6580
J. Phys. Chem. 1996, 100, 6580-6586
FTIR Spectroscopic Studies of the Mechanisms of the Halogen Atom Initiated Oxidation of Haloacetaldehydes Junyi Chen,* Valerie Young, Valery Catoire,† and Hiromi Niki‡ Centre for Atmospheric Chemistry and Department of Chemistry, York UniVersity, 4700 Keele Street, North York, Ontario, M3J 1P3, Canada ReceiVed: October 19, 1995; In Final Form: December 15, 1995X
Product studies of Cl atom initiated oxidation of ClCH2CHO and Br atom initiated oxidation of BrCH2CHO were conducted by FTIR spectroscopy at 297 ( 2 K in 700 Torr of O2/N2 diluent, using reactant partial pressures in both the torr and millitorr ranges. The halogen atoms initiate reaction via hydrogen abstraction from the aldehydic group (CHO), producing haloacetyl radicals, XCH2CO, where X ) Cl or Br. In 700 Torr of air, the XCH2CO radicals may react via three channels: (i) O2-addition, XCH2CO + O2 f XCH2C(O)O2; (ii) unimolecular dissociation by CsC bond cleavage, XCH2CO (+M) f XCH2 + CO (+M); and (iii) unimolecular dissociation by CsX bond cleavage, XCH2CO (+M) f X + CH2dCdO (+M). All three channels were observed for BrCH2CO radicals, enabling estimation of the branching ratios, but ClCH2CO reacts only by O2-addition. Subsequent reactions of XCH2C(O)O2 radicals lead to XCH2O radical and CO2 formation. The ClCH2O radical reacts with O2 to produce CHClO and HO2, while the BrCH2O radical mainly eliminates the Br atom, based on CH2O detected.
Introduction Reactions of halogen atoms (Br and Cl) with unsaturated hydrocarbons may play important roles in marine and polar tropospheric chemistry.1-7 Recent experiments during the polar sunrise of 1992 by Jobson et al.3 showed that halogen atoms are present in ozone-depleted air and that ozone depletion occurs concurrently with the depletion of light unsaturated hydrocarbons, which are reactive toward halogen atoms. It was speculated that the reaction of halogen atoms with light unsaturated hydrocarbons might represent a chain-terminating step in the catalytic destruction of ozone by halogen atoms, thus limiting the rate of ozone depletion. In order to assess the role of unsaturated hydrocarbons in the ozone depletion episodes observed at polar sunrise, quantitative kinetic and mechanistic data for the halogen atom initiated oxidation of these hydrocarbons are required. In previous studies of halogen atom (Cl and Br) initiated oxidation of H2CdCH2 in 700 Torr of synthetic air,8,9 chloroacetaldehyde (ClCH2CHO) and bromoacetaldehyde (BrCH2CHO) were observed to be the major primary products. Secondary reactions of these haloacetaldehydes were postulated to explain other observed products such as CO, CO2, CH2O, CHBrO, and CHClO. Some kinetic data for the haloacetaldehydes exist. The rate constant k(Br + BrCH2CHO) ) (1.83 ( 0.11) × 10-13 cm3 molecule-1 s-1 was determined by Yarwood et al.8 using the relative rate method with the reaction Br + HCtCH as reference. The rate constant k(Cl + ClCH2CHO) was reported to be (1.5 ( 3) × 10-11 cm3 molecule-1 s-1 by Scollard et al.10 using the relative rate method with the reaction Cl + C6H5CH3 as the reference and (4.3 ( 0.2) × 10-11 cm3 molecule-1 s-1 by Yarwood et al.8 using the reaction Cl + HCtCH as the reference. However, there have been no quantitative product studies of the halogen atom initiated oxidation of haloacetaldehydes. The Cl atom initiated reaction of CH3CHO was previously studied by Niki et al.11 They concluded that the Cl + CH3* To whom correspondence should be addressed. † Present address: CNRS/Laboratoire de Physique et Chimie de l’Environnement, 45071 Orleans Cedex 2, France. ‡ Deceased on April 1, 1995. X Abstract published in AdVance ACS Abstracts, February 15, 1996.
0022-3654/96/20100-6580$12.00/0
CHO reaction was predominantly initiated via aldehydic hydrogen abstraction, producing the CH3CO radical and HCl, and that methyl hydrogen abstraction, leading to formation of the CH2CHO radical and HCl, contributed less than 1% of the total CH3CHO reacted. Presented here are the results from FTIR-based product studies of the Cl and Br atom initiated oxidation of ClCH2CHO and BrCH2CHO, respectively, with and without the presence of NO2, in 700 Torr total pressure of O2 /N2 diluent, at 297 ( 2 K. Experimental Section Experiments with reactant partial pressures in the torr range (O2-free experiments) were performed in a Pyrex photochemical reactor/IR absorption cell (50 cm long, 1 m path length, 1 L volume) with KBr windows, surrounded by six fluorescent lamps (GES30T8/CW, λ g 400 nm for visible photolysis, or GEF30T8/BLB, λ g 300 nm for UV photolysis). Spectra were collected over the frequency range 500-4000 cm-1 in 110 s at 1/ cm-1 resolution by coadding 25 scans, using a Mattson FTIR 8 spectrometer (Galaxy 4326C) with a KBr beam splitter and a liquid N2 cooled Hg-Cd-Te detector. Experiments with reactant partial pressures in the millitorr range (experiments in the presence of O2) used our in-house developed long-path FTIR facility, which has been described previously.12 Briefly, a Pyrex cell (200 cm long, 180 m path length, 140 L volume) with KBr windows, surrounded by 26 fluorescent lamps (GTEF40/CW, λ g 400 nm, or GEF40T12/ BLB, λ g 300 nm), served as the photochemical reactor/IR absorption cell. The IR spectra were recorded in the frequency region 500-3700 cm-1 using a Ge-coated beam splitter and a liquid He cooled CuGe detector. The interferometer was operated at a resolution of 1/16 cm-1, and 16 coadded interferograms were recorded in 90 s. Br atoms were generated by the visible photolysis (λ g 400 nm) of Br2 (Aldrich, 99.5%+), and Cl atoms were generated by the UV photolysis (λ g 300 nm) of Cl2 (Aldrich, 99%+). N2 (99.999%, O2 < 3 ppm, total hydrocarbon < 1 ppm), O2 (99.995%, total hydrocarbon < 0.1 ppm), and synthetic air (total hydrocarbon < 0.1 ppm) were obtained from Liquid Carbonic, Inc. © 1996 American Chemical Society
Halogen Atom Initiated Oxidation of Haloacetaldehydes
Figure 1. Spectral data in the frequency region 500-3100 cm-1 obtained from the photolysis of a mixture containing ClCH2CHO (15 mTorr) and Cl2 (15 mTorr) in 700 Torr of air: (A) before irradiation; (B) after 50 s irradiation; (C) difference spectrum ) [(B) - (A)] × 1.5. The peak at 1751 cm-1 has been truncated to ln(I0/I) ) 0.9. The numbers in parentheses are product concentrations in millitorr.
Reactant BrCH2CHO was synthesized by the hydrolysis of bromoacetaldehyde dimethyl acetal (BrCH2CH(OCH3)2) in the presence of H2SO4.8 More specifically, a mixture of 7 mL of BrCH2CH(OCH3)2 (Aldrich, 97%), 3.5 mL of H2O, and 4 drops of concentrated H2SO4 was warmed to around 370 K and stirred vigorously. The resultant clear solution, containing BrCH2CHO, CH3OH, and H2SO4, was pumped for a few minutes at room temperature to remove most of the CH3OH and H2O. The vapor over the concentrated solution was dried over P2O5 and then collected and diluted with 700 Torr N2. This synthesized BrCH2CHO was stable up to one week in N2 diluent. Commercially available ClCH2CHO (Aldrich, 50% solution in water) was dried in the same manner as described for BrCH2CHO. The purity of the haloacetaldehydes was confirmed by FTIR (cf. Figures 1A and 4A). Most reactant and product species were identified and quantified by reference to the infrared spectra of pure compounds acquired previously in our laboratory. No commercial sources were found for peroxychloroacetyl nitrate (ClCH2C(O)OONO2, PClAN), peroxybromoacetyl nitrate (BrCH2C(O)OONO2, PBrAN), bromoacetyl bromide (BrCH2CBrO), peroxychloromethyl nitrate (ClCH2OONO2, PClMN), and peroxybromomethyl nitrate (BrCH2OONO2, PBrMN). These products were identified on the basis of literature information and quantified by material balance, as described later in this work. The uncertainties in the quantitative analysis of the reactant conversions and product yields were generally estimated to be about 5-10%, based on the visual precision of the IR spectra subtraction. For very low product yields however, the uncertainties were estimated from the detection limit, which was about 0.002-0.005 Torr in the short cell (1 m path length) and 0.0040.008 mTorr in the long cell (180 m path length).
J. Phys. Chem., Vol. 100, No. 16, 1996 6581
Figure 2. Spectral data in the frequency region 500-3100 cm-1 obtained from the photolysis of a mixture containing ClCH2CHO (6 mTorr), NO2 (3 mTorr), and Cl2 (38 mTorr) in 700 Torr of air: (A) before irradiation; (B) after 30 s irradiation; (C) difference spectrum ) [(B) - (A)] × 1.5. The numbers in parentheses are product concentrations in millitorr.
Prior to irradiation, the reactant mixtures were kept in the reaction cells in the dark for about 10 min to ensure thorough mixing. During this period, neither changes in the reactant concentrations nor the formation of products was observed. A mixture containing BrCH2CHO and air was irradiated for 1 min under visible light, and a mixture containing ClCH2CHO and air was irradiated for 1 min under UV light; no changes in the reactant concentrations were observed. Results and Discussion (i) Cl + ClCH2CHO Reactions. Initial experiments were conducted with mixtures containing ClCH2CHO (0.7-1.0 Torr) and Cl2 (0.4-1.0 Torr) in 700 Torr of N2. After UV irradiation, IR bands characteristic of ClCH2CClO, CHClO, and CO were identified. The yields of these products, after consumption of 0.18 Torr of ClCH2CHO (21% conversion), were ClCH2CClO (0.17 Torr), CHClO (0.006 Torr), and CO (0.005 Torr). These products accounted for 97% of the carbon balance (see Table 1, run A). The observation of ClCH2CClO as the predominant product is consistent with the occurrence of reaction 1a, H-abstraction from the aldehydic hydrogen, followed by reaction 2,
XCH2CHO + X f XCH2CO + HX f XCHCHO + HX XCH2CO + X2 f XCH2CXO + X
(1a) (1b) (2)
where X ) Cl. Cl2CHCHO would be formed by a methyl hydrogen abstraction (reaction 1b), followed by reaction 3.
6582 J. Phys. Chem., Vol. 100, No. 16, 1996
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TABLE 1: Yields of the Cl Initiated Reaction of ClCH2CHO run Aa [ClCH2CHO]0 (Torr) [Cl2]0 (Torr) [NO2]0 (Torr) irradiation time (s) ∆[ClCH2CHO] (Torr) ∆[NO2] (Torr)
run Bb
0.83 0.43
15 × 15 × 10-3
4 0.18
20 1.64 × 10-3
10-3
Product Yield (%)c ClCH2CClO 94 ( 6d CHClO 3 ( 2 81 ( 6 CO 3 ( 2 11 ( 2 CO2 95 ( 5 ClCH2C(O)OONO2 (PClAN) ClCH2OONO2 (PClMN) carbon balance (%) 97 ( 7 93 ( 8
run Cb 6 × 10-3 38 × 10-3 3 × 10-3 30 1.80 × 10-3 2.15 × 10-3 3(2 3(2 7(2 88 ( 8 8(2
a Diluent consists of 700 Torr of N2. b Diluent consists of 560 Torr of N2 and 140 Torr of O2. c The percentage yields were based on the conversion of ClCH2CHO. d Uncertainties are based on visual precision of subtractions or detection limit (see text).
XCHCHO + X2 f X2CHCHO + X
(3)
However, no Cl2CHCHO was identified above the detection limit (about 0.005 Torr), so its yield is less than 2% of the ClCH2CHO conversion. A similar preference for aldehydic hydrogen abstraction was reported by Niki et al.11 for the Cl + CH3CHO reaction. No dark decay of the product ClCH2CClO was observed. The minor product CO (3%) may be produced via the unimolecular dissociation of ClCH2CO radical, reaction 4.
XCH2CO (+M) f XCH2 + CO (+M)
(4)
CCl2H2, which is expected to result from Cl2 reaction with the ClCH2 radical, was not observed. On the basis of the detection limit (about 0.005 Torr), the yield of CCl2H2 was estimated to be less than 2%. However, the observed CHClO may result from the reaction of chlorinated radicals such as ClCH2 with O2 impurities in the reactor. In this series of “O2-free” experiments, the reactor could be evacuated to 30 mTorr (remaining pressure presumably laboratory air), and there was less than 2 mTorr of O2 in the N2 used. Therefore, up to 8 mTorr of O2 could be present in the reactor, and reaction 4 cannot be discounted on the basis of the absence of CCl2H2. We can estimate a lower limit for the channel ratio k2[Cl2]/ k4. The extent of reaction 2 is determined by the yield of ClCH2CClO. Assuming that all the remaining ClCH2CO was consumed by reaction 4, k2[Cl2]/k4 g 94%/6% ) 15.6 at [Cl2] ) 0.43 Torr. Thus reaction 4 (if it occurs) is minor compared with reaction 2 at the Cl2 concentrations (0.4-1.0 Torr) normally used in the O2-free experiments. This result is in agreement with that obtained by Wallington et al., who have recently completed a similar study of the reaction of ClCH2CHO + Cl in N2 with the partial pressure of Cl2 > 0.5 Torr.13 By analogy to the Cl initiated oxidation of CH3CHO in the presence of O2,11 following H-abstraction, O2-addition to ClCH2CO produces the corresponding peroxychloroacetyl radical.
XCH2CO + O2 (+M) f XCH2C(O)O2 (+M)
(5)
Figure 1 displays infrared absorbance spectra obtained in the UV photolysis of a mixture containing Cl2 (15 mTorr) and ClCH2CHO (15 mTorr) in 700 Torr of air. Parts A and B of Figure 1 were recorded before and after 50 s of UV irradiation. The y-axis of the difference spectrum (Figure 1C) has been expanded 1.5 times for clarity. After 1.64 mTorr of ClCH2-
CHO reacted, identified products were CHClO (1.31 mTorr), CO (0.19 mTorr), CO2 (1.56 mTorr), H2O2 (0.03 mTorr), and HCl (1.55 mTorr); their yields are given in Table 1, run B. These products are consistent with the following reactions of the peroxyhaloacetyl radicals:
XCH2C(O)O2 + RO2 f XCH2C(O)O + RO + O2 (6) XCH2C(O)O (+M) f XCH2 + CO2 (+M)
(7)
XCH2 + O2 (+M) f XCH2O2 (+M)
(8)
XCH2O2 + RO2 f XCH2O + RO + O2
(9)
XCH2O + O2 f CHXO + HO2
(10)
2HO2 f H2O2 + O2
(11)
where RO2 ) XCH2C(O)O2 or XCH2O2 and RO ) XCH2C(O)O or XCH2O. On the basis of the high yield of CHClO, reaction 10 is the main degradation channel for ClCH2O radicals in 700 Torr of air.14 After subtracting spectra of the identified products from the spectrum shown in Figure 2C, infrared peaks at 3592 (OsH stretch), 1776 (CdO stretch), 1447 (CsC stretch), 1118 (CsO stretch), and 788 (CsCl stretch) were evident in the residual spectrum. By analogy with infrared spectral observations of Cl + CH3CHO in air,11 the residual spectra are tentatively assigned to ClCH2C(O)OH and/or ClCH2OOH and/or ClCH2C(O)OOH, produced via reactions 12, 13, and 14. The total yield of these three molecules is estimated from the carbon balance to be e7%.
XCH2C(O)O2 + HO2 f XCH2C(O)OH + O3
(12)
XCH2O2 + HO2 f XCH2OOH + O2
(13)
XCH2C(O)O2 + HO2 f XCH2C(O)OOH + O2 (14) Although O3 was not observed in the infrared spectrum, reaction 12 cannot be discounted. Wall reactions and halogen atom reactions consume O3 quickly, and at low concentrations its broad spectral shape with fine structure can be difficult to distinguish from baseline noise. In contrast to the O2-free experiments, no ClCH2CClO was identified in 700 Torr of air; its yield is less than 2%. Competition between reactions 5 and 2 leads to a decrease in ClCH2CClO yield with increasing O2 concentration. To gain further insight into the mechanism of Cl + ClCH2CHO reactions, the effect of changing the diluent O2 concentration was tested. Experiments were carried out with ClCH2CHO (0.5-0.9 Torr), O2 (0.2-20 Torr), and Cl2 (0.4-0.6 Torr) diluted in N2 to 700 Torr of total pressure. Expression I results from ratioing the rates of reactions 5 and 2.
{∆[ClCH2CHO] - [ClCH2CClO]}[Cl2]/[ClCH2CClO] ) (k5/k2)[O2] (I) ∆[ClCH2CHO] represents the amount of ClCH2CHO converted; [ClCH2CClO] represents the yield of Cl2-reaction products (reaction 2). Assuming that reactions 2 and 5 account entirely for the ClCH2CHO lost, ∆[ClCH2CHO] - [ClCH2CClO] represents the O2-addition products (reaction 5). On the basis of expression I, the rate constant ratio for O2-addition vs Cl2reaction of ClCH2CO radicals was derived from the O2 dependent experiments to be k5/k2 ) 0.16 ( 0.05. This ratio is
Halogen Atom Initiated Oxidation of Haloacetaldehydes
J. Phys. Chem., Vol. 100, No. 16, 1996 6583
TABLE 2: Comparison of IR Bonds and Absorptivities for CH3C(O)OONO2,14 ClCH2C(O)OONO2, and BrCH2C(O)OONO2 CH3C(O)OONO2a
a
ClCH2C(O)OONO2
BrCH2C(O)OONO2
assignment
freq, cm-1
intb
freq, cm-1
intb
freq, cm-1
intb
CsH stretch CdO stretch NO2 asym stretch NO2 sym stretch CsO stretch CsH rock OsO stretch NO2 scissor CsX stretch
3031 1841 1741 1302 1163 991 930 794
∼0.1 1.3 ( 0.2 4.1 ( 0.5 1.5 ( 0.2 1.9 ( 0.3 0.2 ( 0.1 0.3 ( 0.1 1.5 ( 0.2
3031 1835 1749 1301 1213 1059 929 790 637
∼0.1 0.8 ( 0.2 3.5 ( 0.7 2.1 ( 0.4 0.5 ( 0.1 1.1 ( 0.2 0.4 ( 0.1 1.6 ( 0.3 0.1
3028 1833 1746 1300 1195 1054 929 792 612
∼0.1 0.8 ( 0.2 4.3 ( 0.9 1.8 ( 0.4 0.6 ( 0.1 0.8 ( 0.2 0.4 ( 0.1 1.2 ( 0.3 0.4 ( 0.1
Reference 14. b The absorptivities are given in Torr-1 m-1 (log base 10).
greatly decreased the yields of CHClO, CO, and CO2 (see Table 1, run C). As shown in Figure 2C, the infrared spectrum exhibited the characteristic peaks of peroxy nitrate compounds. We assign this spectrum to ClCH2C(O)OONO2 (peroxychloroacetyl nitrate, PClAN) and to ClCH2OONO2 (peroxychloromethyl nitrate, PClMN) by comparison with the formation mechanisms and IR spectra of CH3C(O)OONO2 (PAN)16 and of CH3OONO2 (peroxymethyl nitrate, PMN),17 respectively:
XCH2C(O)O2 + NO2 (+M) f XCH2C(O)OONO2 (+M) (15) XCH2O2 + NO2 (+M) f XCH2OONO2 (+M) (16)
Figure 3. Infrared spectra of CH3C(O)OONO2, ClCH2C(O)OONO2, and BrCH2C(O)OONO2. The numbers are IR frequencies in inverse centimeters.
consistent with a value of k(CH3CO + Cl2)/k(CH3CO + O2) ) 0.13 ( 0.01 reported by Kaiser and Wallington.15 We earlier derived the channel ratio k2[Cl2]/k4 g 15.6 at [Cl2] ) 0.43 Torr from the O2-free experiments. A lower limit for the channel ratio k5[O2]/k4, in 700 Torr of air, is then estimated to be 8.1 × 102, which means that the unimolecular dissociation of ClCH2CO radicals to ClCH2 + CO (reaction 4) is negligible in the experiments carried out in 700 Torr of air. Thus the CO (11% yield) observed in this set of experiments does not come from reaction 4 but rather from secondary reactions of products such as CHClO. Indeed, the sum of the CO and CHClO yields (92 ( 8%) is equal within uncertainties to the CO2 yield (95 ( 5%). To confirm the formation of peroxy radicals by reactions 5 and 8, experiments were performed in air with 2-3 mTorr of NO2 present. Parts A and B of figure 2 show infrared spectra recorded before and after 30 s of UV irradiation of a mixture containing Cl2 (38 mTorr), NO2 (3 mTorr), and ClCH2CHO (6 mTorr) in 700 Torr of air. Figure 2C corresponds to the difference spectrum (2B - 2A) expanded 1.5 times. The addition of NO2 into the Cl + ClCH2CHO reaction mixture
By analogy with the behavior of PAN and PMN, PClAN is expected to be more stable than PClMN. The two weak peaks at 1730 and 1297 cm-1 that were found to decrease during a 10 min dark decay experiment were thus assigned to PClMN. ClONO and ClNO2 were also detected as minor products of ClCH2CHO oxidation in the presence of NO2. After subtracting the spectra of CHClO, CO, CO2, ClONO, ClNO2, and PClMN from Figure 2C, we assigned the residual spectrum to PClAN. The latter compared with that of PBrAN and PAN16 is presented in Figure 3, with the frequencies and absorptivities of the IR bands given in Table 2. The yields of PClAN and PClMN were derived from the carbon and nitrogen balances of the dark decay experiments, as shown in Table 1 (run C). Because the addition of NO2 to the reaction mixtures effectively “traps” the haloperoxy radicals (ClCH2C(O)O2 and ClCH2O2), the drastic decrease of CHClO, CO and CO2 yields in the presence of NO2 pinpoints their source as the haloperoxy radicals. (ii) Br + BrCH2CHO. O2-free experiments were conducted with mixtures containing Br2 (0.4-0.6 Torr) and BrCH2CHO (0.5-0.8 Torr) in 700 Torr of N2. After visible irradiation, IR bands characteristic of BrCH2CBrO, ketene (CH2dCdO), CHBrO, and CO were identified. The yields of these products, at a BrCH2CHO conversion of 0.11 Torr, were BrCH2CBrO (0.09 Torr), CH2dCdO (0.01 Torr), and CO (0.01 Torr) and accounted for about 95% of the carbon balance, as written in Table 3 (run D). BrCH2CBrO was the major product observed, and no Br2CHCHO was identified spectroscopically. Similar to the Cl + ClCH2CHO experiments, Br + BrCH2CHO is predominantly initiated by H-abstraction from the aldehydic group. Our detection limit for Br2CHCHO is 0.005 Torr, so its yield was less than 5% of the BrCH2CHO consumed by reaction 1. Analogous to ClCH2CO radicals, CO was probably produced via the unimolecular dissociation of BrCH2CO radicals. It would seem that Br + BrCH2CHO and Cl + ClCH2CHO occur by the same reaction pathways, save that the weaker CsBr bond of the BrCH2CO radical may cleave, forming ketene.
6584 J. Phys. Chem., Vol. 100, No. 16, 1996
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TABLE 3: Yields in the Br Initiated Reaction of BrCH2CHO run Da [BrCH2CHO]0 (Torr) [Br2]0 (Torr) [NO2]0 (Torr) irradiation time (s) ∆[BrCH2CHO] (Torr) ∆[NO2] (Torr)
run Eb
0.53 0.45
16 × 15 × 10-3
5 0.11
25 1.15 × 10-3
10-3
Product Yield (%)c BrCH2CBrO 83 ( 5 CH2dCdO 8(2 5(2 CH2O 57 ( 4 CHBrO 4(2 HC(O)OH e0.5 CO 7 ( 2 66 ( 4 CO2 52 ( 3 BrCH2C(O)OONO2 (PBrAN) BrCH2OONO2 (PBrMN) carbon balance (%) 95 ( 6 95 ( 7
run Fb 3 × 10-3 45 × 10-3 2 × 10-3 20 0.31 × 10-3 0.41 × 10-3 6.5 ( 2
35 ( 3 3(1 64 ( 6 35 ( 5
a Diluent consists of 700 Torr of N . b Diluent consists of 560 Torr 2 of N2 and 140 Torr of O2. c The percentage yields were based on the d Uncertainties are based on visual precision conversion of BrCH2CHO. of subtractions or detection limit (see text).
BrCH2CO (+M) T CH2dCdO + Br (+M)
(17, -17)
In our recent investigation of the Br-initiated oxidation of CH2dCdO,18 fast dark wall reactions between Br2 and CH2dCdO producing BrCH2CBrO were observed in the short cell (1 m path length). Thus, the channel ratio of reactions 2 and 4 for BrCH2CO radicals cannot be calculated, because BrCH2CBrO can be produced not only by reaction 2 but also by dark wall reaction between CH2dCdO and Br2. Figure 4 shows infrared absorbance spectra obtained in the visible photolysis of a mixture containing Br2 (15 mTorr) and BrCH2CHO (16 mTorr) in 700 Torr of air. Parts A and B of Figure 4 were recorded before and after 25 s of visible irradiation. In the difference spectrum (Figure 4C) the y-axis has been expanded 2 times for clarity. Besides CHBrO, CO, CO2, H2O2, and HBr, which are similar to the products observed in the ClCH2CHO + Cl2 system, CH2dCdO, CH2O, and HC(O)OH were also identified. After subtracting spectra of the identified products from Figure 7C, the residual spectra show infrared peaks at 3588 (OsH stretch), 2850 (CsH stretch), 1785 (CdO stretch), 1446 (CsC stretch), and 1058 (CsO bend) cm-1 and are tentatively assigned to BrCH2C(O)OH and/or BrCH2OOH and/or BrCH2C(O)OOH, formed via reactions 12, 13, and 14. From the carbon balance, the yield of these residual products was about 5% (see Table 3, run E). To trap peroxy radicals, experiments with 2-3 mTorr of NO2 present were carried out. The main products of 0.31 mTorr of BrCH2CHO conversion were 0.20 mTorr of BrCH2C(O)OONO2 (peroxybromoacetyl nitrate, PBrAN) and 0.11 mTorr of BrCH2OONO2 (peroxybromomethyl nitrate, PBrMN). Again, PBrAN is presumed to be more stable than PBrMN, and by observing which peaks diminished over time in the absence of light, the IR spectra of PBrAN and PBrMN were separated. The infrared spectrum of PBrAN compared with that of PClAN and PAN16 is presented in Figure 3, with the IR band frequencies and absorptivities given in Table 2. The IR spectrum of PBrMN showed peaks at 1733, 1295, 930, and 791 cm-1, which are consistent with those observed for this compound in our recent CH3Br experiment.19 The yields of the peroxynitrate products in Table 3 (run F) were obtained from the carbon and nitrogen balances. CO (0.11 mTorr), CH2dCdO (0.02 mTorr), CO2 (0.01 mTorr), HBr (0.30 mTorr), and nitrogen-containing
Figure 4. Spectral data in the frequency region 600-3200 cm-1 obtained from the photolysis of a mixture containing BrCH2CHO (16 mTorr) and Br2 (15 mTorr) in 700 Torr of air: (A) before irradiation; (B) after 25 s irradiation; (C) difference spectrum ) [(B) - (A)] × 2. The peak at 1746 cm-1 has been truncated to ln(I0/I) ) 0.9. The numbers in parentheses are product concentrations in millitorr.
products such as BrNO2, HONO, and HNO3 were also identified. No CH2O was observed above the detection limit (0.005 mTorr). The addition of NO2 to the reaction mixtures effectively “traps” the haloperoxy radicals (BrCH2C(O)O2 and BrCH2O2) by forming peroxynitrates and drastically decreases the yields of CHBrO, CO2, CH2O, and HC(O)OH. The yield of CH2dCdO (6.5%) in the Br + BrCH2CHO reaction is unaffected by the addition of NO2, which confirms that CH2dCdO is formed not from BrCH2C(O)O2 but rather by the decomposition of its precursor, BrCH2CO (reaction 17). It is interesting to see that there was still 35% CO produced after the addition of NO2 and that the yield of CO was equal to that of BrCH2OONO2 (35%). This result suggests that most of the CO and BrCH2 were formed from the unimolecular decay of BrCH2CO radicals (reaction 4). Unlike ClCH2CO, BrCH2CO may undergo unimolecular decomposition (reactions 4 and 17) as well as O2-addition (reaction 5) in air. Experiments were carried out with O2 partial pressure varied from 50 to 700 Torr in mixtures containing Br2 (50-55 mTorr), BrCH2CHO (5-7 mTorr), and NO2 (3-4 mTorr) diluted by N2 to 700 Torr of total pressure. In the 50 Torr of O2 experiments, the yields of CO and BrCH2OONO2 increased to about 70% from the yields in 700 Torr of air; the yield of BrCH2C(O)OONO2 decreased to 20%; and the yield of CH2dCdO was about 7%. In the 700 Torr of O2 experiments, the yields of CO and BrCH2OONO2 decreased to about 15%; the yield of BrCH2C(O)OONO2 increased to about 80%; and no CH2dCdO was identified. These results further confirmed the competition between channels 4, 5, and 17 for BrCH2CO radicals.
Halogen Atom Initiated Oxidation of Haloacetaldehydes To estimate the channel ratio k5[O2]/k4, we assumed that (i) all the BrCH2C(O)O2 radicals were trapped by NO2; (ii) the secondary reactions of the produced BrCH2C(O)OONO2 were negligible; and (iii) all the CO was produced from reaction 4. Thus, from the product ratio [PBrAN]/[CO], the channel ratio k5[O2]/k4 was estimated to be 1.8 for the BrCH2CO radical in 700 Torr of air. This value definitely represents a lower limit because small amounts of CO2 (3%), which are not accounted for in this simplified calculation, probably represent O2-addition products. In contrast, a lower limit k5[O2]/k4 ) 8.1 × 102 was calculated for ClCH2CO. It is interesting to compare the effect of halogen substitution on the dissociation of the acetyl radical, CH3CO. No literature kinetic data has been found for reactions 5 and 4 for either ClCH2CO or BrCH2CO radicals. Assuming that the rate constants k5 for ClCH2CO and BrCH2CO radicals are similar to that for CH3CO + O2, namely 5 × 10-12 cm3 molecule-1 s-1,20 upper limits for the rate constants k4 for ClCH2CO and BrCH2CO radicals are e2.5 × 104 s-1 and e1.3 × 107 s-1, respectively. The unimolecular decay rate of ClCH2CO and BrCH2CO radicals via reaction 4 is thus much greater than that of CH3CO radical, which is 0.9 s-1 at room temperature and atmospheric pressure.21 Assuming that the pre-exponential factor A ) 4.4 × 1011 s-1 derived for the CH3CO radical 21 is a reasonable approximation for ClCH2CO and BrCH2CO radicals in the equation k ) Ae-Ea/RT, the activation energies Ea of reaction 4 are g9.9 and g6.2 kcal/mol for ClCH2CO and BrCH2CO radicals, respectively. Given that Ea(CH3CO) ) 15.9 kcal/mol,21 it appears that the substitution of a hydrogen by a halogen atom weakens the CsC bond of the CH3CO radical. The effect is more pronounced for Br atom than for Cl atom substitution. In the long cell (180 m path length), the concentrations of the reactants and products were 30 to 150 times lower and the surface-volume ratio was about 6 times smaller than in the short cell, so that the dark wall reaction between CH2dCdO and Br2 was negligible. However, CH2dCdO is very reactive toward Br atoms and returns to BrCH2CO, with a rate constant k-17 ) 1.3 × 10-11 cm3 molecule-1 s-1.18 So, the rate of reaction 17 is not accurately reflected by the observed yield of CH2dCdO (6.5%). Using a simple model including reactions 4, 5, and 17,22 k17 ) 1.1 × 107 s-1 best fits the observed yields of CO, CH2dCdO, and PBrAN. Recall that k4 for BrCH2CO was estimated as e1.3 × 107 s-1. Thus, reactions 4 and 17 are similarly swift unimolecular decomposition pathways for BrCH2CO. Also, reaction thermodynamics for these two reactions are similar: the product heats of formation for reactions 4 and 17 are 14.0 and 15.4 kcal/mol, respectively.23 Using our derived rate constants k4, k17, and k5[O2], about 50% of the BrCH2CO radicals react via reaction 5 (O2-addition) in 700 Torr of air, while reaction 4 (C-C bond cleavage) and reaction 17 (BrsC bond cleavage) each account for about 25%. During a 6 min dark decay period following irradiation of the Br2/BrCH2CHO/NO2 reaction mixture, about 30% of the PBrMN dissociated to CH2O, CHBrO, HNO3, and NO2. The dark decay of halogen atom substituted peroxy methyl nitrates reproduces the substituted peroxy methyl radical XCH2O2 and NO2,24 as in reaction -16:
XCH2OONO2 (+M) f XCH2O2 + NO2 (+M)
(-16)
These BrCH2O2 radicals return to their interrupted reaction pathways, undergoing reaction 9 to produce BrCH2O. The dark decay experiment suggested that there are two degradation channels for BrCH2O radicals, reactions 10 and 18, forming CHBrO and CH2O, respectively. Cleavage of the
J. Phys. Chem., Vol. 100, No. 16, 1996 6585
Figure 5. Yield of CH2O against conversion of BrCH2CHO. The dotted curve is observed from experiments; the straight line is after correcting for secondary reactions with Br atoms.
CsBr bond (reaction 18) produces CH2O in the Br + BrCH2CHO system, a reaction not observed in the Cl + ClCH2CHO system.
BrCH2O (+M) f CH2O + Br (+M)
(18)
As shown by the curve in Figure 5, for experiments in 700 Torr of air without NO2, the percentage yield of CH2O decreased with the conversion of BrCH2CHO due to secondary reactions between Br and CH2O. Correction for the secondary reaction of CH2O was made using the rate constants k(Br + BrCH2CHO)8 ) 1.83 × 10-13 cm3 molecule-1 s-1 and k(Br + CH2O)25 ) 1.1 × 10-12 cm3 molecule-1 s-1 by using the method suggested by Atkinson et al.26 In Figure 5, the slope of the straight line gives the corrected yield of CH2O to be 67 ( 6%, based on the total conversion of BrCH2CHO. It is more informative to consider the yield of CH2O, and thus the importance of reaction 18, in terms of BrCH2O radicals converted rather than BrCH2CHO converted. Because reactions 12, 13, 14, and 17 should not produce BrCH2O radicals, the yields of their products (i.e., BrCH2C(O)OH, BrCH2OOH, BrCH2C(O)OOH, and CH2dCdO) can be subtracted from the conversion of BrCH2CHO to estimate that (90 ( 7)% of the converted BrCH2CHO led to BrCH2O radicals. Thus, the yield of CH2O in 700 Torr of air at 297 ( 2 K is approximately (74 ( 7)%, based on the conversion of BrCH2O radicals. This result is consistent with that derived in our recent Cl + CH3Br experiments, in which a lower limit of 79% was obtained for reaction 18.19 Thus, the major fate of BrCH2O radicals in air is to decompose by Br-elimination (reaction 18), producing CH2O. Several previous studies have reported that the reaction with O2 (reaction 10) was the primary fate of BrCH2O radicals, while the Br-elimination was a minor channel.27,28 The disagreement as to the mechanism of BrCH2O radical degrada-
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Chen et al. a major product of the Br initiated oxidation of C2H4, leads ultimately to the regeneration of the halogen atom Br. Acknowledgment. The authors wish to thank the AFEAS, the NSERC, and British Gas/Consumers Gas for financial support and G. W. Harris and O. Melo for critical review of the manuscript. C.V.F. Weldon is acknowledged for preparation of the manuscript. References and Notes
Figure 6. Schematic for halogen atom initiated reactions of haloacetaldehydes.
tion and its implications for the atmospheric degradation of CH3Br have been discussed in our recent study of the degradation mechanism of BrCH2O radicals.19 Conclusions This FTIR-based product study of the halogen atom initiated oxidation of haloacetaldehydes shows that the reactions proceed via the initial abstraction of the aldehydic hydrogen (CHO) to produce XCH2CO radicals and HX. Under atmospheric conditions, the predominant reaction for the ClCH2CO radical is O2addition. However, for the BrCH2CO radical, two thermal decomposition channels, namely cleavage of the CsC bond (reaction 4) and the BrsC bond (reaction 17), compete efficiently with O2-addition (reaction 5). The product yields of reactions 4, 17, and 5 were determined to be 35%, 6.5%, and 64%, respectively, from the NO2 experiments. After correcting for the fast reaction between Br and CH2dCdO, the channel ratio is estimated to be about 25:25:50 for reactions 4, 17, and 5. In the presence of NO2, ClCH2C(O)OONO2, BrCH2C(O)OONO2, ClCH2OONO2, and BrCH2OONO2 were identified, which confirmed the formation of the corresponding peroxy radicals. XCH2O radicals were produced in both systems by reactions of XCH2C(O)O2 radicals. The only observed degradation channel for ClCH2O radicals was reaction with O2 to produce CHClO and HO2 radicals. In addition to the O2 channel, unimolecular dissociation producing CH2O and Br atoms was also observed for BrCH2O radicals, with about 74% of the BrCH2O radical decay occurring via this unimolecular channel. A summary of the reaction mechanisms for the Cl atom initiated reaction of ClCH2CHO and the Br atom initiated reaction of BrCH2CHO derived from this work is given in Figure 6. It shows that the reaction of Cl atoms with ClCH2CHO, a major product of the Cl initiated oxidation of C2H4, may be a chain-terminating step in the catalytic destruction of ozone by Cl atoms. However, the reaction of Br atoms with BrCH2CHO,
(1) Singh, H. B.; Kasting, J. F. J. Atmos. Chem. 1988, 7, 261. (2) McConnell, J. C.; Henderson, G. S. The Tropospheric Chemistry of Ozone in the Polar Regions. In The Tropospheric Chemistry of Ozone in the Polar Regions; Niki, H., Becker, K. H., Eds.; NATO ASI Series I: Global Environmental Change; Springer-Verlag: Berlin Heidelberg, 1993; Vol. 7, p 89. (3) Jobson, B. T.; Niki, H.; Yokouchi, Y.; Bottenheim, J.; Hopper, F.; Leaitch, R. J. Geophys. Res. 1994, 99, 25355. (4) Lee, F. S. C.; Rowland, F. S. J. Phys. Chem. 1977, 81, 684. (5) Payne, W. A.; Nava, D. F.; Brunning, J.; Stief, L. J. J. Geophys. Res. 1986, 91, 4097. (6) Barrie, L. A.; Bottenheim, J. W.; Schnell, R. C.; Crutzen, P. J.; Rasmussen, R. A. Nature 1988, 344, 138. (7) Bottenheim, J. W.; Gallant, A. J.; Brice, K. A. Geophys. Res. Lett. 1986, 13, 113. (8) Yarwood, G.; Peng, N.; Niki, H. Int. J. Chem. Kinet. 1992, 24, 369. (9) Barnes, I.; Bastian, V.; Becker, K. H.; Overath, R.; Zhu, T. Int. J. Chem. Kinet. 1989, 21, 499. (10) Scollard, D. J.; Treacy, J. J.; Sidebottom, H. W.; Balestra-Garcia, C.; Laverdet, G.; LeBras, G.; MacLeod, H.; Teton, S. J. Phys. Chem. 1993, 97, 4683. (11) Niki, H.; Maker, P. D.; Savage, C. M.; Breitenbach, L. P. J. Phys. Chem. 1985, 89, 588. (12) Niki, H.; Maker, P. D.; Savage, C. M.; Breitenbach, L. P. J. Mol. Struct. 1980, 59, 1. (13) Wallington, T. J.; Ball, J. C.; Straccia, A. M.; Hurley, M. D.; Kaiser, E. W.; Dill, M. J. Phys. Chem., in press. (14) (a) Sanhueza, E.; Heicklen, J. J. Phys. Chem. 1975, 79, 7. (b) Niki, H.; Maker, P. D.; Savage, C. M.; Breitenbach, L. P. Int. J. Chem. Kinet. 1980, 12, 1001. (c) Kaiser, E. W.; Wallington, T. J. J. Phys. Chem. 1994, 98, 5679. (15) Kaiser, E. W.; Wallington, T. J. J. Phys. Chem. 1995, 99, 8669. (16) Niki, H.; Maker, P. D.; Savage, C. M.; Breitenbach, L. P. Int. J. Chem. Kinet. 1985, 17, 525. (17) Niki, H.; Maker, P. D.; Savage, C. M.; Breitenbach, L. P. Chem. Phys. Lett. 1978, 55, 289. (18) Chen, J.; Niki, H. FTIR Studies of the Br Atom Initiated Oxidation of CH2dCdO; unpublished experimental data, 1994. (19) Chen, J.; Catoire, V.; Niki, H. Mechanistic Study of the BrCH2O Radical Degradation in 700 Torr Air. Chem. Phys. Lett. 1995, 245, 519. (20) Atkinson, R.; Baulch, D. L.; Cox, R. A.; Hampson, R. F., Jr.; Kerr, J. A.; Troe, J. J. Phys. Chem. Ref. Data 1989, 18, 881. (21) Baldwin, P. J.; Canosa-Mas, C. E.; Frey, H. M.; Walsh, R. Int. J. Chem. Kinet. 1987, 19, 997. (22) Braun, W.; Herron, J. T.; Kahaner, D. K. Int. J. Chem. Kinet. 1988, 20, 51. (23) Data from the following publications were used for the thermodynamic calculations: (a) Tschuikov-Roux, E.; Paddison, S. Int. J. Chem. Kinet. 1987, 19, 15. (b) Cox, J. D., Wagman, D. D., Medvedev, V. A., Eds. CODATA, Key Values for Thermodynamics; Hemisphere Publishing Corporation: New York, 1989. (c) Handbook of Chemistry and Physics, 75th ed.; Lide, D. R., editor-in-chief; CRC Press: Boca Raton, FL, 1995. (24) Niki, H.; Maker, P. D.; Savage, C. M.; Breitenbach, L. P. Chem. Phys. Lett. 1979, 61, 100. (25) DeMore, W. B.; Sander, S. P.; Howard, C. J.; Howard, C. J.; Ravishankara, A. R.; Golden, D. M.; Kolb, C. E.; Hampson, R. F.; Kurylo, M. J.; Molina, M. J. Chemical Kinetics and Photochemical Data for Use in Stratospheric Modeling; Evaluation 11, NASA/JPL, 1994. (26) Atkinson, R.; Aschmann, S. M.; Carter, W. P. L.; Winer, A. M.; Pitts, J. N., Jr. J. Phys. Chem. 1982, 86, 4563. (27) Nielsen, O. J.; Munk, J.; Locke, G.; Wallington, T. J. J. Phys. Chem. 1991, 95, 8714. (28) Locke, G.; Treacy, J. J.; Sidebottom, H. W.; Percival, C. J.; Wayne, R. P. In Proceedings of the Sixth European Symposium on PhysicoChemical Behaviour of Atmospheric Pollutants, Varese, Italy, 1993, p 405.
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