Kinetics and mechanism for the thermal chlorination of chloroform in the gas phase: Inclusion of HCl elimination from CHCl₃

Abstract

Abstract HCl elimination from chloroform is shown to be the lowest energy channel for initiation in the thermal conversion of chloroform to CCl 4 , with chlorine gas in the temperature range of 573–635 K. Literature data on this reaction is surveyed and we further estimate its kinetic parameters using ab initio and density functional calculations at the G3//B3LYP/6‐311G(d,p) level. Rate constants are estimated and reported as functions of pressure and temperature using quantum RRK theory for k ( E ) and master equation analysis for fall‐off. The high‐pressure limit rate constant of this channel is k (CHCl 3 → 1 CCl 2 + HCl) = 5.84 × 10 40 × T −8.7 exp(−63.9 kcal/mol/ RT ) s −1 , which is in good agreement with literature values. The reactions of 1 CCl 2 with itself, with CCl 3 , and with CHCl 3 are incorporated in a detailed mechanistic analysis for the CHCl 3 + Cl 2 reaction system. Inclusion of these reactions does not significantly change the mechanism predictions of Cl 2 concentration profiles in previous studies (Huybrechts, Hubin, and Van Mele, Int J Chem Kinet 2000, 32, 466) over the temperature range of 573–635 K; but Cl 2 , CHCl 3 , C 2 Cl 6 species profiles are significantly different at elevated temperatures. Inclusion of the 1 CCl 2 + Cl 2 → CCl 3 + Cl reaction (abstraction and chain branching), which is found to have dramatic effects on the ability of the model to match to the experimental data, is discussed. © 2003 Wiley Periodicals, Inc. Int J Chem Kinet 35: 647–660, 2003