Alkyl-substituted cyclic ethers are intermediates formed during low-temperature oxidation and are derived from unimolecular reactions of hydroperoxyalkyl radicals (Q̇OOH). Accounting for detailed consumption reactions of cyclic ethers in chemical kinetics mechanisms is critical to accurately modeling the balance of reactions that Q̇OOH radicals undergo, as the former are isomer-specific proxies for the latter. Substituted oxiranes are one class of cyclic ethers commonly produced during low-temperature combustion, and vary according to length and stereochemical orientation of the alkyl substituents. To enable the first, direct speciation measurements of alkyloxiranes derived from n-pentane, and due to lack of commercial availability, novel organic syntheses were executed to produce cis– and trans– isomers of 2-ethyl-3-methyloxirane. Jet-stirred reactor (JSR) experiments were also conducted on n-propyloxirane to provide a comprehensive determination on the influence of substituents and stereochemistry on reaction mechanisms, which revealed a propensity for carbonyl formation. Potential energy surfaces were also mapped for all carbon-centered radicals to assess the extent to which isomerization and ring-opening reactions are favored in the formation of carbonyls. In addition, an initial chemical kinetics mechanism was produced for both n-propyloxirane and 2-ethyl-3-methyloxirane using RMG and employed to simulate species profiles.

Speciation measurements were conducted for n-propyloxirane and cis-/trans– 2-ethyl-3-methyloxirane oxidation in a JSR at 1.1 atm, from 600 – 950 K, at 2 s residence time and stoichiometric conditions using an initial concentration of 2500 ppm. Isomer-resolved species profiles were quantified using tandem vacuum-ultraviolet absorption spectroscopy and electron-impact mass spectrometry. Species such as ketones, aldehydes, cyclic ethers, and alkenes were observed from each set of oxidation experiments.058 For all alkyloxiranes, carbonyl species (aldehydes and ketones) were major products and are formed either via rearrangement reactions or reactions of resonance-stabilized radicals, which result from low-lying ring-opening reactions on the potential energy surface for the Ṙ radicals. The species produced, including 2-pentanone, 3-pentanone, pentanal, and 2-methylbutanal, are of specific interest owing to facile reactions with ȮH and other radicals and, in the case of aldehydes, production of n-alkyl radicals that can subsequently proceed via Q̇OOH-mediated, chain-branching reactions.