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 k_a are obtained, k_a(c-DCE)= 1.6 × 10⁹ s^–1; k_a(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, k_a was determined using radiochlorine to be 4 × 10⁸ s^–1 and k₂ to be 3 × 10¹⁰ dm³ mol^–1 s^–1. Pressure dependence of k_a 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 k_a < 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.