The Mercury Photo-Sensitized Decomposition of Methyl Chloride

A study has been made of the mercury photo‐sensitized decomposition of methyl chloride over a pressure range of 10 mm to 640 mm, a temperature range of 72°C to 328°C, and a threefold variation in light intensity. The stoichiochemical equation for the reaction at 250°C and 160‐mm pressure is $3{\mathrm{CH}}_3\mathrm{Cl}=\mathrm{HCl}\text{+0.97}{\mathrm{CH}}_4\text{+0.49}{\mathrm{C}}_2{\mathrm{H}}_4{\mathrm{Cl}}_2\text{+0.41}{\mathrm{Hg}}_2{\mathrm{Cl}}_2\text{+0.24}{\mathrm{C}}_2{\mathrm{H}}_4\text{+0.18}{\mathrm{CH}}_2=\mathrm{CHCl}\text{+0.15}\mathrm{C}\text{+0.07}{\mathrm{C}}_2{\mathrm{H}}_6\text{+0.02}{\mathrm{H}}_2\text{+0.01}{\mathrm{C}}_2{\mathrm{H}}_2$ . At high pressures, the following mechanism is shown to be consistent with the experimental results: $$\begin{array}{c}\mathrm{Hg}{\text{(6}}^3{\mathrm{P}}_1\mathrm{)+}{\mathrm{CH}}_3\mathrm{Cl}=\mathrm{Hg}{\text{(6}}^1{\mathrm{S}}_0\mathrm{)+}{\mathrm{CH}}_3+\mathrm{Cl}\\ {\mathrm{CH}}_3+{\mathrm{CH}}_3\mathrm{Cl}={\mathrm{CH}}_4+{\mathrm{CH}}_2\mathrm{Cl}\\ \mathrm{Cl}+{\mathrm{CH}}_3\mathrm{Cl}=\mathrm{HCl}+{\mathrm{CH}}_2\mathrm{Cl}\\ {\mathrm{CH}}_2\mathrm{Cl}+{\mathrm{CH}}_2\mathrm{Cl}={\mathrm{C}}_2{\mathrm{H}}_4{\mathrm{Cl}}_2\\ \mathrm{Hg}+{\mathrm{CH}}_2\mathrm{Cl}=\mathrm{HgCl}+{\mathrm{CH}}_2\\ \mathrm{HgCl}+\mathrm{HgCl}\mathrm{\hspace{0.17em}\hspace{0.17em}=}{\mathrm{Hg}}_2{\mathrm{Cl}}_2\\ {\mathrm{CH}}_2+{\mathrm{CH}}_2\mathrm{\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}=}{\mathrm{C}}_2{\mathrm{H}}_4\end{array}$$ The first of these reactions is shown to have a quantum efficiency of unity. From the effect of added CO₂ on the reaction, an approximate value of σ²=5.7×10⁻¹⁶ cm² has been calculated for the quenching cross section of methyl chloride.