Rate Coefficients for the Gas-Phase Reactions of Hydroxyl Radicals

Article pubs.acs.org/JPCA

Rate Coefficients for the Gas-Phase Reactions of Hydroxyl Radicals with a Series of Methoxylated Aromatic Compounds Amélie Lauraguais,†,‡,§ Iustinian Bejan,†,∥ Ian Barnes,† Peter Wiesen,† and Cécile Coeur*,‡,§ †

Faculty C − Department of Physical & Theoretical Chemistry, University of Wuppertal, Gauss Strasse 20, D-42119 Wuppertal, Germany ‡ Laboratoire de Physico-Chimie de l’Atmosphère (LPCA), EA 4493, Université du Littoral Côte d’Opale, 32 Avenue Foch, 62930 Wimereux, France § Université Lille Nord de France, Lille, France ∥ Faculty of Chemistry, “Al. I. Cuza” University, Iasi, Romania ABSTRACT: Rate coefficients for the reactions of hydroxyl radicals (OH) with a series of oxygenated aromatics (two methoxybenzene and five methoxyphenol isomers) have been obtained using the relative kinetic method in 1080 and 480 L photoreactors at the University of Wuppertal, Germany. The experiments were realized at 295 ± 2 K and 1 bar total pressure of synthetic air using in situ Fourier transform infrared spectroscopy for the chemical analysis. The following rate coefficients (in units of cm3 molecule−1 s−1) were determined: methoxybenzene (anisole), (2.08 ± 0.21) × 10−11; 1methoxy-2-methylbenzene, (4.56 ± 0.50) × 10−11; 2-methoxyphenol (guaiacol), (5.40 ± 0.72) × 10−11; 3-methoxyphenol, (6.93 ± 0.67) × 10−11; 4-methoxyphenol, (5.66 ± 0.55) × 10−11; 2-methoxy-4methylphenol, (7.51 ± 0.68) × 10−11; 2,3-dimethoxyphenol, (7.49 ± 0.81) × 10−11; and 2,6-dimethoxyphenol (syringol), (8.10 ± 0.98) × 10−11. The rate coefficients for the reactions of OH with 2,3-dimethoxyphenol and 1-methoxy-2-methylbenzene are first time measurements. The rate coefficients determined in this work are compared with previous determinations reported in the literature and also with the values estimated using a structure−activity relationship method. A comparison is performed between the OH rate coefficients obtained for methoxylated aromatics with those of other substituted aromatics in order to understand the influence of the type, number, and position of the different substituents on the reactivity of aromatics toward OH. In addition, a comparison is made between the OH and Cl rate coefficients for the compounds. The principal atmospheric sink of these methoxylated aromatic compounds during daytime is their reaction with OH radicals. The corresponding lifetimes for reaction with OH radicals and Cl atoms are 2−8 and 11−50 h, respectively.

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INTRODUCTION Air pollution is a very serious issue with global implications. It has been shown that in urban areas 20% of the nonmethane hydrocarbons (NMHC) are aromatic hydrocarbons (AH).1 They are known to contribute substantially to the formation of photooxidants2,3 and secondary organic aerosols (SOA)1,4,5 with huge implications for human health.6 Nowadays, environmental policies are being formulated in many countries which encourage the use of renewable energy sources to help to reduce dependence on the ever-diminishing sources of fossil fuels. Biomass combustion is one of the major alternatives; however, biomass burning is also a predominant source of atmospheric particles which are recognized today to have considerable impacts on human health,7 regional and global air quality,8 and climate.9,10 The term “biomass burning” encompasses natural fires, human-initiated burning of vegetation, and residential combustion of wood.11−15 The pyrolysis of wood lignin generates methoxyphenols (mainly 2-methoxyphenol, 2,6-dimethoxyphenol, and their derivatives).11−16 These © 2015 American Chemical Society

oxygenated aromatic compounds can be distributed between the particle and gas phases; however, at typical atmospheric temperatures, they are principally present in the gas phase.11,12,14 Recent studies have exhibited their high reactivity toward hydroxyl radicals (OH)17,18 and chlorine atoms (Cl)19 and also their significant capability to form SOA.18,20 Methoxyphenol derivatives containing unsaturated substituents can also react with ozone, and those with carbonyl groups may be photolyzed by sunlight.21 In addition, the reaction of methoxyphenols with nitrate radicals (NO3) is likely to be fast and could contribute to the atmospheric sinks of these compounds during the night.21 We report relative kinetic room-temperature determinations of the rate coefficients for the reactions of the OH radical with methoxybenzene (MB), 1-methoxy-2-methylbenzene (1-M-2Received: April 3, 2015 Revised: May 19, 2015 Published: May 20, 2015 6179

DOI: 10.1021/acs.jpca.5b03232 J. Phys. Chem. A 2015, 119, 6179−6187

Article

The Journal of Physical Chemistry A

600 cm−1. The White system was run at 82 traverses, with an overall optical path length of 484.7 ± 0.8 m. IR spectra were recorded with a resolution of 1 cm−1 using a Thermo Nicolet Nexus FT-IR spectrometer equipped with a liquid-nitrogencooled mercury−cadmium−telluride (MCT) detector. Duran Glass Reactor (480 L). The reactor consists of a cylindrical vessel in borosilicate glass, which is 3 m long and has an internal diameter of 0.45 m. Both ends are closed by aluminum flanges, which contain ports for the introduction of bath gases and reactants into the chamber. A capacitance manometer and a mixing fan are mounted on one of the flanges. The reactor can be irradiated by 20 superactinic lamps (Philips TLA 40 W/05, λmax = 360 nm, 300 ≤ λ ≤ 450 nm) which are distributed symmetrically around the chamber. The reactor can be evacuated to a pressure of 10−3 mbar by means of a turbo-molecular pump backed by a double-stage rotary fore pump. A White-type mirror system mounted inside the reactor is set to an overall optical path length of 51.6 m and is coupled to an FTIR spectrometer (Nicolet 6700) for the monitoring of infrared spectra at a resolution of 1 cm−1. Experimental Procedure. For the relative rate coefficient investigations on the reactions of OH with the MAs, the photolysis of CH3ONO in the presence of NO and air was used as the OH radical source:

MB), 2-methoxyphenol (2-MP), and a series of structurally related compounds including 3-methoxyphenol (3-MP), 4methoxyphenol (4-MP), 2-methoxy-4-methylphenol (2-M-4MP), 2,3-dimethoxyphenol (2,3-DMP), and 2,6-dimethoxyphenol (2,6-DMP). With the exception of 1-methoxy-2methylbenzene and 2,3-dimethoxybenzene, single room-temperature determinations of the rate coefficients for the reactions of OH with the other compounds have been reported previously in the literature. Coeur-Tourneur et al.17 have reported room-temperature rate coefficients for the reactions of OH with MB, 2-MP, 3-MP, 4-MP, and 2-M-4-MP determined in a 8 m3 Plexiglas reaction chamber using a relative kinetic technique in combination with gas chromatography flame ionization detection for the chemical analysis after collection of samples on Tenax sorbent. Lauraguais et al.18 have reported a rate coefficient for the reaction of OH with 2,6-DMP also obtained in the same 8 m3 reactor using the same kinetic and chemical analysis technique. Finally, Perry et al.22 have reported a rate coefficient for the reaction of OH with MB obtained with the flash photolysis−resonance fluorescence technique. The objective of the present work was to validate the previous single determinations of the rate coefficients using a different chemical analysis technique, i.e., in situ Fourier transform infrared (FTIR) spectroscopy, and also extend the kinetic database for substituted phenolic compounds. The rate coefficients determined in the present work are collated with those from previous studies and also values calculated with a structure−activity relationship (SAR) method.23 The OH reactivity of the investigated methoxylated aromatics (MA) is also compared with that of other substituted aromatic compounds in order to understand the influence on reactivity exerted by the type, number, and position of the different substituents on the aromatics. Finally, atmospheric lifetimes of the MAs with respect to reaction with hydroxyl radicals and chorine atoms are calculated and compared.

CH3ONO + hν → CH3O + NO

(1)

CH3O + O2 → HO2 + HCHO

(2)

HO2 + NO → NO2 + OH

(3)

The duration of the experiments was roughly between 20 and 30 min. Experiments were performed on mixtures of the MA compound under study, a reference compound, and the OH radical precursor CH3ONO in synthetic air at atmospheric pressure and 295 ± 2 K. Small amounts of NO were also added to the chamber to suppress the formation of ozone and enhance the formation of OH radicals. Generally, 30 and 60 interferograms were coadded per recorded infrared spectrum for the studies performed in the 480 and 1080 L reactor, respectively, resulting in around 1 min of scanning. Typically, 5 spectra were monitored in the dark prior to the experiment and 15 spectra with the lamps switched on. In the presence of OH radicals the aromatics and references in the gas mixture will be consumed by the following reactions:

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EXPERIMENTAL SECTION The experiments were performed in a 1080 L quartz glass reaction chamber (QUAREC) and a 480 L borosilicate glass reaction chamber at 295 ± 2 K and 1 bar total pressure of synthetic air. Detailed descriptions of the reactors are available in the literature,24,25 and only the principal specifications for each reactor are given below. QUAREC (1080 L). The reactor consists of a central flange which allows the connection of two quartz glass tubes; it has an overall length of 6.2 m and an internal diameter of 0.47 m. Two aluminum flanges close the reactor at both ends. These flanges contain a number of inlet ports and can be evacuated to a pressure of 10−3 mbar by a turbo-molecular pump system. Reactant homogeneity within the reactor is ensured by three mixing fans with Teflon blades, which are mounted inside the reactor. Irradiation is provided by 32 superactinic fluorescent lamps (Philips TL 05/40 W, λmax = 360 nm, 300 < λ < 480 nm) and 32 low-pressure mercury lamps (Philips TUV40W, λmax = 254 nm), which are placed regularly around the chamber. The lamps can be switched individually, which permits the variation of intensity of the light and consequently also the photolysis frequency and radical production rate within the reaction vessel. The reactor is instrumented with a White type multiplereflection mirror system, with a base length of 5.91 ± 0.01 m, for sensitive in situ long path absorption monitoring of reactants and products in the infrared spectral range of 4000−

MA + OH → Products (kMA )

(4)

Reference + OH → Products (k ref )

(5)

The relative kinetic method relies on the assumption that both the MA and reference are removed solely by reaction with hydroxyl radicals. Preliminary experiments performed in the absence of the radical precursor showed that photolysis of both the studied MAs and reference compounds used in this work was negligible. The tests also showed that while wall losses were negligible for the reference compounds, for the MAs they had to be taken into account in the kinetic data analysis (the wall deposition accounted for 7−20% of the overall loss of the MAs; the highest values being measured in the 480 L chamber). MA( +wall) → Products (k wall)

(6)

Kinetic treatment of reactions 4−6 leads to the following relationship: 6180


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The Journal of Physical Chemistry A ln

[MA]0 [reference]0 k − k wall(t − to) = MA ln [MA]t k ref [reference]t

(I)

where kMA, kref ,and kwall are the rate coefficients for reactions 4, 5 ,and 6, respectively, and the subscripts 0 and t indicate concentrations at the beginning of the reaction, to, and at time t, respectively. Plots of the kinetic data in the form of eq I should yield straight lines with zero intercept and slope kMA/kref. The typical initial concentrations used in the experiments were as follows: MAs and reference compounds, 1.5 ppmV in the 1080 L reactor and 4.5 ppmV in the 480 L reactor; CH3ONO, 2 ppmV in the 1080 L reactor and 10 ppmV in the 480 L reactor; and NO, 0.5 ppmV in the 1080 L reactor and 4 ppmV in the 480 L reactor. The compounds used in this study, their manufacturer, and the manufacturer stated purity were: MB (Acros, 99%); MMB (Acros, 99%); 2-MP (Aldrich, 99%); 3-MP (Alpha Aesar, 97%); 2-M-4-MP (Alpha Aesar, 98%); 2,3-DMP (Aldrich, 99%); 2,6-DMP (Aldrich, 99%); 2-methyl-2-butene (Messer Griesheim, 99%); 1,3-butadiene (Messer Griesheim, 99%); propene (Messer Griesheim, 99.95%); 1-butene (Messer Griesheim, 99%); NO (Messer Griesheim, 99%); and synthetic air (Air Liquide, 99.999%). Methyl nitrite (CH3ONO) was prepared using a method described by Winzor26 and has been used with success previously as precursor for OH radicals in the OH-radical mediated oxidation of more complicated aromatic systems.27,28

Figure 2. Relative rate coefficient plot for the reaction of OH radicals with 1-methoxy-2-methylbenzene (1-M-2-MB). 1-Butene (pink circles) and propene (red triangles) have been used as references. For clarity, data acquired with 1-butene have been displaced vertically by 0.3 units.

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RESULTS AND DISCUSSION The rate coefficient for the reaction of OH with each methoxylated aromatic has been determined relative to that of two or three reference compounds and 3−5 experiments have been performed for each reference compound. The reference compounds used in this study and their 298 K rate coefficients with OH radicals were (k ref, in units of cm3 molecule−1 s−1): propene,29 (2.9 ± 0.6) × 10−11; 1butene,21 (3.15 ± 0.2) × 10−11; 1,3-butadiene,30 (6.93 ± 0.48) × 10−11; and 2-methyl-2-butene,21 (8.72 ± 0.2) × 10−11. The kinetic data obtained for each MA are plotted according to eq I in Figures 1−8. All the plots show good linearity with near zero intercept. The slopes (kMA/kref) derived from the plots and the OH rate coefficients determined from the slopes are

Figure 3. Relative rate coefficient plot for the reaction of OH radicals with 2-methoxyphenol (2-MP). Propene (red triangles), 1,3-butadiene (blue squares), and 2-methyl-2-butene (green diamonds) have been used as references. For clarity, data acquired with propene and 2methyl-2-butene have been both displaced vertically by 0.1 units.

Figure 4. Relative rate coefficient plot for the reaction of OH radicals with 3-methoxyphenol (3-MP). 1-Butene (pink circles) and propene (red triangles) have been used as references. For clarity, data acquired with 1-butene have been displaced vertically by 0.3 units. Figure 1. Relative rate coefficient plot for the reaction of OH radicals with methoxybenzene (MB). 1-Butene (pink circles) and propene (red triangles) have been used as references. For clarity, data acquired with 1-butene have been displaced vertically by 0.3 units.

summarized in Table 1. Values of the rate coefficients from previous determinations as well as those estimated using the 6181


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Figure 8. Relative rate coefficient for the reaction of OH radicals with 2,6-dimethoxyphenol (2,6-DMP). 1-Butene (pink circles) and 1,3butadiene (blue squares) have been used as references. For clarity, data acquired with 1-butene have been displaced vertically by 0.3 units.

Figure 5. Relative rate coefficient plot for the reaction of OH radicals with 4-methoxyphenol (4-MP). 1,3-Butadiene (blue squares) and propene (red triangles) have been used as references. For clarity, data acquired with propene have been displaced vertically by 0.3 units.

in good agreement. Therefore, we prefer to quote final rate coefficients for the reactions which are arithmetic means of the set of the values obtained for that reaction. The indicated error is calculated as the square root of the sum of the squares of the error for each rate coefficient determination with each reference compound. The errors stated in the ratio kMA/kref are twice the standard deviation arising from the linear regression analysis of the data plotted in Figures 1−8. The errors presented for kMA also include the errors for the reference rate coefficients. In the studies of Coeur-Tourneur et al.17 and Lauraguais et al.,18 the errors given for the rate coefficients did not take into account the uncertainties of the reference reaction. Therefore, we have chosen to recalculate these errors and include that of the reference rate coefficient in order to make a more meaningful comparison between the different values. Comparison with Literature Data. To our knowledge, this work represents the first measurement of rate coefficients for the reactions of 1-M-2-MB and 2,3-DMP with hydroxyl radical. However, the rate coefficients obtained for the other MAs can be compared with previous determinations. CoeurTourneur et al.,17 obtained rate coefficients for the reactions of OH radicals with MB, 2-MP, 3-MP, 4-MP, and 2-M-4-MP while Lauraguais et al.18 determined rate coefficient for the reaction of 2,6-DMP with OH radicals. Both studies were performed in a large Plexiglas reaction chamber using a relative kinetic method and gas chromatography with flame ionization detection (GC-FID) for reactant monitoring. Perry et al.22 determined a rate coefficient for the reaction of OH with MB using the flash photolysis−resonance fluorescence technique. The rate coefficient obtained in this work for the reaction of OH radical with MB is in excellent agreement with the values reported by Coeur-Tourneur et al.17 and Perry et al.22 and also with those predicted by the SAR. For the reaction of 1-M-2MB, the rate coefficient is slightly more than twice that of MB. This relatively large increase in the reactivity due to an additional methyl group to MB is not given by the SAR; only a modest increase in the rate coefficient of about 20% is predicted, giving a value which is roughly equivalent to the sum of the rate coefficients for the reactions of OH with MB and toluene. The SAR obviously does not correctly handle resonance electronic effects of the substituents. Unfortunately, experimental measurements of the rate coefficients for the reactions of OH radical with the other 1-methoxy-methyl-

Figure 6. Relative rate coefficient for the reaction of OH radicals with 2-methoxy-4-methylphenol (2-M-4-MP). 1-Butene (pink circles) and propene (red triangles) have been used as references. For clarity, data acquired with 1-butene have been displaced vertically by 0.3 units.

Figure 7. Relative rate coefficient plot for the reaction of OH radicals with 2,3-dimethoxyphenol (2,3-DMP). 1-Butene (pink circles) and 1,3-butadiene (blue squares) have been used as references. For clarity, data acquired with 1-butene have been displaced vertically by 0.3 units.

structure−activity relationship developed by Kwok and Atkinson23 are also shown. As can be seen from Table 1, the OH rate coefficients determined for each MA compound using several references are 6182


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Table 1. Summary of the Rate Coefficients for the Reaction of Methoxylated Aromatics (MAs) with OH Radicals at 295 ± 2 K and Associated Atmospheric Lifetimes Regarding Their Reaction with OH Radicals compound MB 1-M-2-MB 2-MP

3-MP 4-MP

2-M-4-MP 2,3-DMP

2,6-DMP

reference propenef 1-butenef propenef 1-butenef propenef 1,3butadienef 2-methyl-2butenef propeneg 1,3butadieneg propenef 1-butenef propenef 1,3butadienef propenef 1-butenef 1,3butadienef 1-butenef 1,3butadienef 1-butenef

kMA/krefa 0.78 0.60 1.48 1.53 1.68 0.74

± ± ± ± ± ±

0.03 0.03 0.06 0.08 0.04 0.01

kMAb (×10−11 cm3 molecule−1 s−1)

kMA(average) (×10−11 cm3 molecule−1 s−1)

kMAc (SAR) (×10−11 cm3 molecule−1 s−1)

kMAd (literature) (×10−11 cm3 molecule−1 s−1)

τMAe (h)

± ± ± ± ± ±

2.08 ± 0.21

2.23

2.20 ± 0.2417 1.96 ± 0.2422

8.2

4.56 ± 0.50

2.73

5.40 ± 0.72

2.98

2.26 1.89 4.29 4.82 4.87 5.16

0.13 0.16 0.27 0.40 0.27 0.37

0.75 ± 0.02

6.55 ± 0.21

1.64 ± 0.04 0.79 ± 0.02

4.76 ± 0.26 5.50 ± 0.42

3.2

9.80 ± 1.5817

2.5

6.93 ± 0.67 5.66 ± 0.55

2.98

9.50 ± 1.8917

3.0

2.53 ± 0.07 2.43 ± 0.08 1.07 ± 0.06

7.34 ± 0.41 7.67 ± 0.54 7.42 ± 0.64

7.51 ± 0.68

3.98

9.45 ± 1.8217

2.3

2.40 ± 0.03 1.23 ± 0.08

7.56 ± 0.49 8.52 ± 0.81

2.44 ± 0.07

7.67 ± 0.54

0.07 0.09 0.05 0.03

6.82 7.04 5.19 6.12

± ± ± ±

7.53 ± 1.2817

0.41 0.54 0.45 0.47

2.35 2.24 1.79 0.88

± ± ± ±

3.7

20.1

7.49 ± 0.81

20.2

8.10 ± 0.98

16.5

2.3

9.66 ± 1.5818

2.1

a

kMA = kmethoxylated aromatic ; kref = kreference. bThe indicated errors are a combination of the error of the reference rate coefficient and twice the standard deviation arising from the linear regression analysis. cCalculated using a structure−activity relationship.23 dPrevious determinations available in the literature; the superscript gives the literature reference number. The indicated errors have been recalculated to take into account the uncertainties on the reference compounds. eLifetime in hours: τMA = 1/kMA[OH], where the 12 h daily average [OH] = 1.6 × 106 molecules cm−3.31 fThe experiments were performed in the 480 L reactor. gThe experiments were performed in the 1080 L reactor.

benzene isomers are not available in the literature. If they become available they will help to show what influence the relative positions of the two groups will have on the reactivity of the different isomers toward OH and also possibly give some insight into the electronic resonance effects of the substituents. For the three methoxyphenol isomers studied in this work, the trend in the OH rate coefficients is the same as that observed by Coeur-Tourneur et al.17 Although the values obtained in this work are systematically lower than those of Coeur-Tourneur et al. by around 30%, they agree within the combined error limits given for the determinations (noting that the uncertainties in the Coeur-Tourneur et al. values have been recalculated to take into account the errors of the reference reactions). A relative kinetic method has been used in this study and also the study of Coeur-Tourneur et al.;17 however, there are several differences in the analytical methods used in the studies. In this study, the compounds were monitored in situ in the infrared, whereas in the study of Coeur-Tourneur et al.17 the measurements were performed off-line with the samples adsorbed on Tenax and subsequently desorbed and measured by GC-FID. Both methods used multiple reference compounds for the measurements; however, it is unfortunate that a common reference compound was not used in both studies because this might have given some indication as to the cause for the differences in the rate coefficient values. Although the uncertainties in the OH rate coefficients for the reference compounds add to the uncertainty in the rate coefficients obtained for the reaction of OH with the methoxyphenols, the internal consistency in the results with different reference

compounds in both this and the study of Coeur-Tourneur et al. suggest that the choice of rate coefficient is probably not the reason for the differences. The SAR method under predicts the rate coefficients for the reactions of OH with 2-MP and 4-MP. With our results, the prediction is good to less than a factor of 2, and considering the large uncertainties in the SAR method for aromatic compounds, this level of agreement can probably be considered to be satisfactory. The SAR, however, over predicts the rate coefficient for the reaction of OH with 3-MP in our case by a factor of 3 and for the measurement of Coeur-Tourneur et al.17 by a factor of 2. Of the three methoxyphenol isomers, 3MP is the one with the largest number of activated sites for OH electrophilic addition; therefore, it should be the most reactive toward OH. Its rate coefficient with OH is, however, only ∼20% higher than those of the two other isomers with OH. The rate coefficient for the reaction of OH with 2-M-4-MP is also 30% smaller than that reported by Coeur-Tourneur et al.,17 but both values agree within the range of uncertainties. As discussed above for the methoxyphenol isomers, the reason for this difference is presently not known. The SAR method again under predicts the OH rate coefficient, and the agreement between prediction and the measurement of the rate coefficient, as determined in this study, is less than a factor of 2. The SAR predicts a higher reactivity of ∼25% for the reaction of OH with 2-M-4-MP compared to 2-MP, which is approximately what is observed in the measurements. The rate coefficient for the reaction of 2,3-dimethoxyphenol with OH radical is slightly lower than for its reaction with 2,66183


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Table 2. Comparison of the Rate Coefficients for the Reactions of a Range of Methylated, Methoxylated, and Hydroxylated Aromatic Compounds with OH Radicals and Cl Atoms compound

kOH (×10−11 cm3 molecule−1 s−1)

methylbenzene (toluene) methoxybenzene (anisole) hydroxybenzene (phenol) 1,2-dimethylbenzene (o-xylene) 1,3-dimethylbenzene (m-xylene) 1,4-dimethylbenzene (p-xylene) 1-methoxy-2-methylbenzene (2-methylanisole) 2-methoxyphenol (guaiacol) 3-methoxyphenol 4-methoxyphenol 2-methylphenol (o-cresol) 3-methylphenol (m-cresol) 4-methylphenol (p-cresol) 2,3-dimethylphenol 2,4-dimethylphenol 2,5-dimethylphenol 2,6-dimethylphenol 3,4-dimethylphenol 3,5-dimethylphenol 2-methoxy-4-methylphenol 2,3-dimethoxyphenol 2,6-dimethoxyphenol (syringol) 1,2-dihydroxy-3-methylbenzene 1,2-dihydroxy-4-methylbenzene

0.60 2.08 2.70 1.36 2.31 1.43 4.56 5.44 6.93 5.66 4.30 5.9 5.0 8.3 7.4 8.8 6.7 8.3 11.4 7.51 7.49 8.10 20.5 15.6

ref 1 this 1 1 1 1 this this this 17 33 33 33 34 34 34 34 34 34 this this this 35 35

work

work work work

work work work

kCl (×10−11 cm3 molecule−1 s−1)

ref

6.2a 10.7 19.3 14.0a 13.5a 14.4a 12.0 29.7 29.6 28.6 11.3 30.3 31.3 17.7b 17.7b 17.7b 17.7b 31.7c 31.7c 33.5 47.3 27.1 64.9 64.3

36 19 37 36 36 36 19 19 19 19 38 38 38

19 19 19 39 39

a

Rate coefficient is the average of all reported values. bEstimation from the sum of the rate coefficients for Cl with ortho-cresol and toluene; orthocresol has been chosen for the estimation to take hydrogen bonding between OH and CH3 into consideration. cEstimation from the amount of the rate coefficient for Cl with phenol and twice that for Cl with toluene.

the monosubstituted aromatics listed in Table 2, the activation of the ring toward electrophilic OH addition by the different substituent groups takes the order −CH3 ≪ −OCH3 < −OH. However, if we look at the rate coefficients for the reactions of OH with disubstituted aromatics such as o-xylene (1,2dimethylbenzene; 1.36 × 10−11 cm3 molecule−1 s−1);1 2methylanisole (1-methoxy-2-methylbenzene; 4.56 × 10−11 cm3 molecule−1 s−1); and o-cresol (1-hydroxy-2-methylbenzene; 4.30 × 10−11 cm3 molecule−1 s−1),33 the effect of the substituents on electrophilic addition of OH to the aromatic ring takes the order −CH3 ≪ −OH < −OCH3. Moreover, with the exception of 3,5-dimethylphenol, all the available rate coefficients for the reactions of OH with the isomers of dimethylphenol and dimethoxyphenol are between 7 × 10−11 and 9 × 10−11 cm3 molecule−1 s−1, suggesting that for these trisubstituted aromatics the activating effects of the methyl and methoxy groups are quite similar. The rate coefficient for OH with 3,5-dimethylphenol is somewhat higher than that for the other isomers, and this is probably due to the fact that this is the isomer with the greatest activation toward electrophilic OH radical reaction. Moreover, the recent computational study of Sandhiya et al.40 indicates that reaction with OH could also occur via H atom abstraction from the aromatic ring of the 3,5dimethylphenol isomer. They argue that the methyl substituent placed in the third position acts as a para director and leads the OH radical to react essentially with the H atom attached to C4atom of the aromatic ring. Rate coefficients are unavailable for the reactions of OH with 3,5-dimethoxyphenol and 3-methyl-5methoxyphenol, but on the basis of the available kinetic information, we would expect them to be just as reactive toward OH as 3,5-dimethylphenol.

dimethoxyphenol; however, the values can be considered the same within the experimental error limits. The rate coefficient obtained in this study for the reaction of OH with 2,6dimethoxyphenol is somewhat lower than that reported by Coeur-Tourneur et al.17 The SAR overestimates the OH rate coefficient for both 2,3- and 2,6-DMP and predicts that 2,3DMP should react faster with OH than 2,6-DMP. The presence of the second methoxy group in 2,3-DMP and 2,6-DMP has led to increases of ∼28% and 33% in the OH rate coefficient, respectively, compared to that for 2-MP. For di- or trisubstituted aromatics, the SAR method over or under estimates the rate coefficients by factors of 2 or 3. This observation has already been reported for multifunctional organic compounds,17,32 and more OH rate coefficient determinations of polyfunctional aromatics, ideally in combination with theoretical studies, need to be made to improve the SAR rate coefficient predictive method for aromatics. Trends in Reactivity. To obtain a better understanding of the atmospheric reactivity of MAs, the rate coefficients regarding their reactions with hydroxyl radicals have been compared with those for other substituted aromatics. The rate coefficients for the reactions of OH with a variety of methyl-, methoxy-, and hydroxy-substituted aromatics are indicated in Table 2. The corresponding rate coefficients for the reaction of the compounds with chlorine atoms, when available in the literature, are also given in Table 2. The reaction of the OH radical with aromatic compounds is known to operate mainly by OH addition to the aromatic ring with relatively minor contributions from H atom abstraction from any hydrogen-containing ring substituents.1 As can be inferred from the rate coefficients for the reactions of OH with 6184


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with Cl, where 2-methylphenol has the lowest reactivity because of H atom bonding between the hydroxyl and methyl substituent. The reactivity of Cl toward 3-MP and 4-MP isomers are approximately the same, whereas that of 2-MP is more than a factor of 2 lower because of the hydrogen bonding.38 Although for the reaction of Cl with 2,6-DMP steric hindrance has been invoked to explain its relatively low reactivity toward Cl, in the case of 2-MP hydrogen bonding between the CH3 and OH substituents is more likely to be the major reason for the reduced reactivity of 2-MP toward Cl compared to the other isomers. This hydrogen bonding effect has been taken into consideration when estimating the rate coefficients for the reactions of the dimethylphenol isomers with Cl given in Table 2. Atmospheric Lifetimes of Methoxylated Aromatics. The rate coefficients determined in this study can be used to calculate the atmospheric lifetime of methoxylated aromatic compounds regarding their reactions with OH. Assuming a 12 h average value for [OH] = 1.6 × 106 molecules cm−3,31 lifetimes of the compounds for their reaction with OH have been calculated and are listed in Table 1. The lifetimes for the methoxybenzenes and methoxyphenols are in the ranges of 4− 8 h and 2−3 h, respectively. In the recent work of Lauraguais et al.,19 rate coefficients for the reactions of Cl atoms with a series of MA compounds were reported. Using a daytime concentration of Cl atoms of 5 × 104 atoms cm−3 (ref 41) as representative for elevated levels in the marine boundary layer and some polluted urban regions, they calculated atmospheric lifetimes of 46−50 h for methoxybenzenes and 11−21 h for methoxyphenols. These data indicate that although the rate coefficients for the reactions of Cl atoms with MAs are 2−5 times higher than those with OH radicals, the principal diurnal sink of these oxygenated aromatics is their reaction with OH even in areas with elevated levels of Cl. It is known that the reactions of NO3 with phenol1 and cresols1 are in the range of (0.4−1.5) × 10−11 cm3 molecule−1 s−1, so it seems reasonable to assume that the reaction of methoxyphenols with NO3 could also be an important sink for these compounds during the night. To the best of our knowledge there are currently no rate coefficients for the reactions of NO3 radicals with methoxyphenols. However, Liu et al.16 have studied the heterogeneous reaction of nitrate radicals with particulate methoxyphenols and shown that these reactions lead to nocturnal lifetimes of about 3 h for methoxyphenols. A rapid reaction of NO3 with methoxyphenols would involve an efficient nighttime oxidation of these compounds in both the gas and particle phases. On the basis of the potentially high contribution to their nighttime oxidation, kinetic and mechanistic investigations on the gas-phase reactions of nitrate radicals with methoxyphenols seem warranted.

In light of the observations above, it can be concluded that it is very difficult to estimate the OH rate coefficients of polysubstituted aromatics if only the activating effects of the different substituent groups on the aromatic ring are taken into account. Other factors apparently have to be considered, such as steric factors and possibly intramolecular bonding between substituents, which are not included in the SAR method. Such effects can at least partially explain why this semiempirical method does not provide accurate predictions of the rate coefficients for the reaction of OH with polysubstituted aromatics. Comparison between OH and Cl Reactions with Aromatics. In contrast to OH radicals, which react mainly by addition to the aromatic ring, Cl reacts with aromatics via H atom abstraction from hydrogen-containing substituents on the ring or in some cases, via Cl ipso addition to a ring substituent such as −NO2.38 It is well-established that the rate coefficients for the reaction of Cl with alkanes and alkenes are an order of magnitude higher or more than those of the corresponding reactions with OH and that the reaction mechanisms are similar. As illustrated in Table 2, the Cl atom is still more reactive toward the listed aromatic compounds than OH even though the reaction mechanisms are completely different. The magnitude of the difference in reactivity is, however, in most cases much less than that observed between the reactions of Cl and OH with alkanes and alkenes.1 Although the rate coefficients for the reactions of OH radicals and Cl atoms with aromatics show the same tendency, i.e., with the exception of nitroaromatics,27,38 the reactivity increases with the number of substituents on the aromatic ring,28 the trends in reactivity, because of the different mechanism, do not always exactly parallel one another. This is due to a number of factors including the position of the substituents on the aromatic ring, hydrogen bonding, and steric factors and is illustrated here in a few examples. The reactivity of 2,3-DMP and 2,6-DMP toward OH radicals is very similar to kOH = (7.49 ± 0.81) × 10−11 and (8.10 ± 0.98) × 10−11 cm3 molecule−1 s−1, respectively. This is not the case for their reactivity toward chlorine atoms where 2,3-DMP has a rate coefficient (in 10−11 cm3 molecule−1 s−1 units) of 47.3 ± 1.06 and 2,6-DMP a rate coefficient of 27.1 ± 0.61; i.e., the 2,6isomer is about 1.7 times less reactive than the 2,3-isomer. Lauraguais et al.19 have suggested that the difference in the reactivity toward Cl atoms between the two dimethoxyphenol isomers can be explained by steric factors. In 2,6-DMP access of Cl to the H atom of the hydroxyl group is relatively hindered, which could explain its lower reactivity toward Cl than that of 2,3-DMP. In the case of the OH reaction, these steric effects have no influence as the 3 sites for the OH addition on 2,6DMP and 2,3-DMP are unhindered and are similarly activated by the −OH and −OCH3 substituents. The difference between the reactivity of OH and Cl toward the aromatics can also be influenced by the position of the different substituent groups. For instance, the reactivity of the methoxyphenol isomers toward OH takes the order k(2‑MP) < k(3‑MP) < k(4‑MP) while the reactivity for all these methoxyphenols toward Cl is very similar. The same observation can be made for the xylene isomers; whereas their OH rate coefficients vary according to the position of the methyl groups, Wang et al.36 did not notice any significant differences in the Cl rate coefficients for the xylene isomers. Finally, if we look at the methylphenol isomers (cresols), the reactivity of the isomers toward OH is close in contrast to that

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AUTHOR INFORMATION

Corresponding Author

*Tel: +33 321996405. Fax: +33 321996401. E-mail: coeur@ univ-littoral.fr. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the Nord-Pas de Calais Regional Council and the University of Wuppertal, which provided the 6185


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experimental facilities for the work. I.B. acknowledges the European Commission for support through the EUROCHAMP2 project.

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Rate Coefficients for the Gas-Phase Reactions of Hydroxyl Radicals

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