Vacuum Ultraviolet Photolysis of Ethane: Molecular Detachment of Hydrogen

Abstract

The primary process of direct ethane photolysis by Xe radiation (1470 A and 1295 A) was studied at room temperature. The hydrogen and methane isotopic compositions from a mixture of C₂H₆—C₂D₆ and from CH₃CD₃ were measured mass‐spectrometrically. The results show that, contrary to the previously proposed mechanism, almost all (>95%) of the hydrogen is formed intramolecularly, and preferentially from the same carbon atom. Methane is also formed by a molecular process. Then the primary processes may be written as $${\mathrm{C}}_2{\mathrm{H}}_6\to {\mathrm{CH}}_3\mathrm{CH}+{\mathrm{H}}_2$$ $${\mathrm{C}}_2{\mathrm{H}}_6\to {\mathrm{C}}_2{\mathrm{H}}_4+{\mathrm{H}}_2$$ $${\mathrm{C}}_2{\mathrm{H}}_6\to {\mathrm{CH}}_4+{\mathrm{CH}}_2.$$ The absence of C₂D₆ in the products of the photolysis of CH₃CD₃ places an upper limit on [open phi]CD₃/[open phi]CD₃H+CH₃D of about unity assuming all methyl radicals form ethane by recombination. The effect of the product ethylene on the composition of hydrogen isotopes from C₂H₆—C₂D₆ mixtures was examined, since any atomic hydrogen produced may be reacting rapidly with ethylene. The sample was purified and very low conversion (0.01%) was made. The result shows almost no change in the composition of the hydrogen isotopes in the products, indicating that the hydrogen forming step is largely molecular. It is suggested that higher hydrocarbons may be produced by the following reactions $${\mathrm{CH}}_2+{\mathrm{C}}_2{\mathrm{H}}_6\to {\mathrm{C}}_3{\mathrm{H}}_8$$ $${\mathrm{CH}}_3\mathrm{CH}+{\mathrm{C}}_2{\mathrm{H}}_6\to {\mathrm{C}}_4{\mathrm{H}}_{10}.$$