Kinetics and mechanism of chloroprene cycloaddition reactions

Abstract

The thermal dimerization of chloroprene, containing diphenylpicrylhydrazyl as polymerization inhibitor, has been studied in the temperature range 25–60°C. Cyclobutane- and cyclohexene-derivatives are formed in comparable yield; the products were characterized as 1,2-dichloro-1,2 divinyl-cyclobutane, together with 1-chloro-4 (α-chlorovinyl)-cyclohexene-1, and 2-chloro-4(α-chloro-vinyl)-cyclohexene-1. Small quantities of 1,6-dichloro-cyclo-octadiene 1 : 5 are formed from part of the cyclobutane-derivative (Cope rearrangement). Dilatometric studies have been used to measure the overall rate and activation energy of the dimerization process, and indicate that the overall reaction is second order. The kinetics of formation of the individual dimers have been investigated by using a pyrolysis-GLC technique to analyze the reaction products; the energy of activation for formation of the cyclobutane derivative is identical with that for formation of the cyclohexene derivative, and the corresponding entropies of activation are similar. From a consideration of the kinetic parameters, the positions of the chlorine atoms in the products, and the absence of either solvent or photochemical effects, a general mechanism for the dimerization has been proposed. It is postulated that the transition state is formed by one-centre attack, and that the three possible types of attack (1,1; 1,4; 4,4) all occur to give intermediates which are probably diradical in character and which possess similar free energy. Published work on other cycloaddition systems provides some support for the mechanism suggested.