Activated chloroethyl radicals in the chlorination of 1,2-dichloroethylenes

Abstract

The addition of a chlorine atom to dichloroethylene (DCE) gives an activated trichloroethyl radical ACl^*, which may decompose to produce a mixture of cis- and trans-DCE or be deactivated to give a normal trichloroethyl radical which subsequently reacts with chlorine to form tetrachloroethane. Between 35 and 133°C, ACl^* decomposes to give 78% cis-DCE, and 22% trans-DCE. Deactivation occurs with comparable efficiency with Cl₂, DCE, C₃H₈ and CO₂. Assuming that every collision with Cl₂ is efficient the rate constant for unimolecular decomposition of ACl^* is k_a= 2.5 × 10⁹ sec^–1. By competing the chlorination of DCE with that of propane the absolute rate constants for addition of Cl to cis- and trans-DCE were obtained: k_2c= 8 × 10¹⁰ exp [–190/1.99T] mole^–1 l. sec^–1, and k_2t= 3 × 10¹⁰ exp [+170/1.99T] mole^–1 l. sec^–1. It is deduced that internal rotation in both ACl^* and the transition state complex ACl^{[graphic ommitted]} must be effectively unhindered.