J. Phys. Chem. A 2010, 114, 11645–11650
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Rate Coefficients for the Gas-Phase Reaction of Hydroxyl Radicals with 2-Methoxyphenol (Guaiacol) and Related Compounds Ce´cile Coeur-Tourneur,*,† Andy Cassez,† and John C. Wenger‡ Laboratoire de Physico-Chimie de l’Atmosphe`re (LPCA), EA 4493, CNRS, UniVersite´ du Littoral Coˆte d’Opale, 32 AVenue Foch, 62930 Wimereux, France, and Department of Chemistry and EnVironmental Research Institute, UniVersity College Cork, Cork, Ireland ReceiVed: July 29, 2010; ReVised Manuscript ReceiVed: September 2, 2010
2-Methoxyphenol (guaiacol) and its derivatives are potential marker compounds for wood smoke emissions in the atmosphere. To investigate the atmospheric reactivity of this type of compounds, rate coefficients for their reactions with hydroxyl (OH) radicals have been determined at 294 ( 2 K and 1 atm using the relative rate method with gas chromatography for chemical analysis. The rate coefficients (in units of cm3 molecule-1 s-1) are: 2-methoxyphenol, (7.53 ( 0.41) × 10-11; 3-methoxyphenol, (9.80 ( 0.46) × 10-11; 4-methoxyphenol, (9.50 ( 0.55) × 10-11; 2-methoxy-4-methylphenol, (9.45 ( 0.59) × 10-11; and methoxybenzene, (2.20 ( 0.15) × 10-11. The estimated atmospheric lifetime for 2-methoxyphenol is ∼2 h, indicating that it is too reactive to be used as a tracer for wood smoke emissions. The reactivity of the methoxyphenols is compared with other substituted aromatics and interpreted in relation to the type, number, and positions of the different substituents on the aromatic ring. The atmospheric implications of the reactions are also discussed. Introduction
Experimental Section
Methoxyphenols form a significant component of biomass burning emissions arising from natural fires, human-initiated burning of vegetation, and residential wood combustion.1–5 They are produced from the pyrolysis of wood lignin and mainly consist of derivatives of 2-methoxyphenol (guaiacol) and 2,6-dimethoxyphenol (syringol).1–5 Under typical ambient temperatures, 2-methoxyphenol and its derivatives are found almost exclusively in the gas phase2–4 and can therefore be chemically transformed via gasphase atmospheric degradation processes. As a result, the atmospheric lifetimes and hence the effectiveness of methoxyphenols as stable molecular tracers for wood smoke emissions are governed by the rates of their atmospheric degradation. The gas-phase atmospheric degradation of methoxyphenols is controlled by reaction with hydroxyl (OH) radicals during the day and nitrate (NO3) radicals at night.6 For methoxyphenol derivatives containing unsaturated substituents, reaction with ozone can also be important, whereas derivatives containing the carbonyl functional group may also be susceptible to direct photolysis by sunlight.6 The rate coefficient for the reaction of methoxybenzene with OH radicals has been reported by Perry et al.;7 however, to the best of our knowledge, there is no available information in the literature on the atmospheric reactivity of methoxyphenols. The aim of this work was to determine rate coefficients for the reaction of OH radicals with 2-methoxyphenol and a series of structurally related compounds including 3-methoxyphenol, 4-methoxyphenol, 2-methoxy-4methylphenol, and methoxybenzene. The reactivity of these methoxyphenols is compared with that of other aromatic compounds, and the rate coefficients are also used to calculate atmospheric lifetimes for reaction with OH radicals. * To whom correspondence should be addressed. Tel: +33 321996405. Fax: +33 321996401. E-mail:
[email protected]. † Universite´ du Littoral Coˆte d’Opale. ‡ University College Cork.
Experiments were performed in a 8 m3 (2 m × 2 m × 2 m) evacuable Plexiglas reaction chamber at room temperature (294 ( 2 K) and atmospheric pressure (1 atm) of dry purified air (relative humidity 3-6%). The chamber is illuminated with 10 fluorescent tubes (Philips TLD 58W/965 58 W) and fitted with a fan to ensure homogeneous mixing of reactants. Before each experiment, the chamber is flushed for at least 12 h with purified air (Dominick Hunter LGCAD 140). A detailed description of the chamber and its operation is provided elsewhere.8,9 Rate coefficients for the reactions of the methoxylated aromatics with OH radicals were determined using a relative rate method in which the decay rates of the compounds were measured relative to that of a reference organic compound. The organic compounds were introduced to the reaction chamber using an inlet system in which measured amounts of the substances were gently heated in a small flow of purified air. Photolysis of methyl nitrite was used as the OH radical source:
CH3ONO + hν
(λ ) 300-450 nm) f CH3O + NO
CH3O + O2 f HO2 + HCHO HO2 + NO f NO2 + HO Methyl nitrite was generated by the dropwise addition of sulfuric acid to a stirred solution of sodium nitrite in methanol and flushed into the chamber with purified air. Nitric oxide was also added to the chamber via a gas-tight syringe to prevent the formation of ozone and thus nitrate radicals, which may also react with the methoxylated aromatics and reference compounds.10 The initial mixing ratios of the different reactants were (in ppmv): [methoxylated aromatic] ) (0.5-2.5); [reference] ) (0.4-0.7), and [NO] ) (1.0-1.2). After allowing 20 min for mixing of the reactants, the lights were turned on to initiate the generation of OH and oxidation of the organic compounds. Reactions were typically
10.1021/jp1071023 2010 American Chemical Society Published on Web 10/04/2010
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Figure 1. Relative rate plot for the reaction of OH radicals with methoxybenzene at 294 ( 2 K. For reasons of clarity, one of the plots has been displaced vertically by 0.5 units.
performed for 1.5-4 h. Experiments were performed on the methoxylated aromatics separately using two reference compounds. At least two (and in most cases three) runs were carried out with each methoxylated aromatic-reference combination. We investigated the possibility of wall loss and photolysis of the methoxylated aromatics and reference compounds by leaving the compounds in the chamber for up to 5 h and monitoring their decay in the presence and absence of light. Wall loss and photolysis rates for the methoxylated aromatics and reference compounds (β-pinene, 1,2-dimethylbenzene, 1,4-dimethylbenzene, 3-methylphenol, and 1,3,5-trimethylbenzene) were found to be negligible. The concentrations of the methoxylated aromatics and reference compounds were determined by gas chromatography with flame ionization detection (Perkin-Elmer, Turbomatrix-GC-FID). Samples were collected for 1-6 min on stainless steel tubes filled with Tenax TA (60-80 mesh) at regular intervals (6-30 min) before and during the experiments. Typically 8-10 samples were collected during each experiment. The contents of the tubes were then thermally desorbed onto a 30 m DB-5 Megabore fused silica capillary column held at 313 K for 5 min and then programmed to 523 K at 5 K min-1. The compounds used in this study, their manufacturer, and stated purity were methoxybenzene (Acros, 99%), 2-methoxyphenol (Alpha Aesar, 98%), 3-methoxyphenol (Alpha Aesar, 97%), 4-methoxyphenol (Alpha Aesar, 98%), 2-methoxy-4-methylphenol (Alpha Aesar, 98%), β-pinene (Fluka, 99%), 1,2-dimethylbenzene (Merck, 99%), 1,4-dimethylbenzene (Merck, 99%), 3-methylphenol (Merck, 99%), 1,3,5-trimethylbenzene (Acros, 99%), NO (Air Liquide, 99.9%), sulfuric acid (VWR 50% in water), methanol (VWR, 99.8%), and sodium nitrite (Acros, 98%). Results We determined rate coefficients for the reactions of the methoxylated aromatics with OH by comparing the rates of decay of the reactants relative to that of selected reference compounds:
methoxylated aromatic + OH f products
(1)
reference + OH f products
(2)
Kinetic treatment of these reactions yields the following relationship
ln
[methoxylated aromatic]0 k1 [reference]0 ) ln [methoxylated aromatic]t k2 [reference]t
(I)
where k1 and k2 are the rate coefficients for reactions 1 and 2 and the subscripts 0 and t indicate concentrations at the start of the reaction and at time t, respectively. Plots in the form of eq I should yield a straight line with zero intercept and slope k1/k2. The rate of reaction of each methoxylated aromatic was compared with that of two reference compounds. The reference compounds used in this study and their rate coefficients for reaction with OH (k2, in units of cm3 molecule-1 s-1) were: 1,2-dimethylbenzene, 1.36 × 10-11; 1,4-dimethylbenzene 1.43 × 10-11; 1,3,5-trimethylbenzene, 5.67 × 10-11; β-pinene, 7.89 × 10-11; 3-methylphenol, 5.88 × 10-11. The reference rate coefficients for the alkylbenzenes and β-pinene were obtained from the evaluations of Calvert et al.,10,11 whereas the value for 3-methylphenol was taken from the recent work of Coeur-Tourneur et al.8 Data generated from the reactions were plotted in the form of eq I and show good linearity with near zero intercepts (Figures 1-5). A summary of the slopes derived from the plots and the calculated OH rate coefficients is provided in Table 1. The indicated errors on k1 are twice the standard deviation arising from the least-squares fit of the data and do include the uncertainty in the reference rate coefficients, k2. Rather than take average values of the rate coefficients determined using the different reference compounds, we have chosen to derive more representative values for k1 by combining the data obtained for each methoxylated aromatic into one plot represented by eq II:
ln
[methoxylated aromatic]0 [reference]0 k2 ) k1 ln [methoxylated aromatic]t [reference]t
(II)
These plots are also linear, with near-zero intercepts and shown in Figures S1-S5 (Supporting Information).
Reaction of Hydroxyl Radicals with 2-Methoxyphenol
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Figure 2. Relative rate plot for the reaction of OH radicals with 2-methoxyphenol at 294 ( 2 K. For reasons of clarity, one of the plots has been displaced vertically by 0.5 units.
Figure 3. Relative rate plot for the reaction of OH radicals with 3-methoxyphenol at 294 ( 2 K. For reasons of clarity, one of the plots has been displaced vertically by 0.5 units.
Discussion Comparison with Literature Data. This study represents the first determination of the rate coefficients for the reaction of OH radicals with the methoxyphenols; therefore, a comparison with the literature is not possible. However, there is good agreement between the values obtained using the different reference compounds. The value obtained for methoxybenzene is in reasonable agreement with the only literature value of (1.96 ( 0.24) × 10-11 cm3 molecule-1 s-1 obtained at 299 K.7 The rate coefficient data obtained in this work can also be compared with the values calculated using the structure activity relationship (SAR) developed by Kwok and Atkin-
son.12 The approach uses experimentally derived data to assign a partial rate coefficient to each reactive site in the molecule and is utilized in the AOP WIN rate coefficient calculator operated by the U.S. Environmental Protection Agency.13 The calculated rate coefficients are also listed in Table 1. Comparison of the predicted and measured rate coefficients indicates that there is only good agreement for methoxybenzene, whereas the predicted values for the methoxyphenol compounds are different from the experimentally determined values by up to a factor of three. The poor performance of the SAR method has been noted for other multifunctional organic compounds14 and appears to be due to inaccurate representation of the electronic effects
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Figure 4. Relative rate plot for the reaction of OH radicals with 4-methoxyphenol at 294 ( 2 K. For reasons of clarity, one of the plots has been displaced vertically by 0.5 units.
Figure 5. Relative rate plot for the reaction of OH radicals with 2-methoxy-4-methylphenol at 294 ( 2 K. For reasons of clarity, one of the plots has been displaced vertically by 0.5 units.
of different functional groups on reactivity. This highlights the need for experimental determination of rate coefficients for multifunctional organic compounds. Trends in Reactivity. In an attempt to rationalize the experimental results for the methoxylated aromatics, it is useful to compare their reactivity with other substituted aromatic compounds. A summary of the measured rate coefficients for the reaction of OH radicals with a range of methyl- and hydroxyl-substituted aromatic hydrocarbons is provided in Table 2. The reaction of OH radicals with aromatic compounds may proceed either by addition of OH to the aromatic ring or by H-atom abstraction. For benzene, H-atom abstraction is unfavorable because of the particularly stable nature of the aromatic ring, and OH addition is by far the dominant reaction pathway.
Similarly, for methylbenzene (toluene), the addition of OH to the aromatic ring dominates, with H-atom abstraction from the methyl group accounting for only ∼6% of the reaction. The rate coefficient for methylbenzene is almost five times larger than the rate coefficient for benzene and is attributed to the presence of the methyl group, which donates electron density inductively to the aromatic ring and activates the ortho and para positions toward addition of the electrophilic OH radical.10 Similarly, for hydroxybenzene (phenol), only ∼9% of the overall reaction is thought to occur via H-atom abstraction from the -OH group, with OH addition to the ortho position accounting for at least 80% of the reactivity.15 However, phenol is considerably more reactive toward OH radicals than methylbenzene because the hydroxyl group donates electron density
Reaction of Hydroxyl Radicals with 2-Methoxyphenol
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TABLE 1: Rate Coefficients for the Reaction of OH with the Methoxylated Aromatics at 294 ( 2 K and 1 atm and the Associated Atmospheric Lifetimes reference
k1/k2a
kOHb
kOH(average)b,c
kOH(predicted)b,d
τOHe
1,2-dimethylbenzene 1,4-dimethylbenzene 1,3,5-trimethylbenzene β-pinene 1,3,5-trimethylbenzene 3-methylphenol 1,3,5-trimethylbenzene 3-methylphenol 1,3,5-trimethylbenzene 3-methylphenol
1.87 ( 0.09 1.38 ( 0.10 1.37 ( 0.19 0.94 ( 0.05 1.76 ( 0.07 1.55 ( 0.09 1.76 ( 0.15 1.43 ( 0.18 1.83 ( 0.15 1.49 ( 0.14
2.54 ( 0.12 1.98 ( 0.15 7.78 ( 1.23 7.44 ( 0.42 10.00 ( 0.42 9.13 ( 0.54 9.96 ( 0.85 8.40 ( 1.11 10.36 ( 0.82 8.74 ( 0.82
2.20 ( 0.15
2.23
7.9
7.53 ( 0.41
2.98
2.3
compound methoxybenzene 2-methoxyphenol (guaiacol) 3-methoxyphenol 4-methoxyphenol 2-methoxy-4-methylphenol
9.80 ( 0.46
20.0
1.8
9.50 ( 0.55
2.98
1.8
9.45 ( 0.59
3.98
1.8
a Indicated errors are twice the standard deviation arising from the linear regression analysis and do not include the uncertainty in the reference rate coefficients. b Units of 10-11 cm3 molecule-1 s-1. c Determined using plots based on eq II (see the text). d Calculated using US EPA AOP WIN software package.13 e Lifetime in hours. τOH ) 1/kOH[OH], where 12 h daily average [OH] ) 1.6 × 106 molecules cm-3.20
TABLE 2: Comparison of the Rate Coefficients for the Reactions of OH with a Range of Hydroxylated, Methoxylated, and Methylated Aromatic Compounds compound
kOHa
reference
methylbenzene (toluene) methoxybenzene hydroxybenzene (phenol) 1,2-dimethylbenzene (o-xylene) 1,3-dimethylbenzene (m-xylene) 1,4-dimethylbenzene (p-xylene) 2-methoxyphenol (guaiacol) 3-methoxyphenol 4-methoxyphenol 2-methylphenol (o-cresol) 3-methylphenol (m-cresol) 4-methylphenol (p-cresol) 1,2-dihydroxybenzene (catechol) 2,4-Dimethylphenol 2,5-Dimethylphenol 2,6-Dimethylphenol 3,4-Dimethylphenol 3,5-Dimethylphenol 2-Methoxy-4-methylphenol 1,2-Dihydroxy-3-methylbenzene 1,2-Dihydroxy-4-methylbenzene
0.60 2.20 2.70 1.36 2.31 1.43 7.53 9.80 9.50 4.3 5.9 5.0 10.4 7.4 8.8 6.7 8.3 11.4 9.45 20.5 15.6
10 this work 10 10 10 10 this work this work this work 8 8 8 18 17 17 17 17 17 this work 19 19
a
In units of 10-11 cm3 molecule-1 s-1.
to the aromatic ring via resonance effects, which are stronger than the inductive effects of the methyl group. Methoxybenzene contains the -OCH3 group, which also donates electron density to the aromatic ring via resonance effects. The reactivity of methoxybenzene is slightly lower than that of hydroxybenzene, indicating that the activating effects of the substituents toward electrophilic addition of OH to the aromatic ring are in the order -OH > -OCH3 . -CH3, in line with expectations.16 Because there have been no product studies of the reaction of OH with methoxybenzene, the contribution of H-atom abstraction from the -OCH3 group to the overall reactivity is not known. However, according to the SAR method, which appears to work well for methoxybenzene, this pathway is expected to account for
