Reaction of Methyl-d3 Radicals with Isobutane, Isobutane-2-d, and Propane

The relative rates of abstraction by CD₃ radicals of primary, secondary, and tertiary H atoms have been measured. Using a standard rate constant for abstraction of D from acetone‐d₆, the following rate constants are obtained: $$\begin{array}{ccc}& \mathrm{\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}k(}{\mathrm{mole}}^{\text{−1}}{\mathrm{cc\; sec}}^{\text{−1}})& \\ {\mathrm{CD}}_3+{\mathrm{CD}}_3{\mathrm{COCD}}_3\to {\mathrm{CD}}_4+{\mathrm{CD}}_2{\mathrm{COCD}}_3& {\text{3.5×10}}^{10}{T}^{\frac{1}{2}}\exp \text{(−11 100/RT),}& \text{(2)}\\ {\mathrm{CD}}_3\mathrm{+(}{\mathrm{CH}}_3{)}_3)\mathrm{CD}\to {\mathrm{CD}}_4\mathrm{+(}{\mathrm{CH}}_3{)}_3\mathrm{C}& {\text{0.88×10}}^{10}{T}^{\frac{1}{2}}\exp \text{(−9200/RT),}& \text{(3)}\\ {\mathrm{CD}}_3\mathrm{+(}{\mathrm{CH}}_3{)}_3\mathrm{CD}\to {\mathrm{CD}}_3\mathrm{H}+{\mathrm{CH}}_2–\mathrm{CD}({\mathrm{CH}}_3{)}_2& {\text{4.38×10}}^{10}{T}^{\frac{1}{2}}\exp \text{(−11 100/RT),}& \text{(4)}\\ {\mathrm{CD}}_3\mathrm{+(}{\mathrm{CH}}_3{)}_3\mathrm{CH}\to {\mathrm{CD}}_3\mathrm{H}\mathrm{+(}{\mathrm{CH}}_3{)}_3\mathrm{C}& {\text{0.70×10}}^{10}{T}^{\frac{1}{2}}\exp \text{(−7600/RT).}& \text{(10)}\end{array}$$ Isotope effects obtained are, for secondary H and D in propane $$k_{\mathrm{H}}{\mathrm{/k}}_{\mathrm{D}}\text{=1.04}\exp \text{(1400/RT),}$$ and for tertiary H and D in isobutane $$k_{\mathrm{H}}{\mathrm{/k}}_{\mathrm{D}}\text{=0.80\hspace{0.17em}}\exp \text{(1600/RT).}$$ These isotope effects are within experimental error of that given by the general expression for the deuterium isotope effect previously proposed $$k_{\mathrm{H}}{\mathrm{/k}}_{\mathrm{D}}=\exp \text{(1500/RT),}$$ and are in excellent correspondence with the theoretical treatment of Johnston and Rapp.