Free radical mechanism of the Cl2 addition to acetylene

Abstract

The free radical mechanism for the addition of Cl2 to acetylene in the gas phase has been studied. The structures and energies of reactants, transition states and products were determined through abinitio calculations of the stationary points on the potential-energy surface (PES) for the interaction of these two molecules. Using BD(T)/6-311+G(2df,2p)//CASSCF(6,6)/6-31G(d,p) level of theory, the reaction rate for the initiation step (Cl2+C2H2→Cl+C2H2Cl) was estimated as 10-18 l mol-1 s-1 (at 298.15 K). This leads to the formation of a small quantity of Cl and C2H2Cl radicals, the chain propagators, and the following steps will only occur to an appreciable extent after an induction period, which generates a measurable amount of these radicals. The following steps were studied at the UCCSD(T)/6-311+G(2df,2p)//UMP2/6-31G(d,p) level of theory. The propagation reaction C2H2+Cl→C2H2Cl occurs with an activation energy of -1.22 kcal mol-1, and produces a radical C2H2Cl, where the two hydrogens are on opposite sides of the molecule (trans-isomer). This reaction has a rate constant 2.85×1010 l mol-1 s-1 at 298.15 K. The interconversion of the two isomers of the C2H2Cl radical (cis–trans) is very fast, with a rate constant 4.75×1010 s-1 and so these species can be considered to be in equilibrium. The rate constants for the reaction C2H2Cl+Cl2→C2H2Cl2+Cl, where the products trans- and cis-1,2-dichloroethylenes are formed, are 1.95×1010 and 3.63×109 l mol-1 s-1, respectively, and those for the two polymerization reactions C2H2+C2H2Cl→C2H2C2H2Cl are ca. 102 l mol-1 s-1. Hence, the latter reactions will not compete with the formation of C2H2Cl2, and the polymerization products will not be produced in meaningful amounts. Analysis of the kinetics data gives 97.3% of the trans-1,2-dichloroethylene and 2.7% of the cis-1,2-dichloroethylene products.