Cymenes are monoterpene derivatives composed of a benzene ring with iso-propyl and methyl substituents, and the proximity of the two alkyl groups enables relevant analysis into the ortho-effect of polysubstituted aromatics, in which low-temperature autoignition is more facile for ortho-substitution. The initial steps of ROO-related reactions from Cl-initiated oxidation of ortho-, meta– and para-cymene were studied at low pressure (8 Torr) over the temperature range 450–750 K using multiplexed photoionization mass spectrometry (MPIMS). Ratios of cyclic ether formation (related to chain-propagation and OH formation) relative to HO2-loss (related to chain-termination) were measured to characterize the ortho-effect as a function of temperature. The main results are twofold:
Cyclic ether yields in m– and p-cymene oxidation are negligible below 650 K, indicating minimal chain-propagation. In contrast, o-cymene exhibits significant cyclic ether formation, attributed to the favorable 6- and 7-membered-ring transition states in the formation of hydroperoxyalkyl (QOOH) intermediates from peroxy radicals (ROO). The cyclic ether/HO2-loss ratio in o-cymene oxidation is defined as unity at 450 K and decreases to ∼0.10 at 700 K. The ratio converges to ∼0.10 at 700 K for the three cymene isomers, indicating an upper limit of temperature for the ortho-effect. Photoionization spectra of cyclic ether formation from o-cymene oxidation indicate a competition between both resonance- and non-resonance-stabilized initial radicals at the lower range of temperature. With increasing temperature, cyclic ether formation shifts from pathways involving resonance-stabilized initial radicals towards pathways solely from non-resonance-stabilized initial radicals because of back-dissociation of weakly bound ROO adducts.