US EPA

8417155 

Journal Article 

Unimolecular reactions of chemically activated C2H5Br, 1,2-C2H4Br2, and 1,2-C2H4BrCl and the reaction of methylene with CH2Br2 and CH2BrCl 

Johnson, RL; Setser, DW 

1967 

1 

Journal of Physical Chemistry
ISSN: 0022-3654 

71 

13 

4366-4383 

English 

10.1021/j100872a033 

The unimolecular hydrogen halide elimination reactions from chemically activated C2H5Br, 1,2-C2H4Br2, and 1,2-C2H4BrCl have been studied. The molecules were produced in the gas phase by the combination reactions of methyl and chloro- or bromo-substituted methyl radicals which were generated at 25° by the H and Br or Cl abstraction reactions of CH2 from CH2Br2 or CH2BrCl. The measured nonequilibrium rate constants for C2H5Br, 1,2-C2H4Br2, and 1,2-C2H4BrCl were 7.1 × 109, 2.1 × 109, and 0.53 × 109 sec-1, respectively. No evidence was found for Br rupture from these chemically activated molecules. The measured rate constants are 2-10 times larger than for the corresponding chloroethane molecules. RRKM calculations were done for each of these molecules and the results are compared to the experimental rate constants. It was found that the model formerly developed for the four-centered elimination reaction of HCl from chloroethane when adapted to C2H5Br gave good agreement with experimental results. The important feature for explaining the diminished rate constants between the chloro- and bromoethanes is the lower critical energy for the latter. Some general discussion is given concerning nonequilibrium competitive unimolecular reactions as applied to C2H4BrCl, and a brief comment upon the behavior of thermal activation Arrhenius parameters in the falloff region is also included.