H2-Transfer Reactions in the Gas-Phase Radiolysis of Hydrocarbons

Abstract

The radiolysis of cyclohexane has been investigated in the presence of varying concentrations of acetylene, ethylene, propylene, butene, and cyclopropane. On the basis of a number of observations, it is concluded that, in all cases, the H₂‐transfer reaction $${\mathrm{C}}_n{\mathrm{H}}_m+{\mathrm{cyclo-C}}_6{\mathrm{H}}_{12}^{+}\to {\mathrm{C}}_n{\mathrm{H}}_{\text{m+2}}+{\mathrm{C}}_6{\mathrm{H}}_{10}^{+}$$ does take place. No extensive rearrangement occurs in the collision complex. For instance, a transfer of H₂ to CD₃CDCD₂ results exclusively in the formation of CD₃CDHCD₂H, while an H₂ transfer to (CD₂)₃ leads to the formation of CD₂HCD₂CD₂H. Relative rate constants for the transfer of an H₂ molecule to CH₃CHCH₂, 1‐C₄H₈, iso‐C₄H₈, 2‐C₄H₈, C₂H₄, and C₂H₂ are, respectively, 1.00, 0.68, 0.27, 0.10, 0.11, and 0.072. Similar variations in the relative rate constants of the H₂⁻‐transfer reaction $${\mathrm{C}}_n{\mathrm{H}}_m^{+}+{\mathrm{cyclo-C}}_6{\mathrm{H}}_{12}\to {\mathrm{C}}_n{\mathrm{H}}_{\text{m+2}}+{\mathrm{C}}_6{\mathrm{H}}_{10}^{+}$$ are noted. The following additional information was derived in the course of this study: (1) When cyclopentane or n‐pentane is substituted for cyclohexane in the reaction mixture, the H₂‐transfer reaction again occurs. Under comparable experimental conditions, the probability of the H₂‐transfer reaction is proportional to the yield of the parent ion as derived from the 70‐eV mass‐spectral cracking patterns. (2) An increase in the pressure of cyclopentane, from 12.7 to 210 mm, leads to approximately a 50% increase in the yield of the parent ion of cyclopentane. (3) The parent cyclohexane ion undergoes a charge transfer to NO at a rate which is comparable to the rates of the H₂‐transfer reactions. O₂ does not interact with the parent ion. (4) The cyclopropane‐d₆ parent ion reacts with cyclo‐C₆H₁₂ to form CD₃CDHCD₂H, indicating that this ion acquires the propylene ion structure prior to or during reaction.