C13 Isotope Effect in the Thermodecomposition of Ethyl Bromide

The 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: $$\begin{array}{cc}{\mathrm{CH}}_3{\mathrm{CH}}_2\mathrm{Br}\to {\mathrm{CH}}_2={\mathrm{CH}}_2+\mathrm{HBr}& k_1,\\ {\mathrm{CH}}_3{\mathrm{C}}^{*}{\mathrm{H}}_2\mathrm{Br}\to {\mathrm{CH}}_2={\mathrm{C}}^{*}{\mathrm{H}}_2+\mathrm{HBr}& k_2,\\ {\mathrm{C}}^{*}{\mathrm{H}}_3{\mathrm{CH}}_2\mathrm{Br}\to {\mathrm{C}}^{*}{\mathrm{H}}_2={\mathrm{CH}}_2+\mathrm{HBr}& k_3.\end{array}$$ At 400°C, the C¹² enrichment of the first fraction of ethylene from decomposition of the ethyl bromide is $$S_0{\text{≡1+ε}}_0=\frac{{\text{2k}}_1}{k_2{\mathrm{+k}}_3}\text{=1.0079±0.0006,}$$ 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.