Environ. Sci. Technol. 1997, 31, 2421-2427
Gas Phase Reaction of Alkenes with Ozone: Formation Yields of Primary Carbonyls and Biradicals ERIC GROSJEAN AND DANIEL GROSJEAN* DGA, Inc., 4526 Telephone Road, Suite 205, Ventura, California 93003
The gas phase reaction of ozone with alkenes plays a major role in tropospheric chemistry including urban air quality. The reaction of trans-2,2-dimethyl-3-hexene and 2,4dimethyl-2-pentene with ozone has been studied at ambient temperature and p ) 1 atm of air (RH ) 55% ( 10%) with sufficient cyclohexane added to scavenge the hydroxyl radical. Carbonyl products have been identified, and their formation yields are reported. The results are compared to those previously obtained for 35 other alkenes under the same conditions. For these alkenes, the sums of the formation yields of the primary carbonyls are close to the value of 1.0 that is consistent with the following reaction mechanism: O3 + R1R2CdCR3R4 f R(R1COR2 + R3R4COO) + (1 - R)(R1R2COO + R3COR4), where R1COR2 and R3COR4 are the primary carbonyls and R1R2COO and R3R4COO are the corresponding biradicals. For 26 nonsymmetrical alkenes, the coefficients R range from 0.28 to 0.82 and indicate preferential formation of the more substituted biradicals and of the biradicals that bear the less bulky substituents. The results are directly relevant to the atmospheric chemistry of alkenes and to their role in oxidant and aerosol formation.
Introduction The reaction of alkenes with ozone in the liquid phase has been the object of numerous studies, and the major features of the reaction mechanism are reasonably well understood (1). The reaction of alkenes with ozone in the gas phase has also received much attention (2) but is still incompletely understood. Contributing to difficulties in elucidating the reaction mechanism is the fact that the hydroxyl radical (OH) forms as a reaction product. Evidence for the formation of OH may be found in the low-pressure studies of Herron and co-workers (3-7) for several simple alkenes and, in studies carried out at atmospheric pressure, in the work on Niki et al. (8) for 2,3-dimethyl-2-butene, Hatakeyama et al. (9) for cyclohexene, and Grosjean (10) for 2-methyl-2-butene. More recently, Atkinson and co-workers have reported OH formation yields for the reaction of ozone with several alkenes (1113). The hydroxyl radical reacts with alkenes ca. 4-5 orders of magnitude faster than ozone does (14) and also reacts rapidly with carbonyls, especially aldehydes, that are major products of the ozone-alkene reaction. Thus, the “apparent” products of the ozone-alkene reaction are likely to include a complex mixture of ozone-alkene, OH-alkene, and OHcarbonyl reaction products, thereby complicating the interpretation of experimental results. In order to minimize “interferences” from OH, several investigators have studied the ozone-alkene reaction in the presence of an OH scavenger. Niki et al. (8) added formal* Corresponding author telephone: (805) 644-0125; fax: (805) 6440142.
S0013-936X(97)00075-8 CCC: $14.00
1997 American Chemical Society
dehyde and acetaldehyde as OH scavengers in three of their ozone-2,3-dimethyl-2-butene experiments. Grosjean et al. (15) have reported carbonyl and carboxylic acid products of the reaction of eight alkenes with ozone in the presence of cyclohexane added to scavenge OH. Cyclohexane has also been used to scavenge OH in recent reports of carbonyl formation yields in the reaction of ozone with a number of alkenes (13, 16-22). Scavengers for OH, including cyclohexane (22-25), n-octane (26), and 2-propanol (22), have also been employed in recent kinetic studies of the alkeneozone reaction. Laboratory experiments that involve the use of a scavenger for OH are not unlike the polluted atmosphere, where most of the hydroxyl radical that forms in the reaction of ozone with a given alkene is “scavenged” by reactions of OH with carbon monoxide, aromatics, paraffins, aldehydes, other alkenes, and other pollutants. In this article, we report carbonyl formation yields in the gas phase reaction of ozone with the two alkenes trans-2,2dimethyl-3-hexene and 2,4-dimethyl-2-pentene with sufficient cyclohexane added to scavenge OH. The two alkenes were selected from structural considerations to complement those studied earlier in our laboratory (16-21). trans-2,2Dimethyl-3-hexene is a nonsymmetrical 1,2-disubstituted alkene R1CHdCHR2: these alkenes have not been studied previously. The alkene 2,4-dimethyl-2-pentene is a trisubstituted alkene, only three of which have been studied previously (10, 16, 20). The major carbonyl products have been identified, and the relevant reaction mechanisms are outlined. We examine, for the 37 alkenes studied to date, the formation yields of the major carbonyl products and their relative abundance as a function of the nature and number of the alkene substituents. For all alkenes studied, measured carbonyl formation yields are consistent with the simple reaction mechanism:
R1R2CdCR3R4 + O3 f R(R1COR2 + R3R4COO) + (1 - R)(R1R2COO + R3COR4) (1) where R1COR2 and R3COR4 are the carbonyl products (hereafter referred to as primary carbonyls) and R1R2COO and R3R4COO are the corresponding carbonyl oxide biradicals (hereafter referred to as biradicals). We also examine the value of the coefficient R as a function of the nature and number of the substituents.
Experimental Methods The experimental methods employed in this study have been described in detail (16-21, 27-29), and only a brief summary is given below. Cyclohexane (Aldrich, stated purity g 99.9%), trans-2,2-dimethyl-3-hexene (Wiley Organics, stated purity ) 99%), and 2,4-dimethyl-2-pentene (ChemSampCo, stated purity ) 99.6%) were used without further purification. Mixtures of ozone (initial concentrations 62-263 ppb), alkene (initial concentrations 0.5-2.6 ppm), and cyclohexane [400 ppm, initial cyclohexane/ozone concentration ratio ) (1.56.4) × 103] were allowed to react in the dark in 3.7-3.9 m3 FEP Teflon chambers at ambient temperature and p ) 1 atm of purified humid air (RH ) 55% ( 10%). The concentration of ozone was measured by ultraviolet photometry using a calibrated continuous analyzer. Once ozone had been consumed (O3 e 3 ppb), samples were collected using C18 cartridges coated with 2,4-dinitrophenylhydrazine (DNPH) and the carbonyl products were analyzed by liquid chromatography as their DNPH derivatives. Validation aspects of the sampling and analytical protocols have been described elsewhere (27-29). Control experiments involving ozone
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R-hydroxylalkyl radical with oxygen. The relevant reactions, discussed previously for other alkenes that bear the isopropyl substituent (17, 18), are reaction 2 followed by
TABLE 1. Carbonyl Formation Yields in Ozone-Alkene-Cyclohexane Experiments carbonyl
formation yieldb
trans-2,2-Dimethyl-3-hexenea formaldehyde 0.036 ( 0.001 acetaldehyde 0.187 ( 0.017 propanal 0.335 ( 0.005 trimethylacetaldehyde 0.845 ( 0.024 methylglyoxal 0.077 ( 0.011 cyclohexanonec 0.167 ( 0.017 unidentifiedd d 2,4-Dimethyl-2-pentenee formaldehyde acetone 2-methyl propanal cyclohexanonec methyl glyoxal unidentified f
0.298 ( 0.016 0.196 ( 0.013 0.861 ( 0.030 0.175 ( 0.002 0.257 ( 0.008 f
a Three experiments with initial alkene concentrations of 0.50, 2.60, and 0.50 ppm, respectively, and initial ozone concentrations of 66, 263, and 62 ppb, respectively. b Carbonyl formed, ppb/reacted ozone, ppb. Mean ( one standard deviation of data for three experiments. Overall method uncertainties (calibration, accuracy, sample recovery, replicate injections, co-located samples, etc.) are discussed in refs 27-29. c Product of the OH-cyclohexane reaction. d Retention time of DNPH derivative relative to that of formaldehyde-DNPH ) 0.90; peak height of DNPH derivative relative to that of trimethylacetaldehyde-DNPH ) 0.026. e Three experiments with initial alkene concentrations of 2.0, 2.0, and 1.0 ppm, respectively, and initial ozone concentrations of 204, 214, and 92 ppb, respectively. f Retention times of DNPH derivatives relative to that of formaldehyde-DNPH ) 0.89, 3.21, and 6.13; peak heights relative to that of the formaldehyde-DNPH e 0.04.
alone, ozone-cyclohexane mixtures, and carbonyls alone in purified, humid air have also been described (16-18, 23). Cyclohexanone-DNPH and trimethylacetaldehyde-DNPH were resolved using water:acetonitrile:tetrahydrofuran eluent (29). Quantitative analysis involved the use of external standards as described previously (27, 28).
Results and Discussion Carbonyl Products of the Reaction of Ozone with trans2,2-Dimethyl-3-hexene and with 2,4-Dimethyl-2-pentene. Carbonyl products formed in the ozone-trans-2,2-dimethyl3-hexene-cyclohexane and ozone-2,4-dimethyl-2-pentenecyclohexane experiments are listed in Table 1. Carbonyl formation yields were calculated as in previous work (16-21) as the ratios carbonyl formed (ppb)/reacted ozone (ppb) and are also listed in Table 1. Cyclohexanone, a product of the OH-cyclohexane reaction, indicates that OH forms as a product of the reaction of ozone with both alkenes. As discussed earlier (15, 16), the OH-alkene and OH-carbonyl reactions make negligible contributions to the carbonyl formation yields listed in Table 1. For both alkenes, the primary carbonyls that are expected to form via reaction 1 were identified as major products, i.e. 2-methylpropanal and acetone from 2,4-dimethyl-2-pentene
(CH3)2CHCHdC(CH3)2 + O3 f (CH3)2CHCHO + (CH3)2COO + (CH3)2CHCHOO + CH3COCH3 (2) and propanal and trimethylacetaldehyde from trans-2,2dimethyl-3-hexene
(CH3)3CCHdCHCH2CH3 + O3 f (CH3)3CCHO + CH3CH2CHOO + (CH3)3CCHOO + CH3CH2CHO (3) Acetone may also form from the biradical (CH3)2CHCHOO in a sequence of reactions that involves formation of an unsaturated hydroperoxide, rearrangement of the peroxide into an hydroxycarbonyl, carbon-carbon bond scission leading to an R-hydroxyalkyl radical, and reaction of the
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(CH3)2CHCHOO f (CH3)2CdC(H)OOH* f (CH3)2C(OH)CHO* (4a) (CH3)2C(OH)CHO* f HCO + (CH3)2COH
(4b)
(CH3)2COH + O2 f HO2 + CH3COCH3
(4c)
The other nonprimary carbonyl observed to form from 2,4dimethyl-2-pentene, formaldehyde, is formed in a similar sequence of reactions from the biradical (CH3)2COO (1521), i.e., reaction 2 followed by
(CH3)2COO f CH2dC(CH3)OOH* f CH2OHC(O)CH3* (5a) CH2OHC(O)CH3* f CH2OH + CH3CO
(5b)
CH2OH + O2 f HO2 + HCHO
(5c)
Similarly, acetaldehyde forms from trans-2,2-dimethyl-3hexene by reactions involving the biradical CH3CH2CHOO, i.e., reaction 3 followed by
CH3CH2CHOO f CH3CHdCH(OOH)* f CH3CH(OH)CHO* (6a) CH3CH(OH)CHO* f HCO + CH3CHOH
(6b)
CH3CHOH + O2 f HO2 + CH3CHO
(6c)
Methylglyoxal was observed to form from both alkenes. As discussed previously for other alkenes that lead to the biradicals CH3CH2CHOO and (CH3)2COO (15-21), the relevant reactions are, for (CH3)2COO, reaction 5a followed by
CH2OHC(O)CH3* f CH2OHC(O)CH3 and/or H2 + CH3COCHO (7) and, for CH3CH2CHOO, reaction 6a followed by
CH3CH(OH)CHO* f CH3CHOHCHO and/or H2 + CH3COCHO (8) As noted before (27), the reaction of DNPH with hydroxycarbonyls and with the corresponding R-dicarbonyls lead to the same DNPH derivatives, i.e. those of the R-dicarbonyls. As a result, our measured formation yields for methylglyoxal may be those of (methylglyoxal + hydroxyacetone) in reaction 7 and of (methylglyoxal + 2-hydroxypropanal) in reaction 8. This leaves formaldehyde (small yield of ca. 0.04 from trans2,2-dimethyl-3-hexene) as the only measured carbonyl whose formation is not explained by reactions 2-8. The sums of the formation yields of the two primary carbonyls are 1.18 ( 0.03 (one standard deviation) for trans2,2-dimethyl-3-hexene and 1.06 ( 0.03 for 2,4-dimethyl-2pentene. These values are close to that of 1.0, which is consistent with the reaction mechanism summarized by reaction 1. For 2,4-dimethyl-2-pentene, acetone forms as a primary carbonyl in reaction 2 and also from the biradical (CH3)2CHCHOO in reactions 4a-c. Thus, the sum of the formation yields of the primary carbonyls is overstated by the contribution of the biradical (CH3)2CHCHOO to the measured acetone concentration. This contribution can be estimated using data for other alkenes that also lead to the biradical (CH3)2CHCHOO, i.e. 3-methyl-1-butene (21) and trans-2,5-dimethyl-3-hexene (18). For these two alkenes the formation yields of acetone from (CH3)2CHCHOO are 0.15
TABLE 2. Primary Carbonyl Formation Yields in Ozone-Alkene-Cyclohexane Experiments alkenes symmetrical alkenes R1R2CdCR1R2 ethylene (40% cis + 60% trans)-2-butene trans-3-hexene cis-3-hexene trans-4-octene cis-4-octene trans-5-decene cis-5-decene trans-2,5-dimethyl-3-hexeneb 2,3-dimethyl-2-butene (cis + trans)-3,4-dimethyl-3-hexenec monosubstituted alkenes RCHdCH2 propene 1-butene 1-pentene 1-hexene 1-heptene 1-octene 1-decene 3-methyl-1-butene 3-methyl-1-pentene 4-methyl-1-pentene 3,3-dimethyl-1-butene vinylcyclohexane styrene 1,1-disubstituted alkenes R1R2CdCH2 2-methylpropene 2-methyl-1-butene 2-methyl-1-pentene 2-ethyl-1-butene 2,3-dimethyl-1-butene 3-methyl-2-i-propyl-1-butene 2,3,3-trimethyl-1-butene methylene cyclohexane trisubstituted alkenes R1CHdCR2R3 2-methyl-2-butene 2,4,4-trimethyl-2-pentene 2,4-dimethyl-2-pentene 3,4-diethyl-2-hexene nonsymmetrical 1,2-disubstituted alkenes trans-2,2-dimethyl-3-hexene
formation yieldsa
sum of formation yields
formaldehyde acetaldehyde propanal propanal butanal butanal pentanal pentanal 2-methylpropanal acetone 2-butanone
1.03 ( 0.07 1.15 ( 0.10 1.01 ( 0.05 1.02 ( 0.08 1.15 ( 0.03 1.21 ( 0.02 1.09 ( 0.06 1.21 ( 0.03 1.40 ( 0.08b 1.01 ( 0.05 1.16 ( 0.06
1.03 ( 0.07 1.15 ( 0.10 1.01 ( 0.05 1.02 ( 0.08 1.15 ( 0.03 1.21 ( 0.02 1.09 ( 0.06 1.21 ( 0.03 1.40 ( 0.08b 1.01 ( 0.05 1.16 ( 0.06
formaldehyde acetaldehyde formaldehyde propanal formaldehyde butanal formaldehyde pentanal formaldehyde hexanal formaldehyde heptanal formaldehyde nonanal formaldehyde 2-methyl propanal formaldehyde 2-methyl butanal formaldehyde 3-methyl butanal formaldehyde trimethylacetaldehyde formaldehyde cyclohexyl methanal formaldehyde benzaldehyde
0.78 ( 0.02 0.52 ( 0.03 0.63 ( 0.03 0.35 ( 0.02 0.50 ( 0.01 0.50 ( 0.02 0.50 ( 0.01 0.54 ( 0.02 0.51 ( 0.01 0.51 ( 0.05 0.48 ( 0.03 0.47 ( 0.02 0.53 ( 0.01 0.49 ( 0.01 0.50 ( 0.04 0.51 ( 0.03 0.39 ( 0.01 0.63 ( 0.01 0.44 ( 0.01 0.71 ( 0.05 0.32 ( 0.01 0.67 ( 0.01 0.47 ( 0.04 0.62 ( 0.03 0.34 ( 0.05 0.64 ( 0.07
1.30 ( 0.03
0.60 ( 0.02
0.98 ( 0.04
0.64 ( 0.04
1.00 ( 0.02
0.50 ( 0.01
1.04 ( 0.02
0.48 ( 0.01
1.02 ( 0.05
0.50 ( 0.03
0.95 ( 0.04
0.50 ( 0.04
1.02 ( 0.02
0.52 ( 0.02
1.01 ( 0.05
0.49 ( 0.05
1.02 ( 0.02
0.38 ( 0.01
1.15 ( 0.05
0.38 ( 0.02
0.99 ( 0.01
0.32 ( 0.01
1.09 ( 0.06
0.43 ( 0.10
0.98 ( 0.09
0.35 ( 0.06
formaldehyde acetone formaldehyde 2-butanone formaldehyde 2-pentanone formaldehyde 3-pentanone formaldehyde 3-methyl-2-butanone formaldehyde 2,4-dimethyl-3-pentanone formaldehyde 3,3-dimethyl-2-butanone formaldehyde cyclohexanone
0.95 ( 0.10 0.34 ( 0.03 0.66 ( 0.06 0.35 ( 0.01 0.62 ( 0.02 0.32 ( 0.01 0.58 ( 0.01 0.43 ( 0.01 0.66 ( 0.01 0.37 ( 0.01 0.61 ( 0.08 0.43 ( 0.03 0.64 ( 0.03 0.35 ( 0.02 0.60 ( 0.05 0.55 ( 0.01
1.29 ( 0.10
0.74 ( 0.10
1.01 ( 0.06
0.65 ( 0.08
0.94 ( 0.02
0.66 ( 0.03
1.01 ( 0.02
0.58 ( 0.01
1.03 ( 0.02
0.64 ( 0.02
1.03 ( 0.08
0.59 ( 0.09
0.99 ( 0.04
0.65 ( 0.04
1.15 ( 0.05
0.52 ( 0.05
acetaldehyde acetone trimethylacetaldehyde acetone 2-methylpropanal acetone acetaldehyde 4-ethyl-3-hexanone
0.69 ( 0.02 0.30 ( 0.01 0.84 ( 0.04 0.19 ( 0.01 0.86 ( 0.03 0.20 ( 0.01 0.71 ( 0.04 0.29 ( 0.03
0.99 ( 0.02
0.69 ( 0.02
1.03 ( 0.04
0.82 ( 0.05
1.06 ( 0.03
0.82 ( 0.04
0.99 ( 0.05
0.71 ( 0.06
propanal trimethylacetaldehyde
0.34 ( 0.01 0.84 ( 0.02
1.18 ( 0.03
0.28 ( 0.01
primary carbonyls
coefficient r
a Versus ozone consumed ( one standard deviation. Data are from this study and from refs 16-22. b Stated purity ) 95%. GC-MS analysis indicated that the sample contained 92% trans-isomer, 5% cis-isomer, and 3% of impurities including 3,4-dimethyl-2,4-hexadiene (18). c 36% cisisomer and 64% trans-isomer from GC-MS analysis (18).
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TABLE 3. Literature Data for Primary Carbonyl Products of the Ozone-Alkene Reaction alkene
primary carbonyls
ethylene
formaldehyde
propene
formaldehyde acetaldehyde formaldehyde acetaldehyde acetaldehyde acetone
trans-2-butene 2,3-dimethyl-2-butene 1-pentene
1-hexene
1-heptene
1-octene
2,3-dimethyl-1-butene
styrene
2-vinylpyridine
formaldehyde butanal formaldehyde butanal formaldehyde pentanal formaldehyde pentanal formaldehyde hexanal formaldehyde hexanal formaldehyde heptanal formaldehyde heptanal formaldehyde 3-methyl-2-butanone formaldehyde 3-methyl-2-butanone formaldehyde benzaldehyde formaldehyde benzaldehyde formaldehyde 2-pyridine carboxaldehyde
carbonyl formation yield
sum of formation yields
OH scavenger
reference
1.00
none none none none cyclohexane, CO cyclohexane none
30 31 32c,d 32c,e 33 16, 18 30
1.30 ( 0.03g
cyclohexane
16
1.14 ( 0.09
none HCHO, CH3CHO cyclohexane cyclohexane
30 8 16 13
1.001 ( 0.016
cyclohexane
17
1.09 ( 0.11
cyclohexane
13
1.037 ( 0.024
cyclohexane
17
1.12 ( 0.10
cyclohexane
13
1.023 ( 0.051
cyclohexane
17
1.05 ( 0.09
cyclohexane
13
0.949 ( 0.037
cyclohexane
17
1.17 ( 0.09
cyclohexane
13
1.032 ( 0.016
cyclohexane
17
0.78
none
34
0.980 ( 0.086
cyclohexane
21
1.14 ( 0.14
none
34
0.114a
0.661 ( 0.92a 0.80b, 0.89a 0.80b, 1.07a 0.648 ( 0.022a 1.027 ( 0.066b 0.62a,f 0.38a,f 0.780 ( 0.015b,g 0.520 ( 0.026b 0.899 ( 0.075 1.07b,h 1.006 ( 0.049b 0.595 ( 0.055a 0.541 ( 0.065a 0.505 ( 0.003b 0.496 ( 0.016b 0.575 ( 0.057a 0.518 ( 0.095a 0.501 ( 0.006b 0.536 ( 0.023b 0.533 ( 0.049a 0.582 ( 0.078a 0.511 ( 0.014b 0.512 ( 0.049b 0.519 ( 0.054a 0.527 ( 0.070a 0.476 ( 0.029b 0.473 ( 0.023b 0.776 ( 0.071a 0.391 ( 0.050a 0.663 ( 0.010b 0.369 ( 0.012b 0.37 ( 0.05a 0.41 ( 0.05a 0.340 ( 0.050b 0.640 ( 0.070b 0.34 ( 0.05a 0.80 ( 0.09a
a Yield reported as carbonyl formed/reacted alkene. b Yield reported as carbonyl formed/reacted ozone. c Estimated from Figure 1 in ref 32. d In dry air. e In humid air. f Yield derived from computer simulation of experimental data (30). g Includes contribution of biradical CH3CHOO to formaldehyde, see text and ref 16. h From runs K, L, and M in Table 2 of ref 8.
and 0.20, respectively (18, 21), with an average of 0.175. Thus, the fraction of the measured acetone formation yield that is contributed by the biradical (CH3)2CHCHOO in the reaction of ozone with 2,4-dimethyl-2-pentene is estimated to be small, i.e. less than or equal to (0.196 × 0.175) ) 0.034. In turn, the “corrected” sum of the formation yields of the primary carbonyls is 1.02 vs 1.06 measured. Formation Yields of Primary Carbonyls in OzoneAlkene-Cyclohexane Experiments. Formation yields of the primary carbonyls are listed in Table 2 for 37 alkenes (including the two alkenes studied in this work) that have been studied in this laboratory under the same conditions, i.e. at ambient temperature, p ) 1 atm of purified humid air (RH ) 55% ( 10%) and with cyclohexane added to scavenge OH. The alkenes listed in Table 2 include 11 symmetrical alkenes R1R2CdCR1R2, 13 monosubstituted alkenes RCHdCH2, eight 1,1-disubstituted alkenes R1R2CdCH2, one nonsymmetrical disubstituted alkene R1CHdCHR2, and four trisubstituted alkenes R1CHdCR3R4. Also listed in Table 2 are the sums of the formation yields of the primary carbonyls. For all but a few alkenes (see discussion below), the sums of the formation yields of the primary carbonyls are close to the value of 1.0, which is consistent with reaction 1. Measured sums of primary carbonyl formation yields are 0.94-1.10 for 26 alkenes, 1.1-1.2 for six alkenes, 1.2-1.3 for four alkenes, and 1.4 for one alkene. These results provide strong supportive evidence for the hypothesis that the gas phase reaction of ozone with alkenes is, under conditions that are relevant to the atmosphere, consistent with the simple mechanism summarized by reaction 1. Differences between measured and actual carbonyl formation yields may result from (a) the presence of reactive
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alkene impurities and (b) the contribution of biradical reactions to the measured concentration of the primary carbonyl(s). The purity of the symmetrical alkene trans-2,5dimethyl-3-hexene (stated purity 95%) was only 92% as measured by GC-MS, and thus the observed primary carbonyl formation yield may be less than the measured value, i.e. (1.40 ( 0.08) × 0.92 ) 1.29 ( 0.07. Biradical reactions may lead to the same carbonyl that also forms as a primary carbonyl in two ways, one involving the reaction biradical f carbonyl + O(3P), followed by O(3P) + O2 + M f O3 + M, and the other involving the reaction sequence biradical f peroxide f carbonyl, as is shown in the following paragraph for propene and 2-methylpropene. As indicated by the data obtained to date (16-21), the contribution of biradical reactions to the measured primary carbonyl yield is small and has little or no impact on the discussion presented in the following sections. For propene and 2-methylpropene, the primary carbonyl product formaldehyde also forms as a product of the reactions of the biradicals CH3CHOO and (CH3)2COO, respectively. Formation of formaldehyde from (CH3)2COO has been described earlier, see reactions 5a-c. Formaldehyde forms from CH3CHOO in a similar manner:
CH3CHOO f CH2dC(H)OOH* f CH2OHCHO*
(9a)
CH2OHCHO* f HCO + CH2OH
(9b)
CH2OH + O2 f HO2 + HCHO
(9c)
Estimates of the contributions of the biradicals CH3CHOO and (CH3)2COO to the measured formaldehyde from propene
TABLE 4. Relative Formation Yields of the Biradicals HCHOO, RCHOO, and R1R2COO in Ozone-Alkene-Cyclohexane Experiments RCHOO vs H2COO (1-alkenes) R ) n-alkyl methyl ethyl n-propyl n-butyl
0.60 0.64 0.50 0.48
n-pentyl n-hexyl
0.50 0.50
n-octyl
0.52
R1R2COO vs H2COO (1,1-disubstituted alkenes)
R ) branched-chain
r
R1
r
(0.57)a isopropyl isobutyl sec-butyl tert-butyl
0.49 0.38 0.38 0.32
cyclohexyl C6H5-
0.43 0.35
methyl methyl methyl ethyl methyl methyl isopropyl cyclo-(CH2)5
R1CHOO vs R2CHOO (1,2-disubstituted alkenes) R1
tert-butyl
a
methyl ethyl n-propyl ethyl isopropyl tert-butyl isopropyl
r 0.74 (0.68)a 0.65 0.66 0.58 0.64 0.64 0.59 0.52
R2R3COO vs R1CHOO (trisubstituted alkenes)
R2
r
ethyl
0.28
R1
R2, R3
r
methyl isopropyl tert-butyl ethyl
methyl, methyl methyl, methyl methyl, methyl ethyl, -CH(C2H5)2
0.69 0.82 (0.84)a 0.82 0.71
The value in parentheses is corrected for contribution of biradical reactions to the primary carbonyl, see text.
and 2-methylpropene, respectively, have been reported previously by comparison of data for these two alkenes to data for other alkenes that also lead to the biradicals CH3CHOO and (CH3)2COO (16). The sums of the formation yields of primary carbonyls thus “corrected” are 1.22 ( 0.03 for propene (vs 1.30 ( 0.03 measured) and 1.08 ( 0.11 for 2-methylpropene (vs 1.29 ( 0.10 measured). Few studies are available for comparison with our results. The literature data compiled in Table 3 (8, 13, 30-34) were obtained using experimental conditions, initial concentrations, OH scavengers (or absence thereof), carbonyl measurement methods, and/or definitions of carbonyl formation yields (vs alkene or vs ozone) that were different from those employed in our laboratory. Results that agree with ours are those of Neeb et al. (31) for formaldehyde from ethylene, those of Horie and Moortgat (30) for (formaldehyde + acetaldehyde) from propene, those of Niki et al. (8) for acetone from 2,3-dimethyl-2-butene, and those of Atkinson et al. (13) for 1-pentene, 1-hexene, 1-heptene, 1-octene, and 2,3dimethyl-1-butene. Relative Formation Yields of Primary Carbonyls. The formation yields of the primary carbonyls are also the formation yields of the corresponding biradicals (see reaction 1). The reaction of ozone with alkenes is thought to involve 1,3-dipolar electrophilic addition at the unsaturated carboncarbon bond followed by decomposition of the 1,2,3trioxolane adduct into two carbonyls and two biradicals as is shown in reaction 1. For trans-2,2-dimethyl-3-hexene, R ) 0.28 ( 0.01, the data indicate preferential formation of the biradical CH3CH2CHOO + trimethylacetaldehyde as compared to the biradical (CH3)3CCHOO + propanal, i.e. preferential formation of the biradical that bears the less bulky substituent by scission of the O2-O3 bond. O H
R2
1
C C2H5
O 2
O
3
C
C(CH3)3 H
0.72(C2H5CHOO + (CH3)3CCHO) (10)
As discussed previously for 1-alkenes that bear bulky substituents (21), this preferential decomposition may indicate that the O2-O3 bond is weaker than the O1-O2 bond, due to the steric hindrance of the tert-butyl substituent. For 2,4dimethyl-2-pentene, R ) 0.82 ( 0.04, the data indicate preferential formation of the biradical (CH3)2COO + 2-methylpropanal as compared to the biradical (CH3)2CHCHOO
+ acetone, i.e. preferential formation of the more substituted biradical. Thus, the 1,2,3-trioxolane decomposes preferentially by scission of the O1-O2 bond: O H (CH3)2CH
1
C
O 2
O
3
C
CH3 CH3
0.82((CH3)2COO + (CH3)2CHCHO) (11)
As discussed previously for 1,1-disubstituted alkenes (20), this preferential decomposition may indicate that the O2-O3 bond is stronger than the O1-O2 bond due to the larger inductive effect of the two methyl groups as compared to that of the isopropyl substituent. It is of interest to examine the influence of the number and nature of the alkyl substituents on the decomposition of the 1,2,3-trioxolane adduct. For this purpose, we have compiled in Table 2 the coefficients R measured in this study and those measured previously for alkenes under the same conditions (16-22). For symmetrical alkenes, reaction 1 can be written as follows:
R1R2CdCR1R2 + O3 f R1COR2 + R1R2COO
(12)
For symmetrical alkenes the O1-O2 and O2-O3 bonds are identical and R ) 0.50 by definition. Experimental data for the 11 symmetrical alkenes studied yield Rav ) 0.565, or, if the high value for trans-2,5-dimethyl-3-hexene is excluded, see earlier discussion, Rav ) 0.55. For nonsymmetrical alkenes, as a convention we set the coefficient R to be associated with the more substituted biradical, i.e. with the least substituted primary carbonyl and, in the case of equally substituted biradicals, with the biradical that bears the larger substituent(s), i.e. with the primary carbonyl that bears the smaller substituent(s). Thus for 1-alkenes, 1,1-disubstituted alkenes, trisubstituted alkenes, and nonsymmetrical 1,2-disubstituted alkenes, respectively, reaction 1 can be written as follows:
RCHdCH2 + O3 f R(HCHO + RCHOO) + (1 - R)(RCHO + H2COO) (13) R1R2CdCH2 + O3 f R(HCHO + R1R2COO) + (1 - R)(R1COR2 + H2COO) (14)
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R1CHdCR2R3 + O3 f R(R1CHO + R2R3COO) + (1 - R)(R2COR3 + R1CHOO) (15) R1CHdCHR2 + O3 f R(R1CHO + R2CHOO) + (1 - R)(R2CHO + R1CHOO) (16) The coefficients R compiled in Table 2 have been organized in Table 4 according to the type of biradicals formed, i.e. H2COO vs monosubstituted biradicals RCHOO (13 alkenes), H2COO vs disubstituted biradicals R1R2COO (eight alkenes), monosubstituted biradicals R1CHOO vs R2CHOO (one alkene), and monosubstituted biradicals R1CHOO vs disubstituted biradicals R2R3COO (four alkenes). The coefficients R for 26 nonsymmetrical alkenes range from 0.28 to 0.82. Within that range, the coefficient R appears to be consistent with the competing influences of substituent inductive and steric effects. For 1-alkenes that bear n-alkyl groups, i.e. from propene (CH3CHOO) to 1-decene (n-C8H17CHOO) the data indicate little or no preferential formation of the monosubstituted biradical RCHOO as compared to H2COO, i.e. R ) ca. 0.5. This trend is similar to that observed for the corresponding ozone-alkene gas phase reaction rate constants (2, 23-26, 35) and, by analogy with the kinetic data (35), may indicate that substituent inductive effect and substituent steric hindrance, which both increase with increasing size of the n-alkyl substituent, offset each other at ambient temperature. For 1-alkenes that bear branched-chain substituents (e.g. 3,3dimethyl-1-butene, R ) tert-butyl) or ortherwise bulky substituents (e.g. styrene, vinylcyclohexane), the coefficient R is lower than that for 1-alkenes with n-alkyl substituents. For these alkenes, the data indicate modest preferential formation of H2COO + RCHO as compared to RCHOO + HCHO. For 1,1-disubstituted alkenes, the values of R indicate preferential formation of the disubstituted biradical R1R2COO + HCHO as compared to H2COO + R1COR2. The coefficient R is lower for the 1,1-disubstituted alkenes that bear bulky substituents, e.g. R ) 0.52 for methylene cyclohexane. As for 1-alkenes, the trend in R vs substituent steric hindrance for 1,1-disubstituted alkenes is similar to that observed for the corresponding gas phase ozone-alkene reaction rate constants (25). For trisubstituted alkenes the data indicate preferential formation of the disubstituted biradical R2R3COO + R1CHO as compared to the monosubstituted biradical R1CHOO + R2COR3. As for 1-alkenes and 1,1-disubstituted alkenes, the formation yield of the monosubstituted biradical RCHOO decreases with increasing substituent size, e.g. compare R ) 0.69 for (CH3)2COO vs CH3CHOO to R ) 0.82 for (CH3)2COO vs (CH3)3CCHOO or for (CH3)2COO vs (CH3)2CHCHOO. For the one nonsymmetrical 1,2-disubstituted alkene studied, the data show, consistent with data for other alkenes, preferential formation of the biradical that does not bear a bulky substituent. While not all types of biradical pairs have been studied (for example, data are available for only one pair of monosubstituted biradicals and no data are available for tetrasubstituted alkenes that lead to two different disubstituted biradicals), the coefficients R measured for 26 alkenes are consistent with the hypothesis that the relative formation yields of the two biradicals reflect the competing influences of substituent inductive and steric effects. Atmospheric Implications. Results for 37 alkenes show that the gas phase reaction of ozone with alkenes, when studied under conditions relevant to the atmosphere, is consistent with the simple mechanism summarized by reaction 1. The relative abundance of the two primary carbonyls and of the two biradicals that form in reaction 1 varies with the number and nature of the alkyl substituents. For the 26 nonsymmetrical alkenes studied, the coefficient R ranges from 0.28 to 0.82. Measurements of the coefficient
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R provide information on the formation yields of the biradicals whose subsequent reactions lead to carboxylic acids, carbonyls, free radicals, carbon monoxide, and other products (1-22, 30-34). The ozone-alkene reaction plays a major role in the formation of photochemical oxidants (36) and the production of secondary organic aerosols (37) which have a significant impact on urban air pollution and regional air quality (38). Our results contribute to a better understanding of the atmospheric chemistry of alkenes and may also serve as input to update and expand the description of the ozonealkene reaction in computer kinetic models (36) that are being used to describe ozone formation and other features of urban and regional air quality.
Acknowledgments This work was supported by internal R & D funds, DGA, Inc., Ventura, CA, and was presented in part at the International Chemical Congress of Pacific Basin Societies, Honolulu, HI, December 17-22, 1995 (Paper No. 570), and at the 89th Annual Meeting of the Air and Waste Management Association, Nashville, TN, June 23-28, 1996 (Paper 96-WA62A.03). Ms. Brenda Brennan prepared the draft and final versions of the manuscript.
Literature Cited (1) Bailey, P. S. Ozonation in Organic Chemistry. Vol. 1. Olefinic Compounds; Academic Press: New York, 1978. (2) Atkinson, R.; Carter, W. P. L. Chem. Rev. 1984, 84, 437. (3) Herron, J. T.; Huie, R. E. J. Am. Chem. Soc. 1977, 99, 5430. (4) Herron, J. T.; Huie, R. E. Int. J. Chem. Kinet. 1978, 10, 1019. (5) Martinez, R. I.; Herron, J. T. J. Phys. Chem. 1987, 91, 946. (6) Martinez, R. I.; Herron, J. T. J. Phys. Chem. 1988, 92, 4644. (7) Martinez, R. I.; Herron, J. T.; Huie, R. E. J. Am. Chem. Soc. 1981, 103, 3807. (8) Niki, H.; Maker, P. D.; Savage, C. M.; Breitenbach, L. P.; Hurley, M. D. J. Phys. Chem. 1987, 91, 941. (9) Hatakeyama, S.; Tanonaka, T.; Weng, J.; Bandow, H.; Takagi, H.; Akimoto, H. Environ. Sci. Technol. 1985, 19, 935. (10) Grosjean, D. Environ. Sci. Technol. 1990, 24, 1428. (11) Atkinson, R.; Aschmann, S. M.; Arey, J.; Shorees, B. J. Geophys. Res. 1992, 97, 6065. (12) Atkinson, R.; Aschmann, S. M. Environ. Sci. Technol. 1993, 27, 1357. (13) Atkinson, R.; Tuazon, E. C.; Aschmann, S. M. Environ. Sci. Technol. 1995, 29, 1860. (14) Atkinson, R. J. Phys. Chem. Ref. Data, Monogr. 2, 1994, 216 pp. (15) Grosjean, D.; Grosjean, E.; Williams, E. L., II. Environ. Sci. Technol. 1994, 28, 186. (16) Grosjean, E.; de Andrade, J. B.; Grosjean, D. Environ. Sci. Technol. 1996, 30, 975. (17) Grosjean, E.; Grosjean, D. Environ. Sci. Technol. 1996, 30, 1321. (18) Grosjean, E.; Grosjean, D. Environ. Sci. Technol. 1996, 30, 2036. (19) Grosjean, E.; Grosjean, D.; Seinfeld, J. H. Environ. Sci. Technol. 1996, 30, 1038. (20) Grosjean, E.; Grosjean, D. J. Atmos. Chem. 1996, 24, 141. (21) Grosjean, E.; Grosjean, D. Atmos. Environ. 1996, 30, 4107. (22) Grosjean, E.; Grosjean, D. Int. J. Chem. Kinet. 1997 (in press). (23) Grosjean, E.; Grosjean, D. Int. J. Chem. Kinet. 1995, 27, 1045. (24) Grosjean, E.; Grosjean, D. Int. J. Chem. Kinet. 1996, 28, 461. (25) Grosjean, E.; Grosjean, D. Int. J. Chem. Kinet. 1996, 28, 911. (26) Greene, C. R.; Atkinson, R. Int. J. Chem. Kinet. 1992, 24, 803. (27) Grosjean, E.; Grosjean, D. Int. J. Environ. Anal. Chem. 1995, 61, 47. (28) Grosjean, E.; Grosjean, D. Int. J. Environ. Anal. Chem. 1995, 61, 343. (29) Grosjean, E.; Grosjean, D.; Fraser, M. P.; Cass, G. R. Environ. Sci. Technol. 1996, 30, 2687. (30) Horie, O.; Moortgat, G. K. Atmos. Environ. 1991, 25A, 1881. (31) Neeb, P.; Horie, O.; Moortgat, G. K. Chem. Phys. Lett. 1995, 246, 150. (32) Horie, O.; Neeb, P.; Limbach, S.; Moortgat, G. K. Geophys. Res. Lett. 1994, 21, 1523. (33) Thomas, W.; Zabel, F.; Becker, K. H.; Fink, E. H. A Mechanistic Study of the Ozonolysis of Ethene. In Tropospheric Oxidation Mechanisms; Air Pollution Research Report 54; Commission of the European Communities: Brussels, 1995; pp 315-320.
(34) Tuazon, E. C.; Arey, J.; Atkinson, R.; Aschmann, S. M. Environ. Sci. Technol. 1993, 27, 1832. (35) Treacy, J.; El Hag, M.; O’Farrell, D.; Sidebottom, H. Ber. Bunsenges. Phys. Chem. 1992, 96, 422. (36) Carter, W. P. L. Atmos. Environ. 1990, 34A, 481. (37) Grosjean, D. Atmos. Environ. 1992, 26A, 953. (38) National Research Council. Rethinking the Ozone Problem in Urban and Regional Air Pollution; National Academy Press: Washington, DC, 1991.
Received for review January 29, 1997. Revised manuscript received April 7, 1997. Accepted April 10, 1997.X ES970075B
X
Abstract published in Advance ACS Abstracts, June 15, 1997.
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