The C12mdash;C13fractionation factor in the decomposition of gaseous ethyl bromide has been measured from 350mdash;450deg;C, using samples of natural isotope abundance. The rate constants are defined as follows:CH3CH2Brrarr;CH2equals;CH2plus;HBrk1,CH3C*H2Brrarr;CH2equals;C*H2plus;HBrk2,C*H3CH2Brrarr;C*H2equals;CH2plus;HBrk3.At 400deg;C, the C12enrichment of the first fraction of ethylene from decomposition of the ethyl bromide isS0equiv;1plus;egr;0equals;2k1k2plus;k3equals;1.0079plusmn;0.0006,with a temperature coefficient of mdash; 2.8times;10hyphen;5/deg;C.The primary and secondary isotope effects are defined, respectively, as bgr;=k1/k2mdash;1 and ggr;=k1/k3mdash;1; thus, to a good approximation, bgr;+ggr;=2egr;0. According to theory, bgr;ggr;ge;0, so that egr;0bgr;le;2egr;0and egr;0ggr;ge;0. From the data of 400deg;C one then obtains as an upper limit (k1/k2)maxle;1.0159plusmn;0.0012. This is significantly lower than the valuek1/k2ge;1.036 calculated for the rupture of an isolated Cmdash;Br bond. The present results, therefore, favor a mechanism involving the direct intramolecular elimination of HBr.
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