OH Radical Initiated Photooxidation of 2-Ethoxyethanol under

The products formed by the hydroxyl radical-initiated oxidation of 2-ethoxyethanol (CH3CH2OCH2CH2OH) have been investigated by irradiating synthetic a...
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Environ. Sci. Technol. 1996, 30, 3385-3391

OH Radical Initiated Photooxidation of 2-Ethoxyethanol under Laboratory Conditions Related to the Troposphere: Product Studies and Proposed Mechanism KONRAD STEMMLER, WOLFGANG MENGON, AND J. ALISTAIR KERR* EAWAG, Swiss Federal Institute for Environmental Science and Technology, ETH Zu ¨ rich, CH-8600 Du ¨ bendorf, Switzerland

The products formed by the hydroxyl radical-initiated oxidation of 2-ethoxyethanol (CH3CH2OCH2CH2OH) have been investigated by irradiating synthetic air mixtures containing the substrate, methyl nitrite, and nitric oxide at ppm levels in a Teflon bag reactor at room temperature. The decay of reactants and the formation of products were monitored by gas chromatography and mass spectrometry. The major products ethyl formate [HC(O)OCH2CH3], ethylene glycol monoformate [HC(O)OCH2CH2OH], ethylene glycol monoacetate [CH3C(O)OCH2CH2OH], and ethoxyacetaldehyde [CH3CH2OCH2C(O)H] give a quantitative mass balance with the decay of the substrate molecule. The yields of these products were 34 ( 10%, 36 ( 7%, 7.8 ( 2.4%, and 24 ( 13%, respectively, in terms of percent of 2-ethoxyethanol removed by the OH radical. The product distribution is explained by a mechanism involving initial OH attack at the three CH2 groups in 2-ethoxyethanol followed by the subsequent reactions of the resulting alkyl and alkoxy radicals. The decomposition reactions of the alkoxy radicals from 2-ethoxyethanol, which can take place either by C-C or C-O bond breaking, involve preferential C-C cleavage rather than C-O cleavage. Rate coefficients at room temperature for the reactions of OH radicals with ethoxyacetaldehyde and 2-methyl-1,3-dioxolane (CH3CHOCH2CH2O, a minor product) have been determined to be 16.6 × 10-12 and 9.4 × 10-12 cm3 molecule-1 s-1, respectively.

Introduction Large quantities of ethylene glycol ethers are used in paints, coatings, inks, cleaners, and polishes. In addition, they * Corresponding author fax: 44 121 472 8067; e-mail address: [email protected].

S0013-936X(96)00348-3 CCC: $12.00

 1996 American Chemical Society

find wide application as solvents, chemical intermediates, brake fluids, and jet fuel additives. Because of their high solvency power, ethylene glycol ethers are used to replace greater quantities of less efficient solvents. Ethylene glycol ethers are frequently used as solvent couplers of many organic liquids in water-based formulations of coatings and cleaners. In 1991, the total demand for glycol ethers in western Europe was 502 kt (1). Toxicological studies (2) have shown the ethylene glycol ethers to be reproductive toxins and toxic to rapidly dividing cell systems such as bone marrow, testes, thymus, and spleen. The short-chain ethylene glycol ethers, for instance, 2-ethoxyethanol, are known to cause teratogenicity and foetotoxicity. Recent studies indicate that the dermal absorption of ethylene glycol ethers from the vapor phase can contribute significantly to their total uptake. The toxicity of 2-ethoxyethanol has been related to ethoxyacetic acid, which has been identified as the main metabolite after inhalation of 2-ethoxyethanol. There is very little experimental information on the atmospheric fate of these molecules apart from a few studies of some OH rate coefficients (3-6). These studies indicate atmospheric lifetimes of less than 1 day for the members of the series of ethylene glycol monomethyl to monobutyl ethers. The main degradation pathway of these compounds in the troposphere is expected to proceed by H-atom abstraction from C-H bonds by the OH radical and to involve the intermediate radicals, alkyl (R), alkyl peroxy (ROO), and alkoxy (RO) (7). The present product and mechanistic study of the OH radical-initiated photooxidation reactions of 2-ethoxyethanol was undertaken to obtain quantitive data on the detailed pathways for the atmospheric breakdown of such molecules. One of the simplest members of the ethylene glycol ether series was selected for this initial study, which we are presently extending to higher members of the series.

Experimental Section Materials. The synthetic air (Pangas) was a mixture of 20% O2 and 80% N2, which was passed through a tube containing molecular sieves and activated carbon prior to use. Nitric oxide (5000 ppm) in N2 was supplied by Carbagas. Methyl nitrite was prepared from the reaction of methanol with nitrous acid (8) and was transferred to the high vacuum line, purified by bulb-to-bulb distillation, and stored in the dark at 77 K. Ethylene glycol monoformate was synthesized (9, 10) by ozonizing 8 cm3 (0.115 mol) of 1,3-dioxolane in 50 cm3 of ethyl acetate at -78 °C for 3 h (rate of ozone addition: -0.7 mmol/min). Excess ozone was removed by flushing the solution with nitrogen. The solvent was removed by evaporation, and the residue was distilled under reduced pressure. The purity of the product was checked by GC/FID and GC/MS and found to be >97% (impurity, ethylene carbonate). Ethylene glycol monoformate was stored at 77 K prior to use since a previous sample had shown a tendency to form ethylene glycol and ethylene glycol diformate. Ethylene glycol monoacetate was prepared by the same procedure as for the corresponding formate, from 4 cm3 of 2-methyl-1,3-dioxolane (0.045 mol) in 50 cm3 of ethyl acetate and purified by vacuum distillation. The purity was found to be >97% (impurity,

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ethylene carbonate). Ethoxyacetaldehyde was prepared by the method of Hatch and Nesbitt (11) by oxidizing 4.5 g of glycerol R-ethyl ester (0.037 mol) with 8.5 g of periodic acid (0.037 mol) in 30 cm3 of water at room temperature. The product was extracted from the reaction mixture with diethyl ether and dried over sodium sulfate. The solvent was removed in vacuum, and the residue was purified by liquid column chromatography. The purity of the product was checked by GC/FID and GC/MS and found to be >95% (impurity, ethanol). The product was stored at 77 K to avoid polymerization (12), which was not observed. The calibration responses of mixtures of ethoxyacetaldehyde in 200 dm3 of air were corrected for the ethanol impurity observed in these mixtures. The intermediate glycerol R-ethyl ester was prepared from 3-chloro-1,2-propanediol and sodium ethoxide, following the procedure of Davies et al. (13) and purified by vacuum distillation. The following chemicals were used without further purification other than bulb-to-bulb distillation: 2-ethoxyethanol (Merck, >99.5%), 2-methyl-1,3-dioxolane (Fluka, >98%), ethyl glycolate (Aldrich, 98%), ethoxyacetic acid (Fluka, 98%), hydroxyacetaldehyde dimer (Fluka, 98%), 1-hexanol (Fluka, >99%), and ethyl formate (Merck, >98%). Apparatus. The product studies were carried out in an experimental system that has been described previously (14-16). Hydroxyl radicals were produced from the photolysis of methyl nitrite in air containing NO:

CH3ONO + hν f CH3O + NO

(1)

CH3O + O2 f CH2O + HO2

(2)

HO2 + NO f OH + NO2

(3)

The experiments were performed at room temperature (297 ( 3 K) and at atmospheric pressure (750 ( 10 Torr) in a 200-dm3 Teflon bag surrounded by 16 black lamps (Philips L20/05), which provide UV radiation in the region of 350450 nm. The reaction chamber was covered with a black cloth to prevent prephotolysis of reactants and two electrical fans helped to maintain a uniform reaction temperature during the irradiation of the reactants. Reactant mixtures were prepared by sweeping measured amounts of 2-ethoxyethanol and methyl nitrite vapors from a calibrated volume into the Teflon bag with a stream of synthetic air. Pressures were measured with a capacitance manometer, MKS Baraton 220C. The bag was filled with up to 200 dm3 of synthetic air using mass flow controllers and a stop watch, and nitric oxide was added to the mixture from a cylinder containing 5000 ppm of NO in N2 to minimize the formation of O3. Once the gas mixture was prepared, the bag was agitated and left to stand for about 1 h to thoroughly mix the reactants. The bag mixture was irradiated for up to 2 h during which time samples were periodically removed and analyzed. Product Analyses. The decay of the starting material, 2-ethoxyethanol, was measured by gas chromatography (Carlo Erba HGRC 5300) with a flame ionization detector (FID). Vapor samples were injected into the gas chromatograph via a 3-cm3 stainless steel gas sampling valve, and the gas chromatograph was equipped with a 30-m DBWax fused silica column (J&W Scientific) with 0.32 mm internal diameter and 0.25 µm film thickness operated with temperature programming from 313 to 393 K at a heating rate of 20 K min-1.

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The products ethyl formate and 2-methyl-1,3-dioxolane were measured by preconcentration on Tenax TA adsorption tubes. An air sample of 212 cm3 was withdrawn from the bag at a flow rate of 53 cm3 min-1 through an adsorption tube (100 mg of Tenax TA, Chrompack). The contents of the tubes were thermally desorbed (Carlo Erba TDAS 5000) at 473 K for 10 min in a stream of helium and reconcentrated by means of a cold trap at 123 K. The trap was rapidly heated to 523 K to inject the sample onto a capillary column (J&W Scientific, 30 m, 0.25 mm i.d., 1 µm film DB-1701). The column was kept at 313 K for 1 min and then heated to 393 K at a heating rate of 5 K min-1 and finally to 473 K at 10 K min-1. The column was connected to a glass splitting device that was attached via fused silica restrictions to a FID detector and to a mass spectrometer detector (Fisons MD800) operated in the total ion mode. Calibration of these compounds was carried out with the FID detector. Several samples were drawn through two Tenax TA adsorption tubes in series that were both analyzed in the same way to check for breakthrough of the compounds in the adsorption tubes. The products ethylene glycol monoformate and ethylene glycol monoacetate were measured as described above for 2-ethoxyethanol but with a column heating rate of 10 K min-1. The product ethoxyacetaldehyde was withdrawn from the bag using a 3-cm3 sampling valve and was separated from the other compounds on the DB-1701 column kept at 353 K for 4 min. After the elution of ethoxyacetaldehyde, the column was rapidly heated to 433 K to purge higher boiling materials. Detection was performed with a mass spectrometer detector (Fisons MD800) operated in the single ion mode recording ions with m/z 45.2 ( 0.5 and m/z 59.2 ( 0.5. Calibrations were carried on the ion mass trace m/z 59.2, which corresponds to the base peak of ethoxyacetaldehyde. All of the above products were calibrated by preparing at least four bag mixtures containing standards in air at mixing ratios comparable to those in the experiments and by analyzing with the same procedures as in the experiments to obtain response factors from a linear least square fit of the data. Rate Coefficients of OH Reactions with Products. Rate coefficients for the gas phase reaction of the OH radical with ethoxyacetaldehyde and 2-methyl-1,3-dioxolane were determined by the relative rate method (6) in which the decays of the test compounds (TST) and the reference compound, 1-hexanol (REF), were measured in the presence of OH radicals:

OH + TST f products

(4)

OH + REF f products

(5)

Assuming that there are no loss processes of the test and reference compounds other than by reaction with OH radicals, the rate expressions for these processes can be integrated and combined to give

ln ([TST]0/[TST]t) ) (kTST/kREF) ln ([REF]0/[REF]t) (6) where the subscripts 0 and t indicate concentration at the beginning of the experiment and at time t, respectively. A plot of ln ([TST]0/[TST]t) versus ln ([REF]0/[REF]t) thus yields the rate coefficient ratio kTST/kREF. Synthetic air mixtures (200 dm3) containing methyl nitrite, NO, the test compound, and the reference compound were prepared in the Teflon

bag reactor. The bag mixture was irradiated for approximately 2 h, during which time samples of the mixture were periodically removed via the 3-cm3 sampling valve, and the test and the reference compound were analyzed by gas chromatography (Carlo Erba HGRC 5300) with a flame ionization detector (FID) equipped with the DB-WAX fused silica column operated, as described above, with temperature programming from 313 to 393 K. Product Interferences from the Reactions of Primary Products with OH Radicals. In this series of experiments, the formation of ethyl formate from the OH radical-initiated photooxidation of ethoxyacetaldehyde was studied in the presence of high mixing ratios of NO or NO2. Bag mixtures containing ethoxyacetaldehyde, methyl nitrite, NO, and NO2 in 200 dm3 of air were irradiated as described above. Ethyl formate and ethoxyacetaldehyde were analyzed by injection, via the gas sampling valve, onto the DB-Wax column operated at 313 K for 5 min and then heated with a rate of 20 K min-1 to 453 K. Single-point calibrations were carried out for both of these compounds using a standard gas mixture prepared as above.

Results Three experiments were carried out to measure principally the product yields of ethyl formate and 2-methyl-1,3dioxolane. The initial mixing ratios of the reactants were as follows: 2-ethoxyethanol, 2.1-2.7 ppm; methyl nitrite, 2.5-11 ppm; NO, 5-12.5 ppm. The following compounds were observed and identified by retention time and/or mass spectra using pure standards: methyl nitrite, ethyl formate, 2-methyl-1,3-dioxolane, ethoxyacetaldehyde, 2-ethoxyethanol, ethylene glycol monoformate, and ethylene glycol monoacetate. Additionally, we observed methyl nitrate, benzaldehyde, toluene, and an unknown compound, probably a methylated silane. The last three compounds were found to be artifacts of the analytical system. Benzaldehyde and toluene can be formed if Tenax TA is exposed to ozone (17). The observed methyl nitrate was formed mainly from the photolysis of methyl nitrite and was therefore not calibrated. It was not possible to quantify ethylene glycol monoformate and ethylene glycol monoacetate under these conditions, since the thermal desorption of these compounds was not complete, probably owing to their high boiling points. Ethoxyacetaldehyde was not stable under the given adsorption-desorption conditions in the presence of NOx-methyl nitrite-air mixtures and gave rise to an unidentified decomposition product. In pure air, ethoxyacetaldehyde appears to be stable under the given analytical conditions. A typical concentration versus time profile is shown in Figure 1a for the starting compound 2-ethoxyethanol and for the two calibrated products 2-methyl-1,3-dioxolane and ethyl formate. No breakthrough of these products was observed using two Tenax TA tubes in series. Ethylene glycol monoformate and ethylene glycol monoacetate were analyzed in three separate runs with the following initial mixing ratios in the bag reactor: 2-ethoxyethanol, 2.6-3.1 ppm; methyl nitrite, 4.1-5.6 ppm; NO, 6 ppm. The following compounds were observed and identified by retention time and/or in an additional experiment with initial mixing ratios of 2-ethoxyethanol of about 20 ppm by connecting the column to a MS detector: methyl nitrite, ethyl formate, methyl nitrate, 2-methyl-1,3dioxolane, ethoxyacetaldehyde, 2-ethoxyethanol, ethylene

FIGURE 1. Reactant and product profiles of mixing ratio versus time: Panel a shows data from experiment 1: (O) 2-ethoxyethanol, (() ethyl formate, (0) 2-methyl-1,3-dioxolane (right-hand scale); panel b shows data from experiment 4: (O) 2-ethoxyethanol, (9) ethylene glycol monoformate, (×) ethylene glycol monoacetate; panel c shows data for experiment 7: (O) 2-ethoxyethanol, (2) ethoxyacetaldehyde (right-hand scale).

glycol monoformate, and ethylene glycol monoacetate. Additionally, two minor products were observed but not identified. A typical concentration versus time profile is shown in Figure 1b for 2-ethoxyethanol and the two calibrated products ethylene glycol monoformate and ethylene glycol monoacetate. The analysis of ethoxyacetaldehyde was performed in three separate runs with the following initial conditions: 2-ethoxyethanol, 2.3-3.4 ppm; methyl nitrite, 5 ppm; NO, 5-10 ppm. A typical concentration versus time profile is shown in Figure 1c for 2-ethoxyethanol and ethoxyacetaldehyde.

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TABLE 1

Room Temperature Rate Coefficients for Reaction of OH Radicals with Oxygenated Compounds Relevant to This Study compound

rate coefficient (10-12 cm3 molecule-1 s-1)

method,b reference

2-ethoxyethanola ethyl formate 2-methyl-1,3-dioxolanea ethoxyacetaldehydea ethylene glycol monoformate ethylene glycol monoacetate

14.5 1.02 9.4 16.6 4.9 5.5

RR (1-hexanol) (6) FP-RF (18) RR (1-hexanol), this work RR (1-hexanol), this work estimation, see text estimation, see text

a The ratio k(OH + oxygenate)/(k(OH + hexanol) was used in correcting for secondary OH reactions. b FP-RF, flash photolysis-resonance fluorescense; RR, relative rate method; reference compound in parentheses.

be significantly slower than the reaction of 2-ethoxyethanol with OH, and therefore the secondary removal is of less importance. The rate coefficients for ethylene glycol monoformate and ethylene glycol monoacetate, which are shown in Table 1, were estimated from the structureactivity relationship of Atkinson (20) by taking literature data for the OH reactions at room temperature with ethyl formate (18) and ethyl acetate (19), respectively, and replacing the CH3 groups in these molecules by a HOCH2 group. The yield of each product was then corrected for losses due to reactions with OH radicals, according to the method of Atkinson et al. (21). Measured yields of the respective product were multiplied with the correction factor F to obtain the fractions f leading to these products according to FIGURE 2. Sample plots of ln ([TST]0/[TST]t) for the reaction of OH with the following test compounds (TST): (×) ethoxyacetaldehyde, (9) 2-methyl-1,3-dioxolane versus ln ([hexanol]0/[hexanol]t).

Since the products also react with the OH radical, their measured concentrations were corrected for this secondary reaction in obtaining a quantitative assessment of the importance of each reaction channel. The rate coefficients of the OH radical reactions with ethoxyacetaldehyde and 2-methyl-1,3-dioxolane were determined in separate relative rate measurement involving 1-hexanol as the reference compound. The initial mixing ratios in the bag reactor were as follows: 2-methyl-1,3-dioxolane, 3.5 ppm; ethoxyacetaldehyde, 5.7 ppm; hexanol, 1.7-1.8 ppm; NO, 15 ppm; and methyl nitrite, 9-17 ppm. The ln-ln plots of the present data according to eq 6 are shown in Figure 2. The rate coefficient ratios derived from the slope of a linear least square fit of the data are as follows: k(OH + 2-methyl1,3-dioxolane)/k(OH + hexanol) ) 0.75 ( 0.06, k(OH + ethoxyacetaldehyde)/k(OH + hexanol) ) 1.33 ( 0.06. The error limits correspond to 95% confidence intervals. These rate coefficient ratios were converted to absolute rate coefficients on the basis of the evaluated value (7) for k(OH + hexanol) ) 12.5 × 10-12 cm3 molecule-1 s-1 at 298 K and are shown in Table 1. The estimated uncertainty of both absolute rate coefficients is about (50% mainly due to uncertainties in the rate coefficient of the reference compound with the OH radical. However, the absolute rate coefficients were not needed in this particular case since the rate coefficient of 2-ethoxyethanol was measured by the competitive method against 1-hexanol in the same system using the same analytical conditions (6). The reactions of ethylene glycol monoformate and ethylene glycol monoacetate with the OH radical were expected to

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F[prod] ) f∆[EE]

(7)

and ∆[EE] ) [EE]0 - [EE]t, where [EE] refers to the concentration of 2-ethoxyethanol and the subscripts denote the time at which this was measured. F is the correction factor for the secondary removal of the respective reaction product. f is the fraction of 2-ethoxyethanol that forms this product in the OH radical-initiated reaction sequence. The correction factor F was calculated using eq 8 along with the rate coefficients in Table 1

F)

(

)(

)

1 - [EE]t/[EE]0 kEE - kprod kEE ([EE]t/[EE]0)kprod/kEE - [EE]t/[EE]0

(8)

where kprod and kEE denote room temperature rate coefficients for the reactions of OH radicals with the reaction product or with 2-ethoxyethanol, respectively. The correction factors increase with the extent of reaction and with the increasing magnitude of the rate coefficients of the reactions of the respective products with the OH radical. The fractions f can be found from the slopes of the data plotted according to eq 7. Figure 3 shows the data from Figure 1a-c corrected for secondary OH removal, and the maximum values of the correction factors F (corresponding to the largest extent of reaction) are given in the caption to Figure 3. The results for all the experiments are given in Table 2. Ethyl formate can be formed via two routes from OH radical attack on ethoxyacetaldehyde as shown in Scheme 1. The OH may abstract one of the H atoms in the R-position to the carbonyl group (channel A) to form a radical that subsequently goes on to produce mainly ethyl formate. The second pathway (channel B) is initiated by the

TABLE 2

Correcteda Yieldsb of Major Products from Photooxidation of 2-Ethoxyethanol experiment

ethyl formate (%)

1 2 3 4 5 6 7 8 9

44.1 45.0 39.7

mean valuesc

43 ( 5

2-methyl1,3-dioxolane (%)

ethylene glycol monoformate (%)

ethylene glycol monoacetate (%)

ethoxyacetaldehyde (%)

4.3 3.6 2.3 37.2 32.7 38.0

7.5 7.2 8.8 27.4 26.0 17.2

3.4 ( 2.5

36 ( 7

7.8 ( 2.4

24 ( 13

a

Corrected for secondary OH radical attack (see text). b Expressed as percent of 2-ethoxyethanol decomposed. c Error limits are for 95% confidence intervals and include the errors in analytical responses, the scatter of results, and the worst-case estimates of the errors in correcting for secondary OH reactions.

SCHEME 1

FIGURE 3. Plots of the mixing ratios of (O) ethyl formate (correction factor F < 1.07), (+) ethylene glycol monoformate (F < 1.34), (() ethoxyacetaldehyde (F < 3.70), (2) ethylene glycol monoacetate (F < 1.38), (b) 2-methyl-1,3-dioxolane (F < 1.82) corrected for the reaction with the OH radical against the mixing ratios of 2-ethoxyethanol reacted with the OH radical, corresponding to the data shown in Figure 1a-c.

abstraction of the aldehydic H atom and includes the formation of an acyl peroxy radical. The intermediary acyl peroxy radical is thought to react either with NO to form a carbonate radical, which eliminates CO2 and is expected to form ethyl formate, or to react with NO2, leading to a peroxy acyl nitrate (PAN). This PAN compound may act as a temporary sink for the acyl peroxy radical at room temperature. The yield of ethyl formate is therefore expected to depend on the NO/NO2 ratio, which changes significantly during the course of an experiment. Since the rate coefficients for the reaction of the acyl peroxy radical with NO and NO2 and for the thermal decomposition of the PAN are unknown, as is the variation of the NO/NO2 ratio during an experiment, it is not possible to model this effect. To estimate the importance of this product interference under conditions of high and low NO/NO2 ratios, two bag mixtures were prepared containing 8-8.5 ppm ethoxyacetaldehyde and 10 ppm methyl nitrite. In the first experiment, 40 ppm NO was added initially to minimize formation of the peroxy acyl nitrate (PAN) and hence increase the yield of ethyl formate. In a second experiment, 40 ppm NO2 was added initially in order to force the formation of the PAN compound. In these two experiments, the observed yields of ethyl formate were 43 ( 5% and 53 ( 10%, respectively.

Discussion Scheme 2 shows the reaction sequence proposed following H atom abstraction at the three CH2 groups of 2-ethoxyethanol. In this scheme, alkyl peroxy radicals have been omitted for clarity. Potential isomerization products from 1,4-H or 1,5-H atom shifts in RO radicals and reactions of RO2 with NO to form organic nitrates are neglected. The observed major products are highlighted in boxes and with the exception of ethyl formate, they are explained by unique reactions of the alkyl radical 3 and the alkoxy radicals 4 and 5. Ethyl formate is, however, produced in significant amounts from secondary reactions of ethoxyacetaldehyde under the present experimental conditions. No evidence for the formation of butyl glycolate and hydroxyacetaldehyde was found by comparing the responses of liquid and gaseous standards of these compounds with the results of the analyses. We conclude that the possible reaction channels of the alkoxy radicals 4 and 5 that could lead to these products are of minor importance

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SCHEME 2

FIGURE 4. Plot of the total observed mixing ratio of major products against the mixing ratio of 2-ethoxyethanol reacted with the OH radical, corresponding to the data shown in Figure 1a-c. The dotted lines are the error limits (95% confidence intervals) arising from the errors in the analytical response factors. The narrow straight line corresponds to 100% product yield.

(>1%). The absence of butyl glycolate is in agreement with the finding (22) that the reaction of O2 with β-hydroxy alkoxy radicals containing more than two carbon atoms has never been observed and appears to be of minor importance. The reaction channels leading to the formation of hydroxyacetaldehyde involve both a decomposition process by breakage of the ether C-O bond in alkoxy radicals 4 or 5 to form either hydroxyacetaldehyde directly or to form the 2-oxy ethanol radical, which has been shown by Niki et al. (23) to form ∼21% of hydroxyacetaldehyde. The present results are in line with the available product studies of the nonbranched ethers dimethyl ether (24) and diethyl ether (15, 25) and of the linear side chain in methyl tertbutyl ether (24, 26, 27) and ethyl tert-butyl ether (25, 28) where decomposition via C-O breakage in the resulting alkoxy radicals is of minor importance. The R-hydroxy alkyl radical 3 is expected to form ethoxyacetaldehyde in a direct reaction with oxygen (29) and consequently does not produce an R-hydroxy alkoxy radical that could form ethoxyacetic acid, which was not found in the product analyses. The observed formation of small amounts of 2-methyl1,3-dioxolane is proposed to involve the alkyl radical 1, on the basis of its structural relationship to the observed dioxolane. Its also seems likely that O2 may be involved in such a conversion with the production of HO2 so that the overall reaction is represented as

The decomposition fragment radicals CH3 and HOCH2 are believed to lead mainly to the formation of formaldehyde (22, 30), and the fragment radical CH3CH2O will form mainly acetaldehyde (22). Measurements of these two aldehydes were not carried out since they are expected to be produced from a variety of sources, such as photolysis of methyl nitrite and from reactions of the initial products with OH radicals.

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Figure 4 represents the stoichiometric balance between the formation of the major products and the consumption of the reactant. The fits of the data corresponding to Figure 1a-c are plotted as the sum of the mixing ratio of products, uncorrected for secondary OH attack, versus the mixing ratio of 2-ethoxyethanol removed. At low conversions, the observed major products balance the loss of 2-ethoxyethanol within the experimental errors, indicating that all the important degradation routes have been accounted for by the proposed mechanism. The slight deviation in the balance from linearity at higher conversion ratios is probably due to losses of products by secondary OH reactions. The bulk of the observed products react significantly slower than 2-ethoxyethanol and are therefore thought to accumulate during the degradation of this glycol ether mainly as ethyl formate and ethylene glycol monoformate. The toxicologically more relevant species 2-ethoxyethanol and ethoxyacetaldehyde have both lifetimes of about 1 day with respect to OH reactions based on an average atmospheric concentration of OH of 106 radicals cm-3 at ambient temperatures. Secondary Reactions of OH Radicals with Products. The rate coefficients of OH radicals with ethoxyacetaldehyde and 2-methyl-1,3-dioxolane were measured since they are expected to be of the same order of magnitude as that of their precursor 2-ethoxyethanol, and hence their loss via OH radical reaction is important. It should be noted that the structure-activity relationship (SAR) prediction (20) for rate coefficients of OH radicals appears to overestimate the values of cyclic polyethers and for several hetero atom substituted acetaldehydes. The measured rate coefficients for 2-methyl-1,3-dioxolane of 9.4 × 10-12 cm3 molecule-1 s-1 may be compared to the literature data (19) for 1,3dioxane and 4-methyl-1,3-dioxane of 9.15 and 11.3 × 10-12 cm3 molecule-1 s-1, respectively, which are the most structurally related compounds for which data are available. The result for ethoxyacetaldehyde, 16.6 × 10-12 cm3 molecule-1 s-1, may be compared with the value of hydroxyacetaldehyde (31) of 9.9 × 10-12 cm3 molecule-1 s-1. Both aldehydes react only slightly faster than the structurally related alcohols, ethylene glycol [7.7 × 10-12

cm3 molecule-1 s-1 (19)] and 2-ethoxyethanol [14.5 × 10-12 cm3 molecule-1 s-1 (6)]. As shown in Scheme 1, ethyl formate can be formed from the photooxidation of ethoxyacetaldehyde via two routes, and a preliminary investigation of the photooxidation of ethoxyacetaldehyde was undertaken. The measured yields of ethyl formate in presence of high initial mixing ratios of NO and NO2 respectively were combined and give an estimated yield of 48 ( 20% by neglecting the influence of the NO/NO2 ratio. This approximation was necessary since the variations of the NO/NO2 ratios during the experiments were unknown.. The overall yield of ethyl formate formed from 2-ethoxyethanol is thus significantly affected by this secondary channel. An estimate of the ethyl formate yield formed as a primary product from 2-ethoxyethanol via alkoxy radical 5 was derived by subtracting the calculated secondary formation from the total observed formation of ethyl formate and resulted in a value of ∼34 ( 10%. The given uncertainty brackets both sets of conditions, i.e., high and low ratios of NO/NO2. Finally, the distribution of major products from the photooxidation of 2-ethoxyethanol may be compared with the SAR predictions of Atkinson (20) with respect of the reactivity of the different reaction sites in 2-ethoxyethanol toward the initial OH attack. The SAR predicts the following fractions of the total reactivity of the groups in CH3CH2OCH2CH2OH, starting on the left-hand side of the molecule: 0.8% for the primary carbon group; 36%, 44%, and 18% for the secondary carbon groups; and 0.8% for the hydroxy group. In the present study, only products from attack on the secondary carbon groups were observed. The sum of the corrected yields of 2-methyl-1,3-dioxolane, ethylene glycol monoformate, and ethylene glycol monoacetate yields 47 ( 8% and indicates the importance of OH abstraction at the first secondary carbon group. The corrected primary yield of ethyl formate represents the abstraction from the second CH2 group that was estimated above to be 34 ( 10%. The corrected yield of ethoxyacetaldehyde was found to be 24 ( 13%, which corresponds to the OH attack at the third CH2 group. Thus, the observed product distributions are in reasonable agreement with the SAR predictions.

Acknowledgments The authors thank the Schweizerische Nationalfonds zur Fo¨rderung der wissenschaftlichen Forschung for financial support and Dr. C. Geel of Dow Europe, Horgen, Switzerland, for supplying technical information on glycol ethers.

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Received for review April 16, 1996. Revised manuscript received June 14, 1996. Accepted June 17, 1996.X ES960348N X

Abstract published in Advance ACS Abstracts, September 15, 1996.

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