Kinetics and Mechanism of the Reaction of Methacrolein with Chlorine Atoms in 1−950 Torr of N₂or N₂/O₂Diluent at 297 K

Abstract

The rate constant of the reaction of Cl atoms with methacrolein (k(1)) has been measured relative to that of Cl with propane (k(2)) or cyclohexane (k(6)) at ambient temperature and pressures varying from 1-950 Torr. The experiments were carried out by irradiation (350 nm) of Cl(2)/methacrolein/propane or cyclohexane mixtures in N(2) or N(2)/O(2) diluent at ambient temperature in a spherical (500 cm(3)) Pyrex reactor (GC/FID analyses) or a 140 L FTIR smog chamber. The measured relative rate constant ratios in the GC/FID experiments were k(1)/k(2) = 1.464 +/- 0.015(2sigma) in N(2) and k(1)/k(2) = 1.68 +/- 0.03(2sigma) in N(2)/O(2) diluent (O(2) > 20,000 ppm). No pressure dependence was observed over the range studied in N(2) (120-950 Torr) using the GC/FID. In the FTIR/smog chamber experiments values of k(1)/k(6) = 0.645 +/- 0.032, 0.626 +/- 0.037, 0.586 +/- 0.026, and 0.479 +/- 0.024 were measured in 700, 100, 10, and 1 Torr, respectively, of N(2) diluent. Using k(2) = (1.4 +/- 0.2) x 10(-10) and k(6) = (3.3 +/- 0.5) x 10(-10) high pressure limiting rate constants of k(1) = (2.05 +/- 0.3) x 10(-10) [GC/FID] and (2.13 +/- 0.34) x 10(-10) [FTIR] cm(3) molecule(-1) s(-1) were determined. In experiments using the GC/FID reactor with N(2) diluent the following products (molar yields) were observed: 2,3-dichloro-2-methylpropanal [(47.2 +/- 8)% excluding error in calibration]; methacryloyl chloride [(22.9 +/- 2)%]; and 2-chloromethylacrolein [(2.3 +/- 0.8)%]. Addition of 200 ppm O(2) (with Cl(2) = 5000 ppm) resulted in a sharp reduction of the 2,3-dichloro-2-methylpropanal yield (to approximately 2%) with an accompanying appearance of chloroacetone [yield = (55 +/- 7)% decreasing to (44 +/- 7)% in air diluent]. The methacryloyl chloride yield was 23% for [O(2)]/[Cl(2)] ratios from 0 to 0.2 but decreased to near zero as the O(2)/Cl(2) ratio was increased to approximately 400. The variation in methacryloyl chloride yield with the O(2)/Cl(2) ratio in the initial mixture allowed an approximate measurement of the rate constant for the reaction of the methacryloyl radical with O(2) relative to that with Cl(2) (k(O(2))/k(Cl(2)) = 0.066 +/- 0.02). In experiments using the FTIR reactor in 700 Torr of N(2) diluent, methacryloyl chloride [(26 +/- 3)%] and HCl [(27 +/- 3)%] were observed as products. In 700 Torr of air diluent, the observed products were: chloroacetone [(44 +/- 5)%], CO(2) [(27 +/- 3)%], HCl [(21 +/- 3)%], and HCHO [(14 +/- 2)%], and CH(3)C(O)CH(2)OH (3-4%). The observation of CH(3)C(O)CH(2)OH indicates the presence of OH radicals in the system. At atmospheric pressure and 297 K, the title reaction proceeds [(24.5 +/- 5)%] via abstraction of the aldehydic hydrogen atom, [(2.3 +/- 0.8)%] via abstraction from the -CH(3) group, and approximately [(47 +/- 8) %] via addition to the CH(2)=C < double bond with most of the addition occurring at the terminal carbon atom (uncertainties represent statistical 2sigma). The results are discussed with respect to the literature data.

Atmospheric Reaction of Cl + Methacrolein: A Theoretical Study on the Mechanism, and Pressure- and Temperature-Dependent Rate Constants

“…As shown in Figure 10, the rate constant increases significantly with the pressure increasing from 1 to 10 Torr, and there is no discernible pressure dependence when P > 100 Torr. This finding is in good agreement with the experiment observations of Kaiser et al 28 …”

“…This is in agreement with the experimental observations of Kaiser et al 28 So we will focus on the addition reaction of the CC double bond, the aldehyde-H and methyl-H abstraction reactions in the following kinetic calculation.…”

“…28 Canosa-Mas et al found that the branching ratio for the hydrogen abstraction is 0.18. 25 At room temperature and atmospheric pressure, the overall rate constant of the Cl + MACR reaction measured by experiments ranged from 2.09 × 10 −10 to (3.2 ± 0.5) × 10 −10 cm 3 molecule −1 s −1 .…”

Laboratory studies of organic peroxy radical chemistry: an overview with emphasis on recent issues of atmospheric significance

“…Birdsall et al51 also provide evidence for the formation of OH in the reaction of HO 2 with bicyclic peroxy radicals formed in the oxidation of aromatics, e.g.While some of this discrepancy may be related to artefactually high OH measurements in isoprenedominated environments,135 a great deal of recent effort has been expended on understanding the chemistry of isoprene-derived peroxy radicals, particularly in pristine regions of the atmosphere, where their fate is controlled in large part by reactions with HO 2 and by unimolecular reactions (see Section 3.8).While initial speculation regarding the missing HO x recycling chemistry centered on reactions of HO 2 with RO 2 radicals derived from reaction of OH with isoprene, 18 OH production appears to be minimal in these (and related) cases 123,136. Reactions of HO 2 with peroxy species derived from reaction of OH with methyl vinyl ketone and methacrolein, the two main isoprene oxidation products, appear likely to generate OH, however,124,137 and thus likely play a minor role in OH radical production / HO x recycling. Further studies of OH production from the reactions of HO 2 with the complex suite of organic radicals derived from other biogenic…”

Rate constants for the gas‐phase reactions of chlorine atoms with 1,4‐cyclohexadiene and 1,5‐cyclooctadiene at 298 K

“…3, our study shows a systematic increase in the observed relative rate ratio for the reaction of Cl atom with cyclohexadiene, with increasing concentration of oxygen. Very recently, during the measurement of the rate constant of the Cl atom reaction with methacrolein with reference to propane, addition of 2-20% oxygen was found to increase the relative ratio by a statistically significant amount [20]. Such difference in the rate constant in the presence of air was suggested to be due to interference of the OH reaction, generated during the reactions of peroxy radicals with HO 2 , as observed in the Cl-initiated oxidation of acetaldehyde [21,22].…”