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.