KrF laser-induced thermal decomposition of 1,2-dichloroethane

Abstract

The thermal decomposition of 1,2-dichloroethane to give vinyl chloride and hydrogen chloride has been reinvestigated at 703 K and the major features of this free-radical chain reaction found previously have been confirmed. A direct comparison of the pulsed KrF laser-induced process (248 nm) in the same static reactor at this temperature showed that a substantial increase in rate occurs even at low pulse-repetition rates (2 Hz).

In an extended study of the laser-induced reaction at temperatures where thermal reaction is negligible (476–577 K), quantum yields of vinyl chloride formation have been measured to be between 3000 and 13 000. Quantum yields decreased slightly with increasing pressure and showed an inverse square-root dependence on initial chlorine atom concentration. The major by-products were identified as ethene, ethyne, 1,2,3,4-tetrachlorobutane and 1,1,2-trichloroethane.

At 577 K and 150 Torr, the quantum yield of ethene production is close to unity, indicating that under these conditions reaction (2) is essentially complete. C₂H₄Cl₂+hν(248 nm)→ CH₂ClCH^*₂+ Cl (1), CH₂ClCH^*₂→ C₂H₄+ Cl (2)

Computer modelling of a complete reaction mechanism involving the propagation steps (3) and (4) Cl + C₂H₄Cl₂→ C₂H₃Cl₂+ HCl (3), C₂H₃Cl₂→ C₂H₃Cl + Cl (4) was used to predict quantum yields of both vinyl chloride and the minor products.