Rate Constants for the Reactions of Oh Radicals with 1,2,4,5- Tetramethylbenzene, Pentamethylbenzene, 2,4,5-Trimethylbenzaldehyde, 2,4,5-Trimethylphenol, and 3-Methyl-3-Hexene-2,5-Dione and Products of OH + 1,2,4,5-tetramethylbenzene

Article abstract of DOI:10.1021/jp400323n

Using a relative rate method, rate constants have been measured for the reactions of OH radicals with 1,2,4,5-tetramethylbenzene, pentamethylbenzene, 2,4,5-trimethylbenzaldehyde, 2,4,5-trimethylphenol and 3-methyl-3-hexene-2,5- dione at 298 ± 2 K and atmospheric pressure of air. The rate constants obtained (in units of 10^-11 cm³ molecule^-1 s^-1) were: 1,2,4,5-tetramethylbenzene, 5.55 ± 0.34; pentamethylbenzene, 10.3 ± 0.8; 2,4,5-trimethylbenzaldehyde, 4.27 ± 0.39; 2,4,5-trimethylphenol, 9.75 ± 0.98; and 3-methyl-3-hexene-2,5-dione, 9.4 ± 1.1. The following first-generation products were identified from the OH + 1,2,4,5-tetramethylbenzene reaction in the presence of NO: biacetyl, methylglyoxal, 3-methyl-3-hexene-2,5-dione, and 2,4,5-trimethylbenzaldehyde. The measured molar formation yields for 3-methyl-3-hexene-2,5-dione and 2,4,5-trimethylbenzaldehyde were 45 ± 9% and 3.3 ± 0.7%, with that for 3-methyl-3-hexene-2,5-dione being extrapolated to low NO₂ concentrations where the OH-1,2,4,5- tetramethylbenzene adducts react only with O₂. Biacetyl appeared to be formed as both a first- and second-generation product, and a first-generation formation yield of 9 ± 3% was derived. The relative formation yield of methylglyoxal was ～0.8 of that for 3-methyl-3-hexene-2,5-dione, indicating that methylglyoxal and 3-methyl-3-hexene-2,5-dione are coproducts. H-atom abstraction from OH + 1,2,4,5-tetramethylbenzene is estimated to account for 3.7 ± 0.8% of the overall OH radical reaction. On the basis of the current understanding of the mechanism of the OH-aromatic adduct + O₂ reaction, the observed formation of biacetyl indicates that some ipso addition of OH occurs for OH + 1,2,4,5-tetramethylbenzene.

Full text of DOI:10.1021/jp400323n

Article
pubs.acs.org/JPCA
Rate Constants for the Reactions of OH Radicals with 1,2,4,5-
Tetramethylbenzene, Pentamethylbenzene, 2,4,5-
Trimethylbenzaldehyde, 2,4,5-Trimethylphenol, and 3‑Methyl-3-
hexene-2,5-dione and Products of OH + 1,2,4,5-Tetramethylbenzene
Sara M. Aschmann, Janet Arey,* and Roger Atkinson*
Air Pollution Research Center, University of California, Riverside, California 92521, United States
S
* Supporting Information
ABSTRACT: Using a relative rate method, rate constants have been measured
for the reactions of OH radicals with 1,2,4,5-tetramethylbenzene, pentamethyl-
benzene, 2,4,5-trimethylbenzaldehyde, 2,4,5-trimethylphenol and 3-methyl-3-
hexene-2,5-dione at 298
constants obtained (in units of 10−11 cm3 molecule−1 s−1) were: 1,2,4,5-
tetramethylbenzene, 5.55 0.34; pentamethylbenzene, 10.3 0.8; 2,4,5-
trimethylbenzaldehyde, 4.27 0.39; 2,4,5-trimethylphenol, 9.75 0.98; and 3-
methyl-3-hexene-2,5-dione, 9.4 1.1. The following first-generation products
2 K and atmospheric pressure of air. The rate
were identified from the OH + 1,2,4,5-tetramethylbenzene reaction in the
presence of NO: biacetyl, methylglyoxal, 3-methyl-3-hexene-2,5-dione, and 2,4,5-trimethylbenzaldehyde. The measured molar
formation yields for 3-methyl-3-hexene-2,5-dione and 2,4,5-trimethylbenzaldehyde were 45 9% and 3.3 0.7%, with that for 3-
methyl-3-hexene-2,5-dione being extrapolated to low NO2 concentrations where the OH-1,2,4,5-tetramethylbenzene adducts
react only with O2. Biacetyl appeared to be formed as both a first- and second-generation product, and a first-generation
formation yield of 9 3% was derived. The relative formation yield of methylglyoxal was ∼0.8 of that for 3-methyl-3-hexene-2,5-
dione, indicating that methylglyoxal and 3-methyl-3-hexene-2,5-dione are coproducts. H-atom abstraction from OH + 1,2,4,5-
tetramethylbenzene is estimated to account for 3.7 0.8% of the overall OH radical reaction. On the basis of the current
understanding of the mechanism of the OH-aromatic adduct + O2 reaction, the observed formation of biacetyl indicates that
some ipso addition of OH occurs for OH + 1,2,4,5-tetramethylbenzene.
reactions of OH radicals with the tetramethylbenzenes or
pentamethylbenzene. In this work we have measured rate
INTRODUCTION
■
Aromatic hydrocarbons account for ∼20% of nonmethane
volatile organic compounds in urban atmospheres.1 In the
troposphere, alkyl-substituted benzenes react with OH radicals,
by H-atom abstraction from the C−H bonds of the alkyl
substituents and by OH radical addition to the aromatic ring to
form OH-aromatic adducts.1,2 Kinetic data are available for the
reactions of OH radicals with a number of aromatic
hydrocarbons,1,2 and kinetic and product studies show that
H-atom abstraction accounts for <10% of the overall OH
radical reaction for toluene, the xylenes, and the trimethylben-
zenes at room temperature and below.1−4 For these
methylbenzenes, the electrophilic OH radical addition pathway
therefore dominates at room temperature and below. For alkyl-
substituted benzenes, OH radical addition occurs preferentially
ortho- and para- to the substituent alkyl group(s), with the
room temperature rate constants for OH radical addition
correlating well with the sum of the electrophilic substituent
constants, Σσ+.5,6
constants at 298
2 K for the gas-phase reactions of OH
radicals with 1,2,4,5-tetramethylbenzene and pentamethylben-
zene, as well as for 2,4,5-trimethylbenzaldehyde, 2,4,5-
trimethylphenol and 3-methyl-3-hexene-2,5-dione, observed
or potential reaction products from OH + 1,2,4,5-tetramethyl-
benzene. In addition, products formed from the OH + 1,2,4,5-
tetramethylbenzene reaction in the presence of NO have been
investigated, and formation yields of 2,4,5-trimethylbenzalde-
hyde, methylglyoxal, biacetyl and 3-methyl-3-hexene-2,5-dione
obtained.
EXPERIMENTAL METHODS
■
Experiments were carried out in a ∼7500 L volume Teflon
chamber at 298 2 K and ∼735 Torr of dry purified air. The
chamber is equipped with black lamps for irradiation and a
Teflon-coated fan to ensure rapid mixing of reactions during
introduction into the chamber. OH radicals were generated by
the photolysis of CH3ONO at >300 nm, and NO was present
While there have been numerous kinetic studies of the
reaction of OH radicals with benzene, toluene, the xylenes and
the trimethylbenzenes1 and a rate constant for the reaction of
OH radicals with hexamethylbenzene has been reported,7 no
published kinetic data are currently available concerning the
Received: January 10, 2013
Revised: February 21, 2013
Published: March 18, 2013
© 2013 American Chemical Society
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The Journal of Physical Chemistry A
Article
to suppress formation of O3 and hence of NO3 radicals. All
irradiations were carried out at a light intensity corresponding
to an NO2 photolysis rate of 0.14 min−1.
Dark decays were determined by monitoring, over periods of
several hours, the concentrations of 1,3,5-trimethylbenzene,
1,2,4,5-tetramethylbenzene, pentamethylbenzene, 2,4,5-trime-
thylbenzaldehyde and 2,4,5-trimethylphenol in the chamber in
dry air in the dark. Because of large losses of 2,4,5-
trimethylphenol observed in the postreaction replicate analyses
(see below), this compound was also monitored in the chamber
in the dark in the presence of CH3ONO and NOx.
Rate Constant Measurements. Rate constants for the
reactions of OH radicals with 1,2,4,5-tetramethylbenzene,
pentamethylbenzene, 2,4,5-trimethylbenzaldehyde and 2,4,5-
trimethylphenol were measured using a relative rate method in
which the concentrations of the aromatics and a reference
compound were measured in the presence of OH radicals.8
Under conditions that the aromatic and reference compound
are removed by reaction with OH radicals and the aromatic
may also undergo dark decay
Detection and Quantification of Selected Products
from OH + 1,2,4,5-Tetramethylbenzene. Irradiations of
CH3ONO−NO−1,2,4,5-tetramethylbenzene−air and
CH3ONO−NO−1,2,4,5-tetramethylbenzene−1,3,5-trimethyl-
benzene−air mixtures (the latter being the experiments used to
measure the rate constant for OH + 1,2,4,5-tetramethylben-
zene) were carried out, with 1,2,4,5-tetramethylbenzene and
selected products being measured by GC-FID analyses of gas
samples collected onto Tenax solid adsorbent as described
above for the kinetic studies. The initial CH3ONO and NO
concentrations (molecules cm−3) were ∼2.4 × 1014 each or
∼4.8 × 1013 each, and the initial 1,2,4,5-tetramethylbenzene
concentrations were (2.33−2.76) × 1013 molecules cm−3.
Additionally, for combined gas chromatography−mass
spectrometry (GC-MS) confirmation of products, one
CH3ONO−NO−1,2,4,5-tetramethylbenzene−air irradiation
and one CH3ONO−NO−1,2,4-trimethylbenzene−air irradia-
tion were carried out, with initial reactant concentrations
(molecules cm−3) of: CH3ONO and NO, ∼4.8 × 1013 each;
1,2,4,5-tetramethylbenzene, 2.65 × 1013; and 1,2,4-trimethyl-
benzene, ∼3.6 × 1013. A single irradiation was carried out (3
and 4 min for the 1,2,4,5-tetramethylbenzene and 1,2,4-
trimethylbenzene reactions, respectively), and GC-FID analyses
of 1,2,4,5-tetramethylbenzene or 1,2,4-trimethylbenzene were
carried out as described above. Samples were also collected by
exposing a 65 μm polydimethylsiloxane/divinylbenzene solid
phase microextraction (SPME) fiber to the chamber contents
for 20 min prior to reaction, after reaction, and (for the 1,2,4,5-
tetramethylbenzene reaction) after reaction and the sequential
addition of 2,4,5-trimethylbenzaldehyde and 2,4,5-trimethyl-
phenol to the chamber. The exposed SPME fibers were
thermally desorbed and analyzed by GC-MS using an HP-5MS
column interfaced to an Agilent 5973 Mass Selective Detector
operated in positive chemical ionization mode (PCI GC-MS)
with methane as the reagent gas. Samples were also collected
after each reaction onto a 5-channel, 400 mm length annular
denuder (URG-2000−30B5, URG, Chapel Hill, NC) coated
with finely ground XAD-4 resin and, to derivatize carbonyls to
their oximes,9 further coated with O-(2,3,4,5,6-
pentafluorobenzyl)hydroxyl amine (PFBHA) prior to sam-
pling.9 Samples were collected from the chamber at 15 L min−1
for 60 min directly onto the denuder, the entrance of which
extended into the chamber. The denuder samples were
extracted as described previously,9 and the extracts were
analyzed by GC-MS as described above, and by GC-FID using
a 30 m DB-5 column. Each carbonyl group derivatized to an
oxime adds 195 mass units to the compound’s molecular
weight, and methane-PCI gives characteristic protonated
molecules ([M + H]+) and smaller adduct ions at [M + 29]+
and [M + 41]+.9 Derivatized hydroxycarbonyls also exhibit
strong [M + H − H2O]+ fragment ions.8,9
OH + aromatic → products
(1)
OH + reference compound → products
(2)
(3)
aromatic → loss to chamber walls
then
⎛
⎝
⎞
⎠
[aromatic]t
[aromatic]
0
⎜
⎜
⎟
⎟
ln
− k (t − t )
3
0
t
⎛
⎞
[reference compound]t
[reference compound]
k1
k2
0
⎜
⎜
⎟
⎟
=
ln
⎝
⎠
t
(I)
where [aromatic]t and [reference compound]t are the
0
0
concentrations of aromatic and reference compound, respec-
tively, at time t0, [aromatic]t and [reference compound]t are the
corresponding concentrations at time t, and k1, k2 and k3 are the
rate constants for reactions 1, 2 and 3, respectively. Hence a
plot of {ln([aromatic]t /[aromatic]t) − k3(t − t0)} against
0
ln([reference compound]t /[reference compound]t) should be
0
a straight line of slope k1/k2 and zero intercept.
The initial reactant concentrations (molecules cm−3) were:
CH3ONO, ∼2.4 × 1014; NO, ∼2.4 × 1014; and aromatic and
reference compound, ∼2.4 × 1013 each. 1,3,5-Trimethylben-
zene was used as the reference compound, since its rate
constant for reaction with OH radicals is reliably known.1,2
Irradiations were carried out for up to 9 min, resulting in up to
64, 88, 62, 86 and 70% consumption of the initially present
1,2,4,5-tetramethylbenzene, pentamethylbenzene, 2,4,5-trime-
thylbenzaldehyde, 2,4,5-trimethylphenol and 1,3,5-trimethyl-
benzene, respectively. The concentrations of 1,2,4,5-tetrame-
thylbenzene, pentamethylbenzene, 2,4,5-trimethylbenzalde-
hyde, 2,4,5-trimethylphenol and 1,3,5-trimethylbenzene were
measured during the experiments by gas chromatography with
flame ionization detection (GC-FID). Gas samples of 100 cm3
volume were collected from the chamber onto Tenax-TA solid
adsorbent, and subsequently thermally desorbed onto a 30 m
DB-1701 megabore column, temperature programmed from
−40 °C at 8 °C min−1. 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. Three experiments were conducted for
each aromatic compound. Replicate analyses in the dark, after
correcting for any dark decays (see below), showed that the
measurement uncertainties were typically ≤2% for 1,3,5-
trimethylbenzene and 1,2,4,5-tetramethylbenzene and ≤3%
for pentamethylbenzene, 2,4,5-trimethylbenzaldehyde and
2,4,5-trimethylphenol.
For the analyses of samples collected onto Tenax solid
adsorbent, GC-FID response factors for 1,2,4,5-tetramethyl-
benzene and biacetyl were determined by introducing measured
amounts of the chemical into the chamber and conducting
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The Journal of Physical Chemistry A
Article
several replicate GC-FID analyses. Because of difficulties in
quantitatively introducing 2,4,5-trimethylbenzaldehyde and
2,4,5-trimethylphenol into the chamber, and the nonavailability
of 3-methyl-3-hexene-2,5-dione, GC-FID response factors for
these products or potential products were calculated by
combining the measured GC-FID response factor for 1,2,4,5-
tetramethylbenzene with calculated Effective Carbon Numbers
for 1,2,4,5-tetramethylbenzene, 2,4,5-trimethylbenzaldehyde,
2,4,5-trimethylphenol and 3-methyl-3-hexene-2,5-dione, of
10.0, 9.0, 9.0 and 4.9, respectively.10 Note that this assumes
collection and desorption efficiencies for 2,4,5-trimethylbenzal-
dehyde, 2,4,5-trimethylphenol and 3-methyl-3-hexene-2,5-
dione equal to that of 1,2,4,5-tetramethylbenzene.
A single CH3ONO−NO−1,2,4,5-tetramethylbenzene−air
irradiation was conducted with analysis by direct air sampling
atmospheric pressure ionization tandem mass spectrometry
(API-MS/MS), with initial reactant concentrations similar to
those for the corresponding experiment carried out with
sampling onto the PFBHA-coated denuder and GC-MS
analysis (see above), and with two irradiation periods of 1
and 2 min, respectively. API-MS and API-MS/MS analyses
were conducted in positive ion mode as described previously.8
Formation Yield of 3-Methyl-3-hexene-2,5-dione and
its OH Radical Reaction Rate Constant. 3-Methyl-3-
hexene-2,5-dione was identified as a major product from OH
+ 1,2,4,5-tetramethylbenzene by GC-MS analyses of SPME
fiber and PFBHA-coated denuder samples (with the latter
identification being of its dioximes), and was quantified from
GC-FID analyses of samples collected onto Tenax solid
adsorbent. Since no rate constant for the reaction of OH
radicals with 3-methyl-3-hexene-2,5-dione was available, the
approach described by Baker et al.11 was used to derive the
removal rate of 3-methyl-3-hexene-2,5-dione and the formation
yield of 3-methyl-3-hexene-2,5-dione from OH + 1,2,4,5-
tetramethylbenzene. For the reaction sequence
(90%), Aldrich; 2,4,5-trimethylphenol (purity not stated), City
Chemical; O-(2,3,4,5,6-pentafluorobenzyl)hydroxylamine hy-
drochloride (99+%), Alfa Aesar; and NO (≥99.0%), Matheson
Gas Products. Methyl nitrite was prepared and stored as
described previously.8,9,12
RESULTS
■
Dark Decays. Dark decays were investigated by monitoring
the concentrations of the compounds in the chamber in the
dark for 2.3−6.6 h. There was no evidence for any dark decay
of 1,3,5-trimethylbenzene or 1,2,4,5-tetramethylbenzene, with
<2% loss of 1,3,5-trimethylbenzene over 4.8 h and <3% loss of
1,2,4,5-tetramethylbenzene over 6.6 h (corresponding to k3
<1.3 × 10−6 s−1 in each case). However, pentamethylbenzene,
2,4,5-trimethylbenzaldehyde and 2,4,5-trimethylphenol did
undergo dark decay, with measured decay rates, k3, in dry
purified air of (2.0 1.0) × 10−6s−1, (5.1 1.0) × 10−6 s−1 and
(2.3 0.9) × 10−6 s−1, respectively, where the indicated errors
are two least-squares standard deviations. Over the typical
duration of the experiments to measure the OH radical reaction
rate constants (∼3.0 h from sampling for the last prereaction
analysis to sampling for the replicate post-third irradiation
analysis), these dark decays correspond to 2.2% and 5.5% loss
of pentamethylbenzene and 2,4,5-trimethylbenzaldehyde, re-
spectively.
For 2,4,5-trimethylphenol, replicate postreaction analyses in
irradiated CH3ONO−NO−2,4,5-trimethylphenol−1,3,5-trime-
thylbenzene−air mixtures showed dark decays which were
much more rapid than that measured in dry purified air.
Accordingly, the dark decay of 2,4,5-trimethylphenol was
measured in dry purified air in the presence of CH3ONO
and NOx (i.e., with initial reactant concentration similar to
those used for the kinetic experiments, but with no irradiation).
Under these conditions, the dark decay rate of 2,4,5-
trimethylphenol was (1.7
0.3) × 10−5 s−1, a factor of 7
faster than in dry air in the absence of CH3ONO and NOx.
Correcting for this dark decay rate in the presence of CH3ONO
and NOx resulted in excellent agreement of postreaction
replicate analyses of 2,4,5-trimethylphenol in the kinetic
experiments. GC-MS analyses of a SPME fiber exposed to
2,4,5-trimethylphenol in the presence of CH3ONO and NOx
showed the formation of a product of molecular weight 181,
presumably 2,4,5-trimethyl-6-nitrophenol or 2,4,5-trimethyl-3-
nitrophenol assumed to be formed by heterogeneous reaction
of 2,4,5-trimethylphenol with NO2. Note that NO2 is formed
by thermal oxidation of NO during introduction of NO to the
chamber and subsequently in the dark, as well as by NO-to-
NO2 conversion during CH3ONO−NO−air phootoxidations
of volatile organic compounds from reactions of HO2 and
organic peroxy radicals with NO.2 The observation that the
dark decay rate of 2,4,5-trimethylphenol after irradiation of
CH3ONO−NO−2,4,5-trimethylphenol−1,3,5-trimethylben-
zene−air mixtures was the same within the experimental
uncertainties as in a nonirradiated CH3ONO−NO−2,4,5-
trimethylphenol−1,3,5-trimethylbenzene−air mixture suggests
that while the dark decay rate of 2,4,5-trimethylphenol was
markedly faster in the presence of NOx (presumably NO2), it
was independent of the gas-phase concentration of NOx (or
NO2).
OH + 1,2,4,5‐tetramethylbenzene
→ α3‐methyl‐3‐hexene‐2,5‐dione + other products
(4)
OH + 3‐methyl‐3‐hexene‐2,5‐dione → products
(5)
3‐methyl‐3‐hexene‐2,5‐dione
→ loss of 3‐methyl‐3‐hexene‐2,5‐dione
(6)
then the concentration of 3-methyl-3-hexene-2,5-dione at time t
is given by
[3‐methyl‐3‐hexene‐2,5‐dione]t = A(e−x − e−Bx
)
(II)
where α is the formation yield of 3-methyl-3-hexene-2,5-dione
from reaction 4, A = α[1,2,4,5-tetramethylbenze-
ne]initial{k4[OH]/(k5[OH] + k6)]}, B = (k5[OH] + k6)/
k4[OH], and x = extent of reaction = ln([1,2,4,5-tetramethyl-
benzene]t /[1,2,4,5-tetramethylbenzene]t). Hence a fit of eq II
0
to the experimental data leads to values of A and B and hence
of α and (k5[OH] + k6)/k4[OH].11
CHEMICALS
■
The chemicals used, and their stated purities, were: biacetyl
[2,3-butanedione] (99%), 1,2,4-trimethylbenzene (98%), 1,3,5-
trimethylbenzene (98%), 1,2,4,5-tetramethylbenzene (98%),
pentamethylbenzene (98%) and 2,4,5-trimethylbenzaldehyde
Rate Constants for OH Radical Reactions with 1,2,4,5-
Tetramethylbenzene, Pentamethylbenzene, 2,4,5-Tri-
methylbenzaldehyde and 2,4,5-Trimethylphenol. A
series of CH3ONO − NO − aromatic −1,3,5-trimethylbenzene
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The Journal of Physical Chemistry A
Article
− air irradiations were carried out, and the data obtained are
plotted in accordance with eq I in Figure 1.
Table 1. Rate Constant Ratios k1/k2 or k5/k4 and Rate
Constants k1 or k5 for the Reactions of OH Radicals with
1,2,4,5-Tetramethylbenzene, Pentamethylbenzene, 2,4,5-
Trimethylbenzaldehyde, 2,4,5-Trimethylphenol and 3-
Methyl-3-hexene-2,5-dione at 298 2 K and Atmospheric
Pressure of Air
1011 × k1 or k5
(cm3 molecule−1 s−1
)
a
b
aromatic
k1/k2 or k5/k4
this work
literature
1,2,4,5-
tetramethylbenzene
0.979 0.006
5.55 0.34
pentamethylbenzene
1.81 0.03
10.3 0.8
2,4,5-
0.753 0.015
4.27 0.39
trimethylbenzaldehyde
cd
,
2,4,5-trimethylphenol
1.72 0.04
9.75 0.98
11.7 0.5
12.9 0.8
ce
,
f
g
3-methyl-3-hexene-2,5-
dione
1.70 0.16
9.4 1.1
a
Relative to OH + 1,3,5-trimethylbenzene unless noted otherwise. The
rate constant ratio involving 3-methyl-3-hexene-2,5-dione is referred to
as k5/k4. The data for pentamethylbenzene, 2,4,5-trimethylbenzalde-
hyde and 2,4,5-trimethylphenol have been corrected for dark decay,
using dark decay rates, k3, of 2.0 × 10−6, 5.1 × 10−6 and 1.7 × 10−5 s−1,
respectively (that for 2,4,5-trimethylphenol being in the presence of
CH3ONO and NOx; see text). Corrections for dark decays reduced
the rate constant ratios k1/k2 and rate constants k1 for
pentamethylbenzene, 2,4,5-trimethylbenzaldehyde and 2,4,5-trimethyl-
phenol by 1%, 6% and 8%, respectively, compared to those obtained
neglecting dark decay corrections. The indicated errors are two least-
squares standard deviations; the estimated overall uncertainties are
6% for 1,2,4,5-tetramethylbenzene, 7% for pentamethylbenzene,
9% for 2,4,5-trimethylbenzaldehyde and 10% for 2,4,5-trimethylphe-
Figure 1. Plots of eq I for the reaction of OH radicals with 2,4,5-
trimethylbenzaldehyde, 1,2,4,5-tetramethylbenzene (1,2,4,5-TMB),
2,4,5-trimethylphenol and pentamethylbenzene, with 1,3,5-trimethyl-
benzene as the reference compound. The measured concentrations of
pentamethylbenzene, 2,4,5-trimethylbenzaldehyde and 2,4,5-trimethyl-
phenol have been corrected for dark decay (k3) using our measured
dark decay rates, with that for 2,4,5-trimethylphenol being the decay
rate in the presence of CH3ONO and NOx (see text). The data for
1,2,4,5-tetramethylbenzene, 2,4,5-trimethylphenol and pentamethyl-
benzene have been displaced vertically by 0.10, 0.10, and 0.30 units,
respectively, for clarity. Data are from 3 experiments for each aromatic,
with GC-FID analyses of samples collected onto Tenax solid
adsorbent.
b
nol. Placed on an absolute basis using a rate constant for the reaction
of OH radicals with 1,3,5-trimethylbenzene of k2 = 5.67 × 10−11 cm3
molecule−1 s−1,2 unless noted otherwise. Unless noted otherwise, the
indicated errors are the estimated overall uncertainties (see footnote-
In Figure 1, the measured concentrations of pentamethyl-
benzene, 2,4,5-trimethylbenzaldehyde and 2,4,5-trimethylphe-
nol have been corrected for dark decay using our measured
dark decay rates, with that for 2,4,5-trimethylphenol being the
decay rate in the presence of CH3ONO and NOx. Least-squares
analyses of the data shown in Figure 1 lead to the rate constant
ratios k1/k2 listed in Table 1. These rate constant ratios can be
placed onto an absolute basis using a rate constant of k2(OH +
1,3,5-trimethylbenzene) = 5.67 × 10−11 cm3 molecule−1 s−1,2
and the resulting rate constants k1 are also given in Table 1.
Identification of products from OH + 1,2,4,5-
Tetramethylbenzene. Table 2 lists the products identified
from the OH + 1,2,4,5-tetramethylbenzene reaction. Authentic
standards of 2,4,5-trimethylbenzaldehyde and biacetyl were
available, allowing absolute identification and quantification.
Other products were identified by GC-MS analysis of SPME
fiber samples, GC-MS analysis of extracts of PFBHA-coated
denuder samples (i.e., with carbonyls derivatized to their
oximes), and by positive ion mode API-MS analysis. Since it
was expected that at least some of the products formed from
the previously studied OH + 1,2,4-trimethylbenzene reac-
tion9,14−18 and the OH + 1,2,4,5-tetramethylbenzene reaction
would be identical, the products of the two reactions were
compared. As detailed in the Supporting Information, the
products identified from GC-MS analyses of extracts of
PFBHA-coated denuder samples from the OH + 1,2,4,5-
tetramethylbenzene reaction were: 2,4,5-trimethylbenzalde-
hyde, biacetyl (2,3-butanedione), methylglyoxal, 3-methyl-3-
hexene-2,5-dione [CH3C(O)C(CH3)CHC(O)CH3], and a
hydroxydicarbonyl of molecular weight 116 attributed to
c
(a)) and do not include uncertainties in the rate constant k2. From
d
Bejan et al.,13 at 298 2 K. Measured relative to OH + isoprene, with
the measured rate constant ratio of 1.17 0.05 being placed on an
absolute basis using k(OH + isoprene) = 1.00 × 10−10 cm3 molecule−1
e
s−1.2,4 Measured relative to OH + 1,3-butadiene, with the measured
rate constant ratio of 1.94 0.12 being placed on an absolute basis
using k(OH + 1,3-butadiene) = 6.66 × 10−11 cm3 molecule−1 s−1.2
f
Measured relative to k4(OH + 1,2,4,5-tetramethylbenzene); see text
and Figure 2. The uncertainty is two standard errors of the nonlinear
g
least-squares fit to eq II. Placed on an absolute basis using our
measured rate constant for the reaction of OH radicals with 1,2,4,5-
tetramethylbenzene of k4(OH + 1,2,4,5-tetramethylbenzene) = (5.55
0.34) × 10−11 cm3 molecule−1 s−1. The indicated error is the two
standard errors in the rate constant ratio combined with the cited
uncertainty in the rate constant for OH + 1,2,4,5-tetramethylbenzene.
CH3C(O)CH(OH)C(O)CH3 and/or CH3C(O)C(CH3)-
(OH)CHO and formed as a second-generation product from
OH + 3-methyl-3-hexene-2,5-dione (see Supporting Informa-
tion). As noted in Table 2 and presented in the Supporting
Information, the API-MS and API-MS/MS analyses were
consistent with the formation of these products (with the
exception of methylglyoxal whose [M + H]+ at m/z = 73 would
be obscured by a water cluster ion), and in addition indicated
the formation of products of molecular weight 100, 112, 142,
182 (tentative) and 227.
GC-MS analyses of samples collected onto SPME fibers from
the OH + 1,2,4-trimethylbenzene and OH + 1,2,4,5-
tetramethylbenzene reactions (without derivatization) both
showed the presence of an identical molecular weight 126
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Table 2. Products Identified from the Reaction of OH Radicals with 1,2,4,5-Tetramethylbenzene in the Presence of NO
observed by
a
product molecular weight
product attributed to
GC-MS
API-MS
product quantified by
bc
d
,
72
methylglyoxal
biacetyl
yes
yes
GC-FID denuder
bce
, ,
86
yes
GC-FID Tenax
100
112
yes
2,3-dimethyl-1,4-butenedial
yes
f
bcf
, ,
d
116
CH3C(O)CH(OH)C(O)CH3 and/or CH3C(O)C(CH3)(OH)CHO
3-methyl-3-hexene-2,5-dione
yes
yes
yes
GC-FID denuder
bcg
, ,
126
142
148
150
182
227
yes
GC-FID Tenax
yes
bgh
, ,
2,4,5-trimethylbenzaldehyde
2,3,5,6-tetramethylphenol
yes
yes
GC-FID Tenax
i
i
j
tentative
tentative
tentative
k
yes
a
b
See Figure S2 and associated figure caption in the Supporting Information. Observed as its dioxime or (for 2,4,5-trimethylbenzaldehyde) mono-
c
oxime by GC-MS analyses of extract of PFBHA-coated denuder sample (see Supporting Information). Observed in GC-MS analyses of extract of
the PFBHA-coated denuder sample of OH + 1,2,4-trimethylbenzene reaction (see Supporting Information). Quantified, relative to 3-methyl-3-
d
hexene-2,5-dione, from GC-FID analysis of their dioximes in the extract of the PFBHA-coated denuder sample. Yield placed on an absolute basis
using the quantification of 3-methyl-3-hexene-2,5-dione from GC-FID analyses of samples collected onto Tenax solid adsorbent (without
e
derivatization). Confirmed by retention time matching of dioximes in GC-MS analyses with those of authentic standard introduced into chamber
f
and sampled onto PFBHA-coated denuder. Second-generation product from OH + 3-methyl-3-hexene-2,5-dione (see Supporting Information),
consistent with API-MS analyses after 1 and 3 min of reaction which showed that the molecular weight 116 product increased with extent of reaction
g
h
significantly more rapidly than did 3-methyl-3-hexene-2,5-dione. Observed by GC-MS analysis of sample collected onto SPME fiber. Confirmed by
retention time matching in GC-MS analysis of sample collected onto SPME fiber with that of authentic standard introduced into chamber and
i
similarly sampled. Evidence for presence of a minor molecular weight 150 product in SPME GC-MS analysis, but could not be confirmed due to the
lack of an authentic standard. In the API-MS analysis, a small peak attributed to [150 + H]+ was present, but no MS/MS analysis was conducted.
j
Also present in prereaction API-MS analysis, and the m/z = 183 signal remained approximately constant relative to that of 1,2,4,5-
tetramethylbenzene (i.e., m/z = 134 + 164) during the reaction. This suggests that this molecular weight 182 species was formed in the ion source
k
from 1,2,4,5-tetramethylbenzene (and could have been the unsaturated epoxy-dicarbonyl shown in Scheme 3). Probably a nitrate (see caption to
Figure S2, Supporting Information).
product, and this is assigned to the 3-methyl-3-hexene-2,5-
dione observed in the extracts of the PFBHA-coated denuder
samples. This indicates that 3-methyl-3-hexene-2,5-dione, like
3-hexene-2,5-dione,17 can be analyzed by GC without prior
derivatization. Indeed, GC-FID analyses of samples collected
onto Tenax solid adsorbent showed a GC peak with identical
retention times from both reactions, attributed to 3-methyl-3-
hexene-2,5-dione. As noted in Table 2, 2,4,5-trimethylbenzal-
dehyde, 3-methyl-3-hexene-2,5-dione and biacetyl were then
quantified from GC-FID analyses of samples collected onto
Tenax solid adsorbent. Quantification of methylglyoxal relative
to 3-methyl-3-hexene-2,5-dione was obtained from GC-FID
analyses of their dioximes in the extract of the PFBHA-coated
denuder sample, taking into account the differences in the
ECNs of the various dioximes10,18 and secondary reactions of
these dicarbonyls with OH radicals (see footnote (f) to Table
3). The resulting methylglyoxal formation yield, relative to the
3-methyl-3-hexene-2,5-dione yield measured by Tenax/GC-
FID analysis, is listed in Table 3. The amount of the molecular
weight 116 hydroxydicarbonyl attributed to CH3C(O)CH-
(OH)C(O)CH3 and/or CH3C(O)C(CH3)(OH)CHO which
was present (i.e., uncorrected for any wall losses or for
photolysis and/or reaction with OH radicals), similarly derived
from the GC-FID analyses of the extract of the PFBHA-coated
denuder sample, was ∼45% of the amount of 3-methyl-3-
hexene-2,5-dione calculated to have reacted with OH radicals.
The GC-MS analysis of a postreaction SPME fiber showed
evidence for minor formation of a tetramethylphenol (see
Table 2). There was no conclusive evidence for formation of
2,4,5-trimethylphenol.
hexene-2,5-dione is not commercially available, the removal
rate of 3-methyl-3-hexene-2,5-dione and its formation yield
from OH + 1,2,4,5-tetramethylbenzene were obtained from a fit
of eq II to the experimental data. The initial 1,2,4,5-
tetramethylbenzene concentrations ranged from (2.48−2.76)
× 1013 molecules cm−3, and a plot of the measured 3-methyl-3-
hexene-2,5-dione concentrations, adjusted to [1,2,4,5-tetrame-
thylbenzene]initial = 2.50 × 1013 molecules cm−3, against the
extent of reaction, defined as ln([1,2,4,5-tetramethylbenzene]t /
0
[1,2,4,5-tetramethylbenzene]t), is shown in Figure 2. A
nonlinear least-squares regression analysis leads to A = (1.27
0.22) × 1013 molecules cm−3 and B = (1.70 0.16), where
the indicated errors are two standard errors, and the solid line
in Figure 2 is eq II with these parameters. Replicate
postreaction GC-FID analyses showed no evidence for dark
decay of 3-methyl-3-hexene-2,5-dione. Furthermore, by analogy
with 3-hexene-2,5-dione,17 it is likely that 3-methyl-3-hexene-
2,5-dione photolyzes only slowly, if at all, by black lamps.
Hence it is likely that k6 ≈ 0, and hence that B = k5/k4 = 1.70
0.16 and, combined with k4(OH + 1,2,4,5-tetramethylbenzene)
from Table 1, k5 = (9.4 1.1) × 10−11 cm3 molecule−1 s−1
(Table 1).
From A = α[1,2,4,5-tetramethylbenzene]initial{k4[OH]/
(k5[OH] + k6)]} = 1.27 × 1013 molecules cm−3, the molar
formation yield of 3-methyl-3-hexene-2,5-dione from OH +
1,2,4,5-tetramethylbenzene, α, is 35.6%. This formation yield
was measured in experiments with initial CH3ONO and NO
concentrations of ∼2.4 × 1014 molecules cm−3 each, and with
an average NO2 concentration during the reactions of 1.02 ×
1014 molecules cm−3 (assuming that ([NO] + [NO2]) =
constant, as previously derived from computer calculations of
irradiated CH3ONO−NO−NO2−toluene−air mixtures using a
Formation Yield of 3-Methyl-3-hexene-2,5-dione and
its OH Radical Reaction Rate Constant. Since 3-methyl-3-
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Table 3. First-generation Products Quantified from the
Reaction of OH Radicals with 1,2,4,5-Tetramethylbenzene
in the Presence of NO
10−13 × [NO2]
ava
(molecules cm−3
molar yield
(%)
product
)
b
2,4,5-
9.0−11.9 (10.2)
1.5−2.3 (1.9)
1.5−11.9
3.5 0.3
b
trimethylbenzaldehyde
3.8 0.8
c
3.3 0.7
2,4,5-trimethylphenol
9.0−11.9
<3
d
3-methyl-3-hexene-2,5-
dione
9.0−11.9 (10.2)
1.5−2.3 (1.9)
0
35
43
45
7
8
9
d
e
f
methylglyoxal
1.5
35
9
g
biacetyl (2,3-butanedione)
a
1.5−11.9
3
Range of average NO2 concentrations for the experiments conducted.
The value in parentheses is the average for those experiments.
b
c
Indicated errors are two least-squares standard deviations. For the
Figure 2. Plot of the amounts of 3-methyl-3-hexene-2,5-dione
measured during OH + 1,2,4,5-tetramethylbenzene (1,2,4,5-TMB)
reactions against the extents of reaction, defined as ln([1,2,4,5-
entire data set shown in Figure 5. The indicated error is two least-
squares standard deviations combined with the estimated uncertainty
in the GC-FID response factor for 2,4,5-trimethylbenzaldehyde
relative to that for 1,2,4,5-tetramethylbenzene of 15% (the yield
tetramethylbenzene]t /[1,2,4,5-tetramethylbenzene]t) [eq II]. The
0
○
△ ▲
, ,
differing symbols refer to the 6 experiments carried out ( ,
d
with two least-squares standard deviation errors is 3.3 0.4%). The
□ ◇
▽
in the presence of 1,3,5-trimethylbenzene;
,
,
, in the absence of
3-methyl-3-hexene-2,5-dione formation yields are 43.2
2.8% and
1,3,5-trimethylbenzene). The solid line is the fit to eq II with A = 1.27
× 1013 molecules cm−3 and B = (k5[OH] + k6)/k4[OH] = 1.70.
1,2,4,5-Tetramethylbenzene and 3-methyl-3-hexene-2,5-dione were
measured from GC-FID analyses of samples collected onto Tenax
solid adsorbent (see text).
35.5 1.5% at average NO2 concentrations of 1.9 × 1013 and 10.2 ×
1013 molecules cm−3, respectively, where the errors are two least-
squares standard deviations of the slopes of the plots shown in Figure
3. The indicated errors given in the table are the two least-squares
standard deviations combined with estimated uncertainties in the GC-
FID response factor for 3-methyl-3-hexene-2,5-dione relative to that
for 1,2,4,5-tetramethylbenzene of 15% and in the rate constant ratio
∼2.4 × 1014 and ∼4.8 × 1013 molecules cm−3 each. Plots of the
amount of 3-methyl-3-hexene-2,5-dione formed, corrected for
secondary reactions19 using k4[OH]/(k5[OH] + k6) = 1.70,
against the amounts of 1,2,4,5-tetramethylbenzene reacted are
shown in Figure 3. Excellent straight-line plots are obtained,
indicating that the rate constant ratio k4[OH]/(k5[OH] + k6) is
appropriate. Least-squares analyses leads to the 3-methyl-3-
hexene-2,5-dione formation yields listed in Table 3, together
with the average NO2 concentrations during these two sets of
experiments. As may be expected, the 3-methyl-3-hexene-2,5-
dione formation yield at an average NO2 concentration of 1.02
× 1014 molecules cm−3 obtained from the plot in Figure 3
(35.5%) is essentially identical to that obtained from the same
data using eq II and shown in Figure 2 (35.6%).
Formation Yields of 2,4,5-Trimethylbenzaldehyde,
2,4,5-Trimethylphenol and Biacetyl from OH + 1,2,4,5-
Tetramethylbenzene. 2,4,5-Trimethylbenzaldehyde, 2,4,5-
trimethylphenol and biacetyl were quantified from GC-FID
analyses of samples collected onto Tenax solid adsorbent.
Secondary reactions with OH radicals (plus dark decay, where
applicable) were taken into account as described previously,19
using our measured rate constants for 1,2,4,5-tetramethylben-
zene, 2,4,5-trimethylbenzaldehyde and 2,4,5-trimethylphenol
(Table 1) and a rate constant for OH + biacetyl of 2.3 × 10−13
cm3 molecule−1 s−1.4 The multiplicative correction factors F
increase with the rate constant ratio k(OH + product)/k(OH +
1,2,4,5-tetramethylbenzene) and with the extent of reaction,19
and the maximum values of F were 1.59 for formation of 2,4,5-
trimethylbenzaldehyde, 2.25 for any formation of 2,4,5-
trimethylphenol, and 1.003 for formation of biacetyl (and
hence no corrections for loss to the measured biacetyl
concentrations were made).
k4[OH]/(k5[OH] + k6) of 10%, corresponding to a
8%
uncertainty in the maximum value of the multiplicative correction
e
factor F. Yield at low (zero) NO2 concentration was calculated using
f
eq III. The uncertainty is the estimated overall uncertainty. Derived
from GC-FID analyses of the extract of a PFBHA-coated denuder
sample, relative to a 3-methyl-3-hexene-2,5-dione formation yield of
43.2% and taking into account the slight differences in the ECNs of the
dioximes10,18 and secondary reaction with OH radicals (using a rate
4
constant for OH + methylglyoxal of 1.3 × 10−11 cm3 molecule−1 s−1
and those in Table 1 for 1,2,4,5-tetramethylbenzene and 3-methyl-3-
hexene-2,5-dione). Relative to 3-methyl-3-hexene-2,5-dione =1.0, the
methylglyoxal formation yield was 0.82 0.06, where the error is two
standard deviations from 3 replicate injections. This methylglyoxal
yield is not corrected for expected secondary formation of
methylglyoxal from OH + 3-methyl-3-hexene-2,5-dione as a coproduct
to biacetyl (see text). Correction for secondary formation would result
in a methylglyoxal yield relative to that of 3-methyl-3-hexene-2,5-dione
of ∼0.73 and hence a methylglyoxal yield at 1.5 × 1013 molecules cm−3
g
of NO2 of ∼31%. The biacetyl first-generation formation yields
derived from the initial slopes of the second-order regression fits to the
data shown in Figure 4 (○) were 9.0
2.5% and 8.3
3.8% at
average NO2 concentrations of 1.9 × 1013 and 10.2 × 1013 molecules
cm−3, respectively, where the errors are two least-squares standard
deviations. The cited yield is the weighted average with the indicated
error being the two least-squares standard deviation combined with
estimated uncertainties in the GC-FID response factors for biacetyl
and 1,2,4,5-tetramethylbenzene of 10% and 5%, respectively.
Assumption of 30% biacetyl formation from OH + 3-methyl-3-
hexene-2,5-dione leads to the biacetyl concentrations corrected for
secondary formation (● in Figure 4), and least-squares analyses of
these data results in biacetyl formation yields at average NO2
concentrations of 1.9 × 1013 and 10.2 × 1013 molecules cm−3 of
10.5% and 7.7%, respectively.
detailed reaction mechanism12). As noted above, experiments
were carried out at initial CH3ONO and NO concentrations of
Plots of the amounts of biacetyl and 2,4,5-trimethylbenzal-
dehyde formed against the amounts of 1,2,4,5-tetramethylben-
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Figure 4. Plots of the amounts of biacetyl formed against the amounts
of 1,2,4,5-tetramethylbenzene reacted with OH radicals at average
NO2 concentrations of 1.9 × 1013 molecules cm−3 and 10.2 × 1013
molecules cm−3 (the NO2 concentrations (molecules cm−3) are noted
Figure 3. Plots of the amounts of 3-methyl-3-hexene-2,5-dione
formed, corrected for secondary reactions (see text), against the
amounts of 1,2,4,5-tetramethylbenzene reacted with OH radicals at
13
average NO2 concentrations of ( ) 1.9 × 10 molecules cm−3 and
△
( , ) 10.2 × 10 molecules cm−3 (the NO2 concentrations
13
○
in the figure) , measured biacetyl concentrations, with the solid
□
○
−3
●
curves being second-order regression fits; , measured biacetyl
○
(molecules cm ) are noted in the figure) , 3 experiments carried out
concentrations corrected for secondary formation of biacetyl from OH
□
,
△
in the presence of 1,3,5-trimethylbenzene;
, 3 experiments each,
+ 3-methyl-3-hexene-2,5-dione in 30% yield, with [biacetyl]corr
=
carried out in the absence of 1,3,5-trimethylbenzene. The 3-methyl-3-
hexene-2,5-dione data at an average NO2 concentrations of 1.9 × 1013
molecules cm−3 have been displaced vertically by 5 × 1011 molecules
cm−3 for clarity. 1,2,4,5-Tetramethylbenzene and 3-methyl-3-hexene-
2,5-dione were measured from GC-FID analyses of samples collected
onto Tenax solid adsorbent.
[biacetyl]measured − 0.3([3-methyl-3-hexene-2,5-dione] reacted). The
biacetyl data at an average NO2 concentrations of 1.9 × 1013 molecules
cm−3 have been displaced vertically by 2.0 × 1012 molecules cm−3 for
clarity. 1,2,4,5-Tetramethylbenzene and biacetyl were measured from
GC-FID analyses of samples collected onto Tenax solid adsorbent.
zene reacted from irradiated CH3ONO−NO−1,2,4,5-tetrame-
thylbenzene−air and CH3ONO−NO−1,2,4,5-tetramethylben-
zene−1,3,5-trimethylbenzene−air mixtures are shown in
Figures 4 (biacetyl) and 5 (2,4,5-trimethylbenzaldehyde). In
Figure 4, the measured biacetyl concentrations are shown as the
open circles, and the biacetyl yields increased with the extent of
reaction and second-order regression fits are shown by the
curved lines in Figure 4. It is therefore apparent that biacetyl is
formed as both a first-generation product and as a second-
generation product (almost certainly from OH + 3-methyl-3-
hexene-2,5-dione; see Supporting Information). The initial
slopes of the plots to the measured biacetyl data (the data
shown as the ○ symbols in Figure 4) obtained from the
second-order regression fits are listed in Table 3. As also shown
in Figure 4, correcting the measured biacetyl concentrations for
secondary formation from OH + 3-methyl-3-hexene-2,5-dione
in a 30% molar yield (data shown by the ● symbols in Figure
4) leads to good straight-line plots with slopes similar to the
initial slopes of the uncorrected data. Formation of biacetyl in
∼30% yield from OH + 3-methyl-3-hexene-2,5-dione is
compatible with the simultaneous formation of ≥45% of
CH3C(O)CH(OH)C(O)CH3 and/or CH3C(O)C(CH3)-
(OH)CHO.
Figure 5. Plot of the amounts of 2,4,5-trimethylbenzaldehyde formed,
corrected for reaction with OH radicals (see text), against the amounts
of 1,2,4,5-tetramethylbenzene reacted with OH radicals at average
13
NO2 concentrations of ( ) 1.9 × 10 molecules cm−3 and ( ,
)
□
○
△
10.2 × 1013 molecules cm−3. , 3 experiments carried out in the
○
□
△
, 3 experiments each, carried
presence of 1,3,5-trimethylbenzene;
,
out in the absence of 1,3,5-trimethylbenzene. 1,2,4,5-Tetramethylben-
zene and 2,4,5-trimethylbenzaldehyde were measured from GC-FID
analyses of samples collected onto Tenax solid adsorbent.
The 2,4,5-trimethylbenzaldehyde formation yield obtained
from least-squares analyses of the data shown in Figure 5 is
given in Table 3. Note that the amounts of 2,4,5-
trimethylbenzaldehyde present were low (≤4.3 × 1011
molecules cm−3), resulting in significant experiment-to-experi-
ment variability. However, as noted in Table 3, the 2,4,5-
trimethylbenzaldehyde formation yields at average NO2
concentrations of 1.9 × 1013 and 10.2 × 1013 molecules cm−3
were indistinguishable within the experimental uncertainties. As
noted above, no evidence was obtained from GC-MS analyses
for the formation of 2,4,5-trimethylphenol, and an upper limit
to its formation yield of <3% was derived based on the areas of
a peak in the GC-FID analyses of Tenax samples at the
retention time of 2,4,5-trimethylphenol.
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addition, H-atom abstraction accounts for <10% of the overall
reaction for benzene, toluene, the xylenes, the trimethylben-
zenes and 1,2,4,5-tetramethylbenzene and hence the use of the
overall reaction rate constants instead of the addition rate
constants makes no significant difference (the difference would
be the size of the symbols in Figure 6, or less). The correlation
proposed by Atkinson23 and Kwok and Atkinson6 from a
regression analysis of 66 aromatic compounds, of log10k =
−11.71 − 1.34Σσ+ (with k being the rate constant for OH
radical addition to the aromatic ring, in cm3 molecule−1 s−1
units), is shown by the solid line in Figure 6.
For the restricted series of aromatic compounds represented
in Figure 6, the rate constants predicted from the least-squares
regression line are uniformly a factor of 1.6 higher than those
predicted from the correlation proposed by Atkinson and
Kwok6 and Atkinson23 from use of a much larger database. This
is within the factor of 2 uncertainty of the structure−reactivity
predictions,6 and within two least-squares standard deviations
of the fit to the data shown in Figure 6 (which is a factor of 1.74
at Σσ+ = 0.0).
Products and Mechanism of OH + 1,2,4,5-Tetrame-
thylbenzene. As shown in Scheme 1, the initial reaction of
OH radicals with 1,2,4,5-tetramethylbenzene is expected1−3 to
proceed by H-atom abstraction from the substituent CH3
groups and by addition to the aromatic ring to form two
isomeric OH-aromatic adducts. Because of the symmetric
nature of 1,2,4,5-tetramethylbenzene, H-atom abstraction from
the CH3 groups forms the 2,4,5-trimethylbenzyl radical, and the
subsequent reactions of this radical in the presence of NO are
shown in Scheme 2, leading to the formation of 2,4,5-
trimethylbenzaldehyde together with a lesser amount of 2,4,5-
trimethylbenzyl nitrate via reactions 7a, 7b and 8.
DISCUSSION
■
Rate Constants for OH Radical Reactions. These are the
first reported rate constants for the reaction of OH radicals with
2,4,5-trimethylbenzaldehyde and 3-methyl-3-hexene-2,5-dione.
Our rate constant for OH + 2,4,5-trimethylphenol is 17−24%
lower than the rate constants measured by Bejan et al.,13 who
used a similar relative rate method with isoprene and 1,3-
butadiene as the reference compounds. This is reasonable
agreement considering the occurrence of wall losses in both
studies (stated to account for 20−28% of the measured total
loss rate in the Bejan et al.13 study compared to 7−14% in the
present study), and the present observation of a markedly
higher dark loss rate in the presence of CH3ONO and NOx
which was not mentioned by Bejan et al.13 Our rate constants
for 1,2,4,5-tetramethylbenzene and pentamethylbenzene agree
well with those presented in graphical form by Alarcon et
al.20,21 Our rate constant for OH + 3-methyl-3-hexene-2,5-
dione is a factor of 1.6−2.3 higher than those we have measured
for the reactions of OH radicals with cis- and trans-3-hexene-
2,5-dione,22 consistent with the 3-position methyl group
enhancing the reactivity of 3-methyl-3-hexene-2,5-dione
compared to 3-hexene-2,5-dione.6
Our present rate constants for 1,2,4,5-tetramethylbenzene,
pentamethylbenzene and 2,4,5-trimethylphenol, together with
the room temperature rate constants for the overall reactions of
OH radicals with benzene,2 toluene,2 the xylenes,2 the
trimethylbenzenes2 and hexamethylbenzene,7 are plotted in
Figure 6 in the form log10k against the sum of the electrophilic
substituent constants, Σσ+ (see reference 6 for details of how
the values of Σσ+ are calculated). There is a reasonable
correlation between log10k and Σσ+, as expected for an
electrophilic reaction, and the least-squares regression fit to
the data plotted is shown as the dashed line in Figure 6. While
the rate constants plotted should really be those for OH radical
(CH3)3C6H2CH2OO• + NO
→ (CH3)3C6H2CH2O• + NO2
(7a)
(CH3)3C6H2CH2OO• + NO → (CH3)3C6H2CH2ONO2
(7b)
(CH3)3C6H2CH2O• + O2 → (CH3)3C6H2CHO + HO2
(8)
While 2,4,5-trimethylbenzaldehyde was observed and quanti-
fied, 2,4,5-trimethylbenzyl nitrate was not observed (the lack of
an authentic standard hindered analysis). However, rate
constant ratios k7b/k7 [= k7b/(k7a + k7b)] can be derived from
previously reported product yields for formation of benzyl
nitrate and benzaldehyde from OH + toluene (k7b/k7 = 0.11
0.0424 and 0.12
0.0312), and methylbenzyl nitrates and
tolualdehydes from the OH + o-, m- and p-xylene reactions
(k7b/k7 = 0.23
0.10, 0.16
0.08 and 0.10
0.03,
respectively25). Using a weighted average of k7b/k7 = 0.12
(with an estimated uncertainty of 50%), then the yield of
2,4,5-trimethylbenzyl nitrate from OH + 1,2,4,5-tetramethyl-
Figure 6. Plot of log10k against the sum of the electrophilic substituent
constants, Σσ+, where k is the overall rate constant for reaction of OH
radicals with a series of aromatic hydrocarbons and 2,4,5-
benzene is estimated to be 0.4
0.2%. Combined with the
2,4,5-trimethylbenzaldehyde yield of 3.3 0.7%, this results in
an estimated fraction of the overall OH + 1,2,4,5-
tetramethylbenzene reaction proceeding by H-atom abstraction
○
trimethylphenol (see text). , benzene, toluene, o-, m- and p-xylene,
and 1,2,3-, 1,2,4- and 1,3,5-trimethylbenzene, with 298 K rate
from the CH3 groups of 3.7
0.8%. This corresponds to a
constants from Atkinson and Arey2 or (the limiting high-pressure
partial rate constant for H-atom abstraction per CH3
substituent group in 1,2,4,5-tetramethylbenzene of (5.1
1.2) × 10−13 cm3 molecule−1 s−1 at 298 2 K, a factor of ∼1.5
higher than the H-atom abstraction rate constant in the OH +
toluene reaction at 298 K.4
rate constant for OH + benzene) Atkinson;3 , 1,2,4,5-tetramethyl-
●
benzene (1,2,4,5-TMB), pentamethylbenzene and 2,4,5-trimethylphe-
△
nol, with the rate constants measured here (Table 1); and
,
hexamethylbenzene, with the room temperature rate constant from
Berndt and Boge.7
̈
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Scheme 1
Scheme 2
Because of symmetry, only two OH-aromatic adducts can be
formed, after OH radical addition to the 1- (or 2-, 4- or 5-)
position (ipso addition) and after addition to the 3- (or 6-)
position (Scheme 1). The OH-aromatic adducts can react with
O2 and NO2,26 with the reaction with O2 dominating for
atmospheric conditions.18,26 Based on literature studies,18,27−35
the reactions of OH-aromatic adducts with O2 appear to
proceed as shown in Scheme 3 for the adduct formed after OH
radical addition at the 3- (or 6-) position.
At low NO concentrations, this OH-1,2,4,5-tetramethylben-
zene adduct is predicted to form 2,3,5,6-tetramethylphenol, an
unsaturated epoxy-1,6-dicarbonyl, 3-methyl-3-hexene-2,5-dione
+ methylglyoxal, and the bicyclic nitrate formed from the minor
channel of the bicyclic peroxy radical (A in Scheme 3) + NO
reaction.34 At higher NO concentrations, reaction of NO with
the OH-1,2,4,5-tetramethylbenzene-O2 peroxy radical may
•
lead to formation of a diunsaturated 1,6-dicarbonyl (Scheme 3).
While lack of a standard precluded identification and
quantification of 2,3,5,6-tetramethylphenol, formation yields
of di- or trimethylphenols from the OH + m-xylene, p-xylene
and 1,2,4- and 1,3,5-trimethylbenzene reactions are relatively
low,16,17 ranging from ∼2% for 1,2,4-trimethylbenzene to 13%
for p-xylene. The product yields listed in Table 3 indicate that
the methylglyoxal and 3-methyl-3-hexene-2,5-dione yields at 1.9
× 1013 molecules cm−3 NO2 are similar (to within a factor of
1.2−1.4; see footnote f to Table 3) and hence suggest that
methylglyoxal and 3-methyl-3-hexene-2,5-dione are coproducts.
While 2,3-dimethyl-1,4-butenedial, the expected coproduct to
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Scheme 3
Scheme 4
biacetyl (Scheme 4), was not observed by GC analyses, this is
consistent with previous analyses of extracts of PFBHA-coated
denuder samples from OH radical-initiated reactions of o-
xylene and 1,2,4-trimethylbenzene where 2,3-dimethyl-1,4-
butenedial was also not observed (noting that for these
reactions 2,3-dimethyl-1,4-butenedial was not the only potential
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coproduct to glyoxal (o-xylene reaction) or methylglyoxal
(1,2,4-trimethylbenzene reaction)).9 As noted in Table 2, API
MS and MS/MS analyses showed evidence for a minor product
(relative to 3-methyl-3-hexene-2,5-dione, see Figure S2,
Supporting Information) attributed to 2,3-dimethyl-1,4-bute-
nedial.
Thus, if the combinations of 1,2-dicarbonyl + unsaturated
1,4-dicarbonyl formed by the mechanism shown in Schemes 3
and 4 are correct, formation of biacetyl from OH + 1,2,4,5-
tetramethylbenzene shows that initial OH radical addition to
the equivalent 1-, 2-, 4- and 5-position carbon atoms in the
aromatic ring, bonded to methyl substituent groups, does occur
and that addition of OH radicals to the four equivalent ipso
positions accounts for ≥(9 3)% of the overall reaction. This
conclusion then supports the kinetic analyses of OH radical
decays following the pulsed formation of OH radicals in the
presence of trimethylbenzenes, tetramethylbenzenes and
pentamethylbenzene recently reported by Bohn and Zetzsch37
and Alarcon et al.20,21 In these studies, triexponential decays
were observed,20,21,37 consistent with the formation of two (or
more) OH-aromatic adducts, and for OH + 1,3,5-trimethyl-
benzene, the tetramethylbenzenes, and pentamethylbenzene
thus implies the occurrence of ipso addition.20,21,37 Obviously,
ipso addition must also occur in OH + hexamethylbenzene,26,38
and the room temperature rate constant for OH +
hexamethylbenzene7 is consistent with the reaction proceeding
primarily by OH addition to the aromatic ring (Figure 6).
Recently, Loison et al.38 have used a discharge-flow reactor with
mass spectrometric detection and observed both the OD-
hexamethylbenzene adduct and the pentamethylbenzyl radical,
and concluded that H-atom abstraction from OH +
The formation yield measured here for 3-methyl-3-hexene-
2,5-dione increased with decreasing NO2 concentration, as
previously observed for formation of 1,2-dicarbonyls and
unsaturated 1,4-dicarbonyls from other aromatic hydro-
carbons.17,18 Assuming that 1,2-dicarbonyls and unsaturated
1,4-dicarbonyls are formed only from the reaction of OH-
18
aromatic adducts with O2
OH‐aromatic + O2 → Ydicarbonyl + other products
(9)
OH‐aromatic + NO2 → products
(10)
then
dicarbonyl yield = fYk9[O2]/(k9[O2] + k10[NO2])
(III)
where k9 and k10 are the rate constants for reactions 9 and 10,
respectively, f (= 0.96) is the fraction of the overall OH radical
reaction proceeding by OH radical addition to form the OH-
aromatic adducts, and Y is the yield of the dicarbonyl being
considered from reaction 9. Extrapolation of our measured 3-
methyl-3-hexene-2,5-dione yields given in Table 3 to NO2
concentrations applicable to ambient atmospheres (effectively,
extrapolation to [NO2] = 0) using eq III results in the
formation yield f Y also listed in Table 3. The NO2
concentration at which k9[O2] = k10[NO2] is calculated to be
3.6 × 1014 molecules cm−3 from the 3-methyl-3-hexene-2,5-
dione data (i.e., a mixing ratio of 15 parts-per-million NO2), of
generally similar magnitude to the corresponding NO2
concentrations at which the OH-m-xylene and OH-1,3,5-
trimethylbenzene adduct plus O2 and NO2 reactions are of
equal importance in air.18,26
The products observed and quantified here, using the 3-
methyl-3-hexene-2,5-dione (extrapolated to zero NO2 concen-
tration) and biacetyl yields for formation of 3-methyl-3-hexene-
2,5-dione plus methylglyoxal and of biacetyl plus its expected
coproduct 2,3-dimethyl-1,4-butenedial, respectively, account for
57% of the overall reaction pathways and products formed from
OH + 1,2,4,5-tetramethylbenzene under conditions where the
OH-1,2,4,5-tetramethylbenzene adducts react with O2. The
remaining products are expected to include bicyclic nitrates,
2,3,5,6-tetramethylphenol and epoxides such as that shown in
Scheme 3, with the yields of the bicyclic nitrates and 2,3,5,6-
tetramethylphenol being expected to be low.16,17,34 The lack of
observation of measurable concentrations of 2,4,5-trimethyl-
phenol shows that dealkylation via an addition−elimination
reaction does not occur to any significant extent, as previously
observed for the OH + m-xylene and OH + p-cymene
reactions.36
hexamethylbenzene accounts for 11.1
reaction at 298 K.
3.6% of the overall
ASSOCIATED CONTENT
■
S
* Supporting Information
Figures S1 and S2 and reporting the results of GC-MS analyses
of extracts from PFBHA-coated denuder samples from OH +
1,2,4-trimethylbenzene and OH + 1,2,4,5-tetramethylbenzene
reactions, the API-MS analysis of OH + 1,2,4,5-tetramethyl-
benzene, and a discussion of the reaction mechanisms of OH +
3-hexene-2,5-dione and OH + 3-methyl-3-hexene-2,5-dione.
This material is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*J.A.: e-mail janet.arey@ucr.edu; telephone (951) 827-3502.
R.A.: e-mail ratkins@mail.ucr.edu; telephone (951) 827-4191.
Present Address
J.A.: Also Department of Environmental Sciences, University of
California, Riverside, California 92521.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors thank the U.S. Environmental Protection Agency
(Grant R833752) for supporting this research. While this work
has been supported by this Agency, the results and content of
this publication do not necessarily reflect the views and opinion
of the Agency. J.A. thanks the University of California
Agricultural Experiment Station for partial salary support. The
authors thank Prof. Cornelius Zetzsch and Dr. Paulo Alarcon
(Atmospheric Chemistry Research Laboratory, University of
Bayreuth, Germany) for discussions that prompted this study.
Of particular interest is that OH radical addition to the 3- (or
6-) position leads only to the 1,2-dicarbonyl + unsaturated 1,4-
dicarbonyl combination 3-methyl-3-hexene-2,5-dione + meth-
ylglyoxal (Scheme 3). As shown in Scheme 4, the analogous
reactions of the bicyclic radical formed after OH radical
addition to the 1- (or 2, 4- or 5-) position leads to either 3-
methyl-3-hexene-2,5-dione + methylglyoxal or 2,3-dimethyl-
1,4-butenedial + biacetyl.
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