2114
J. Phys. Chem. 1996, 100, 2114-2116
Room Temperature Rate Coefficients for the Reactions of OH Radicals with Some Monoethylene Glycol Monoalkyl Ethers Konrad Stemmler, David J. Kinnison, and J. Alistair Kerr* EAWAG, Swiss Federal Institute for EnVironmental Science and Technology, ETH Zu¨ rich, CH-8600 Du¨ bendorf, Switzerland ReceiVed: July 19, 1995; In Final Form: October 26, 1995X
Rate coefficients for the reactions of OH radicals with monoethylene glycol monoalkyl ethers have been determined by the competitive method at 297 ( 3 K. Relative to the value kOH+hexanol ) 12.5 × 10-12 cm3 molecule-1 s-1, the rate coefficients are as follows (in units of 10-12 cm3 molecule-1 s-1): methoxyethanol, 10.8 ( 1.1; ethoxyethanol, 14.5 ( 0.4; propoxyethanol, 16.4 ( 0.7; butoxyethanol, 19.4 ( 2.0. In a second set of experiments relative rate coefficients were measured for the same series of glycol ethers (TST) against heptanol. In all cases the values of kTST/kheptanol divided by kTST/khexanol yielded values of kheptanol/khexanol consistent with our experimentally determined value of kheptanol/khexanol ) 1.24 ( 0.08. The results are discussed with respect to literature data and structure activity relationships, and the tropospheric lifetimes of the glycol ethers are estimated.
I. Introduction Glycol ethers are used extensively as solvents and as chemical intermediates and undergo evaporative losses to the atmosphere. There is very little experimental information on the atmospheric fate of these molecules apart from a few studies of some OH rate coefficients.1-3 The present study of the OH rate coefficients for the short series of ethylene glycol monomethyl to monobutyl ethers, ROCH2CH2OH, was undertaken (i) as a forerunner of product and mechanistic studies of the photooxidations of this type of molecule under atmospherically related conditions and (ii) to extend the data base of OH radical rate coefficients and hence to refine structure activity relationships. The OH radical rate coefficients have been studied by the competitive technique involving measurement of the rates of consumption of the substrate molecules by OH radicals relative to the rates of consumption in separate experiments with both hexanol and heptanol as reference compounds. II. Experimental Section Apparatus. The relative rate measurements were carried out in an experimental system that has been described previously.4-6 Hydroxyl radicals were produced from the photolysis of methyl nitrite in air:
CH3ONO + hν f CH3O + NO
(1)
CH3O + O2 f CH2O + HO2
(2)
HO2 + NO f OH + NO2
(3)
III. Results
The kinetic experiments were carried out at room temperature (297 ( 3 K) and at atmospheric pressure (760 ( 10 Torr) in a 200 dm3 Teflon bag surrounded by 16 black lamps (Philips L20/ 05). These lamps provided 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. The mixtures were prepared by sweeping measured amounts of methyl nitrite, glycol ether, and X
the reference compound from a calibrated volume into the Teflon bag with a stream of synthetic air. Pressures were measured with a capacitance manometer, MKS Baraton 220C. In general, the bag was filled up to 200 dm3 with synthetic air and nitric oxide was added to minimize the formation of O3 and NO3 radicals. Once the gas mixture was prepared, the bag was agitated and left to stand for 1 h to thoroughly mix the reactants. The bag mixture was irradiated for approximately 2 h during which time samples of the mixture were periodically removed and the glycol ether and the reference compound analyzed 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 DB-Wax fused silica column (J&W Scientific) with a 0.32 mm internal diameter and a 0.25 µm film thickness operated with temperature programming from 313 to 393 K. Materials. The synthetic air (Pangas) was a mixture of 20% O2 and 80% N2. Nitric oxide (998 ( 2 ppm) in N2 was supplied by Carbagas. Methyl nitrite was prepared from methanol and nitrous acid.7 It was transferred to the high-vacuum line, purified by bulb-to-bulb distillation, and stored in the dark at 77 K. The following chemicals were used without further purification other than bulb-to-bulb distillation: 2-methoxyethanol (Fluka, >99.5%), 2-ethoxyethanol (Merck, >99.5%), 2-propoxyethanol (Eastman, 98%), 2-butoxyethanol (Scheller, >99%), hexanol, and heptanol (Fluka, >99%).
Abstract published in AdVance ACS Abstracts, January 1, 1996.
0022-3654/96/20100-2114$12.00/0
The relative rate measurements of OH reactions with the glycol ethers (TST) were carried out at least 3 times against hexanol (REF1) as the reference substance:
OH + TST f products
(4)
OH + REF1 f products
(5)
In order to test for internal consistency, each compound was also reacted against a second reference substance, heptanol (REF2), in three separate experiments:
OH + REF2 f products © 1996 American Chemical Society
(6)
Rate Coefficients for the Reactions of OH Radicals
J. Phys. Chem., Vol. 100, No. 6, 1996 2115 TABLE 1: Rate Coefficient Ratiosa Measured in the Present Work and Internal Consistency Check test substrate (TST)
(kTST/kREF1)b
(kTST/kREF2)b
ratiob,c kREF2/kREF1
CH3OCH2CH2OH C2H5OCH2CH2OH C3H7OCH2CH2OH C4H9OCH2CH2OH
0.864 ( 0.086 1.159 ( 0.034 1.309 ( 0.055 1.55 ( 0.16
0.725 ( 0.062 0.921 ( 0.098 1.083 ( 0.081 1.30 ( 0.15
1.19 ( 0.16 1.25 ( 0.14 1.21 ( 0.10 1.19 ( 0.18 (1.24 ( 0.08)d
Error limits are for 95% confidence intervals. b REF1 ) hexanol; REF2 ) heptanol. c See text. d Ratio measured in this work. a
are consumed solely by the OH radical, and furthermore, it indicates that complications arising from product interference in the gas chromatographic analyses are unlikely to be significant. The rate coefficient ratios derived from eq III were converted to absolute rate coefficients on the basis of the evaluated value8 of kOH+hexanol ) 12.5 × 10-12 cm3 molecule-1 s-1 at 298 K and are shown in Table 2. The quoted errors are for 95% confidence limits and reflect only the precision of the results. The error limits in the evaluated rate coefficient for hexanol8 add a further 35% to the uncertainty in the absolute rate coefficients reported in this study. Figure 1. Sample plots of ln([TST]0/[TST]t) for the reaction of OH with following test compounds (TST): (×) 2-methoxyethanol, (0) 2-ethoxyethanol, (9) 2-propoxyethanol, and (O) 2-butoxyethanol vs ln([REF]0/[REF]t) for hexanol.
The ratios of rate coefficients for the reactions of OH with hexanol and heptanol were measured directly in four individual runs. Assuming that there are no loss processes of the test (TST) and reference compounds (REF) other than the reaction with OH radicals, the rate expressions for these processes
-d[TST]/dt ) kTST[OH][TST]
(I)
-d[REF]/dt ) kREF[OH] [REF]
(II)
can be integrated and combined to give the following equation
ln([TST]0/[TST]t) ) (kTST/kREF)ln([REF]0/[REF]t)
(III)
where the subscripts 0 and t indicate concentrations at the beginning of the experiment and at time t, respectively. A plot of ln ([TST]0/[TST]t) vs ln([REF]0/[REF]t) thus yields the rate coefficient ratio kTST/kREF. The synthetic air mixtures contained 17-30 ppm of methyl nitrite and 20 ppm of nitric oxide. The concentrations of the glycol ethers were in the range 1.5-3.5 ppm, and the concentrations of the reference alcohols hexanol and heptanol were in the ranges 1.7-2.6 and 0.6-0.9 ppm, respectively. The rate coefficient ratios were determined from plots according to eq III by a linear least-squares fit. Typical plots of the present data are shown in Figure 1 for the glycol ether-hexanol measurements. The rate coefficient ratios found in this study and the results of the internal consistency test are shown in Table 1, where the errors quoted are for 95% confidence limits. The ratio kREF2/ kREF1 shown in the fourth column of Table 1 was calculated in each case by dividing the measured ratio kREF2/kTST by the measured ratio kREF1/kTST. The experiments carried out by having heptanol compete against hexanol yielded a measured ratio of kREF2/kREF1 ) kheptanol/khexanol ) 1.24 ( 0.08. For each of the glycol ethers studied the ratio kREF2/kREF1 calculated from the two sets of relative rate measurements is equal within experimental error to the directly measured ratio. This level of internal consistency, taken together with the linear ln-ln plots with zero intercepts, makes it seem reasonable to assume that the substrates in these experiments
IV. Discussion There have been no previous experimental determinations of the ratio kheptanol/ khexanol, which we have used as a check for internal consistency in this study, but a value of kheptanol/ khexanol ) 1.10 ( 0.54 can be calculated from the absolute rate coefficients recommended by Atkinson.9 This calculated value overlaps with our measured value of kheptanol/ khexanol ) 1.24 ( 0.08 within the recommended error limits, which are large for the ratio calculated from the absolute rate coefficients compared to those for the directly measured ratio. To the best of our knowledge, there are only three other studies reported for the reaction of OH with glycol ethers,1-3 which are summarized in Table 2. By comparison of the present data to the literature values, some of the results show good consistency while in other cases the agreement is rather poor. Since all of the data reported in Table 2 show reasonable precision, it is clear that the lack of agreement between the different measurements indicates that systematic errors are present in some or all of the measurements. For methoxyethanol the data from Dagaut et al.2 and Porter et al.3 are in good agreement with the present result. The situation is less satisfactory for ethoxyethanol where our result is in line with that of Hartmann et al.,1 but both are ≈25% lower than the absolute rate coefficient of Dagaut et al.,2 which is supported by the preliminary report of absolute and relative data by Porter et al.3 More work is needed on the ethoxyethanol reaction to resolve the discrepancy. No previous data have been reported for propoxyethanol. For butoxyethanol our rate coefficient is close to the mean of the results of Hartmann et al.1 and Dagaut et al.,2 but again, the spread of results is less than satisfactory. In support of the accuracy of the present data, we point out that the rate coefficients show good internal consistency. The fact that our data are close to those predicted by the structure activity relationship (SAR) developed by Atkinson9 (see Table 2) may be fortuitous since the SAR does not account for longrange activating effects. A comparison of the reactivity of OH toward glycol ethers, alkanes, alcohols, and alkyl ethers is shown in Table 3. The rate coefficients for the reaction of OH with each series of compounds are compared by calculating the difference in the rate coefficients between the selected member of the series and the first member of the series, i.e., R ) CH3. It can be seen from Table 3 that the effect of the glycol group appears to
2116 J. Phys. Chem., Vol. 100, No. 6, 1996
Stemmler et al.
TABLE 2: Measured and Predicted Rate Coefficients for the Reactions of OH Radicals with Glycol Ethers at Room Temperature ka test substrate (TST)
this workb,c
SAR predictiond
literature datae
CH3OCH2CH2OH
10.8 ( 1.1
11.2
C2H5OCH2CH2OH
14.5 ( 0.4
15.6
C3H7OCH2CH2OH C4H9OCH2CH2OH
16.4 ( 0.7 19.4 ( 2.0
18.2 19.6
ref
techniquef
12.5 ( 0.7 11 ( 1 12 ( 3 18.7 ( 2.0 19 ( 1 21 ( 1
Dagaut et al.2 Porter et al.3 Hartmann et al.1 Dagaut et al.2 Porter et al.3
FPRF RRg LPRF FPRF RRg PLP-LIF
14 ( 3 23.6 ( 1.6
Hartmann et al.1 Dagaut et al.2
LPRF FPRF
a 10-12 cm-3 molecule-1 s-1. b based on kOH+hexanol ) 12.5 × 10-12 cm3 molecule-1 s-1. c Error limits are for 95% confidence intervals. d Calculated as in ref 9. e Errors are taken from references. f FPRF ) flash photolysis resonance fluorescence; LPRF ) laser photolysis resonance fluorescence; PLP-LIF ) pulsed laser photolysis-laser induced fluorescence; RR ) relative rate. g (n-C3H7)2O or (n-C4H9)2O was used as a reference compound.
TABLE 3: Comparison of the Reactivities in the Linear Aliphatic Chains of Monoethylene Glycol Monoalkyl Ethers and of Alkanes, Alcohols, and Ethersa,b R
∆k in RCH3c
∆k in ROHd
∆k in RORe
CH3 C2H5 C3H7 C4H9 C5H11 C6H13 C7H15
0 0.89 ( 0.18 2.26 ( 0.45 3.68 ( 0.92 5.6 ( 1.3 6.9 ( 1.7 8.4 ( 1.7
0 2.33 ( 0.46 4.6 ( 1.6 7.6 ( 2.7 10.6 ( 3.7 11.6 ( 4.1 12.8 ( 4.5
0 5.1 ( 1.8 7.8 ( 3.1 12.9 ( 4.5 15.1 ( 5.3
∆k in ROCH2CH2OHf 0 3.7 ( 1.4 5.6 ( 2.2 8.6 ( 3.4
a All in units of 10-12 cm3 molecule-1 s-1. b OH rate coefficients are taken from the evaluated data of Atkinson,8 and the errors are from this evaluation. c ∆k ) k(RCH3) - k(C2H6). d ∆k ) k(ROH) k(CH3OH). e ∆k ) 0.5[k(ROR) - k(CH3OCH3)]. f ∆k ) k(ROCH2CH2OH) - k(CH3OCH2CH2OH). The error limits include the evaluated error of the reference compound.
the glycol group on the alkyl C-H bonds is expected to decrease with increasing substitution of the alkyl group up to approximately C5. The glycol group does show a long-range effect, operating over four carbon atoms, that, as previously suggested,10,11 cannot be explained by an inductive effect or by thermodynamic arguments. Unfortunately, no OH reaction rate data for higher (>C4) monoethylene glycol monoalkyl ethers are available to test the expected decreasing influence of the glycol group. The reaction of glycol ethers with OH is expected to be the main removal process in the atmosphere. Other processes such as photolysis, reaction with O3, reaction with NO3, and hydrolysis, as mentioned previously,2 are expected to be unimportant compared to the removal of the glycol ethers by OH. Based on an average OH atmospheric concentration of 106 molecule cm-3 and the rate coefficients for hydroxyl attack reported here, the lifetimes of methoxy-, ethoxy-, propoxy-, and butoxyethanol are 25.7, 19.2, 16.9, and 14.3 h, respectively. Acknowledgment. The authors thank the Schweizerische Nationalfonds zur Fo¨rderung der wissenschaftlichen Forschung for financial support of this work and Dr. C. Geel of Dow Europe, Horgen, Switzerland for supplying samples of the glycol ethers. References and Notes
Figure 2. Plots (best fits by eye) of the increase in the OH rate coefficients at room temperature with increasing aliphatic chain length for a series of alkanes (9), alcohols (+), ethers (b), and monoethylene glycol monoalkyl ethers (2).
activate the alkyl chain to a greater extent than an alcohol functional group but to a lesser degree than an ether functional group, which is perhaps surprising. A plot of the increase in the rate coefficients for the reaction of OH with the chosen organics, at ambient temperature, against their respective number of carbon atoms is shown in Figure 2. The plot for the reaction of OH with alkanes is effectively linear, and the curvature in the other plots is ascribed to the effect of the functional group. With increasing alkyl substitution the effect of the functional group is reduced and reaches a point where the increase in the OH rate coefficient is the same as that observed for an alkane. By analogy with ethers and alcohols,10 the activating effect of
(1) Hartmann, D.; Gedra, A.; Rha¨sa, D.; Zellner, R. Proceedings of the European Symposium on Physico-Chemical BehaVior of Atmospheric Pollutants; Riedel: Dordrecht, The Netherlands, 1987. (2) Dagaut, P.; Liu, R.; Wallington, T. J.; Kurylo M. J. J. Phys. Chem. 1989, 93, 7838. (3) Porter, E.; Locke, G.; Platz, J.; Treacy, J.; Sidebottom, H.; Mellouki, W.; Te´ton, S.; Le Bras, G. Paper presented at Workshop on Chemical Mechanisms Describing Oxidation Processes in the Troposphere, Valencia, Spain, April 25-28 1995. (4) Kerr, J. A.; Stocker, D. W. J. Atmos. Chem. 1986, 4, 263. (5) Eberhard, J.; Mu¨ller, C.; Stocker, D. W.; Kerr, J. A. Int. J. Chem. Kinet. 1993, 25, 639. (6) Eberhard, J.; Mu¨ller, C.; Stocker, D. W.; Kerr, J. A. EnViron. Sci. Technol. 1995, 29, 232. (7) Noyes, W. A. Organic Synthesis CollectiVe; Blatt, A. H., Ed.; Wiley: New York, 1943; Vol. 2, p 108. (8) Atkinson, R. J. Phys. Chem. Ref. Data, Monogr. 2 1994, 132. (9) Atkinson, R. Int. J. Chem. Kinet. 1987, 19, 799. (10) Nelson, L.; Rattigan, O.; Neavyn, R.; Sidebottom, H.; Treacy, J.; Nielsen, O. J. Int. J. Chem. Kinet. 1990, 22, 1111. (11) Wallington, T. J.; Dagaut, P.; Liu, R.; Kurylo, M. J. Int. J. Chem. Kinet. 1988, 20, 541.
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