Department of Chemistry
Acadia University
Wolfville, NS, Canada B0P 1X0

E-mail: [email protected]

Vinyl bromide has been reported to decompose at temperatures above about 800 K by a molecular process that gives HBr and C₂H₂. We have recently reported experiments in which a second decomposition channel was identified at temperatures below about 800 K [1]. This channel ultimately produces C₂H₃ and Br in a process that is second order in vinyl bromide and has both a smaller preexponential factor and a smaller activation energy than the molecular process. This results in the molecular process masking the free radical one at the higher temperatures. We interpreted this new decomposition channel as a disproportionation reaction, giving the following decomposition channels with their indicated activation energies.

(1) C₂H₃Br + M® C₂H₂ + HBr + M E_a = 220 kJ mol^-1 (our data, empirical analysis)

(2) 2 C₂H₃Br ® C₂H₃ + C₂H₃Br₂ E_a = 150 kJ mol^-1 (our data, empirical analysis)

The work reported here results from the application of kinetic modeling to the pyrolysis of C₂H₃Br and its mixtures with hydrogen and a series of hydrocarbons. This provides a concise set of specific chemical reactions for some of which rate constants can be evaluated at temperatures ranging from approximately 650 K to 750 K.

Kinetic modeling provides rate constants for reactions (1) and (2) above. Since reaction (1) requires collisional energy transfer from the bath gas, the value of k₁ is pressure dependent. The model also permits calculation of the rate constant for the reaction of C₂H₃ with the parent compound which has the following potential channels.

(3) C₂H₃ + C₂H₃Br ® C₄H₆Br

(4) C₄H₆Br ® 1,3-C₄H₆ + Br

Reaction (4) is expected to be rapid [1] in which case reaction (3) is kinetically limiting for 1,3-C₄H₆.

(5) C₂H₃ + C₂H₃Br ® C₂H₄ + C₂H₂Br

(6) C₂H₂Br ® C₂H₂ + Br

Reaction (5) is kinetically limiting for C₂H₄, permitting estimation of the relative importance of the addition and abstraction channels for the reaction of C₂H₃ with vinyl bromide. Reaction (6) is also expected to be rapid [1] in which case the proportion of C₂H₂ produced in the free radical component of the reaction is determined by the routes to C₂H₂Br of which reaction (5) is important. This permits separation of the contributions of reactions (1) and (2) to the decomposition of vinyl bromide. Both C₂H₄ and 1,3-C₄H₆ are produced exclusively by the free radical channel, so the rate constant of reaction (2) can be adjusted to provide the correct total yield of these products.

The addition of H₂ to the reaction mixture produced a large increase in C₂H₄ and a smaller increase in 1,3-C₄H₆ but no change in the yield of C₂H₂. The addition of C₂H₂, the dominant product in the pyrolysis of pure vinyl bromide, produced a modest increase equally in the yields of C₂H₄ and 1,3-C₄H₆. The pyrolysis of mixtures of vinyl bromide with ethylene increased the yield of 1,3-C₄H₆ but produced no change in the yield of C₂H₂. The addition of 1,3-C₄H₆ to the reaction produced a large increase in the yield of C₂H₄ but had virtually no effect on the yield of C₂H₂. These results are used to deduce rate constants for the reactions of C₂H₃ with the additives.

1. Patricia Ann Laws, Bradley D. Hayley, Lori M. Anthony, and John M. Roscoe,
  J. Phys. Chem. A, 105, 1830 (2001).