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Influence of the Ether Functional Group on Ketohydroperoxide Formation in Cyclic Hydrocarbons: Tetrahydropyran and Cyclohexane
The Journal of Physical Chemistry A ( IF 2.7 ) Pub Date : 2019-03-13 00:00:00 , DOI: 10.1021/acs.jpca.8b12510 Jacob C. Davis , Alanna L. Koritzke , Rebecca L. Caravan 1 , Ivan O. Antonov 1 , Matthew G. Christianson , Anna C. Doner , David L. Osborn 1 , Leonid Sheps 1 , Craig A. Taatjes 1 , Brandon Rotavera
The Journal of Physical Chemistry A ( IF 2.7 ) Pub Date : 2019-03-13 00:00:00 , DOI: 10.1021/acs.jpca.8b12510 Jacob C. Davis , Alanna L. Koritzke , Rebecca L. Caravan 1 , Ivan O. Antonov 1 , Matthew G. Christianson , Anna C. Doner , David L. Osborn 1 , Leonid Sheps 1 , Craig A. Taatjes 1 , Brandon Rotavera
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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 [O2]0:[R•]0 > 2000, were used to ensure that R• + O2 → 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 ketohydroperoxides 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 •QOOH + O2 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 R• and •QOOH radicals via concerted C–C and C–O β-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 •QOOH + O2 reactions necessary for ketohydroperoxide formation. Carbon balance calculations reveal that products from ring opening of both R• and •QOOH, at 700 K, account for >70% at 10 Torr and >55% at 1520 Torr. Three pathways are confirmed to contribute to the depletion of •QOOH in tetrahydropyran including (i) γ-•QOOH → pentanedial + •OH, (ii) γ-•QOOH → vinyl formate + ethene + •OH, and (iii) γ-•QOOH → 3-butenal + formaldehyde + •OH. 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.
中文翻译：
醚官能团对环烃中四氢吡喃和环己烷中酮氢过氧化物形成的影响
使用多重光电离质谱法在环己烷和四氢吡喃上进行了光解引发的氧化实验，以评估后一物种中醚官能团对与自燃相关的反应机理的影响。使用[O 2 ] 0：[R • ] 0 > 2000的伪一阶条件来确保R • + O 2→产品是主要反应。在8.0-11.0 eV范围内，使用了来自同步加速器的准连续可调真空紫外光，在两个压力（10和1520 Torr）和三个温度（500、600和700 K）下测量了产品的光电离光谱。在两个物种中都测量了氢过氧化酮的光电离光谱，并且在实验噪声的范围内，其质量定性与n中形成的类似物种的光电离光谱相同。-丁烷氧化。但是，注意到在两种物质之间氢过氧化酮形成的温度依赖性上的差异。而来自环己烷收率明显多达700 K，高于650 K.均未检出在四氢吡喃ketohydroperoxides的差异表明反应机理的变化，由于该醚基，有可能影响所需• QOOH + O 2添加步骤。为了确定开环反应在减少氢过氧化酮中的作用，对四氢吡喃中9种物质的支化级分进行了定量。结果表明，产物由R •和•的单分子分解形成在四氢吡喃中，通过共同的C–C和C–Oβ断裂产生的QOOH自由基明显，而在环己烷氧化中则无意义。得出的主要结论是，在此处的条件下，开环途径降低了• QOOH的本已很低的稳态浓度，这在四氢吡喃的情况下可防止•氢过氧化酮形成所需的• QOOH + O 2反应。碳平衡计算表明，在700 K时，R •和• QOOH的开环产物在10 Torr时占70％以上，在1520 Torr时占55％以上。确认了以下三种途径有助于•四氢吡喃中QOOH的消耗，其中包括（i）γ- • QOOH→五溴二苯醚+• OH，（ii）γ- • QOOH→甲酸乙烯基酯+乙烯+ • OH，和（iii）γ- • QOOH→3-丁烯醛+甲醛+ • OH。比较了导致相似中间体的环己烷氧化反应的类似机理，并根据质谱结果证实没有发生此类开环反应。从与环己烷的比较中可以看出，四氢吡喃中的醚基增加了开环反应的倾向，并抑制了在链支化之前酮氢过氧化物异构体的形成。相反，在环己烷中不存在此类反应，则可形成高达700 K甚至更高温度的酮状氢过氧化物。
更新日期：2019-03-13
中文翻译：
醚官能团对环烃中四氢吡喃和环己烷中酮氢过氧化物形成的影响
使用多重光电离质谱法在环己烷和四氢吡喃上进行了光解引发的氧化实验，以评估后一物种中醚官能团对与自燃相关的反应机理的影响。使用[O 2 ] 0：[R • ] 0 > 2000的伪一阶条件来确保R • + O 2→产品是主要反应。在8.0-11.0 eV范围内，使用了来自同步加速器的准连续可调真空紫外光，在两个压力（10和1520 Torr）和三个温度（500、600和700 K）下测量了产品的光电离光谱。在两个物种中都测量了氢过氧化酮的光电离光谱，并且在实验噪声的范围内，其质量定性与n中形成的类似物种的光电离光谱相同。-丁烷氧化。但是，注意到在两种物质之间氢过氧化酮形成的温度依赖性上的差异。而来自环己烷收率明显多达700 K，高于650 K.均未检出在四氢吡喃ketohydroperoxides的差异表明反应机理的变化，由于该醚基，有可能影响所需• QOOH + O 2添加步骤。为了确定开环反应在减少氢过氧化酮中的作用，对四氢吡喃中9种物质的支化级分进行了定量。结果表明，产物由R •和•的单分子分解形成在四氢吡喃中，通过共同的C–C和C–Oβ断裂产生的QOOH自由基明显，而在环己烷氧化中则无意义。得出的主要结论是，在此处的条件下，开环途径降低了• QOOH的本已很低的稳态浓度，这在四氢吡喃的情况下可防止•氢过氧化酮形成所需的• QOOH + O 2反应。碳平衡计算表明，在700 K时，R •和• QOOH的开环产物在10 Torr时占70％以上，在1520 Torr时占55％以上。确认了以下三种途径有助于•四氢吡喃中QOOH的消耗，其中包括（i）γ- • QOOH→五溴二苯醚+• OH，（ii）γ- • QOOH→甲酸乙烯基酯+乙烯+ • OH，和（iii）γ- • QOOH→3-丁烯醛+甲醛+ • OH。比较了导致相似中间体的环己烷氧化反应的类似机理，并根据质谱结果证实没有发生此类开环反应。从与环己烷的比较中可以看出，四氢吡喃中的醚基增加了开环反应的倾向，并抑制了在链支化之前酮氢过氧化物异构体的形成。相反，在环己烷中不存在此类反应，则可形成高达700 K甚至更高温度的酮状氢过氧化物。
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