Products and Mechanism of the Reaction of Chlorine Atoms with 3-Pentanone in 700−950 Torr of N2/O2 Diluent at 297−515 K

Kinetic and product studies of Cl atoms reactions with a series of branched Ketones

The rate constants for the Cl atom reaction with three branched ketones have been measured at 298±2K and 760Torr using the relative rate method in the absence of NO. The rate constants values obtained (in units of 10^-10cm³/(molecule·sec)) are: k_{(2-methyl-3-pentanone)}=1.07±0.26, k_{(3-methyl-2-pentanone)}=1.21±0.26, and k_{(4-methyl-2-pentanone)}=1.35±0.27. Combining the chemical kinetic data obtained by this study with those reported for other ketones, a revised Structure Activity Relationship (SAR) parameter and R group reactivity (k_R) of R(O)R' and CHx (x=1, 2, 3) group reactivity (k_CHx) toward Cl atoms were proposed. In addition, the products from the three reactions in the presence of NO were also identified and quantified by using PTR-ToF-MS and GC-FID, and the yields of the identified products are: acetone (39%±8%)+ethanal (78%±12%), 2-butanone (22%±2%)+ethanal (75%±10%)+propanal (14%±1%) and acetone (26%±3%)+2-methylpropanal (24%±2%), for Cl atoms reaction with 2-methyl-3-pentanone, 3-methyl-2-pentanone and 4-methyl-2-pentanone, respectively. Based on the obtained results, the reaction mechanisms of Cl atoms with these three ketones are proposed.

Investigation of the Gas-Phase Photolysis and Temperature-Dependent OH Reaction Kinetics of 4-Hydroxy-2-butanone

Hydroxyketones are key secondary reaction products in the atmospheric oxidation of volatile organic compounds (VOCs). The fate of these oxygenated VOCs is however poorly understood and scarcely taken into account in atmospheric chemistry modeling. In this work, a combined investigation of the photolysis and temperature-dependent OH radical reaction of 4-hydroxy-2-butanone (4H2B) is presented. The objective was to evaluate the importance of the photolysis process relative to OH oxidation in the atmospheric degradation of 4H2B. A photolysis lifetime of about 26 days was estimated with an effective quantum yield of 0.08. For the first time, the occurrence of a Norrish II mechanism was hypothesized following the observation of acetone among photolysis products. The OH reaction rate coefficient follows the Arrhenius trend (280-358 K) and could be modeled through the following expression: k4H2B(T) = (1.26 ± 0.40) × 10(-12) × exp((398 ± 87)/T) in cm(3) molecule(-1) s(-1). An atmospheric lifetime of 2.4 days regarding the OH + 4H2B reaction was evaluated, indicating that OH oxidation is by far the major degradation channel. The present work underlines the need for further studies on the atmospheric fate of oxygenated VOCs.

Low temperature (550-700 K) oxidation pathways of cyclic ketones: dominance of HO2-elimination channels yielding conjugated cyclic coproducts

The low-temperature oxidation of three cyclic ketones, cyclopentanone (CPO; C5H8=O), cyclohexanone (CHO; C6H10=O), and 2-methyl-cyclopentanone (2-Me-CPO; CH3-C5H7=O), is studied between 550 and 700 K and at 4 or 8 Torr total pressure. Initial fuel radicals R are formed via fast H-abstraction from the ketones by laser-photolytically generated chlorine atoms. Intermediates and products from the subsequent reactions of these radicals in the presence of excess O2 are probed with time and isomeric resolution using multiplexed photoionization mass spectrometry with tunable synchrotron ionizing radiation. For CPO and CHO the dominant product channel in the R + O2 reactions is chain-terminating HO2-elimination yielding the conjugated cyclic coproducts 2-cyclopentenone and 2-cyclohexenone, respectively. Results on oxidation of 2-Me-CPO also show a dominant contribution from HO2-elimination. The photoionization spectrum of the co-product suggests formation of 2-methyl-2-cyclopentenone and/or 2-cyclohexenone, resulting from a rapid Dowd-Beckwith rearrangement, preceding addition to O2, of the initial (2-oxocyclopentyl)methyl radical to 3-oxocyclohexyl. Cyclic ethers, markers for hydroperoxyalkyl radicals (QOOH), key intermediates in chain-propagating and chain-branching low-temperature combustion pathways, are only minor products. The interpretation of the experimental results is supported by stationary point calculations on the potential energy surfaces of the associated R + O2 reactions at the CBS-QB3 level. The calculations indicate that HO2-elimination channels are energetically favored and product formation via QOOH is disfavored. The prominence of chain-terminating pathways linked with HO2 formation in low-temperature oxidation of cyclic ketones suggests little low-temperature reactivity of these species as fuels in internal combustion engines.