Resonance Stabilization Effects on Ketone Autoxidation: Isomer-Specific Cyclic Ether and Ketohydroperoxide Formation in the Low-Temperature (400–625 K) Oxidation of Diethyl Ketone

Abstract

The pulsed photolytic chlorine-initiated oxidation of diethyl ketone [DEK; (CHCH)C═O], 2,2,4,4-d-DEK [d-DEK; (CHCD)C═O], and 1,1,1,5,5,5-d-DEK [d-DEK; (CDCH)C═O] is studied at 8 torr and 1-2 atm and from 400-625 K. Cl atoms produced by laser photolysis react with diethyl ketone to form either primary (3-pentan-on-1-yl, R) or secondary (3-pentan-on-2-yl, R) radicals, which in turn react with O. Multiplexed time-of-flight mass spectrometry, coupled to either a hydrogen discharge lamp or tunable synchrotron photoionizing radiation, is used to detect products as a function of mass, time, and photon energy. At 8 torr, the nature of the chain propagating cyclic ether + OH channel changes as a function of temperature. At 450 K, the production of OH is mainly in conjunction with formation of 2,4-dimethyloxetan-3-one, resulting from reaction of the resonance-stabilized secondary R with O. In contrast, at 550 K and 8 torr, 2-methyl-tetrahydrofuran-3-one, originating from oxidation of the primary radical (R), is observed as the dominant cyclic ether product. Formation of both of these cyclic ether production channels proceeds via a resonance-stabilized hydroperoxy alkyl (QOOH) intermediate. Little or no ketohydroperoxide (KHP) is observed under the low-pressure conditions. At higher O concentrations and higher pressures (1-2 atm), a strong KHP signal appears as the temperature is increased above 450 K. Definitive isomeric identification from measurements on the deuterated DEK isotopologues indicates the favored pathway produces a γ-KHP via resonance-stabilized alkyl, QOOH, and HOOPOOH radicals. Time-resolved measurements reveal the KHP formation becomes faster and signal more intense upon increasing temperature from 450 to 575 K before intensity drops significantly at 625 K. The KHP time profile also shows a peak followed by a gradual depletion for the extent of experiment. Several tertiary products exhibit a slow accumulation in coincidence with the observed KHP decay. These products can be associated with decomposition of KHP by β-scission pathways or via isomerization of a γ-KHP into a cyclic peroxide intermediate (Korcek mechanism). The oxidation of d-DEK, where kinetic isotope effects disfavor γ-KHP formation, shows greatly reduced KHP formation and associated signatures from KHP decomposition products.

Direct Sulfoxidation of Aromatic Methyl Thioethers with Aryl Halides by Copper-Catalyzed C(sp3)–H Bond Activation

“…To obtain preliminary data for the reaction mechanism study, some additional reactions were conducted according to Scheme 3. The kinetic deuterium isotope effects [14] observed in the control experiments (k H /k D = 3.9) were consistent with the C-H cleavage, being the rate-limiting step (see Supplementary Materials). We also did the two controlled experiments (Scheme 4).…”

Quantitative Detection of Products and Radical Intermediates in Low-Temperature Oxidation of Cyclopentane

“…The high-pressure experiments, conducted with ∼50–500 higher oxygen partial pressure than at 10 Torr, lead to much higher rate coefficients for O 2 addition and much more efficient collisional relaxation of chemically activated radicals. Our previous work on alkane, ether, and ketone oxidation revealed that high- P , high-oxygen conditions in the HPR enhance second-O 2 addition relative to the LPR. ,,− We therefore assigned the m / z = 67, 84, 100, and 116 peaks at 3000 and 7500 Torr to species formed by γ-QOOH + O 2 , as shown in Figure . In addition, the time dependence of m / z = 57 and 82 peaks (marked with asterisks in Figure ) suggests that they are also likely due to DI fragments of second-O 2 addition products; however, we could not definitively assign them.…”

Formation of a Resonance-Stabilized Radical Intermediate by Hydroxyl Radical Addition to Cyclopentadiene

“…Participation of the OH group with the delocalized π electrons may further stabilize the radical. RSRs play a significant chemical role in oxygen-rich gas-phase environments (e.g., interstellar medium, planetary atmospheres, and combustion) ,− due to their resistance to oxidation by molecular oxygen. Reaction of an RSR with O 2 forms a peroxy radical at the expense of localizing the previously delocalized electron, leading to a weak C–O 2 bond.…”