Proton-Transfer Reactions in the Simple Alkanes: Methane, Ethane, Propane, Butane, Pentane, and Hexane

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Proton-transfer reactions from CHO+ and CDO+ to the simple alkanes, methane through hexane, have been investigated in a tandem mass spectrometer. Acetaldehyde is shown to be the molecular source yielding CHO+ reactant with the lowest internal excitation energy. The translational energy dependence observed for the reactions CHO++CH4→CX5++CO, CX5+→CX3++X2 (X=H or D) suggests that the interaction is better represented over the energy range considered as proton stripping rather than as proceeding through an intermediate [CXO+–CX4]* complex. In addition, there is no evidence from isotope effects for a transition in mechanism. A highly simplified model treating the protonated methane species as a collection of closely coupled harmonic oscillators permits application of the classic rate expression for unimolecular decomposition to the dissociation yielding CH3+, and this relation provides a reasonable semiempirical fit to the experimental data. The reaction of CHO+ with ethane at quasithermal energy yields C2H7+ in relatively large abundance. C2H5+ is the major product, but isotopic experiments indicate that the latter is not formed by a direct hydride ion abstraction but instead results from dissociation of the protonated ethane ion. Isotopic evidence also indicates that 1,1 elimination of hydrogen from protonated ethane is more probable by a factor of 1.5 than 1,2 elimination. Further, there is a slight preference for eliminating the added proton even at the lowest kinetic energy, a tendency which is more pronounced at higher energies. The extreme sensitivity to slight changes in internal energy which is demonstrated in the C2H7+ decomposition is shown to account for the failure to detect C2H7+ as an intermediate in the CH3+/CH4 reaction. For alkanes larger than ethane, no protonated molecular ion could be observed, although the reactions of these hydrocarbons with CHO+ can also be reasonably interpreted as proton transfer followed by decomposition. For the higher hydrocarbons a greater number of decomposition paths are evident, and there is an increased tendency to eliminate the added proton at low kinetic energies. Proton addition to the hexanes apparently occurs by a fairly random attack of the protonating agent on the carbon skeleton. The relative intensities of decomposition products in the latter case suggest that the protonated hexanes undergo rearrangement prior to dissociation.