Products of the gas-phase reactions of OH radicals with p-xylene and 1,2,3- and 1,2,4-trimethylbenzene: Effect of NO2 concentration

Article abstract of DOI:10.1021/jp001161s

Aromatic hydrocarbons are important constituents of gasoline fuels, vehicle exhaust, and ambient air in urban areas. In the troposphere, aromatic hydrocarbons, e.g., BTEX and trimethylbenzenes, react essentially only with the hydroxyl (OH) radical. Several product studies of the reaction of OH radicals with BTX and trimethylbenzenes have been conducted, but few of these determined product yields under conditions where the reaction of the OH-aromatic adducts with O₂ clearly dominates. GC was used to measure products of the gas-phase reactions of the OH radical with p-xylene and 1,2,3- and 1,2,4-trimethylbenzene in the presence of varying NO₂ concentrations. Product analyses showed that the ring-cleavage products 2,3-butanedione and 3-hexene-2,5-dione displayed a dependence of their formation yields on the NO₂ concentration, with higher yields from the reactions of the OH-aromatic adducts with O₂ than from their reactions with NO₂. These ring-cleavage products were primary products of the OH-aromatic adduct reactions. Formation yields extrapolated to zero NO₂ concentration should be applicable to ambient atmospheric conditions. The formation yields of 3-hexene-2,5-dione from p-xylene and 1,2,4-trimethylbenzene were comparable to those reported for glyoxal from p-xylene and of methylglyoxal from 1,2,4-trimethylbenzene, thus suggesting that these were coproducts.

Full text of DOI:10.1021/jp001161s

8922
J. Phys. Chem. A 2000, 104, 8922-8929
Products of the Gas-Phase Reactions of OH Radicals with p-Xylene and 1,2,3- and
1
,2,4-Trimethylbenzene: Effect of NO2 Concentration
†
,†,‡,§
and Janet Arey*,†,‡
Heidi L. Bethel, Roger Atkinson,*
Air Pollution Research Center, UniVersity of California, RiVerside, California 92521
ReceiVed: March 28, 2000; In Final Form: July 26, 2000
Products of the gas-phase reactions of the OH radical with p-xylene and 1,2,3- and 1,2,4-trimethylbenzene
have been measured by gas chromatography in the presence of varying concentrations of NO
analyses show that the ring-cleavage products 2,3-butanedione (from 1,2,3- and 1,2,4-trimethylbenzene) and
-hexene-2,5-dione (from p-xylene and 1,2,4-trimethylbenzene) exhibit a dependence of their formation yields
on the NO concentration, with higher yields from the reactions of the OH-aromatic adducts with O than
from their reactions with NO . Furthermore, our data show that these ring-cleavage products are primary
concentration
2
. Our product
3
2
2
2
products of the OH-aromatic adduct reactions. Formation yields extrapolated to zero NO
2
should be applicable to ambient atmospheric conditions (provided that there is sufficient NO that peroxy
radicals react dominantly with NO), and are from p-xylene, p-tolualdehyde, 0.0706 ( 0.0042 (independent
of NO
2
2
concentration), 2,5-dimethylphenol, 0.138 ( 0.016 (independent of NO concentration), and 3-hexene-
2,5-dione, 0.32 (extrapolated); from 1,2,3-trimethylbenzene, 2,3-butanedione, 0.52 (extrapolated); and from
,2,4-trimethylbenzene, 2,3-butanedione, 0.10 (extrapolated) and 3-hexene-2,5-dione, 0.31 (extrapolated). Our
1
formation yields of 3-hexene-2,5-dione from p-xylene and 1,2,4-trimethylbenzene are similar to those reported
for glyoxal from p-xylene and of methylglyoxal from 1,2,4-trimethylbenzene and therefore suggest that these
are coproducts, as expected from reaction schemes presented here.
Introduction
and atmospheric pressure.6,7 The benzyl (or alkyl-substituted
benzyl) radicals formed by the H-atom abstraction pathway react
Aromatic hydrocarbons are important constituents of gasoline
in the atmosphere analogously to alkyl radicals,7 and the
p-methylbenzyl radical formed in reaction 1a reacts in the
presence of O2 and NO (with sufficient NO that organic peroxy
radicals react primarily with NO) to form p-tolualdehyde (p-
CH3C6H4CHO) and p-methylbenzyl nitrate (p-CH3C6H4CH2-
,8
1
-3
1,4
5
fuels,
vehicle exhaust and ambient air in urban areas,
typically accounting for ∼20-25% of present-day reformulated
gasolines2,3 and a similar fraction of nonmethane organic
5
compounds in ambient air in urban areas. In the troposphere,
aromatic hydrocarbons such as benzene, toluene, xylenes,
7
,8
ONO2). The hydroxycyclohexadienyl-type radicals (OH-
aromatic adducts) formed by OH radical addition to benzene,
toluene, and xylenes (reaction 1b) react with O2 and NO2 (but
not with NO), with rate constants for the O2 and NO2 reactions
ethylbenzene, and trimethylbenzenes react essentially only with
the hydroxyl (OH) radical.6
-8
These OH radical reactions
proceed by H-atom abstraction from the C-H bonds of the alkyl
substitutent groups (or, for benzene, from the C-H bonds of
the aromatic ring) and by initial addition of the OH radical to
-
16
-11
3
-1
of (1.8-20) × 10
and (2.5-3.6) × 10
cm molecule
Hence the dominant
-
1
7,9-11
the carbon atoms of the aromatic ring,6-8 as shown for p-xylene.
s , respectively, at room temperature.
reaction of the OH-aromatic adducts in the troposphere (even
in polluted urban atmospheres) is with O2. However, in
laboratory studies carried out at elevated NOx concentrations,
the reaction of the OH-aromatic adducts with NO2 can be
12
significant, and possibly even dominant (for benzene, toluene,
and o- and p-xylene the reactions of the OH-aromatic adducts
with O2 and NO2 are of equal importance at room temperature
and atmospheric pressure of air for NO2 concentrations of
1
3
-3 9,11,12
∼
(3.6-13) × 10 molecules cm
). Therefore, the
products formed, and their yields, as determined from laboratory
product studies may not be applicable to atmospheric conditions.
Numerous product studies of the reactions of OH radicals
with benzene, toluene, the xylenes, and trimethylbenzenes
7
,8,12-42
have been carried out.
However, relatively few of these
The H-atom abstraction pathway (1a) generally accounts for
studies determined product yields under conditions where
e10% of the overall OH radical reaction at room temperature
reaction of the OH-aromatic adducts with O clearly domi-
2
1
2,14,30,31,38,39,41
nated,
and product yields have been specifically
*
Authors to whom correspondence should be addressed.
Also Interdepartmental Program in Environmental Toxicology.
Also Department of Environmental Sciences.
Also Department of Chemistry.
measured as a function of the NO2 concentration in only four
†
1
2,30,31,40
‡
§
studies.
In this work, we have measured the formation
yields of 2,3-butanedione (biacetyl) and/or 3-hexene-2,5-dione
1
0.1021/jp001161s CCC: $19.00 © 2000 American Chemical Society
Published on Web 09/01/2000

Reactions of OH Radicals with p-Xylene and Trimethylbenzene
J. Phys. Chem. A, Vol. 104, No. 39, 2000 8923
from the OH radical-initiated reactions of p-xylene and 1,2,3-
and 1,2,4-trimethylbenzene, and of p-tolualdehyde and 2,5-
dimethylphenol from p-xylene, as a function of NO2 concentra-
tion.
Results
A series of CH3ONO-NO-p-xylene-air, CH3ONO-NO-
,2,3-trimethylbenzene-air, and CH3ONO-NO-1,2,4-tri-
1
methylbenzene-air irradiations were carried out with varying
initial CH3ONO and NO concentrations (ranging from ∼2.2 ×
Experimental Section
13
14
-3
1
0
to ∼2.2 × 10 molecules cm each). As noted above,
NO and initial NO2 concentrations were monitored by a
chemiluminescence analyzer and, because methyl nitrite and
organic nitrates are measured by this NO-NO2-NOx analyzer
as “NO2”, the NO2 concentrations during the experiments were
estimated assuming that ([NO] + [NO2]) ) constant during the
The experimental methods used were similar to those
12,30,31
described previously.
Experiments were carried out at 298
(
2 K and 740 Torr total pressure of air (at ∼5% relative
humidity) in a 7900 L Teflon chamber with analysis by gas
chromatography with flame ionization detection (GC-FID) and
combined gas chromatography-mass spectrometry (GC-MS).
Irradiation was provided by two parallel banks of blacklamps,
and the chamber was fitted with a Teflon-coated fan to ensure
rapid mixing of reactants during their introduction into the
chamber. Hydroxyl radicals were generated in the presence of
NO by the photolysis of methyl nitrite in air at wavelengths
3
0,31
irradiations.
This has been shown by computer model
3
0
calculations of these systems and is consistent with the overall
photolysis of methyl nitrite: CH3ONO + hν (+ O2) f HCHO
+
OH + NO2 followed by OH + NO2 (+ M) f HNO3 (+ M).
For each experiment, the average NO2 concentration, [NO2]av
n
3
0,31
was calculated from [NO2]av ) (∑ [NO2]i)/n.
i)1
The products identified and quantified by GC-FID and GC-
MS in this work (by matching of GC retention times and mass
spectra with those of authentic standards) were p-tolualdehyde,
1
2
>
300 nm, and NO was added to the reactant mixtures to
suppress the formation of O3 and hence of NO3 radicals.
-
3
The initial reactant concentrations (in molecules cm units)
2,5-dimethylphenol, and 3-hexene-2,5-dione from p-xylene, 2,3-
13
were as follows: CH3ONO, (2.2-22.7) × 10 ; NO, (2.0-21.2)
butanedione from 1,2,3-trimethylbenzene, and 2,3-butanedione
and 3-hexene-2,5-dione from 1,2,4-trimethylbenzene. These
reaction products also react with OH radicals and carbonyls may
1
3
13
×
10 ; NO2, (0.12-8.0) × 10 ; and aromatic hydrocarbons,
13
(
2.22-2.62) × 10 (in all experiments the initial CH3ONO and
NO concentrations were approximately equal). Irradiations were
carried out for 3-15 min (p-xylene), 1.5-8 min (1,2,3-
trimethylbenzene), and 1-8 min (1,2,4-trimethylbenzene),
resulting in up to 41%, 53%, and 51% reaction of the initially
present p-xylene, 1,2,3-trimethylbenzene, or 1,2,4-trimethyl-
benzene, respectively. The concentrations of the aromatic
hydrocarbons and reaction products were measured during the
6-8,45
also photolyze.
Photolysis of cis-3-hexene-2,5-dione in the
presence of cyclohexane (to scavenge any OH radicals present)
showed a <3% loss of 3-hexene-2,5-dione over a 12 min
irradiation period. However, in agreement with previous stud-
2
9,45
ies,
cis/trans photoisomerization occurred with conversion
of 12% of the cis isomer to the trans isomer over the 12 min
irradiation period employed. Because we observed ∼10% trans
isomer to be initially present (possibly formed by cis/trans
isomerization during the thermal desorption and analysis
procedure), we choose to sum the cis and trans isomer products
and report them as “3-hexene-2,5-dione”. We have recently
shown that the photolysis rate of 2,3-butanedione in the Teflon
chamber under the conditions used in this work is (1.25 ( 0.56)
3
experiments by GC-FID. Gas samples of 100 cm volume were
collected from the chamber onto Tenax-TA solid adsorbent, with
subsequent thermal desorption at ∼225 °C onto a DB-1701
megabore column in a Hewlett-Packard (HP) 5710 GC, initially
held at 0 °C and then temperature programmed to 200 °C at 8
-
1
°
C min . In addition, gas samples were collected onto Tenax-
-
3
-1
TA solid adsorbent for GC-MS analyses, with thermal desorp-
tion onto a 30 m DB-5MS fused silica capillary column in an
HP 5890 GC interfaced to a HP 5970 mass selective detector
operating in the scanning mode. GC-FID response factors were
determined as described previously.43 NO and initial NO2
concentrations were measured using a Thermo Environmental
Instruments, Inc., Model 42 NO-NO2-NOx chemiluminescence
analyzer.
× 10 min , where the indicated error is two least-squares
46
standard deviations. For the e8 min irradiation times employed
in the 1,2,3- and 1,2,4-trimethylbenzene reactions (where 2,3-
butanedione was observed as a product), this corresponds to a
1.0 ( 0.5% loss of 2,3-butanedione due to photolysis which is
within the analytical uncertainties and hence neglected. Cor-
rections for the OH radical reactions with the products were
4
7
calculated as described previously, using rate constants at 298
K (in units of 10
-
12
3
-1 -1
7
7
6
cm molecule s ) of p-xylene, 14.3;
1
3
An irradiation of a cis-3-hexene-2,5-dione (2.20 × 10
molecules cm )-air mixture was also carried out in the
7
1
,2,3-trimethylbenzene, 32.7; 1,2,4-trimethylbenzene, 32.5;
-
3
4
8
p-tolualdehyde, 17.3 (estimated ); 2,5-dimethylphenol, 80.0;
15
-3
presence of 3.5 × 10 molecules cm of cyclohexane (to
scavenge any OH radicals formed) at the same light intensity
as used for the product studies, for up to 12 min to investigate
the importance of photolysis of cis-3-hexene-2,5-dione during
the CH3ONO-NO-aromatic hydrocarbon-air irradiations.
7
2
,3-butanedione, 0.238; and 3-hexene-2,5-dione, 60 (based on
the literature rate constants for the cis and trans isomers of 6.3
-
11
-11
3
-1 -1
7,45
×
10
and 5.3 × 10
cm molecule s , respectively,
and using the fractions of cis and trans isomers typically
observed during the reactions). The multiplicative correction
factors, F, increase with the rate constant ratio k(OH + product)/
The chemicals used, and their stated purities, were as
follows: cyclohexane (high purity solvent grade), American
Burdick and Jackson; 2,3-butanedione (biacetyl) (99%), 2,5-
dimethylphenol (99+%), p-tolualdehyde (97%), 1,2,3-trimeth-
ylbenzene (90%), 1,2,4-trimethylbenzene (98%), and p-xylene
47
k(OH + aromatic hydrocarbon) and with the extent of reaction.
The maximum values of the multiplicative factors F were as
follows: for the p-xylene reactions, 1.38 for p-tolualdehyde,
3.44 for 2,5-dimethylphenol, and 2.69 for 3-hexene-2,5-dione;
for the 1,2,3-trimethylbenzene reactions, <1.01 for 2,3-butane-
dione; and for the 1,2,4-trimethylbenzene reactions, <1.01 for
2,3-butanedione and 1.92 for 3-hexene-2,5-dione. Because of
the negligible amount of photolysis of the diketones, no
corrections for photolysis were made, nor were corrections made
for the OH radical reaction with 2,3-butanedione.
(
99+%), Aldrich Chemical Co.; and NO (g99.0%), Matheson
gas Products. Methyl nitrite was prepared as described by Taylor
44
et al. and stored at 77 K under vacuum. A synthesized sample
of cis-3-hexene-2,5-dione was kindly donated by Drs. Harvey
E. Jeffries and Kenneth G. Sexton of the University of North
Carolina at Chapel Hill.

8924 J. Phys. Chem. A, Vol. 104, No. 39, 2000
Bethel et al.
Figure 1. Plots of the amounts of p-tolualdehyde and 2,5-dimeth-
ylphenol formed, corrected for reaction with the OH radical (see text),
against the amounts of p-xylene reacted with the OH radical. The entire
Figure 3. Plot of the 3-hexene-2,5-dione formation yields from the
OH radical-initiated reaction of p-xylene against the average NO2
concentration, [NO ] .
2
av
2
data set, obtained at average NO concentrations varying in the range
1
3
-3
(
0.90-8.16) × 10 molecules cm , are shown in these plots. The
aldehyde or 2,5-dimethylphenol formation yields on the NO2
concentration was observed. However, for 3-hexene-2,5-dione
the formation yield decreased with increasing NO2 concentra-
tion, as shown in Figure 3. The formation yields of the products
identified and quantified are given in Table 1.
data for 2,5-dimethylphenol have been displaced vertically by 4.0 ×
1
1
-3
1
0 molecules cm for clarity.
As shown in Table 2, our present formation yield for
p-tolualdehyde of 7.06 ( 0.42% is in general agreement with
2
5,31,41
previous literature values
1
which are in the range 7.0-
29
0.3% (Becker and Klein have also reported a p-tolualdehyde
yield obtained in the presence of NOx of 10%, but few
experimental details were reported). Our present and previous31
studies show that within the experimental errors the p-tolual-
dehyde formation yield is independent of the NO2 concentration
1
3
-3
over the range (0.9-24) × 10 molecules cm . This is
anticipated, because in the presence of NO the p-methylben-
zylperoxy radical formed after O2 addition to the p-methylbenzyl
radical reacts with NO to form either the p-CH3C6H4CH2O˙
radical plus NO2 (reaction 2a) or p-methylbenzyl nitrate (reaction
2
3
b), with the p-CH3C6H4CH2 O˙ radical reacting with O2 (reaction
7
,8
) to form p-tolualdehyde.
CH C H CH O O˙ + NO f CH C H CH O˙ + NO
2
(2a)
3
6
4
2
3
6
4
2
CH C H CH O O˙ + NO f CH C H CH ONO (2b)
3
6
4
2
3
6
4
2
2
Figure 2. Plots of the amounts of 3-hexene-2,5-dione formed, corrected
for reaction with the OH radical (see text), against the amounts of
p-xylene reacted with the OH radical at differing average NO
2
CH
3
C
6
H
4
CH
2
O˙ + O
2
f CH
3
C
6
H
4
CHO + HO
2
(3)
-3
concentrations (in molecules cm units). The data at [NO
2
-3
]
av ) (4.15-
1
3
13
4
.76) × 10 and (0.90-2.62) × 10 molecules cm have been
While the p-CH3C6H4CH2 O˙ radical is also expected to react
with NO2 to form p-methylbenzyl nitrate (reaction 4)
11 11 -3
displaced vertically by 2.0 × 10 and 4.0 × 10 molecules cm
,
respectively, for clarity.
CH C H CH O˙ + NO f CH C H CH ONO
(4)
3
6
4
2
2
3
6
4
2
2
p-Xylene. GC-MS and GC-FID analysis of irradiated CH3-
ONO-NO-p-xylene-air mixtures showed the formation of
p-tolualdehyde, 2,5-dimethylphenol, and 3-hexene-2,5-dione (cis
and trans, with the trans/cis ratio increasing with the extent of
reaction probably due to cis,trans photoisomerization as observed
in the photolysis experiment). Plots of the amounts of p-
tolualdehyde and 2,5-dimethylphenol formed, corrected for
reaction with the OH radical, against the amounts of p-xylene
reacted are shown in Figure 1, and analogous plots for the
formation of 3-hexene-2,5-dione are shown in Figure 2. Within
the experimental uncertainties, no dependence of the p-tolu-
rate constants for reactions 3 and 4 for alkoxy radicals formed
-
15
-11
3
from alkanes are ∼(8-10) × 10
and ∼3 × 10
cm
-
1
-1
7
molecule s , respectively. Assuming similar rate constants
for the corresponding reactions of the p-CH C H CH O˙ radical,
3
6
4
2
14
-3
at an NO concentration of 2.4 × 10 molecules cm reaction
2
4 is calculated to contribute 12-15% of the p-CH C H CH O˙
3
6
4
2
radical loss processes.
Our 2,5-dimethylphenol formation yield is in reasonable
31
agreement with the previous data of Atkinson et al. and Smith
4
1
et al., given the high reactivity of 2,5-dimethylphenol and the

Reactions of OH Radicals with p-Xylene and Trimethylbenzene
J. Phys. Chem. A, Vol. 104, No. 39, 2000 8925
TABLE 1: Products Identified and Quantified, and Their
Formation Yields, from the OH Radical-Initiated Reaction
of p-Xylene in the Presence of NOa
that ring-retaining products do not account for more than a small
fraction of the products formed from reactions of OH-aromatic
adducts with NO2, and hence that ring cleavage must also occur.
Klotz et al.52,53 have measured OH radical reaction rate
1
0-13[NO2]av
molecules cm-3)
3-hexene-2,5-
dione
(
p-tolualdehyde 2,5-dimethylphenol
0.0706 ( 0.0042 0.138 ( 0.016
-
11
3
-1 -1
constants of (7.4-7.6) × 10
cis- and trans,trans-2,4-hexadienedial and 1.18 × 10
molecule for trans,trans-2-methyl-2,4-hexadienedial. If
cm molecule
s
for trans,
0
0
2
2
2
4
4
7
8
.90-8.16
5
3
-10
3
cm
.90
0.295 ( 0.013
0.294 ( 0.003
0.295 ( 0.019
0.273 ( 0.029
0.246 ( 0.006
0.231 ( 0.005
0.200 ( 0.006
0.194 ( 0.001
-1
-1
52
.02
s
.45
one assumes that 3-hexene-2,5-dione is formed by reaction of
the first-generation product CH3C(O)CHdCHC(CH3)dCHCHO
(formed in path C of Scheme 1) with OH radicals [and
.62
.15
.76
.76
48
-10
3
estimating a rate constant for this reaction of 1.2 × 10 cm
.16
-
1 -1
molecule
s
(i.e., similar to that for HC(O)C(CH3)dCHCHd
a
Indicated errors are two least-squares standard deviations. The
CHCHO)], then model calculations using the reactions
estimated uncertainties in the GC-FID response factors for p-xylene
and the products are (5% each.
OH + p-xylene f
R CH C(O)CHdCHC(CH )dCHCHO (5)
corrections necessary for secondary reactions in this and our
3
3
3
1
previous studies. Formation of 2,5-dimethylphenol has also
OH + CH C(O)CHdCHC(CH )dCHCHO f
29
3
3
been reported by Becker and Klein in the presence of NOx
and by Becker et al.32 and Bierbach et al.33 in the absence of
â (3-hexene-2,5-dione + glyoxal) (6)
NOx, but few experimental details were reported (for example,
the extents of reaction were not reported, nor is it clear whether
or not the cited product yields were corrected for secondary
reactions).
OH + 3-hexene-2,5-dione f products
(7)
3
-1 -1
with rate constants (in cm molecule s ) of k5 ) 1.43 ×
, k6 ) 1.2 × 10 , and k7 ) 6.0 × 10 , can be compared
with our measured 3-hexene-2,5-dione concentrations. Figure
clearly shows that our measured 3-hexene-2,5-dione concen-
trations agree very well with predictions based on 3-hexene-
,5-dione being a first-generation product, but differ significantly
-11
-10
-11
1
0
As obvious from Figure 3 and Table 2, the 3-hexene-2,5-
dione formation yield measured here decreases with increasing
NO2 concentration. Our 3-hexene-2,5-dione formation yield is
4
4
1
3
0-40% higher than that reported by Smith et al. but
2
reasonably similar to the reported glyoxal [HC(O)CHO]
2
5,27,41
from predictions based on 3-hexene-2,5-dione being a second-
generation product (i.e., with reactions 5, 6, and 7). Furthermore,
based on the similarity of the literature yields for glyoxal2
to that we measure here for 3-hexene-2,5-dione and the fact
that glyoxal is anticipated to be formed along with 3-hexene-
2,5-dione (path B of Scheme 1), we conclude that glyoxal is
also a first-generation product and the coproduct to 3-hexene-
2,5-dione. Formation of 3-hexene-2,5-dione has also been
yields.
Glyoxal and 3-hexene-2,5-dione are potential
coproducts from one path of the reaction of the OH-p-xylene
5,27,41
adduct with O2 (path B in Scheme 1)8
,16,49-51
and/or from further
reaction of the ring-opened product CH3C(O)CHdCHC(CH3)d
CHCHO (formed as shown in path C of Scheme 1) with OH
radicals.52 Much less information is available concerning the
mechanism of the reactions of OH-aromatic adducts with
NO2,51 and the reaction pathways occurring are not presently
2
9
51
reported by Becker and Klein in the presence of NOx and by
understood in any detail. Calculations carried out by Andino
3
2
33
et al.51 indicate that the pathways involving intermediate
Becker et al. and Bierbach et al. in the absence of NOx, but
(
as noted above) few experimental details were reported.
formation of hydroxynitrocyclohexadiene(s) (path D in Scheme
2
) and leading (for p-xylene) to formation of 2,5-dimethylphe-
1,2,3-Trimethylbenzene. GC-MS and GC-FID analysis of
nol, nitro-p-xylenes, and (not shown in Scheme 2) hydroxynitro-
p-xylenes are exothermic, while formation of an alkoxy radical
irradiated CH3ONO-NO-1,2,3-trimethylbenzene-air mixtures
showed the formation of 2,3-butanedione. Plots of the amounts
of 2,3-butanedione formed against the amounts of 1,2,3-
trimethylbenzene reacted are shown in Figure 5. The 2,3-
plus NO (path E in Scheme 2) was calculated to be endother-
mic.51 However, our present and previous
12,30,31
studies suggest
TABLE 2: Comparison of the Formation Yields for p-Tolualdehyde, 2,5-Dimethylphenol, 3-Hexene-2,5-dione, and Glyoxal from
p-Xylene (in the Presence of NO) Obtained in the Present Study with Literature Data
1
0-13[NO
2
]
product
formation yield
(molecules cm-3)
reference
2
5
p-tolualdehyde
0.08 ( 0.01
1.2 (initial)
2.6-24
Bandow and Washida
31
0
0
0
.0701 ( 0.0103
.103 ( 0.016
Atkinson et al.
Smith et al.41
this work
0.8-1.7 (final)
2.0-8.2
.0706 ( 0.0042
31
2
,5-dimethylphenol
-hexene-2,5-dione
0.188 ( 0.038
0
0
2.6-24
Atkinson et al.
Smith et al.41
this work
.13 ( 0.018
.138 ( 0.016
0.8-1.7 (final)
2.0-8.2
3
0.221 ( 0.04
0.81-1.7 (final)
0.0
Smith et al.41
this work
a,b
0
0
0
.323
.306
.193
b
b
0.90
8.16
∼5-10 (est)
1.2 (initial)
∼5-10 (est)
0.81-1.7 (final)
glyoxal
0.120 ( 0.020
Tuazon et al.21
2
5
0
0
0
.24 ( 0.02
.225 ( 0.039
.394 ( 0.11
Bandow and Washida
2
7
Tuazon et al.
Smith et al.41
a
b
Extrapolated using the regression expression given in footnote b. Calculated from the regression expression shown in Figure 3, of 3-hexene-
-
15
-30
2
-3
2
,5-dione yield ) {0.323 - 1.92 × 10 [NO
2
2 2
] + 3.97 × 10 [NO ] }, where the NO concentration is in molecules cm units.

8926 J. Phys. Chem. A, Vol. 104, No. 39, 2000
Bethel et al.
SCHEME 1
SCHEME 2
Figure 4. Plots of the amounts of 3-hexene-2,5-dione formed against
the amounts of p-xylene reacted with the OH radical: (b) Measured
1
3
3
-hexene-2,5-dione concentrations for [NO
2
]
av ) (0.90-2.62) × 10
-3
molecules cm ; (- - -) calculated concentrations of 3-hexene-2,5-dione
assuming that 3-hexene-2,5-dione is a first-generation product with a
molar yield of 0.29 and reacts with the OH radical with a rate constant
s ; (s) calculated concentrations of
3-hexene-2,5-dione assuming that 3-hexene-2,5-dione is a second-
butanedione formation yield decreased with increasing NO2
concentration, as shown in Figure 6, and the formation yields
are given in Table 3. There is no evidence for 2,3-butanedione
being a second-generation product (i.e., being formed from a
diunsaturated dicarbonyl such as CH3C(O)C(CH3)dC(CH3)-
CHdCHCHO), because the experimental data shown in Figure
-
11
3
-1 -1
of 6.0 × 10
cm molecule
generation product (see text and reactions 5-7) with molar yields of
0
6
.30 and 0.40, and reacts with the OH radical with a rate constant of
-
11
3
-1 -1
.0 × 10 cm molecule
s .
5
show no evidence for a delay in the formation of 2,3-
butanedione. If 2,3-butanedione was a second-generation prod-
uct, the delay in its formation would probably be more
pronounced than that for formation of 3-hexene-2,5-dione from
p-xylene (shown by the calculated lines in Figure 4) because
of the higher reactivity of 1,2,3-trimethylbenzene versus that
of the di-unsaturated dicarbonyl precursor compared to the
corresponding reactivities in the p-xylene reaction system.
Our yield data are compared with literature values in Table
1,2,4-Trimethylbenzene. GC-MS and GC-FID analysis of
irradiated CH ONO-NO-1,2,4-trimethylbenzene-air mixtures
3
showed the formation of 2,3-butanedione and 3-hexene-2,5-
dione, with the trans-/cis-3-hexene-2,5-dione ratio increasing
with the extent of reaction as observed in the p-xylene reaction
(see above). Plots of the amounts of 2,3-butanedione and
3-hexene-2,5-dione formed, corrected for reaction with the OH
radical in the case of 3-hexene-2,5-dione, against the amounts
of 1,2,4-trimethylbenzene reacted are shown in Figures 7 and
8, respectively. The formation yields of both 2,3-butanedione
and 3-hexene-2,5-dione decrease with increasing NO2 concen-
4
and it can be seen that there is reasonable agreement between
our 2,3-butanedione formation yields and those of Bandow and
Washida,26 Tuazon et al. and Atkinson and Aschmann.
27
12

Reactions of OH Radicals with p-Xylene and Trimethylbenzene
J. Phys. Chem. A, Vol. 104, No. 39, 2000 8927
TABLE 4: Comparison of the Formation Yields for
2
,3-Butanedione from 1,2,3-Trimethylbenzene in the
Presence of NO Obtained in the Present Study with
Literature Data
1
0-13[NO
formation yield (molecules cm-3)
2
]
product
,3-butanedione 0.45 ( 0.02
reference
2
1.2 (initial)
Bandow and
Washida
Tuazon et al.
Atkinson and
Aschmann
this work
2
6
2
7
0
0
.316 ( 0.036
.444 ( 0.053
∼5-10 (est)
0.58-2.4
1
2
a,b
0
0
0
.520b
0.0
2.04
9.85
.492
.383b
a
Extrapolated using the regression expression given in footnote b.
Calculated from the regression expression shown in Figure 6, of 2,3-
b
-
15
2 2
butanedione yield ) {0.520 - 1.40 × 10 [NO ]}, where the NO
-
3
concentration is in molecules cm units.
Figure 5. Plots of the amounts of 2,3-butanedione formed against the
amounts of 1,2,3-trimethylbenzene reacted with the OH radical at
-
3
differing average NO
2
concentrations (in molecules cm units). The
13
13
-3
data at [NO
2
]
av ) (4.78-4.96) × 10 and 2.04 × 10 molecules cm
1
1
12
have been displaced vertically by 5.0 × 10 and 1.0 × 10 molecules
-
3
cm , respectively, for clarity.
Figure 7. Plots of the amounts of 2,3-butanedione formed against the
amounts of 1,2,4-trimethylbenzene reacted with the OH radical at
-
3
2
differing average NO concentrations (in molecules cm units). The
1
3
13
data at [NO
0
1
2
]
av ) (4.06-5.28) × 10 , (1.60-2.20) × 10 , and (0.54-
1
3
-3
.82) × 10 molecules cm have been displaced vertically by 4.0 ×
11 11 12 -3
0 , 8.0 × 10 , and 1.2 × 10 molecules cm , respectively, for
clarity.
(
(
O)C(CH3)dCHC(CH3)dCHCHO and CH3C(O)CHdCHC-
CH3)dCHC(O)CH3, respectively). Once again, owing to the
Figure 6. Plot of the 2,3-butanedione formation yields from the OH
radical-initiated reaction of 1,2,3-trimethylbenzene against the average
NO concentration, [NO ] .
2 2 av
increased reactivity of 1,2,4-trimethylbenzene the delay in
second-generation product formation would probably be more
pronounced for the 1,2,4-trimethylbenzene reaction than for
formation of 3-hexene-2,5-dione from p-xylene (calculated lines
in Figure 4).
TABLE 3: Products Identified and Quantified, and Their
Formation Yields, from the OH Radical-Initiated Reaction
of 1,2,3-Trimethylbenzene in the Presence of NOa
Our formation yields are compared with the available
literature data in Table 6. Our present formation yields for 2,3-
butanedione are in reasonable agreement with those of Bandow
0-13[NO2]av
10-13[NO2]av
1
(
molecules cm-3) 2,3-butanedione (molecules cm-3) 2,3-butanedione
2
6
27
41
and Washida, Tuazon et al., and Smith et al., and suggest
that the differences between the literature formation yields2
are at least in part due to the different NO2 concentrations
2
4
4
.04
.78
.96
0.490 ( 0.018
0.463 ( 0.012
0.444 ( 0.026
9.82
9.85
0.384 ( 0.076
0.381 ( 0.026
6,27,41
2
6,27,41
a
present in those studies.
While our 3-hexene-2,5-dione
Indicated errors are two least-squares standard deviations. The
estimated uncertainties in the GC-FID response factors for 1,2,3-
trimethylbenzene and 2,3-butanedione are (5% each.
formation yields are higher by a factor of ∼2 than those reported
4
1
by Smith et al., our yields are slightly lower than the yield of
2
6,27,41
methylglyoxal,
the expected coproduct to 3-hexene-2,5-
tration, as shown in Figure 9, and the formation yields are given
in Table 5. Again there is no evidence for 2,3-butanedione or
dione and which can also be a coproduct to other C6-
1,4-unsaturated dicarbonyls. 3-Hexene-2,5-dione was also ob-
served in low yield by Tagaki et al. in an irradiated NO-
air-1,2,4-trimethylbenzene mixture (the low yield being almost
1
8
3-hexene-2,5-dione being a second-generation product (i.e.,
being formed from diunsaturated dicarbonyls such as CH3C-

8928 J. Phys. Chem. A, Vol. 104, No. 39, 2000
Bethel et al.
TABLE 5: Products Identified and Quantified, and Their
Formation Yields, from the OH Radical-Initiated Reaction
of 1,2,4-Trimethylbenzene in the Presence of NOa
1
0-13[NO
molecules cm-3)
2 av
]
(
3-hexene-2,5-dione
2,3-butanedione
0
0
1
2
4
5
.54
.82
.60
.20
.06
.28
0.273 ( 0.104
0.306 ( 0.062
0.309 ( 0.024
0.329 ( 0.017
0.282 ( 0.015
0.300 ( 0.017
0.265 ( 0.026
0.248 ( 0.033
0.120 ( 0.065
0.0825 ( 0.0049
0.0906 ( 0.0099
0.0969 ( 0.0092
0.0849 ( 0.0077
0.0782 ( 0.0095
0.0622 ( 0.0054
0.0545 ( 0.0121
1
1
0.46
2.81
a
Indicated errors are two least-squares standard deviations. The
estimated uncertainties in the GC-FID response factors for 1,2,4-
trimethylbenzene, 2,3-butanedione, and 3-hexene-2,5-dione are (5%
each.
TABLE 6: Comparison of the Formation Yields for
2
1
,3-Butanedione and 3-Hexene-2,5-dione from
,2,4-Trimethylbenzene in the Presence of NO Obtained in
the Present Study with Literature Data
1
0-13[NO2]
Figure 8. Plots of the amounts of 3-hexene-2,5-dione formed, corrected
for reaction with the OH radical (see text), against the amounts of 1,2,4-
product
formation yield (molecules cm-3)
reference
trimethylbenzene reacted with the OH radical at differing average NO
2
2,3-butanedione
0.11 ( 0.01
1.2 (initial)
Bandow and
2
6
-3
Washida
concentrations (in molecules cm units). The data at [NO
2
]
av ) (4.06-
2
7
1
3
13
13
0.048 ( 0.009
∼5-10 (est)
0.9-1.6 (final)
0.0
0.54
12.8
Tuazon et al.
5
.28) × 10 , (1.60-2.20) × 10 , and (0.54-0.82) × 10 molecules
4
1
-3 11 12
0.114 ( 0.024
Smith et al.
this work
cm have been displaced vertically by 5.0 × 10 , 1.0 × 10 , and 2.0
a,b
0
0
0
.102
101 molecules cm , respectively, for clarity.
2
-3
×
b
.100
.053b
3-hexene-2,5-dione 0.161 ( 0.012
0.9-1.6 (final)
Smith et al.41
this work
a,b
0
0
0
.309
0.0
.307b
0.54
.255b
12.8
methylglyoxal
0.37 ( 0.01
1.2 (initial)
Bandow and
2
6
Washida
2
7
0
0
.357 ( 0.017
.44 ( 0.074
∼5-10 (est)
Tuazon et al.
4
1
0.9-1.6 (final)
Smith et al.
a
Extrapolated using the regression expression given in footnote b.
b
Calculated from the regression expressions shown in Figure 9, of 2,3-
-
16
butanedione yield ) {0.102 - 3.83 × 10 [NO
2
]} and 3-hexene-2,5-
]}, where the NO
2
-
16
dione yield ) {0.309 - 4.20 × 10 [NO
2
-
3
concentration is in molecules cm units.
3
-hexene-2,5-dione from p-xylene and 1,2,4-trimethylbenzene
are similar to those reported for glyoxal from p-xylene and of
methylglyoxal from 1,2,4-trimethylbenzene, respectively, and
therefore suggest that these are coproducts as expected from,
for example, path B in Scheme 1.
The formation yields of 2,3-butanedione and 3-hexene-2,5-
dione from the reactions studied here do not change as
dramatically as does that of 2,3-butanedione from o-xylene,12
and yields from the reactions of the OH-aromatic adducts with
NO2 (YNO ) and the rate constant ratios kNO /kO for the reactions
Figure 9. Plots of the 2,3-butanedione and 3-hexene-2,5-dione
formation yields from the OH radical-initiated reaction of 1,2,4-
2
2
2
trimethylbenzene against the average NO
2 2 av
concentration, [NO ] .
of the OH-aromatic adducts with NO2 (kNO ) and O2 (kO )
2
2
certainly due to secondary reactions and photolysis of 3-hexene-
cannot be unambiguously obtained. However, extrapolation of
our yield data to 1/[NO2] ) 0 suggests that the yields are as
follows: formation of 3-hexene-2,5-dione from p-xylene, YO2
2,5-dione).
Atmospheric Implications. Our product analyses show that
the ring-cleavage products 2,3-butanedione and 3-hexene-2,5-
dione exhibit a dependence of their formation yields on the NO2
concentration, as previously observed for the formation of 2,3-
) 0.32 (Table 2) and YNO ∼ 0.15; formation of 2,3-butanedione
2
from 1,2,3-trimethylbenzene, YO ) 0.52 (Table 4) and YNO ∼
2
2
0.30; formation of 2,3-butanedione from 1,2,4-trimethylbenzene,
12
butanedione from the OH radical-initiated reaction of o-xylene.
Our present observations indicate that 2,3-butanedione and
-hexene-2,5-dione are formed as primary products from the
YO2 ) 0.10 (Table 6) and YNO2 ∼ 0.05; and formation of
3-hexene-2,5-dione from 1,2,4-trimethylbenzene, YO ) 0.31
2
3
(Table 6) and YNO ∼ 0.25.
2
reactions of the OH-aromatic adducts studied here with O2 and
with NO2, and in higher yield from the O2 reactions than from
the NO2 reactions. Our yields extrapolated to zero NO2
concentration should be applicable to ambient atmospheric
conditions (provided that there is sufficient NO that peroxy
radicals react dominantly with NO). Our formation yields of
These YO yields suggest that in the atmosphere, the OH
2
radical-initiated reaction of p-xylene leads to the formation of
(with percentage yields) p-tolualdehyde, 7%; p-methylbenzyl
3
1
nitrate, 0.8%; 2,5-dimethylphenol, 13%; 3-hexene-2,5-dione
+ glyoxal, 32%; and 2-methyl-1,4-butenedial + methylglyoxal,
2
1,25,27,41
12%,
thus accounting for ∼65% of the products and

Reactions of OH Radicals with p-Xylene and Trimethylbenzene
J. Phys. Chem. A, Vol. 104, No. 39, 2000 8929
reaction pathways. Formation of C8-di-unsaturated dicarbonyls
(14) Darnall, K. R.; Atkinson, R.; Pitts, J. N., Jr. J. Phys. Chem. 1979,
8
83, 1943.
15) Tagaki, H.; Washida, N.; Akimoto, H.; Nagasawa, K.; Usui, Y.;
Okuda, M. J. Phys. Chem. 1980, 84, 478.
16) Atkinson, R.; Carter, W. P. L.; Darnall, K. R.; Winer, A. M.; Pitts,
J. N., Jr. Int. J. Chem. Kinet. 1980, 12, 779.
(as shown in Scheme 1) and unsaturated epoxydicarbonyls such
(
as
(
(
17) Besemer, A. C. Atmos. EnViron. 1982, 16, 1599.
(18) Tagaki, H.; Washida, N.; Akimoto, H.; Okuda, M. Spectrosc. Lett.
1
982, 15, 145.
19) Atkinson, R.; Carter, W. P. L.; Winer, A. M. J. Phys. Chem. 1983,
7, 1605.
(
must then account for some or all of the remaining products.
Similarly, under atmospheric conditions 1,2,3-trimethylben-
zene is anticipated to form: 2,3-butanedione + CH3C(O)CHd
CHCHO, 52%; methylglyoxal + CH3C(O)C(CH3)dCHCHO,
8
(
20) Dumdei, B. E.; O’Brien, R. J. Nature 1984, 311, 248.
(21) Tuazon, E. C.; Atkinson, R.; Mac Leod, H.; Biermann, H. W.;
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8%;26,27
26,27
1
and glyoxal + CH3C(O)C(CH3)dC(CH3)CHO, 7%,
(22) Shepson, P. B.; Edney, E. O.; Corse, E. W. J. Phys. Chem. 1984,
thus accounting for ∼77% of the products and with ring-
retaining products (trimethylphenols, dimethylbenzaldehydes
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8
8, 4122.
(23) Leone, J. A.; Flagan, R. C.; Grosjean, D.; Seinfeld, J. H. Int. J.
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8, 2531.
(
8
and unsaturated epoxydicarbonyls accounting for some or all
5
of the remaining products. 1,2,4-Trimethylbenzene is anticipated
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41
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(
27) Tuazon, E. C.; Mac Leod, H.; Atkinson, R.; Carter, W. P. L.
2
,3-butanedione + HC(O)C(CH3)dCHCHO, 10%; methylgly-
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(
28) Gery, M. W.; Fox, D. L.; Kamens, R. M.; Stockburger, L. EnViron.
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6,27,41
thus
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accounting for ∼65% of the products and with other ring-
cleavage products (for example, C9- di-unsaturated dicarbonyls
8
and unsaturated epoxydicarbonyls ) accounting for the remaining
products.
(
31) Atkinson, R.; Aschmann, S. M.; Arey, J. Int. J. Chem. Kinet. 1991,
3, 77.
32) Becker, K. H.; Barnes, I.; Bierbach, A.; Kirchner, F.; Thomas, W.;
2
Acknowledgment. The authors gratefully thank the U.S.
Environmental Protection Agency for support of this research
through Grant No. R826371-01-0 (“Ozone and Fine Particle
Formation in California and in the Northeastern United States”)
to the California Institute of Technology, and Grant No.
R825252-01-0 (from the U.S. Environmental Protection Agency-
National Center for Environmental Research and Quality
Assurance). While this research has been funded by Federal
funds from the U.S. Environmental Protection Agency, the
results and content of this publication do not necessarily reflect
the views and opinions of the Agency or the U.S. Government.
The authors also gratefully thank Drs. Harvey E. Jeffries and
Kenneth G. Sexton (University of North Carolina at Chapel Hill)
for the gift of a sample of cis-3-hexene-2,5-dione synthesized
by Dr. Ramiah Sangaiah.
(
Wiesen, E.; Zabel, F. “OH Initiated Oxidation of NMVOC under Variable
NOx Conditions”; EUROTRAC Annual Report 1992, Part 8: LACTOZ,
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(
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