Bromination of Hydrocarbons. I. Photochemical and Thermal Bromination of Methane and Methyl Bromine. Carbon-Hydrogen Bond Strength in Methane

The photochemical bromination of methane was studied in the temperature range 423–503°K and found to proceed through the following chain mechanism: $$\begin{array}{c}\text{(1)\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}}{\mathrm{Br}}_2\mathrm{+h\nu =}\mathrm{Br}+\mathrm{Br}\\ \text{(2)\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}}\mathrm{Br}+{\mathrm{CH}}_4={\mathrm{CH}}_3+\mathrm{HBr}\\ \text{(3)\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}}{\mathrm{CH}}_3+{\mathrm{Br}}_2={\mathrm{CH}}_3\mathrm{Br}+\mathrm{Br}\\ \text{(4)\hspace{0.17em}\hspace{0.17em}}{\mathrm{CH}}_3+\mathrm{HBr}={\mathrm{CH}}_4+\mathrm{Br}\\ \text{(5)\hspace{0.17em}}\mathrm{Br}+\mathrm{Br}\mathrm{+M=}{\mathrm{Br}}_2\mathrm{+M.}\end{array}$$ Bromination of methyl bromide is analogous and from 7.5 to 10 times more rapid in this temperature range. Hydrogen bromide inhibits bromination of methane but not of methyl bromide. Thermal bromination was studied at 570°K and found to follow the same mechanism as photochemical reaction, except that bromine atoms are produced thermally. The activation energy of photochemical bromination of methane is 17.8 kcal./mole and that of methyl bromide is 15.6 kcal./mole. Varying efficiencies of different molecules as third bodies in the homogeneous recombination of bromine atoms are discussed. Configurations of activated complexes have been assigned and by statistical mechanical calculations shown to be reasonable. Activation energies and other data have been combined to arrive at a value for the C–H bond strength in methane of 102 kcal./mole at room temperature.