C13 Isotope Effect in the Thermodecomposition of Ethyl Bromide
1955; American Institute of Physics; Volume: 23; Issue: 1 Linguagem: Inglês
10.1063/1.1740506
ISSN1520-9032
AutoresHenry L. Friedman, Richard B. Bernstein, H. E. Gunning,
Tópico(s)Chemical Reaction Mechanisms
ResumoThe C12—C13 fractionation factor in the decomposition of gaseous ethyl bromide has been measured from 350—450°C, using samples of natural isotope abundance. The rate constants are defined as follows: CH3CH2Br→CH2=CH2+HBrk1,CH3C*H2Br→CH2=C*H2+HBrk2,C*H3CH2Br→C*H2=CH2+HBrk3.At 400°C, the C12 enrichment of the first fraction of ethylene from decomposition of the ethyl bromide is S0≡1+ε0=2k1k2+k3=1.0079±0.0006,with a temperature coefficient of — 2.8×10-5/°C. The primary and secondary isotope effects are defined, respectively, as β=k1/k2—1 and γ=k1/k3—1; thus, to a good approximation, β+γ=2ε0. According to theory, β>γ≥0, so that ε0<β≤2ε0 and ε0>γ≥0. From the data of 400°C one then obtains as an upper limit (k1/k2)max≤1.0159±0.0012. This is significantly lower than the value k1/k2≥1.036 calculated for the rupture of an isolated C—Br bond. The present results, therefore, favor a mechanism involving the direct intramolecular elimination of HBr.
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