Kinetics of the Aqueous Phase Reactions of Atmospherically Relevant Monoterpene Epoxides

Article abstract of DOI:10.1021/acs.jpca.7b09427

Laboratory and field measurements have demonstrated that an isoprene-derived epoxide intermediate (IEPOX) is the origin of a wide range of chemical species found in ambient secondary organic aerosol (SOA). In order to explore the potential relevance of a similar mechanism for the formation of monoterpene-derived SOA, nuclear magnetic resonance techniques were used to study kinetics and reaction products of the aqueous-phase reactions of several monoterpene epoxides: β-pinene oxide, limonene oxide, and limonene dioxide. The present results, combined with a previous study of α-pinene oxide, indicate that all of these epoxides will react more quickly than IEPOX with aqueous atmospheric particles, even under low-acidity conditions. As for α-pinene oxide, the observed products can be mainly rationalized with a hydrolysis mechanism, and no long-lived organosulfate or nitrate species nor species that retain the β-pinene bicyclic carbon backbone are observed. As bicyclic ring-retaining organosulfate and nitrate species have been previously observed in monoterpene-derived SOA, it appears that monoterpene-derived epoxides may not be as versatile as IEPOX in producing a range of SOA species, and other mechanisms are needed to rationalize organosulfate and nitrate formation.

Full text of DOI:10.1021/acs.jpca.7b09427

Article
pubs.acs.org/JPCA
Cite This: J. Phys. Chem. A XXXX, XXX, XXX−XXX
Kinetics of the Aqueous Phase Reactions of Atmospherically
Relevant Monoterpene Epoxides
Diego A. Cortes and Matthew J. Elrod*
́
Department of Chemistry and Biochemistry, Oberlin College, Oberlin, Ohio 44074, United States
ABSTRACT: Laboratory and field measurements have
demonstrated that an isoprene-derived epoxide intermediate
(IEPOX) is the origin of a wide range of chemical species
found in ambient secondary organic aerosol (SOA). In order
to explore the potential relevance of a similar mechanism for
the formation of monoterpene-derived SOA, nuclear magnetic
resonance techniques were used to study kinetics and reaction
products of the aqueous-phase reactions of several mono-
terpene epoxides: β-pinene oxide, limonene oxide, and
limonene dioxide. The present results, combined with a
previous study of α-pinene oxide, indicate that all of these
epoxides will react more quickly than IEPOX with aqueous
atmospheric particles, even under low-acidity conditions. As for α-pinene oxide, the observed products can be mainly rationalized
with a hydrolysis mechanism, and no long-lived organosulfate or nitrate species nor species that retain the β-pinene bicyclic
carbon backbone are observed. As bicyclic ring-retaining organosulfate and nitrate species have been previously observed in
monoterpene-derived SOA, it appears that monoterpene-derived epoxides may not be as versatile as IEPOX in producing a range
of SOA species, and other mechanisms are needed to rationalize organosulfate and nitrate formation.
double bond in α-pinene, and β-pinene oxide, an epoxide
formed by O atom insertion across the exocyclic double bond
in β-pinene, have been observed as minor products in gas-phase
photooxidation experiments.8−18 However, despite the rela-
tively large number of studies that have identified either α- or β-
pinene oxide as products of pinene oxidation, the conditions
under which these products are formed and the quantitative
yields of the products remain uncertain. In addition, there have
been some laboratory studies of the reactivity of α-pinene
oxide, β-pinene oxide, and limonene oxide (although limonene
oxide itself has not been directly detected in an atmospherically
relevant reaction system) with SOA-like systems,11,12,16,19−24
which have suggested that IEPOX-type mechanisms, as well as
other monoterpene epoxide-specific mechanisms, are poten-
tially relevant. Field measurements of monoterpene SOA
species have also been suggestive of the relevance of IEPOX-
type mechanisms.19,25−28 Additionally, it has been demon-
strated that limonene can undergo direct reaction on surfaces
characteristic of atmospheric mineral dust, presumably through
an epoxide intermediate, which then undergoes hydrolysis to
form the low-volatility compound limonene diol.29 Therefore, it
appears that monoterpene epoxides may have atmospheric
relevance in a number of different atmospheric aerosol reaction
contexts.
INTRODUCTION
■
Biogenic volatile organic compounds (BVOCs) are emitted
into the atmosphere in massive quantities, with isoprene and
the monoterpenes comprising the largest portion of the roughly
1000 Tg/yr annual emission budget.1 Once in the atmosphere,
BVOCs undergo reactions with oxidants that can ultimately
perturb the NOx and HOx radical cycles that govern the
formation of tropospheric ozone, a key atmospheric pollutant.2
These reactions can also lead to chemical species that serve as
precursors for secondary organic aerosol (SOA), which is also
known to play a key role in air quality, as well as climate
change.3 In particular, in order for BVOCs to become
incorporated into SOA, they must undergo specific types of
chemical processing that serve to reduce their volatility.4
Because of its paramount importance, the formation
mechanisms for isoprene-derived SOA have received intense
attention. From a plethora of laboratory and field studies, it is
now known that many of the isoprene-derived species found in
SOA are ultimately derived from the gas-phase intermediate,
isoprene epoxydiol (IEPOX).5 Because epoxides readily
undergo nucleophilic attack by common SOA species, such as
water, sulfate, nitrate, and organic nucleophiles,6 IEPOX serves
as a bridge to the formation of the low-volatility polyols,
organosulfates, organonitrates, and oligomers that dominate the
isoprene-derived species observed in ambient SOA.7
In previous work,23 we reported that α-pinene oxide reacts
very quickly in aqueous solutions, even under low-acidity
There have been a number of studies that have either
identified epoxide intermediates formed from the mono-
terpenes or have identified potential SOA products of such
epoxide reactions, or both. For example, α-pinene oxide, an
epoxide formed by O atom insertion across the endocyclic
Received: September 22, 2017
Revised: November 16, 2017
© XXXX American Chemical Society
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conditions. Depending on the acid concentration, a number of
different products from the reaction of α-pinene oxide were
observed. In contrast to some previous laboratory results, no
long-lived organosulfate or organonitrate species were
observed, and no species that retain the α-pinene bicyclic
carbon backbone were observed. Instead, the overall product
distribution could be explained by various rearrangements of
the initial carbocation intermediate formed in the four carbon
ring opening of α-pinene oxide, all of which can be rationalized
by the thermodynamically driven relief of the bicyclic ring strain
in the α-pinene carbon backbone.
In this paper, we extend the methods of our previous study of
α-pinene oxide reactivity to other monoterpene epoxides in
order to further ascertain the overall potential impact of
epoxide intermediates on the formation of monoterpene-
derived SOA. Specifically, we report measurements of the
kinetics and products of the aqueous-phase acid-catalyzed
reactions of β-pinene oxide, limonene oxide (an epoxide
formed by O atom insertion across the endocyclic double bond
in limonene), and limonene diepoxide (a diepoxide formed by
an additional O atom insertion across the exocyclic double
bond in limonene oxide), using nuclear magnetic resonance
(NMR) as the analytic technique. These results are then used
to assess the potential impact of these monoterpene epoxides
on the particle-phase composition of the atmosphere.
Kinetics Method. Kinetics measurements of the aqueous
phase reactions of monoterpene epoxides were made by
1
collecting sequential H NMR spectra over the course of the
experiment and measuring the loss of epoxides. Each
measurement was performed in the same manner: 10 μL of
epoxide was added to a 1 mL aliquot (corresponding to an
initial epoxide concentration of about 0.06 M) of the desired
aqueous solution, and the solution was stirred in a 20 mL vial.
After approximately 1 min of stirring to ensure solution
homogeneity, the entire reaction mixture was loaded into an
NMR tube, and spectral collection was started. Experiments
were performed using a pure D2O solution and DClO4
solutions of differing acidities. A first-order decay rate law
was found to fit the epoxide concentration vs time data, and the
first-order rate constants were determined for each solution.
From the DClO4 solution data, second-order acid-catalyzed
rate constants were also determined.
Chloroform Extraction Method. To identify and quantify
all products formed (including phase-partitioned species that
have limited water solubility), the product species were
extracted from the bulk aqueous reaction mixtures with
CDCl3 (Cambridge Isotope Laboratories, Inc.). A typical
solution was prepared by adding about 10 mg of the aqueous
soluble internal standard DSS (sodium 2,2-dimethyl-2-
silapentane-5-sulfonate, Cambridge Isotope Laboratories, Inc.)
to a 20 mL vial equipped with a Teflon-coated magnetic stir bar
and charged with about 7 mL of a premixed solution containing
the inorganic components (as described in the previous
section). The reaction was then initiated by micropipetting
100 μL of the monoterpene epoxide (corresponding to an
initial epoxide concentration of about 0.09 M) into the vial and
then stirring the mixture vigorously. Reaction workup began by
transferring about a 1 mL aliquot of the aqueous solution into
EXPERIMENTAL SECTION
■
β-Pinene Oxide Synthesis. β-Pinene oxide was prepared
according to a previously published procedure.30 To a mixture
of 21.25 g (0.16 mol) of β-pinene and 50 g of powdered,
anhydrous Na2CO3 in 200 mL of CH2Cl2, stirred in an ice
water bath, 33.5 mL of 39% (0.17 mol) peracetic acid
containing 0.3 g of KC2H3O2 (all obtained from Sigma-
Aldrich) was added dropwise. The mixture was stirred at room
temperature until NMR analysis indicated that the β-pinene
had been consumed. The solution was vacuum filtered and
washed with additional CH2Cl2. The solvent was removed on a
rotary evaporator, and the residue was vacuum distilled. The
yield was 14.2 g (60%), and NMR analysis indicated >95%
purity.
1
an NMR tube and taking a standard H NMR spectrum (in
order to determine the concentrations of the aqueous soluble
species). Next, about 6 mL of CDCl3 was added to the
remaining aqueous solution in the vial, and the resulting
mixture was stirred for 20 min, after which the aqueous and
organic layers (about 6 mL of each) were allowed to separate.
Each layer was then micropipetted into two new 20 mL vials.
Then, 20.0 μL of a second, chloroform-soluble, internal
standard (benzene, ACS spectrophotometric grade, ≥99%,
Sigma-Aldrich) was micropipetted into the vial containing the
organic layer, and the solution was stirred for 2 min. Typically,
Bulk Aqueous Solution Preparation. To explore the
concentration dependence of various atmospherically relevant
species on the reaction kinetics and products of the various
monoterpene epoxides, bulk aqueous solutions were prepared
with varying acid, sulfate, and nitrate concentrations. All
experiments were carried out in deuterated solvents, which
were necessary for NMR locking purposes. For acid-dependent
experiments, deuterated perchloric acid was used (to exclude
nucleophilic addition mechanisms other than hydrolysis). For
sulfate/nitrate concentration dependence experiments, dilute
(0−0.02M) deuterated sulfuric/nitric acid was combined with
sodium sulfate/nitrate (0−1 M). These solutions were
prepared using commercially available 68 wt % DClO4
(Sigma-Aldrich), 96−98 wt % D2SO4 (Sigma-Aldrich), 70 wt
% DNO3 (Sigma-Aldrich), 99.9% D2O (Cambridge Isotope
Laboratories, Inc.), Na2SO4 (Sigma-Aldrich), and NaNO3
(Sigma-Aldrich). In order to initiate the chemical reaction,
the monoterpene epoxide was added to the bulk solution, and
the solutions were stirred for at least 1 min to ensure
homogeneity before any analysis was performed. The
commercially available epoxides used were 97% (+)-cis- and
trans-limonene oxide and limonene dioxide (both from Sigma-
Aldrich).
1
both H and 13C NMR spectra were collected for CDCl3-
extracted solutions. NMR analysis of the remaining aqueous
layer indicated that the CDCl3 extraction was not completely
efficient for the aqueous soluble species. Therefore, the total
product concentrations were determined by adding together
the product concentrations determined from the aqueous and
CDCl3 solutions.
NMR Product Identification and Quantitation Meth-
ods. All NMR spectra were collected on a Varian 400 MHz
1
instrument. In CDCl3 solutions, H and 13C chemical shifts
were calibrated to the solvent at 7.26 and 77.16 ppm,
respectively.31 In D2O solutions, 1H chemical shifts were
calibrated to the solvent at 4.79 ppm. In some cases, DSS was
added to D2O solutions to provide a 13C reference at 0.00 ppm.
Some species were identified by comparison to commercially
available standards: (+)-cis- and trans-limonene oxide, limonene
dioxide, perillyl alcohol, perillyl aldehyde, myrtenal, and
1S,2S,4R-(+)-limonene 1,2-diol (all obtained from Sigma-
Aldrich). However, several other noncommercially available
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Figure 1. NMR assignments (1H referenced to HDO at 4.79 ppm, 13C referenced to CDCl3 at 77.16 ppm) for previously unassigned species.
1
species were identified via previously reported H and 13C
RESULTS AND DISCUSSION
■
CDCl3 NMR data: β-pinene oxide,32 7-hydroxyterpineol,33 and
NMR Assignments. While at least partial NMR chemical
dihydrocarvone.34
shift data for all of the reactants and products studied in this
The relative amounts of reaction products were calculated by
work are available in the literature, most of the literature data
peak integration of unique nuclei for each species, referenced to
are not reported as NMR chemical shift assignments (the
association of individual resonances with specific protons and/
or carbon atoms of the structurally specific chemical species).
Because NMR chemical shift assignments are often necessary to
the nine methyl protons at 0.00 ppm from DSS, the six protons
at 7.34 ppm from benzene, or the six carbon atoms at 128.4
ppm from benzene.
In addition to standard one-dimensional H and 13C NMR
1
sort out the small structural differences that distinguish similar
species (especially for isomers), we used extensive correlation
NMR spectroscopy experiments to determine 1H and 13C
assignments for the reactants β-pinene oxide, cis- and trans-
(+)-limonene oxide, and a mixture of four isomers of limonene
dioxide as well as the β-pinene oxide reaction products, 7-
spectroscopy methods, the following correlation NMR spec-
1
troscopy methods were also used: H−1H correlation spec-
troscopy (COSY), 13C distortionless enhancement by polar-
ization transfer (DEPT), 1H−13C heteronuclear single-quantum
correlation (HSQC), and 1H−13C heteronuclear multiple bond
correlation (HMBC) techniques.35
C
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hydroxyterpineol and perillyl alcohol. The assignments are
given in Figure 1. In the limonene oxide36,37 and limonene
dioxide38,39 cases, partial assignments were previously reported
in the literature, but we believe that all of these previous
assignments have errors due to the difficulty in identifying the
specific isomers present in the commercially available mixtures.
Notably, these previous assignments did not utilize C−H bond
correlation NMR spectroscopies (i.e., DEPT, GHSQC, and
GHMBC), which we found to be critical in making the specific
assignments. In particular, the bond correlation experiments
allowed the various individual resonances to be definitively
assigned to a single isomer. By analogy to limonene oxide, there
should exist a similar cis/trans pair of stereoisomers for
limonene dioxide. However, it is unclear exactly how to assign
all four of the observed isomers of limonene dioxide to specific
isomeric identities. It is possible that the four isomers are due to
two sets of cis/trans pairs, in which the pairs are distinguished
by two different confirmations made possible by a high barrier
to rotation of the exocyclic epoxide group. Because the different
isomers were found to have different reaction rate constants,
the present kinetics experiments allowed for clear assignment of
In order to account for both general acid (water) and
Bronsted acid-catalyzed processes, the following rate equation
was used
d[epoxide]
−
= kGA[H2O][H2O][epoxide]
dt
+
+
+ kH [H ][H2O][epoxide]
(1)
where kGA is the general acid-catalyzed rate constant and kH+ is
the Bronsted acid-catalyzed rate constant. Note that water
appears twice in the general acid-catalyzed portion of the rate
law, once as the catalyst and once as the nucleophile. Because
[H2O] is constant in the experiments, eq 1 reduces to
d[epoxide]
+
+
″
′
−
= kGA[epoxide] + kH [H ][epoxide]
(2)
dt
2
+
where k″GA = kGA[H2O] and kH′ + = kH [H2O]. In the Bronsted
acid-dependent experiments, [H+] is also a constant, which
allows eq 2 to reduce to
d[epoxide]
+
″
″
−
= kGA[epoxide] + kH [epoxide]
dt
1
most H or 13C peaks to a specific isomer. In many cases, due
″
= k [epoxide]
(3)
to the spectral overlap caused by the very similar chemical
environments, we were unable to assign the CH2 units
(positions 2, 5, and 6 in Figure 1) in the six-membered rings
of some species, and it was not possible to assign any of the
where kH″+ = kH′ +[H+] and k″ = kG″A + kH″+. Equation 3 is first-
order in the epoxide concentration, and the epoxide loss
kinetics were analyzed in the framework of this first-order rate
law. kG″A was determined from the first-order loss of epoxide in
neutral solutions (the second term in eq 3 is negligible in this
case such that k″ = k″GA), and k″ values were also determined for
several acidic solutions. A plot of the individual k″ values
against the appropriate H+ concentrations yields k′H+ as the
slope and kG″A as the intercept.
1
nonepoxide H resonances for the limonene diepoxide isomer
mixture. However, the assignments were complete enough (in
particular, the epoxide proton(s) for each isomeric species was
distinguishable) to allow for unambiguous kinetic monitoring
of the reaction of each specific isomer epoxide reactant, as well
as for the identification of the major reaction products.
The various isomers of limonene oxide and dioxide were
found to react slowly enough in D2O to allow NMR
monitoring, and their individual first-order general acid-
catalyzed hydrolysis rate constants (k″GA) were determined. In
previous literature work aimed at developing processes to
separate cis- from trans-limonene oxide, it had been reported
that the cis stereoisomer hydrolyzes more quickly than the trans
form.36,37 In general, cis-epoxides have been found to hydrolyze
faster than trans-epoxides,42 presumably due to the relative
stability of the SN2-type transition state, which is less sterically
hindered for the attacking water nucleophile for the cis isomer.
The presently determined rate constants confirm the earlier
finding; Table 1 indicates that cis-limonene oxide hydrolyzes
almost 200 times faster than trans-limonene oxide. However, it
should be noted that cis-limonene oxide was also found to
hydrolyze at a rate at least 20 times slower than that of either α-
pinene oxide or β-pinene oxide, thus indicating that a large
range of hydrolysis rate constants are possible for isomerically
Rate Constant Measurements. For the reaction of β-
pinene oxide in D2O, no remaining β-pinene oxide reactant was
detected after 5 min of reaction time (the minimum time for
1
sample preparation and the collection of a routine H NMR
spectrum). The sensitivity of the NMR detection method for β-
pinene oxide indicates that about 5% of the initial
concentration of β-pinene oxide is the minimum detectable
amount. Because 5% remaining reactant corresponds to about
three first-order lifetimes, the upper limit on the lifetime of the
β-pinene oxide hydrolysis reaction is therefore about 1.7 min,
or a lower limit hydrolysis rate constant of about 0.010 s−1. In
previous work, an identical lower-limit hydrolysis rate constant
for α-pinene oxide was reported.23 As for α-pinene oxide, the
fact that β-pinene oxide hydrolyzes quickly at neutral pH
suggests that water is acting as a general acid catalyst. In this
general acid catalysis process, one water molecule acts as the
general acid by using one of its acidic hydrogen atoms to form a
bond with epoxide oxygen atom (as opposed to a H+ unit
acting in this role in traditional acid catalysis).40 A second water
molecule then acts as the attacking nucleophile on this
intermediate water−epoxide species. Ultimately, the original
catalytic water molecule is released to the aqueous solution as
the final hydrolysis product is formed. It is possible that the use
of deuterated solutions (D2O specifically in the neutral solution
case) in the present study could lead to the observation of
different rates of reaction than would be observed for the
normal isotope.41
Table 1. Hydrolysis Rate Constants (and 1 Standard
Deviation Error Limits) for Monoterpene Epoxides
species
kG″A (10−6 s−1
)
kH′ + (M−1 s−1
)
α-pinene oxide23
>10000
>10000
486 14
β-pinene oxide
cis-endo-limonene oxide
trans-endo-limonene oxide
cis-endo-limonene dioxide
trans-endo-limonene dioxide
exo-limonene dioxide
2.88 0.35
2.534 0.041
160.3 9.7
8.36 0.15
12.6 1.0
1.249 0.039
However, it is not straightforward to ascertain whether this
effect would lead to slower or faster rates of reaction.
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related monoterpene epoxide reactants. Acid-dependent
measurements were carried out for cis- and trans-limonene
oxide to determine second-order Bronsted acid-catalyzed rate
constants (k′H+), the analysis for which is shown in Figure 2.
These values are given in Table 1 and indicate that the two
stereoisomers have very similar hydrolysis rate constants within
the Bronsted acid-catalyzed mechanism.
relative yields shifted to 94% 7-hydroxyterpineol and 6% perillyl
alcohol and a small amount (<10% absolute yield) of
unidentified aldehydes. One previous study also found that 7-
hydroxyterpineol and perillyl alcohol were the major products
formed in the Lewis acid-catalyzed reaction of β-pinene oxide,33
while another study using Lewis acids in a supported liquid
catalyst method found perillyl alcohol, myrtenal, and myrtenol
as products.44 In the latter study, water was deliberately
excluded from reaction conditions, thus explaining the lack of
detection of the hydrolysis product 7-hydroxyterpineol.
Authentic samples of myrtenal and myrtenol were used to
rule out their formation from β-pinene oxide in the present
study. A previous study reported the NMR assignments for the
bicyclic ring-retaining 1,7-diol (synthesized via palladium-
catalyzed dihydroxylation of β-pinene).45 This is the same
species that would result if β-pinene oxide underwent a bicyclic
ring-retaining hydrolysis reaction. However, there is no
evidence in the present NMR data that β-pinene oxide
produces this bicyclic ring-retaining diol. In fact, to the best
of our knowledge, there appear to be no reports in the synthetic
organic chemistry literature that describe the production of the
1,7-diol from the hydrolysis of β-pinene oxide.
In the Na2SO4 solutions, 7-hydroxyterpineol sulfate was
observed as a transient species (identified via a characteristic 1H
methyl group resonance at 1.52 ppm vicinal to the sulfate
group),23 hydrolyzing to 7-hydroxyterpineol with a rate
constant of about 5 × 10−4 s−1. No organonitrate species
were detected in the experiments involving NaNO3 solutions. It
has been previously reported that the hydrolysis of tertiary
organonitrates proceeds much more quickly (>100×) than the
hydrolysis of tertiary organosulfates like 7-hydroxyterpineol
sulfate.46 With a 7-hydroxyterpineol sulfate hydrolysis lifetime
of 33 min, this previously determined organonitrate to
organosulfate hydrolysis rate ratio predicts that any 7-
hydroxyterpineol nitrate that might have formed from the
reaction of β-pinene oxide under our solution conditions would
undergo hydrolysis in a matter of seconds. A very similar
finding was made for the formation and hydrolysis of the
analogous α-pinene oxide reaction product, sobrerol sulfate,
and the lack of detection of sobrerol nitrate.23 Therefore, the
lack of detection of 7-hydroxyterpineol nitrate is consistent
with the general structure−reactivity relationships that have
been previously reported for organosulfate and organonitrate
hydrolysis kinetics. A mechanism (depicting both the general
acid and Bronsted acid catalyst as “H+” and the nucleophilic
addition reactions as SN1-type for conceptual clarity) ration-
alizing the observed β-pinene oxide reaction products is
presented in Figure 3. All of these mechanistic features are
qualitatively similar to those found for the reactivity of α-pinene
oxide:23 (1) the products are not fully water-soluble at the
reactant concentrations used, (2) a four carbon ring-opened
diol is the major product, (3) a four carbon ring-opened
unsaturated isomerization product is preferentially formed
under higher acid conditions, (4) four ring-opened aldehyde
isomerization products are preferentially formed under higher
acid conditions, (5) a four carbon ring-opened hydroxy sulfate
is quickly hydrolyzed to the major product diol, (6) there is no
evidence for the detection of a similar hydroxy nitrate
(although formation and fast hydrolysis could not be ruled
out), and (7) there is no detection of any bicyclic ring-retaining
products of any kind. Previous computational thermodynamics
calculations for α-pinene oxide23 are consistent with the
observed stability of four carbon ring-opened hydrolysis and
Figure 2. Limonene oxide hydrolysis kinetics.
For limonene dioxide, it was possible to distinguish between
isomers for the endocyclic epoxide proton, and quite different
first-order hydrolysis rate constants were determined for these
two isomers. Because of the expected cis/trans stereoisomerism
in limonene dioxide, the kinetics of the fast-reacting isomer is
assumed to follow from the hydrolysis of the cis-endocyclic
epoxide ring (as labeled in Table 1), and the kinetics of the
slow-reacting isomer is assumed to follow from the hydrolysis
of the trans-endocyclic epoxide ring. While the cis-endo and
trans-endo rate constants for limonene oxide and limonene
dioxide are similar, Table 1 indicates that the presence of the
exo-epoxide functionality serves to decrease the reactivity of the
cis-endo isomer while increasing the reactivity of the trans-endo
isomer. It was not possible to distinguish between isomers for
the exocyclic epoxide proton; therefore, a single hydrolysis rate
constant is reported and is assumed to follow from the
hydrolysis of the exocyclic epoxide ring. Because the trans-
endocylic and exocylic epoxide hydrolysis rate constants are of
similar magnitude, the overall hydrolysis of the various isomers
of limonene dioxide can produce epoxy diol intermediate
species of both types (endocyclic diol and exocyclic epoxide
functionality and vice versa) before eventually producing the
final tetraol products.43 To the best of our knowledge, these are
the first reported measurements of aqueous-phase reaction rate
constants for β-pinene oxide, limonene oxide, and limonene
dioxide.
Reaction Mechanism for β-Pinene Oxide. In D2O
solution, the reaction of β-pinene oxide led to two identified
and quantified four carbon ring-opened products: the
hydrolysis product 7-hydroxyterpineol and the isomerization
product perillyl alcohol. The CDCl3 extraction procedure
indicated that neither product was fully water-soluble (and
perillyl alcohol was not detectable at all in the aqueous phase).
The total relative yields determined were 97% 7-hydroxyterpi-
neol and 3% perillyl alcohol. In 1 M DClO4 solution, the
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Figure 4. Schematic mechanism for the reactions of limonene oxide.
Reaction Mechanism for Limonene Diepoxide. As
mentioned in the NMR Assignments section, commercially
available limonene diepoxide is actually a mixture of four
isomers. Because the hydrolysis kinetics results indicated that
both epoxide rings can open on similar time scales, there are
several intermediate species isomers that are also formed, in
addition to the final hydrolysis products during reaction in
D2O. Because of the spectral complexity of this situation, full
assignments of the intermediates and final products were not
possible. However, the spectra obtained after all epoxide species
were observed to react are consistent with the sole production
of six carbon ring-retained tetraol isomers.43 On the basis of the
results discussed above, a proposed mechanism for the reaction
of limonene dioxide in aqueous solution was constructed and is
depicted in Figure 5.
Figure 3. Schematic mechanism for the reactions of β-pinene oxide.
isomerization products as compared to bicyclic ring-retaining
products and/or organosulfate and nitrate products for β-
pinene oxide.
Reaction Mechanism for Limonene Oxide. In D2O, the
reaction of cis- and trans-limonene oxide led to a single
identified hydrolysis product, limonene-1,2-diol. In 1 M DClO4
solution, the reaction of cis- and trans-limonene oxide led to
two identified and quantified six carbon ring-retaining products:
60% yield of the hydrolysis product limonene-1,2-diol and 40%
yield of the isomerization product dihydrocarvone. A small
amount of an unidentified aldehyde was also observed. The
CDCl3 extraction procedure indicated that neither product was
fully water-soluble (and dihydrocarvone was not detectable at
all in the aqueous phase). Costa et al. previously reported the
formation of these two products (and NMR assignments) from
the Lewis acid-catalyzed reaction of limonene oxide. Compar-
ison of the commercially available standard, 1S,2S,4R-
(+)-limonene 1,2-diol, to the product spectra confirms that
this one specific stereoisomer is formed from the hydrolysis of
both cis- and trans-limonene oxide.
A mechanism rationalizing the limonene oxide reaction
products is presented in Figure 4. Because limonene oxide
contains only a single, relatively stable six-membered ring (as
opposed to the presence of an additional highly strained four-
membered ring in both α- and β-pinene oxide), it is not
surprising that the limonene oxide reaction products retain this
structural motif.
Potential Organosulfate and Nitrate Product Detec-
tion for Limonene Epoxide and Diepoxide Reactions.
1
Figure 6 shows H NMR spectra collected during the reaction
of limonene diepoxide in solutions with 1.0 M NaClO4 (top),
1.0 M Na2SO4 (middle), and 1.0 M NaNO3 (bottom). The 1.0
M NaClO4 solution was chosen as a reference that produces
−
only hydrolysis and isomerization products because ClO4 is a
non-nucleophilic species. Therefore, any difference between the
top spectrum and the middle and bottom spectra could indicate
the presence of organosulfate and/or nitrate species,
respectively. However, the three spectra appear to be nearly
identical, which indicates that these experiments do not provide
any positive evidence for the formation of organosulfates or
nitrates. In addition, because the β-pinene oxide reaction with
sulfate was observed to form 7-terpineol sulfate, only to
hydrolyze at later times, these solutions were also monitored
kinetically. However, no change was observed in these solutions
over the same period that corresponded to the hydrolysis of 7-
terpineol sulfate (about 1 h). Similar conclusions were drawn
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Figure 5. Schematic mechanism for the reactions of limonene dioxide.
species, predominantly the hydrolysis product 7-hydroxyterpi-
neol. More acidic aerosol conditions are predicted to favor the
formation of the isomerization product, perillyl alcohol,
although even at pH = 0, 7-hydroxyterpineol is predicted to
remain the predominant product. 7-Hydroxyterpineol sulfate
was observed to form in solutions with elevated sulfate
conditions. However, because its neutral hydrolysis lifetime
was found to be only 33 min, the present results suggest that 7-
hydroxyterpineol sulfate is likely to hydrolyze quickly in
atmospheric particles. Because no organonitrates of any kind
were observed for the reaction of β-pinene oxide, it is also
unlikely that 7-hydroxyterpineol nitrate has a significant
atmospheric lifetime. These very fast hydrolysis rates stand in
contrast to the hydrolysis of the tertiary organosulfates and
nitrates formed from IEPOX, which have lifetimes of 460 and
0.67 h, respectively.6
The cis and trans isomers of limonene oxide are also
predicted to react in neutral solution environments, with
lifetimes of 34 min and 4.6 days, respectively. Because typical
aerosol lifetimes are estimated to be on the order of 2−10
days,48 even trans-limonene oxide is expected to undergo at
least partial reaction under neutral aerosol conditions. Under
Bronsted acid-catalyzed conditions, the cis and trans isomers
react with similar rate constants. While the determination of
ambient aerosol pH remains a challenging problem,49 recent
estimates for aerosol acidities in Southern California suggest
values of pH = 2−3.50 For example, at pH = 3, the lifetimes for
the cis and trans isomers are calculated to be only 5.8 and 13.3
min, respectively. Therefore, while the general acid-catalyzed
reaction rates for cis- and trans-limonene oxide are very
different, the Bronsted acid-catalyzed mechanisms more likely
to be relevant under atmospheric conditions indicate that both
isomers will react with similar, very fast rates to produce mainly
the hydrolysis product, limonene 1,2-diol. The limonene
dioxide kinetics suggest that the exocyclic limonene monoep-
1
Figure 6. H NMR spectra of limonene diepoxide in (top, black) 1.0
M NaClO4, (middle, blue) 1.0 M Na2SO4, and (bottom, red) 1.0 M
NaNO3 solution.
from experiments with limonene epoxide and the three 1.0 M
solutions. Therefore, the present experiments provide no
positive evidence for the formation of organosulfates or nitrates
from either limonene epoxide or limonene diepoxide. However,
because of the general congestion of these spectra in the region
where protons adjacent to sulfate and nitrate groups are
typically found (3.5−4.0 ppm),47 it is possible that organo-
sulfate and/or nitrate species were produced but could not be
distinguished from the hydrolysis and isomerization prdoucts.
Atmospheric Implications. The present finding that β-
pinene oxide has a very short lifetime in neutral aqueous
solution suggests that it will react virtually instantaneously on
atmospheric aqueous particles to form new particle-phase
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ORCID
oxide (not studied in this work) will probably have similar
reactivity to trans-limonene oxide. Therefore, all possible
limonene epoxides are probably quite reactive under typical
atmospheric conditions, producing mainly polyol products.
Thus, the hypothesis that limonene epoxide intermediates are a
potential explanation for the recent observation of limonene
1,2-diol products in the interaction of limonene with mineral
dust is kinetically feasible.29
Matthew J. Elrod: 0000-0002-1656-8261
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank Dylan Bleier for assistance with the β-pinene oxide
synthesis and Albert Matlin for helpful discussions. This
material is based upon work supported by the National Science
Foundation under Grant Nos. 1153861 and 1559319.
In summary, all monoterpene epoxides studied to date are
characterized by facile general acid-catalyzed and even faster
Bronsted acid-catalyzed reactions at atmospherically relevant
pH values. For comparison, while all of the monoterpene
epoxides are predicted to react with lifetimes of less than 15
min at pH = 3, IEPOX has a much longer lifetime of 7.7 h at
the same pH.51 Therefore, from a reactivity standpoint, the
monoterpene epoxides seem to be excellent candidates as
potential intermediates for monoterpene-derived SOA species.
However, these epoxides are much less water-soluble than
IEPOX,23 which could significantly limit their mass transfer in
the SOA phase. In addition, Drozd et al. found that that α-
pinene oxide reaction was self-limiting due to the formation of a
coating of products, which inhibited further uptake of the
epoxide.24 Further, the fact that no long-lived organosulfate or
nitrate products have been observed in the aqueous-phase
reactions of monoterpene oxides suggests that these species
may not lead to the same range of SOA species as the IEPOX
intermediate does in the case of isoprene-derived SOA. Indeed,
there have been a number of previous reports of long-lived
organosulfates and nitrates derived from α- and β-pinene,19,25,27
including the first reported synthesis of authentic four carbon
ring-retaining organosulfate standards derived from α- and β-
pinene.28 Interestingly, these four carbon ring-retaining organo-
sulfates were not synthesized via an epoxide precursor, and it
was important that the reactions be carried out under
anhydrous conditions. Therefore, it seems that monoterpene-
derived organosulfates found on SOA are probably produced
via a different mechanism than isoprene-derived organosulfates.
Specifically, the available evidence suggests that monoterpene-
derived organosulfates are not produced via epoxide
intermediates (unlike isoprene-derived organosulfates) and
might require low water content conditions for efficient
formation.
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Three different monoterpene epoxides, β-pinene oxide,
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AUTHOR INFORMATION
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Corresponding Author
(13) Eddingsaas, N. C.; Loza, C. L.; Yee, L. D.; Seinfeld, J. H.;
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*E-mail: mjelrod@oberlin.edu.
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