Activated Choroalkyl Radicals in the Chlorination of Olefins

Abstract

Propane has been photochlorinated in competition with cis- and trans-dichloroethylene (c-DCE and t-DCE), 1,1-dichloroethylene (1,1-DCE), trichloroethylene (TrCE) and tetrachloroethylene (TCE) at temperatures between 300 and 350 K and pressures up to 200 Torr (1 Torr = 1.33 × 102 N m–2).

The balance of decomposition to collisional stabilisation of excited chloroalkyl radicals, ACl* formed in reaction (2), A + Cl [graphic ommitted] ACl*, is important in the chlorination of c–DCE, t-DCE and TrCE, but thermal decomposition of the deactivated ACl radical is not. With TCE the opposite occurs, pyrolysis of ACl to A + Cl is important but decomposition of ACl* before deactivation is not.

At 350 K, k2 for c-DCE and t-DCE are identical at 3.5 × 1010 dm3 mol–1 s–1. The fraction of C2H2Cl* decomposing to c–DCE, determined using radiochlorine, is 0.67 and is also independent of the precursor. Assuming unit collision efficiency for deactivation of ACl* the following high pressure values for ka are obtained, ka(c-DCE)= 1.6 × 109 s–1; ka(t-DCE)= 3.0 × 109 s–1. Both values are pressure dependent falling with total pressure in good agreement with predictions from RRKM theory.

Attempts to study 1,1-DCE were unsuccessful due to a rapid heterogeneous reaction which was apparently initiated photochemically.

For TrCE, ka was determined using radiochlorine to be 4 × 108 s–1 and k2 to be 3 × 1010 dm3 mol–1 s–1. Pressure dependence of ka over a limited pressure range agreed with RRKM theory.

The rate of chlorination of TCE relative to propane was independent of total pressure when varied by adding inert gas (SF6) but was strongly dependent upon chlorine pressure especially at higher temperatures. We deduce that ka < 2 × 107 s–1 and k2= 2 × 1010 dm3 mol–1 s–1. The rate constant k4 for pyrolysis of C2Cl5 was obtained from the chlorine dependence as, log10(k4/s–1)=(13.47 ± 0.50)–(15 500 ± 700)K/4.576T, in reasonable agreement with previous work.