A study of the reactive properties of the chlorinated ethylenes

Abstract

The chlorinated ethylenes are a group of six molecules having the basic ethylene structure, but differing in the numbers and locations of chlorine substituents. The primary aim of this work has been to determine how the progressive substitution of chlorines affects the reactive properties of the molecules, especially toward electrophilic attack. This is of particular interest because the chlorinated ethylenes, which are widely distributed in the environment, are known to have various adverse biological effects, including (for some of them) carcinogenicity. There is considerable evidence that the first step in the metabolism of these molecules is epoxidation of the double bond, a process which is believed to involve interaction with an electrophilic oxygen species. Accordingly the electrostatic potential that is produced in the space around each molecule by its nuclei and electrons has been computed. This potential is known to be an effective means for studying electrophilic processes; the initial tendency of an approaching electrophile is to go to those regions of the molecule where the electrostatic potential is most negative. The overall trend found in this work is a very distinct decrease in the sizes and magnitudes of the regions of negative potential as the number of chlorines increases. This holds true for the π-bond regions, and it is also true for the regions around the chlorines themselves. (cis- 1,2-Dichloroethylene is an interesting exception to this trend.) Thus the chlorinated ethylenes are expected to show a general decrease in reactivity toward electrophiles as the number of chlorines increases, as is indeed found experimentally. It is suggested that the marked weakening of the negative chlorine potentials that occurs as the number of chlorines increases may be attributed to the limited capacity of the double bond region to provide electronic charge for inductive withdrawal.