ARTICLE pubs.acs.org/est
Kinetics and Products of the Reaction of OH Radicals with 3-Methoxy-3-methyl-1-butanol Sara M. Aschmann, Janet Arey,† and Roger Atkinson*,†,‡ Air Pollution Research Center, University of California Riverside, Riverside, California 92521, United States
bS Supporting Information ABSTRACT: 3-Methoxy-3-methyl-1-butanol [CH3 OC(CH 3 )2 CH 2 CH 2 OH] is used as a solvent for paints, inks, and fragrances and as a raw material for the production of industrial detergents. A rate constant of (1.64 ( 0.18) 10 11 cm3 molecule 1 s1 for the reaction of 3-methoxy-3-methyl-1-butanol with OH radicals has been measured at 296 ( 2 K using a relative rate method, where the indicated error is the estimated overall uncertainty. Acetone, methyl acetate, glycolaldehyde, and 3-methoxy-3-methylbutanal were identified as products of the OH radical-initiated reaction, with molar formation yields of 3 ( 1%, 35 ( 9%, 13 ( 3%, and 33 ( 7%, respectively, at an average NO concentration of 1.3 10 14 molecules cm3 . Using a 12-h average daytime OH radical concentration of 2 106 molecules cm3 , the calculated lifetime of 3-methoxy-3methyl-1-butanol with respect to reaction with OH radicals is 8.5 h. Potential reaction mechanisms are discussed.
’ INTRODUCTION Volatile organic compounds emitted into the atmosphere can be chemically transformed by photolysis (at wavelengths >290 nm in the troposphere) and reactions with OH radicals, NO3 radicals, Cl atoms, and O3.1 3-Methoxy-3-methyl-1-butanol [CH3OC(CH3)2CH2CH2OH] has been produced in Japan for many years, with a production of 10 000 tonnes in 2002, and is used as a solvent for paints, inks, and fragrances and as a raw material for the production of industrial detergents.2 In this work we report the results of kinetic and product studies of the OH radical-initiated reaction of 3-methoxy-3-methyl-1-butanol. ’ EXPERIMENTAL SECTION Experiments were carried out in ∼7000-L all-Teflon chambers equipped with two parallel banks of blacklamps for irradiation. These chambers contain Teflon-coated fans to ensure rapid mixing of reactants during their introduction into the chamber. All experiments were carried out at 296 ( 2 K in dry purified air at 735 Torr total pressure. OH radicals were generated by the photolysis of CH3ONO at wavelengths >300 nm, and NO was included in the reactant mixtures to suppress the formation of O3 and hence of NO3 radicals. Rate Constant for the OH Radical Reaction. A rate constant for the OH radical reaction was measured using a relative rate technique in which the disappearance rate of 3-methoxy-3methyl-1-butanol was determined relative to that of a reference compound whose rate constant is reliably known. Providing that 3-methoxy-3-methyl-1-butanol and the reference compound were removed only by reaction with OH radicals, OH þ 3-methoxy-3-methyl-1-butanol f products
ð1Þ
OH þ reference compound f products
ð2Þ
r 2011 American Chemical Society
then,
! ½3-methoxy-3-methyl-1-butanolt0 ln ½3-methoxy-3-methyl-1-butanolt ! ½reference compoundt0 k1 ¼ ln k2 ½reference compoundt
ðIÞ
where [3-methoxy-3-methyl-1-butanol]t0 and [reference compound]t0 are the concentrations of 3-methoxy-3-methyl-1butanol and the reference compound, respectively, at time t0, [3-methoxy-3-methyl-1-butanol]t and [reference compound]t are the corresponding concentrations at time t, and k1 and k2 are the rate constants for reactions 1 and 2, respectively. A plot of ln([3-methoxy-3-methyl-1-butanol]t0/[3-methoxy-3-methyl-1butanol]t ) against ln([reference compound]t0 /[reference compound]t ) should then be a straight line of slope k1 /k 2 and zero intercept. Di-n-butyl ether was used as the reference compound, and the initial reactant concentrations (molecules cm3) were CH3ONO and NO, ∼2.4 1014 each; and 3-methoxy-3-methyl-1butanol and di-n-butyl ether, ∼2.4 1013 each. The concentrations of 3-methoxy-3-methyl-1-butanol and di-n-butyl ether were measured during the experiments by gas chromatography with flame ionization detection (GC-FID). Gas samples of 100 cm3 volume were collected onto Tenax solid adsorbent, with subsequent thermal desorption onto a 30-m DB-1701 megabore column initially held at 40 °C and then temperature programmed Received: April 29, 2011 Accepted: July 6, 2011 Revised: June 30, 2011 Published: July 20, 2011 6896
dx.doi.org/10.1021/es201475g | Environ. Sci. Technol. 2011, 45, 6896–6901
Environmental Science & Technology at 8 °C min1. During each experiment the following GC-FID analyses were conducted: at least two replicate analyses prior to reaction, one analysis after each of three irradiation periods and a replicate analysis after the third (and last) irradiation period. Replicate analyses in the chamber in the dark typically agreed to within 3% for both 3-methoxy-3-methyl-1-butanol and di-nbutyl ether. Products of the OH Radical-Initiated Reaction. Products of this reaction were analyzed by GC-FID as described above and by combined gas chromatographymass spectrometry (GC-MS). GC-FID response factors were measured by introducing known amounts of 3-methoxy-3-methyl-1-butanol, acetone, and methyl acetate into the chamber and conducting several replicate GCFID analyses. GC-MS analyses were carried out by thermal desorption of samples collected onto Tenax solid adsorbent or onto solid phase microextraction (SPME) fibers, with analysis by positive chemical ionization GC-MS (PCI GC-MS) using methane as the CI gas in an Agilent 5975 Inert XL Mass Selective Detector with a 60-m DB-5 capillary column (0.25 mm i.d., 0.25 μm phase) operated in the scanning mode. A 65-μm polydimethylsiloxane/divinylbenzene SPME fiber was precoated with O-(2,3,4,5,6-pentafluorobenzyl)hydroxylamine (PFBHA) for on-fiber derivatization of carbonyl compounds,3 with subsequent thermal desorption and GC-MS analysis. Each carbonyl group derivatized to an oxime added 195 mass units to the compound’s molecular weight. Methane-CI gives protonated molecules [M + H]+ and smaller adduct ions at [M + 29]+ and [M + 41]+, and the derivatized carbonyl also exhibits a [M + H 198]+ fragment, where M is the molecular weight of the oxime. In addition, oximes of hydroxycarbonyls have characteristic [M + H H 2 O]+ fragment ions. 3 Glycolaldehyde [HOCH2CHO] was quantified with PFBHAcoated SPME fibers, using the OH radical-initiated reaction of methyl vinyl ketone, which produces glycolaldehyde in 64 ( 8% yield,4 as an in situ source of glycolaldehyde for calibration purposes. After exposing the PFBHA-coated SPME fibers to the chamber contents, the fibers were analyzed by GC-FID after being thermally desorbed at 270 °C onto a 30-m DB-5 megabore column, temperature programmed from 40 to 260 °C at 8 °C min1. Three CH3ONONOmethyl vinyl ketoneair irradiations were interdispersed with CH3ONONO3-methoxy3-methyl-1-butanolair irradiations. The initial concentrations (molecules cm3) were CH3ONO and NO, ∼2.4 1014 each, and 3-methoxy-3-methyl-1-butanol or methyl vinyl ketone, (2.412.70) 1013 each. Irradiations were carried out for 1220 min at 20% of the maximum light intensity, resulting in 4460% reaction of the initially present 3-methoxy-3-methyl-1butanol or methyl vinyl ketone. Chemicals. The sources and purities of the chemicals used were as follows: di-n-butyl ether (99+%), 4,4-dimethoxy-2-butanone (90+%), 3-methoxy-3-methyl-1-butanol (98+%), methyl acetate (99+%), and methyl vinyl ketone (99%), Aldrich; O-(2,3,4,5,6-pentafluorobenzyl)hydroxylamine hydrochloride (99+%), Alfa Aesar; acetone (HPLC grade), Fisher Scientific; and NO (g99.0%), Matheson Gas Products. CH3ONO was prepared as described by Taylor et al.5 and stored at 77 K under vacuum.
’ RESULTS Rate Constant for the OH Radical Reaction. The experimental data from irradiated CH3ONONO3-methoxy-3-methyl1-butanoldi-n-butyl ether (the reference compound)air
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Figure 1. Plot of eq I for the reaction of OH radicals with 3-methoxy-3methyl-1-butanol, with di-n-butyl ether as the reference compound. Combined data from 3 experiments, with analyses by GC-FID.
mixtures are plotted in accordance with eq I in Figure 1. Leastsquares analysis of these data leads to a rate constant ratio k1/k2 = 0.582 ( 0.020, where the indicated error is two least-squares standard deviations. This rate constant ratio is placed on an absolute basis using a rate constant k2 at 296 K of k2(di-n-butyl ether) = 2.81 1011 cm3 molecule1 s1,6 resulting in k1 ¼ ð1:64 ( 0:06Þ 1011 cm3 molecule1 s1 at 296 ( 2 K where the indicated error is two least-squares standard deviations and does not include uncertainties associated with the rate constant k2, which is expected to be ∼ (10%. Including a (10% uncertainty in the rate constant k2 leads to k1 ¼ ð1:64 ( 0:18Þ 1011 cm3 molecule1 s1 at 296 ( 2 K where the indicated error is the estimated overall uncertainty. Products of OH + 3-Methoxy-3-methyl-1-butanol. Positive chemical ionization GC-MS analyses of PFBHA-coated SPME fibers exposed to the chamber contents after reaction showed the presence of at least three products (Figure S1 in Supporting Information), each with two mono-oximes (syn and anti). In addition, the dioximes of glyoxal, (CHO)2, were present, eluting at longer retention times than shown in Figure S1. As discussed in the Supporting Information (see also Figures S2S5), the three products observed as their mono-oximes were attributed respectively to the following: a hydroxycarbonyl of molecular weight 60; two coeluting carbonyl-containing products of molecular weight 84 and 102; and a carbonyl-containing product of molecular weight 116 which also contains a methoxy group. The molecular weight 60 hydroxycarbonyl is attributed to glycolaldehyde [HOCH2CHO], and this was verified by matching retention times and mass spectra of the oximes with those from an OH radical-initiated reaction of methyl vinyl ketone, which is known to form glycolaldehyde as a product.4 The molecular weight 116 carbonyl containing a methoxy group is attributed to 6897
dx.doi.org/10.1021/es201475g |Environ. Sci. Technol. 2011, 45, 6896–6901
Environmental Science & Technology
ARTICLE
Table 1. Products Identified and Their Molar Formation Yields from the OH Radical-Initiated Reaction of 3-Methoxy3-methyl-1-butanol at 296 ( 2 K and at an Average NO Concentration during the Reaction Periods of 1.3 1014 Molecules cm3 product
from GC-FID
molar formation
analysis of
yield (%)
acetone methyl acetate
Tenax solid adsorbent Tenax solid adsorbent
3 ( 1a 35 ( 9a
glycolaldehyde
PFBHA-coated SPME fiber
13 ( 3b
3-methoxy-3-methylbutanal Tenax solid adsorbent
33 ( 7c
a
Indicated errors are two least-squares standard deviations combined with estimated uncertainties in the GC-FID response factors for acetone, methyl acetate, and 3-methoxy-3-methyl-1-butanol of (15%, ( 10%, and (5%, respectively. b From three experiments with a single irradiation period per experiment. A glycolaldehyde yield of 12.6 ( 1.1% was obtained relative to a glycolaldehyde yield of 64% from OH + methyl vinyl ketone, where the error is two standard deviations. Inclusion of the uncertainties in the glycolaldehyde formation yield from OH + methyl vinyl ketone4 and in the GC-FID response factors for 3-methoxy-3methyl-1-butanol and methyl vinyl ketone ((5% each) results in the cited glycolaldehyde formation yield from OH + 3-methoxy-3-methyl-1butanol. c Indicated error is two least-squares standard deviations combined with an estimated uncertainty in the GC-FID response factor for 3-methoxy-3-methylbutanal relative to that for 3-methoxy-3-methyl1-butanol of (20%.
CH3OC(CH3)2CH2CHO, based on the expected reaction mechanism (see below). Samples collected onto Tenax solid adsorbent from an OH + 3-methoxy-3-methyl-1-butanol reaction were analyzed by GCFID on a DB-1701 column (see above) and on a DB-5 column, and by GC-MS on a DB-5 column, allowing the peaks on the GCMS total ion chromatogram to be correlated with those from the GC-FID analysis on the DB-5 column. Acetone and methyl acetate were identified as products of OH + 3-methoxy-3methyl-1-butanol by matching of retention times of authentic standards introduced into the chamber with GC peaks in the GCFID analyses. GC-MS analysis showed the presence of an early eluting molecular weight 74 compound, confirming the formation of methyl acetate. A major GC-MS product peak was consistent with a molecular weight 116 compound which is attributed to CH3OC(CH3)2CH2CHO [3-methoxy-3-methylbutanal], and this corresponded to a GC-FID peak which could be quantified by estimating its GC-FID response factor. Using the Effective Carbon Number concept,7 the FID response factor of CH3OC(CH3)2CH2CHO is estimated to be 4.0/4.5 = 0.89 of that for 3-methoxy-3-methyl-1-butanol. Thus, acetone, methyl acetate, and 3-methoxy-3-methylbutanal were quantified by GC-FID analyses of samples collected onto Tenax solid adsorbent, and glycolaldehyde was quantified by GC-FID analysis of PFBHA-coated SPME fibers. These products also react with OH radicals, and their measured concentrations were corrected for secondary reaction with OH radicals as described previously,8 using rate constants (in units of 1012 cm3 molecule1 s1) of: acetone, 0.18;9 methyl acetate, 0.32;10 glycolaldehyde, 8.0;9 3-methoxy-3-methylbutanal, 23.1 (estimated11,12); methyl vinyl ketone, 20;1,9 and 3-methoxy-3methyl-1-butanol, 16.4 (Table 1). The multiplicative correction factors, F, increase with the rate constant ratio k(OH + product)/ k(OH + reactant) and with the extent of reaction.8 The
Figure 2. Plots of the amounts of acetone, methyl acetate, and 3-methoxy-3-methylbutanal formed, corrected for reaction with OH radicals (see text), against the amounts of 3-methoxy-3-methyl-1butanol reacted with OH radicals. Combined data from 3 (acetone and methyl acetate) or 6 (3-methoxy-3-methylbutanal) experiments, with analyses by GC-FID. The data for methyl acetate have been displaced vertically by 2.0 1012 molecules cm3 for clarity.
maximum multiplicative factors for the formation of acetone, methyl acetate, glycolaldehyde, and 3-methoxy-3-methylbutanal from OH + 3-methoxy-3-methyl-1-butanol were
