Influence of the Ether Functional Group on Ketohydroperoxide Formation in Cyclic Hydrocarbons: Tetrahydropyran and Cyclohexane

Photolytically initiated oxidation experiments were conducted on cyclohexane and tetrahydropyran using multiplexed photoionization mass spectrometry to assess the impact of the ether functional group in the latter species on reaction mechanisms relevant to autoignition. Pseudo-first order conditions, with [O-2](0):[R-center dot](0) > 2000, were used to ensure that R center dot + O-2 -> products were the dominant reactions. Quasi-continuous, tunable vacuum ultraviolet light from a synchrotron was employed over the range 8.0-11.0 eV to measure photoionization spectra of the products at two pressures (10 and 1520 Torr) and three temperatures (500, 600, and 700 K). Photoionization spectra of ketohydroper-oxides were measured in both species and were qualitatively identical, within the limit of experimental noise, to those of analogous species formed in n-butane oxidation. However, differences were noted in the temperature dependence of ketohydroperoxide formation between the two species. Whereas the yield from cyclohexane is evident up to 700 K, ketohydroperoxides in tetrahydropyran were not detected above 650 K. The difference indicates that reaction mechanisms change due to the ether group, likely affecting the requisite (center dot)QOOH + O-2 addition step. Branching fractions of nine species from tetrahydropyran were quantified with the objective of determining the role of ring-opening reactions in diminishing ketohydroperoxide. The results indicate that products formed from unimolecular decomposition of It' and (center dot)QOOH radicals via concerted C-C and C-O beta-scission are pronounced in tetrahydropyran and are insignificant in cyclohexane oxidation. The main conclusion drawn is that, under the conditions herein, ring-opening pathways reduce the already low steady-state concentration of 'QOOH, which in the case of tetrahydropyran prevents (center dot)QOOH + O-2 reactions necessary for ketohydroperoxide formation. Carbon balance calculations reveal that products from ring opening of both R-center dot and (center dot)QOOH, at 700 K, account for >70% at 10 Ton and >55% at 1520 Torr. Three pathways are confirmed to contribute to the depletion of (center dot)QOOH in tetrahydropyran including (i) gamma-center dot QOOH pentanedial + (OH)-O-center dot, (ii) gamma-(center dot)QOOH -> vinyl formate + ethene + (OH)-O-center dot, and (iii) gamma-(center dot)QOOH -> 3-butenal + formaldehyde + (OH)-O-center dot. Analogous mechanisms in cyclohexane oxidation leading to similar intermediates are compared and, on the basis of mass spectral results, confirm that no such ring opening reactions occur. The implication from the comparison to cyclohexane is that the ether group in tetrahydropyran increases the propensity for ring-opening reactions and inhibits the formation of ketohydroperoxide isomers that precede chain branching. On the contrary, the absence of such reactions in cyclohexane enables ketohydroperoxide formation up to 700 K and perhaps higher temperature.