6850
J. Phys. Chem. A 2010, 114, 6850–6860
Kinetics and Mechanism of the Reaction of Methacrolein with Chlorine Atoms in 1-950 Torr of N2 or N2/O2 Diluent at 297 K E. W. Kaiser,*,a,b I. R. Pala,b,c and T. J. Wallington*,d Department of Natural Sciences, UniVersity of Michigan-Dearborn, 4901 EVergreen Road, Dearborn, Michigan 48128, and System Analytics and EnVironmental Science Department, Research and InnoVation Center, Ford Motor Company, Mail Drop RIC-2122, Dearborn, Michigan 48121-2053 ReceiVed: April 13, 2010; ReVised Manuscript ReceiVed: May 17, 2010
The rate constant of the reaction of Cl atoms with methacrolein (k1) has been measured relative to that of Cl with propane (k2) or cyclohexane (k6) at ambient temperature and pressures varying from 1-950 Torr. The experiments were carried out by irradiation (350 nm) of Cl2/methacrolein/propane or cyclohexane mixtures in N2 or N2/O2 diluent at ambient temperature in a spherical (500 cm3) Pyrex reactor (GC/FID analyses) or a 140 L FTIR smog chamber. The measured relative rate constant ratios in the GC/FID experiments were k1/k2 ) 1.464 ( 0.015(2σ) in N2 and k1/k2 ) 1.68 ( 0.03(2σ) in N2/O2 diluent (O2 > 20 000 ppm). No pressure dependence was observed over the range studied in N2 (120-950 Torr) using the GC/FID. In the FTIR/smog chamber experiments values of k1/k6 ) 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 N2 diluent. Using k2 ) (1.4 ( 0.2) × 10-10 and k6 ) (3.3 ( 0.5) × 10-10 high pressure limiting rate constants of k1 ) (2.05 ( 0.3) × 10-10 [GC/FID] and (2.13 ( 0.34) × 10-10 [FTIR] cm3 molecule-1 s-1 were determined. In experiments using the GC/FID reactor with N2 diluent the following products (molar yields) were observed: 2,3-dichloro-2methylpropanal [(47.2 ( 8)% excluding error in calibration]; methacryloyl chloride [(22.9 ( 2)%]; and 2-chloromethylacrolein [(2.3 ( 0.8)%]. Addition of 200 ppm O2 (with Cl2 ) 5000 ppm) resulted in a sharp reduction of the 2,3-dichloro-2-methylpropanal yield (to ∼2%) with an accompanying appearance of chloroacetone [yield ) (55 ( 7)% decreasing to (44 ( 7)% in air diluent]. The methacryloyl chloride yield was 23% for [O2]/[Cl2] ratios from 0 to 0.2 but decreased to near zero as the O2/Cl2 ratio was increased to ∼400. The variation in methacryloyl chloride yield with the O2/Cl2 ratio in the initial mixture allowed an approximate measurement of the rate constant for the reaction of the methacryloyl radical with O2 relative to that with Cl2 (kO2/kCl2 ) 0.066 ( 0.02). In experiments using the FTIR reactor in 700 Torr of N2 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)%], CO2 [(27 ( 3)%], HCl [(21 ( 3)%], and HCHO [(14 ( 2)%], and CH3C(O)CH2OH (3-4%). The observation of CH3C(O)CH2OH 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 -CH3 group, and approximately [(47 ( 8) %] via addition to the CH2dC< double bond with most of the addition occurring at the terminal carbon atom (uncertainties represent statistical 2σ). The results are discussed with respect to the literature data. 1. Introduction The kinetics and mechanism of the reaction of Cl atoms with methacrolein (MACR) and the subsequent reaction of the radicals generated have been reported in four publications.1-4 The potential role of this reaction in the atmospheric chemistry of coastal regions has been discussed in detail previously1-4 and is not repeated here. However, recent measurements suggest that Cl atom reactions also may be significant at polluted noncoastal locations.5 Although the literature rate constants for this reaction are in rough agreement, they do differ by ∼50%. Selected product yields formed by abstraction and addition reactions of Cl atoms with MACR have been measured in air in two studies.2,4 However, * To whom correspondence should be addressed. E-mail: ewkaiser@ comcast.net (E.W.K.),
[email protected] (T.J.W.). a Mailing address: 7 Windham Lane, Dearborn, MI 48120. b University of Michigan-Dearborn. c Current address:
[email protected]. d Ford Motor Company.
no measurement of the chlorinated products formed by the reaction of the resultant alkyl and acyl radicals with Cl2 were reported because the high O2 concentration suppressed their formation. Measurements described herein are performed in the absence of O2, providing a direct method of determining the yield of each reaction channel in the Cl + MACR reaction because the corresponding product chlorides can be observed. The experiments discussed here were carried out at ambient temperature and pressures varying from 1.0 to 950 Torr using two very different reactors. Irradiation of Cl2/MACR/(C3H8 or cyclo-C6H12)/N2 mixtures with 350 nm radiation was used to measure the rate constants and products of reaction 1, which can yield four different radicals:
CH2dC(CH3)CHO + Cl + M f CH2ClC(CH3)CHO + M
10.1021/jp103317c 2010 American Chemical Society Published on Web 06/08/2010
(1a)
Reaction of Methacrolein with Chlorine Atoms
J. Phys. Chem. A, Vol. 114, No. 25, 2010 6851
CH2dC(CH3)CHO + Cl + M f CH2CCl(CH3)CHO + M
(1b) CH2dC(CH3)CHO + Cl f CH2dC(CH3)CO + HCl (1c) CH2dC(CH3)CHO + Cl f CH2dC(CH2)CHO + HCl (1d) The overall rate constant k1 was determined by the relative rate technique using the rate constant of the reaction of Cl with propane (k2)
C3H8 + Cl f C3H7 + HCl
(2)
or cyclohexane (k6) as the reference
c-C6H12 + Cl f C6H11 + HCl
(6)
We chose these references because they can be measured to high precision using the analytical methods available to us, and the rates of their reactions with chlorine atoms are well established, pressure independent, and similar to that of MACR. Reaction of the product radicals from reactions 1a-1d with Cl2 in the absence of O2 forms: 2,3-dichloro-2-methylpropanal via 3a or 3b; methacryloyl chloride via 3c; and 2-chloromethylacrolein via 3d:
CH2ClC(CH3)CHO + Cl2 f CH2ClCCl(CH3)CHO + Cl (3a) CH2CCl(CH3)CHO + Cl2 f CH2ClCCl(CH3)CHO + Cl (3b) CH2dC(CH3)CO + Cl2 f CH2dC(CH3)COCl + Cl (3c) CH2dC(CH2)CHO + Cl2 f CH2dC(CH2Cl)CHO + Cl (3d) Decompositions via elimination of a CH3 or HCO radical potentially compete with reaction 3b as fates for CH2CCl(CH3)CHO radicals. We also determined the yields of the two major organic products formed in the presence of moderate concentrations of O2. These products are methacryloyl chloride, formed in reaction 3c, and chloroacetone formed in reactions discussed later. Low concentrations of 2,3-dichloro-2-methylpropanal and 2-chloromethylacrolein are also observed. The concentrations of the products were measured as a function of O2 concentration for comparison to similar measurements performed on 2-butanone6 and 3-pentanone.7 At high O2 concentration (i.e., air diluent), the methacryloyl chloride is suppressed and other products are formed as determined in FTIR experiments. 2. Experiment 2.1. GC Experiments. A spherical (500 cm3), Pyrex reactor was employed. The reactants and products were analyzed using a gas chromatograph with flame-ionization detector (GC/FID), described elsewhere.8 Limited product identification studies were
carried out using an identical GC equipped with a mass spectrometric detector (GC/MS). The experiments were performed with Cl2 (99.7%)/CH4 (research grade)/MACR (95%)/ propane (research) mixtures in N2 (UHP) or N2/O2 (research) diluent. Freeze/thaw degassing cycles were performed on MACR and Cl2. Methane was used for internal calibration of the GC analysis since it is essentially unreactive toward Cl (kmethane ) 1.0 × 10-13 cm3 molecule-1 s-1)9 when compared to MACR (k1 ∼ 2 × 10-10, see Section 3.1). Propane was added as the reference compound to measure the overall rate coefficient for reaction 1. Chlorine atoms were generated by irradiation of the unreacted mixture with UV light using a single Sylvania F6T5 BLB fluorescent lamp (peak intensity at ∼350 nm). After a chosen irradiation time, a portion of the contents of the reactor was placed directly from the flask into the evacuated GC sample loop and then injected into the GC. The presence of CH4 as an internal calibrant permitted corrections to be made for uncertainty in the precise amount of sample injected. The mixture could then be irradiated in the 500 cm3 flask for additional times followed by additional analyses. Most experiments were carried out at ambient temperature and a total pressure of 700-950 Torr, depending upon the depletion of the sample in the reactor flask. The experiments in N2 were performed at 120-950 Torr to investigate the effect of total pressure on the rate constant. The reactant mole fraction ranges at high pressure were: Cl2 (200-7200 ppm); MACR (70-1600 ppm); CH4 (60-400 ppm); O2 (0-197000 ppm); balance N2. Specific compositions are presented in Tables 1 and 2. A repeat analysis of each irradiated mixture was performed for most experiments after a delay of 30-60 min. The yields were stable to ∼(3% from the mean of the two measurements for products except 2,3-dichloro-2methylpropanal. The yield of this species varied by ∼(10% probably because its vapor pressure is low and sampling losses may be larger and less reproducible (see below). Product identification and calibration are ideally achieved by injecting a known concentration of the pure product into the GC to determine its FID sensitivity and retention time. This is convenient only if the product is commercially available. Commercial samples are available for methacryloyl chloride and chloroacetone. Commercial samples of 2,3-dichloro-2-methylpropanal and 2-chloromethylacrolein are not available. The GC peaks for these two species were identified by analysis of reaction samples using a GC/MS instrument as discussed in the Appendix. This does not allow direct calibration of the sensitivity to the FID detector. However, with species for which pure samples are available in previous experiments, the GC sensitivity per mole fraction of the species in the sample is proportional to the number of nonoxygenated carbon atoms in the species plus a correction for sample loss in the GC injection system.6 The carbonyl group has no response to the FID detector. The sample-loss correction is approximately proportional to its retention time on the GC column as determined by measuring the correction for a variety of available compounds. The estimated sample loss correction was a factor of 1.6 for 2,3dichloro-2-methylpropanal and 1.3 for 2-chloromethylacrolein. The use of a correction factor of 1.6 for a major product species clearly introduces significant uncertainties in its yield as will be discussed later, but no other option was available. As discussed in Section 3.3.1, the estimated yield of 2,3-dichloro2-methylpropanal is similar to that of chloroacetone in the presence of O2. This provides support for the accuracy of this sensitivity correction. It is important to note that uncertainty in the absolute calibration of the 2,3-dichloro-2-methylpropanal
6852
J. Phys. Chem. A, Vol. 114, No. 25, 2010
Kaiser et al.
TABLE 1: Relative Rate and Product Yield Data from Each Irradiation in the Absence of Added O2 Using GC/FIDa MACR (ppm) Cl2 (ppm) P (Torr) tirra (s) C3H8 C/C0 MACR C/C0 677 665 669 678 672 672 783 783 783 1569 1569 1596 1596 225 225 5120 4622 4622 5021 5021 4860 675g 675g
5410 5351 5435 5418 5566 5566 7078 7185 7185 5644 5644 5870 5870 4826 4826 48919 43176 43176 48937 48937 46666 1460 1460
950 900 980 930 980 930 750 750 720 940 890 930 890 950 880 110 115 114 110 105 110 915 890
20 5 10 32 5 15 15 4 8 10 50 15 35 2 4 15 5 12 3 10 7 47 107
0.466 0.854 0.632 0.333 0.862 0.505 0.458 0.882 0.654 0.906 0.428 0.767 0.532 0.856 0.661 0.447 0.745 0.356 0.860 0.450 0.777
0.327 0.796 0.508 0.202 0.801 0.367 0.326 0.827 0.533 0.864 0.284 0.665 0.399 0.778 0.541 0.310 0.657 0.220 0.803 0.315 0.693 0.744 0.463
k1/k2
C4H6Cl2Ob (%) C4H5ClOc (%) C4H5ClOd (%) C3H5ClOe (%)
1.464 1.446 1.476 1.455 1.494 1.467 1.435 1.513 1.482 1.481 1.483 1.538 1.456 1.614 1.484 1.455 1.427 1.466 1.455 1.447 1.453
51.2 49.7 44.1 41.6 45.9 48.8 44.1 32.7 41.6 44.1 43.0 37.0 47.3 43.8 39.7 39f 37f 43f 26f 39f 39f 29.5 30.5
2.7 2.1 2.2 2.9 2.2 2.3 2.7 2.4 2.1 1.8 2.4 1.7 2.3 1.8 2.1 N/A 1.7 2.3 2.2 2.0 1.0 1.3 1.5
23.5 23.0 21.6 23.0 22.8 23.3 22.8 22.5 23.3 23.4 23.9 20.8 23.4 20.5 21.8 22.1 21.9 22.5 22.2 22.1 21.9 23.0 23.0
3.7 2.0 2.3 4.6 2.6 1.7 2.7 5.8 4.2 4.9 2.1 2.0 1.8 7.4 5.9 3.3 4.6 3.8 12.2 7.0 4.3 9.7 5.7
a Initial mole fractions are stated for MACR and Cl2. P is the pressure in the reactor, and tirr is the irradiation time in seconds. Also shown are the fractional consumption (C/C0) of both C3H8 and MACR and the rate constant ratio (k1/k2) calculated from these data. The yields of the products (mole %) corrected for secondary consumption are presented in columns 8-11. b Yield of 2,3-dichloro-2-methylpropanal. c Yield of 2-chloromethylacrolein. d Yield of methacryloyl chloride. e Yield of chloroacetone. f The stated yield of 2,3-dichloro-2-methylpropanal has been corrected for the additional sampling loss observed at low pressure (see Section 3.2.1). g These data points were obtained at a lower Cl2 mole fraction 4 years after the other data in this table. No C3H8 was present in these mixtures.
TABLE 2: Relative Rate and Product Yield Data from Each Irradiation in the Presence of Added O2 Using GC/FIDa MACR (ppm)
Cl2 (ppm)
O2 (ppm)
[O2]/ [Cl2]
P (Torr)
tirra (s)
C3H8 C/C0
MACR C/C0
k1/k2
C4H6Cl2Ob (%)
C4H5ClOc (%)
C4H5ClOd (%)
C3H5ClOe (%)
71 71 71 71 74 673 673 690 690 1400 1400 1530 1530 90 90
207 207 207 205 213 5770 5770 6000 6000 5130 5130 5600 5600 4780 4780
85522 85437 85642 84928 88231 196600 196600 19260 19260 1080 1080 1000 1000 200 200
413 413 414 414 414 34 34 3.2 3.2 0.21 0.21 0.18 0.18 0.04 0.04
760 760 760 760 760 950 890 930 880 930 880 910 880 910 850
120 60 30 20 105 50 90 40 70 10 20 20 25 1 3
0.126 0.582 0.751 0.825 0.208 0.703 0.414 0.716 0.546 0.865 0.718 0.701 0.589 0.852 0.424
0.046 0.385 0.639 0.728 0.072 0.555 0.224 0.575 0.369 0.812 0.591 0.585 0.447 0.784 0.288
1.486 1.763 1.564 1.650 1.676 1.671 1.696 1.656 1.647 1.436 1.588 1.509 1.521 1.519 1.451
N/Af
