism involved induction of asymmetry associated with changes in the sulfite conformation of the symmetrical beta form. The structural and isomeric conversion data combined with the measured Henry's Law Constants (2.7 x 10e-3 alpha; 3.6 x 10e-4 beta) and subcooled liquid water solubilities (3.7 mg/L alpha; 21 mg/L beta) provide an explanation for the observed isomeric compartmentalization. That is, very little of the beta isomer would be expected to be in the air and that which is volatilized is readily removed by water droplets. Furthermore, the beta isomer would predominate in the soil and in runoff.