The Mercury Photo-Sensitized Reactions of Isobutene

An investigation has been made of the reaction of isobutene with mercury (³P₁) atoms at 30°C. The main products of the reaction are propylene, liquid polymer, and acetylene. Smaller amounts of a large number of other products are also formed. The over‐all rate of consumption of isobutene, as well as the rates of formation of propylene, acetylene, and propane are independent of pressure in the region of complete quenching. Furthermore the rate of formation of propylene is just double the rate of formation of acetylene. The maximum quantum yield of consumption of isobutene is 0.59. The principal steps in the mechanism proposed for the reaction are $$\begin{array}{c}\mathrm{i-}{\mathrm{C}}_4{\mathrm{H}}_8+\mathrm{Hg}{(}^3({\mathrm{P}}_1\mathrm{)\to }{\mathrm{C}}_4{\mathrm{H}}_7+\mathrm{H}+\mathrm{Hg}{(}^1{\mathrm{S}}_0\mathrm{),}\\ \mathrm{H}\mathrm{+i-}{\mathrm{C}}_4{\mathrm{H}}_8\to {\mathrm{C}}_4{\mathrm{H}}_9,\\ {\mathrm{C}}_4{\mathrm{H}}_7\mathrm{+i-}{\mathrm{C}}_4{\mathrm{H}}_8\to {\mathrm{C}}_2{\mathrm{H}}_2+{\mathrm{C}}_6{\mathrm{H}}_{13},\\ {\mathrm{C}}_4{\mathrm{H}}_9\mathrm{+i-}{\mathrm{C}}_4{\mathrm{H}}_8\to {\mathrm{C}}_3{\mathrm{H}}_6+{\mathrm{C}}_5{\mathrm{H}}_{11},\\ {\mathrm{C}}_5{\mathrm{H}}_{11}\mathrm{+i-}{\mathrm{C}}_4{\mathrm{H}}_8\to {\mathrm{C}}_3{\mathrm{H}}_6+{\mathrm{C}}_6{\mathrm{H}}_{13}.\end{array}$$ On the above mechanism the initial step would be only about 16 percent efficient. Polymerization is considered to be initiated mainly by the C₆H₁₃ radical. The average molecular weight of the polymer would then be approximately 240.