The Mercury Photosensitized Hydrogenation of Ethylene, Ethylene-d4 and Partially Deuterized Ethylenes

The hydrogenation of ethylene, ethylene-d4 and partially deuterized ethylene photosensitized by excited mercury to λ = 2537A has been studied. It has been found that the rate of hydrogenation of the deutero-compound is more rapid than that of ethylene under comparable conditions. With excess hydrogen, the rate is independent of the ethylene pressure. The data obtained indicate that butane is the important product together with small amounts of ethane relatively more abundant in the case of ethylene-d4. Negligible amounts of methane are formed. The suggested mechanism involves the reactions H+C2H4 = C2H5 followed by C2H5+C2H5 = C4H10 and to a lesser degree, C2H5+C2H5 = C2H6+C2H4. At low hydrogen concentrations, or in absence of hydrogen, a photosensitized polymerization occurs, faster with ethylene than with ethylene-d4. The slower rate of polymerization of the deutero-compound may be explained on the basis of zero-point energy differences between C2H4 and C2D4 in their reactions with free radicals to yield more complex radicals.