Assessing the Potential for the Reactions of Epoxides with Amines on Secondary Organic Aerosol Particles

Article abstract of DOI:10.1021/acs.jpca.5b07852

Nuclear magnetic resonance techniques were used to study the kinetics and products of the reaction of a variety of epoxides with various amines under varying pH conditions. In agreement with a previous finding, the amine-epoxide reactions were found to be

Full text of DOI:10.1021/acs.jpca.5b07852

Article
pubs.acs.org/JPCA
Assessing the Potential for the Reactions of Epoxides with Amines on
Secondary Organic Aerosol Particles
Santino J. Stropoli and Matthew J. Elrod*
Department of Chemistry and Biochemistry, Oberlin College, Oberlin, Ohio 44074, United States
S
* Supporting Information
ABSTRACT: Nuclear magnetic resonance techniques were used to study
the kinetics and products of the reaction of a variety of epoxides with various
amines under varying pH conditions. In agreement with a previous finding,
the amine−epoxide reactions were found to be water-catalyzed and not
directly dependent on the pH of the reaction environment. At pH values
higher than the pKa of the particular amine, the amine−epoxide reactions
were extremely efficient, outcompeting hydrolysis reactions even for
conditions where water was the solvent and the amine was a relatively low-concentration solute. This finding was rationalized
by measurements that showed that the relative nucleophilic strength of amines relative to water was on the order of 1000, while
the nucleophilic strength of protonated amines (which are predominant when pH < pKa) was negligible. The epoxide carbon
substitution environment was found to have a large effect on the measured rate constants (more substituted epoxides had slower
rate constants), while the amine carbon substitution environment had a much more limited effect. While the amine−epoxide rate
constants are large enough such that amine−epoxide reactions may be kinetically feasible for isoprene-derived epoxides and high
amine secondary organic aerosol (SOA) concentrations, most atmospheric amines are expected to be present in protonated form
on the largely acidic SOA found in the atmosphere and thus are expected to be largely unreactive toward epoxides.
epoxide reactions are also efficient under atmospheric
conditions. Indeed, a recent article suggested that amines
and/or ammonia may react with and/or catalyze the reactions
of isoprene-derived epoxides on SOA.21 Further, a previous
article reported that amine−epoxide reactions appear to be
water-catalyzed, rather than acid-catalyzed,22 which might allow
amine−epoxide reactions to occur at a wider range of SOA pH
than the previously identified isoprene-derived epoxide
reactions with water and inorganic nucleophiles.
In this paper, we report pH-dependent measurements of the
reaction kinetics of the aqueous-phase reactions of structurally
varying epoxides with several amines, including potentially
atmospherically relevant ones such as MEA, using nuclear
magnetic resonance (NMR) as the analytic technique. We
report fully isomer-specific NMR assignments of the various
products that are observed, in addition to information on the
kinetic stability of the products. These results are then used to
ascertain the potential atmospheric relevance of these reactions
in the formation of SOA.
INTRODUCTION
■
Secondary organic aerosol (SOA) has been implicated in
human respiratory and cardiovascular disease,1 visibility loss,2
and climate modification.3 Recently, the formation of SOA
from atmospheric amines has been identified as occurring via a
number of chemical processes, such as salt formation,4
oxidation reactions,5 and aqueous-phase reactions.6 Atmos-
pheric amines have both natural and anthropogenic emission
sources, and field measurements indicate that aliphatic amines
are most abundant.7 The major anthropogenic sources of
amines are from agricultural activities and biomass burning.4
New carbon-capture technologies utilize aqueous solutions of
amine compounds, such as monoethanol amine (MEA,
HOCH2CH2NH2), to effect reversible absorption of CO2.
Increasing use of such carbon-capture technologies will likely
lead to increased atmospheric amine concentrations from the
solvents such as MEA itself, as well as amine-based degradation
products.8
Isoprene-derived SOA can make up a major proportion of
atmospheric SOA in many continental locales.9,10 Extensive
studies of SOA formation in the southeastern United States
have identified epoxide intermediates as key species in the
formation of isoprene-derived SOA.11−16 In this mechanism,
SOA species are formed from the isomerization and
nucleophilic addition reactions of common SOA nucleophiles
(in particular, water and sulfate) with epoxides formed from
gas-phase oxidation of isoprene and related species.17−19 In
most cases, these reactions require acid catalysis to be
kinetically facile. Because amines are known to be better
nucleophiles than water,20 it is quite possible that amine−
EXPERIMENTAL SECTION
■
Bulk Aqueous Solution Preparation. Because of the
basicity of amines, solutions compromised of only amine and
water (such solutions will be referred to as the “ambient” pH
systems) are characterized by pH values greater than 7 (in these
cases, the pH is determined by the amine concentration and the
Received: August 12, 2015
Revised: September 18, 2015
© XXXX American Chemical Society
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pKa of the amine). To explore the effects of pH on the
mechanistic pathways and on the kinetics of the reactions, as
well as a range of atmospherically relevant aerosol pH values,
strong acids and bases were sometimes also added to the
solutions to adjust pH. The experimental solutions were
prepared by adding small volumes of epoxide and larger
volumes of amine to D2O, and then, if a nonambient pH was
desired, deuterated perchloric acid or sodium deuteroxide was
additionally added (deuterated solvents were used because of
NMR locking requirements). Experiments were performed
using commercially available chemicals from Sigma-Aldrich
(butylamine (99.5%), diethylamine (≥99.5%), t-butylamine
(98%), ethanolamine (99+%), 4-amino-1-butanol (98%),
glycine (≥99%), aniline (≥99%) 2,3-dimethyl-2,3-epoxybutane
(99%), 2,3-epoxy-2-methylbutane (97%), cis-2,3-epoxybutane
(97%), 1,2-epoxybutane (99+%), 1,2-epoxy-2-methylpropane
(97%), perchloric acid-d solution (68 wt % in D2O)), and
sodium deuteroxide (40% in D2O) from Cambridge Isotope
Laboratories, Inc.
The pH measurements were made with a stainless steel
probe and were calibrated with pH 4.00 and 10.00 buffer
solutions before each measurement. The stated precision of the
probe was 0.3 pH units. Because the solutions were primarily
composed of deuterated species, the use of a pH meter
calibrated for normal isotope species will be in error. However,
a previous study showed that pH and pD values differ by on the
order of 0.3 pH units over the pH range of interest in this
study.23 Because this difference is on the order of the stated
precision of the pH meter, the readings from the pH meter
were not corrected for the effect of deuterated species.
To reduce the kinetic situation for the epoxide loss to that of
a pseudo-first-order process, the amine was added in excess
(amine/epoxide molar ratio = 10:1). The epoxides were added
to the solutions last and stirred for 1−3 min to ensure solution
homogeneity before being transferred to an NMR tube for
analysis. Following preparation, product formation and kinetics
were monitored and recorded using 1H and 13C NMR
spectroscopy.
by inspection of the calculated frequencies. The overall energy
expression for the MG2MS scheme is defined in eq 1:
EG2MS = ECCSD(T)/6−31G(d) + EMP2/6−311+G(2df,2p)
− EMP2/6−31G(d) + HLC
(1)
where HLC is an empirically defined correction term with HLC
= Anα + Bnβ where nα and nβ are the number of α- and β-
electrons, respectively, and the constants A and B are 6.06 and
0.19 mH, respectively (all species investigated were closed
shell; therefore, nα = nβ.). Our previous MG2MS results for
atmospherically relevant systems (including radicals and ions)
indicate that the MG2MS-calculated thermodynamic properties
are typically accurate to within 10 kJ mol−1 for systems similar
to those under study here.28
RESULTS AND DISCUSSION
■
1H NMR Spectral Assignments. The identification of
products and the kinetics of all reactions were determined using
1
the H NMR technique. The epoxide reactants and the diol
hydrolysis products have been previously studied, and their
NMR spectra have been reported,29 thus simplifying the
identification of these potential species in the present study.
The reactant amine NMR spectra were obtained separately by
dissolving each amine in D2O and collecting the resulting
spectrum. To the best of our knowledge, there are no previous
1H NMR literature data for the amine−epoxide addition
products that were observed. Therefore, these products were
1
identified by analyzing the H chemical shifts of the protons
that were initially part of the reactant epoxide ring; these
protons experience larger chemical shift differences upon
nucleophilic addition and experience less spectral overlap
than the other protons that exist in the epoxide reactants.
Figure 1 shows a sample 1HNMR spectrum of 1,2-epoxybutane,
t-butylamine, and the primary amine addition product. The
primary addition product was distinguished from the secondary
addition product by observing difference in chemical shifts of
the protons attached to the primary and secondary epoxide
NMR Technique. All NMR spectra were collected on a
Varian 400 MHz instrument, using built-in autolock and
gradient shim routines before collecting each spectrum. The 1H
chemical shifts were calibrated relative to HDO (4.79 ppm),
and the relative amounts of reaction reactants and products
were calculated by peak integration of unique protons for each
species.
Aliquot NMR Analysis Method. To facilitate a separate
study of water’s potential catalytic role in the amine−epoxide
reactions, a 10:1 molar ratio reaction mixture of t-butylamine
and 1,2-epoxybutane (without D2O) was stirred for 2−3 min
and then stored in a vial at room temperature. Small-volume
aliquots of this solution were periodically withdrawn from the
vial, added to ∼700 μL of D2O, and the resulting mixtures were
1
loaded in to NMR tubes and analyzed using H NMR.
Computational Methods. Geometries (determined at the
B3LYP/6-31G(d,p) level) and energies of the relevant species
were calculated using a modified version of the G2MS
compound method (MG2MS),24 which is a variation on G2
theory.25 The Polarizable Continuum Model (PCM) method26
was used to account for the effects of aqueous solvation on the
reactant and product properties. All calculations were
performed with the Gaussian 03 computational suite.27 Each
stationary point was confirmed as a potential energy minimum
Figure 1. 1H NMR spectrum of the reaction between 1,2-epoxybutane
and t-butylamine in D2O at pH = 12.3.
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bridge carbons. It is known from previous work that adjacent
alcohol groups lead to chemical shifts between 0.7 and 1.0 ppm
higher than the corresponding epoxide chemical shifts.29 For a
primary amine addition product, an alcohol group would be
formed at the secondary carbon (which has one proton) and
for a secondary amine addition product, an alcohol group
would be formed at the primary carbon (which has two
distinguishable protons). In addition to the epoxide reactant
peaks at 3.0 (primary), 2.57 (primary), and 2.78 (secondary)
ppm, Figure 1 shows several new peaks: a single multiplet at 3.6
ppm and two multiplets at chemical shift values of 2.95 and
2.74 ppm. Since there is only one new peak with a correct
alcohol functionality chemical shift value (3.7 ppm) and there
are two peaks at much lower chemical shift (2.95 and 2.74
ppm), the spectrum in Figure 1 indicates the sole presence of
the primary amine addition product. On the basis of this
spectrum, it appears the replacement of epoxide functionality
with amine functionality leads to very small changes in chemical
shift for adjacent protons. Similar analyses were made for other
epoxide systems, and in every case only the primary addition
Figure 2. pH dependence of the logarithm of the pseudo-first-order
rate constant (log k′, units: s−1) for the 1,2-epoxybutane and D2O
reaction.
1
product was observed. The details of the H NMR spectral
assignments for all of the reactants and products (and their
chemical shifts for certain solution compositions) are given in
the Supporting Information.
Since the water concentration itself depends extremely weakly
on pH in this pH region, this mechanism is consistent with
kinetics data shown in Figure 2.
Hydrolysis Catalysis Issues. Most of the amine
nucleophilic addition reactions were studied in aqueous
solution. Since water can also react with epoxides via
nucleophilic addition of water (hydrolysis), there is the
potential for a nucleophile competition situation in the present
systems. For the epoxides studied in the present work, the
kinetics of the hydrolysis of epoxides has been previously
reported and has been shown to undergo an acid-catalyzed
mechanism at low pH.29 In a general review of epoxide
reactivity, Whalen reported that many epoxides undergo base-
catalyzed hydrolysis at high pH, and at intermediate pH, the
ring-opening reaction is neither acid- nor base-catalyzed.30
Therefore, the most general rate law contains concentration
dependences for epoxide, water, and a catalyst:
Water as a Catalyst for Amine−Epoxide Systems. In a
previous study, Azizi and Saidi synthesized various β-amino
alcohols by reaction of epoxides in aqueous amine solutions
and concluded that water was acting as a catalyst, since no
reaction was observed between epoxides and amines in dry
solvents.22 To confirm water’s catalytic role in the amine−
epoxide systems, t-butylamine and 1,2-epoxybutane were mixed
together without any aqueous solvent present, and this mixture
1
was monitored using the aliquot HNMR technique described
in the Experimental Section. No reaction was observed to occur
over a 24 h time period. As proposed by Whelan for epoxide
hydrolysis reactions for epoxides at intermediate pH, we
propose that water is similarly acting as a general acid catalyst
for the nucleophilic addition of amines to epoxides.
rate = k[epoxide][H2O][catalyst]
(2)
Because the experiments are generally performed with excess
water, and the catalyst concentration, by definition, does not
change, the rate law can be expressed as a pseudo-first-order
equation:
Kinetics of the Nucleophilic Addition Reaction of
Amines with Epoxides. In the amine−epoxide systems, water
behaves as both a catalyst and, potentially, as a competing
nucleophile. Figure 3 shows the dependence of the logarithm of
the pseudo-first-order rate constants as a function of pH at high
pH for both the formation of the t-butylamine primary addition
product (from nucleophilic addition of the amine to 1,2-
epoxybutane) as well as the diol product (from nucleophilic
addition of water to 1,2-epoxybutane).
As the pH increases by one unit, the corresponding
logarithm of the pseudo-first-order rate constant for the diol
formation increases by one unit (as expected for a first-order,
base-catalyzed process), while the logarithm of the pseudo-first-
order rate constant for the amine addition product stays
relatively constant. (The slight increase in the amine−epoxide
reaction pseudo-first-order rate constants at high pH is most
likely not due to the role of base catalysis but rather to an
indirect effect of the changing solvent conditions (higher ionic
strength, for example).) Thus, it appears that the rate of
formation of the amine addition product is not characterized by
base catalysis. As will be shown later, this process is similarly
not influenced by any acid-catalyzed process either. Instead, it
appears that the amine−epoxide reactions are characterized by
rate = k′[epoxide]
(3)
where k′ is a pseudo-first-order rate constant that is
proportional to the catalyst concentration. According to
Whelan, at low pH, log(k′) should be a negative linear function
of pH (acid catalysis), while at high pH, log(k′) should be a
positive linear function of pH (base catalysis).
To confirm this catalysis situation for the present epoxides, a
full pH-dependent determination of the hydrolysis rate
constants for 1,2-epoxybutane was performed. Figure 2 shows
how the logarithm of the pseudo-first-order rate constant k′
varies with pH. These data agree with the framework suggested
by Whelan, including an intermediate pH region where the
pseudo-first-order rate constant does not depend on pH. One
possible interpretation of this intermediate pH region data is
that the hydrolysis reaction is uncatalyzed. However, Whelan
suggests that the hydrolysis reaction undergoes general acid
catalysis in this pH region, with water acting as a general acid
(water acts as a proton donor to the epoxide oxygen atom).
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Figure 4. Fraction of unprotonated t-butylamine (black) and aniline
(red) remaining in aqueous solution at various pH values.
Figure 3. pH dependence of the logarithm of the pseudo-first-order
rate constant (log k′, units: s−1) for water +1,2-epoxybutane reaction
(red) and for the t-butylamine +1,2-epoxybutane reaction (black).
These data predict that the t-butylamine nucleophilic
addition to epoxides will be efficient for pH > 10, while the
aniline nucleophilic addition reaction to epoxides will be
efficient for pH > 4. Under the hypothesis that the addition of
amines to epoxides is entirely dependent on the unprotonated
amine concentration, the pseudo-first-order rate constants were
measured at the pH values indicated in Figure 4. At higher pH
values (for example, t-butylamine is 100% unprotonated at pH
= 12.3), the primary amine addition product was the sole
product observed. As pH was lowered, less amine product was
formed, and the diol product from the competing hydrolysis
reaction became observable. At lower pH values (for example, t-
butylamine is more than 99% protonated at pH = 7.7), the
amine addition product was no longer observable, and only the
diol product was observed. Therefore, it is more accurate to
define the pseudo-first-order rate constant for amine−epoxide
reactions as
water acting as a general acid catalyst across the whole range of
measured pH values.
Therefore, the rate law for the reaction for amine−epoxide
reactions is assumed to follow the general form:
rate = k[epoxide][amine][H2O]
(4)
However, because the catalyst water is held at a nearly fixed
concentration (55 M), and the amine concentrations are held
in at least 10-fold excess over the epoxide concentrations, this
rate reduces a similar first-order form to that for hydrolysis
rate = k″[epoxide]
(5)
k″ = [amine][H2O]
(6)
Because these rate equations do not contain any acid or base
terms, it is reasonable to posit that the amine−epoxide
reactions will not be pH-dependent. However, as will be
discussed in the next section, unlike for acid- and base-catalyzed
hydrolysis of epoxides, the concentration of the active
nucleophile species itself, the unprotonated amine, is pH
dependent, whereas the concentration of the hydrolysis
reaction nucleophile, water, is not pH-dependent. This leads
to a fundamentally different pH-dependent kinetics situation
for amine−epoxide reactions as compared to epoxide
hydrolysis.
k″ = [unprotonated amine][H2O]
(8)
To quantitatively establish this relationship, the data were
analyzed in a traditional pseudo-first-order fashion, where k″ is
plotted against the excess concentration (unprotonated amine
in this case) to yield the overall rate constant k as the slope of
the line. The particular analysis for the aniline-1,2-epoxybutane
reaction is given in Figure 5 (the lowest pH data point for
aniline from Figure 4 does not appear in Figure 5 due to the
existence of an immeasurably small rate constant for this pH).
The overall rate constants k (for which the constant water
concentration term is subsumed) for the formation of the
primary amine addition products from 1,2-epoxybutane were
determined to be 1.64 0.11 (1 σ) and 1.56 0.19 × 10−4
M−1 s−1, respectively, for t-butylamine and aniline. Therefore,
within the uncertainty of the measurements, there is no
measurable difference in the rate of reaction of 1,2-epoxybutane
with t-butylamine and aniline, which suggests that the amine
structure does not sensitively affect the rate of the epoxide−
amine reactions.
pH Dependence of Amine-Epoxide Rate Constants. At
moderate and low pH, the protonation equilibrium of amines
becomes relevant:
+
R−NH2 + H+ ↔ R−NH3
(7)
As nucleophilic strength is related to the charge of the
species (the more negatively charged the nucleophile, the
stronger the nucleophile), it is expected that a protonated
amine will be a significantly weaker nucleophile than an
unprotonated amine. Using the pKa for t-butylamine (10.68), a
relatively strongly basic amine, and the pKa for aniline (4.70), a
relatively weakly basic amine,31 and assuming a 1 M total amine
concentration, the fractions of unprotonated amine compared
to total amine concentrations were calculated at experimentally
utilized pH values and are plotted in Figure 4.
Amine Nucleophilic Strength Determination. At high
pH values, all of the epoxide reactant is converted to the
primary amine addition product, and no hydrolysis product is
observed. Figure 2 suggests that over the pH = 8−12 range,
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Figure 5. Pseudo-first-order determination of the overall rate constant
for the reaction between aniline and 1,2-epoxybutane.
Figure 6. Nucleophilic strength determination for aniline with 1,2-
epoxybutane.
wherein the t-butylamine protonation equilibrium is important,
the competing hydrolysis reaction may switch from a general
acid-catalyzed mechanism at the lower pH to a faster base-
catalyzed mechanism at the higher pH. However, since
hydrolysis products are only observed at lower pH values, the
appearance of hydrolysis products at the lower pH values must
be due to a difference in nucleophile concentrations
(unprotonated amine concentrations drop as the pH as
lowered, while water concentrations stay constant) rather
than to a change in mechanism. The relative nucleophilic
strength of the amines as compared to water were calculated
using reactant (X) and product (Y) mole fractions where
monly known as the SN2 mechnanism).33 These two
mechanisms are depicted for the acid-catalyzed case in Figure 7.
In the case of the SN1 mechanism, the stability of the ring-
opened intermediate was used to rationalize structure−
reactivity trends. For example, because it is known that tertiary
carbocations are the most stable type of carbocation
intermediate, the previously cited experimental result that
epoxide hydrolysis is faster for more substituted epoxides was
used to indicate the importance of the SN1 character of tertiary
epoxide reactions. Because of the instability of primary and
secondary carbocations, it is often assumed that primary and
secondary epoxides more likely react via an SN2-type
mechanism. However, the epoxide hydrolysis kinetics literature
indicates that these idealized mechanisms are likely too simple
to explain actual kinetics trends and that some epoxides may
react via a hybrid SN1/SN2-type mechanism.30,34,35 Table 1
contains the results of these experiments for the reactions of
various epoxides with t-butylamine.
It is clear from these data that increasing carbon substitution
on the epoxide ring leads to slower amine nucleophilic addition,
with the case of the ditertiary substituted epoxide (2,3-
dimethyl-2,3-epoxybutane) being completely unreactive on
the time scale (seven months) of the experiments. These
results also suggest the likely importance of SN2 character in the
amine addition reactions; the more steric hindrance around the
epoxide ring, the slower the amine addition reaction (the fact
that primary nucleophilic addition is exclusively observed is also
indicative of the likelihood of an SN2-type mechanism). Note
that because water is a catalyst in these reactions and because it
was not possible to systematically vary water concentrations
while keeping other important parameters constant, it was not
possible to use a rate law determination to directly support the
assertion of the dominance of SN2 character in the mechanism
of these reactions.
(Yamine product/Xunprotonated amine
)
amine nucleophilic strength
water nucleophilic strength
=
(Yhydrolysis product/XH O
)
(9)
2
The Yamine product and Yhydrolysis product values were determined
from NMR integrations, and Xunprotonated amine and XH O values
2
were determined from equilibrium calculations for the relevant
pH values. Figure 6 shows the results of this analysis for aniline,
where the slope corresponds to the relative nucleophilic
strength of aniline as compared to water:
Because of the rigorous relationship between the formation
of the amine product and the unprotonated amine concen-
tration, the regression equation was forced through the origin.
The measured nucleophilic strengths were found to be 3000
and 900 for t-butylamine and aniline, respectively. Even in
comparison to the negatively charged inorganic ions nitrate and
sulfate, which have nucleophilic strengths relative to water
ranging from 5 to 70,17 respectively, it is clear that these amines
are extremely strong nucleophiles.
Epoxide Structure−Reactivity Effects. To explore the
effect of epoxide structure on the kinetics of the amine
nucleophilic addition reaction, other epoxides were selected for
study on the basis of differing carbon substitution on the
epoxide ring. A previous study on the hydrolysis kinetics of
epoxides indicated that as the extent of carbon substitution on
the epoxide ring increased, the hydrolysis rate constants also
increased.32 One of the textbook mechanisms for nucleophilic
addition reactions of epoxides is a sequential ring-opening step
followed by a nucleophilic attack step (commonly known as the
SN1 mechanism), while the other textbook mechanism is a
concerted ring-opening and nucleophilic attack step (com-
Potential Hydrolysis of Amine Addition Products. In
previous work on organosulfates and organonitrates formed
from epoxide reactions, it has been shown that some of these
species are not stable against hydrolysis (nucleophilic
substitution of nitrate or sulfate for water).17,19,36−38 Therefore,
it is of interest to consider whether the amine addition products
are stable against hydrolysis. Table 1 contains the computed
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Figure 7. SN1 (upper) and SN2 (lower) nucleophilic addition mechanisms.
Table 1. t-Butylamine Nucleophilic Addition Rate Constants and Amine Addition Product Hydrolysis Free Energies
free energies of hydrolysis for the various amine addition
products formed from reactions of 1,2-epoxybutane (the data
used to compute these values are given in the Supporting
Information). As was observed for the organosulfates and
organonitrates,38 these calculations generally indicate a higher
thermodynamic propensity for hydrolysis for the most highly
substituted amine products. However, as the computational
method has an accuracy of only ∼10 kJ mol−1, these results are
not unambiguous. Therefore, the amine addition products for
the reaction of 1,2-epoxybutane with t-butylamine (under the
ambient basic conditions for these solutions) monitored for
extended periods of time (three to five months) to
experimentally assess their stability against hydrolysis. None
of the amine addition products were observed to hydrolyze on
these time scales. Therefore, the potential distribution of amine
addition and hydrolysis (diol) products is expected to be
determined solely by nucleophilic competition effects in the
epoxide reactions themselves and not due to the hydrolysis of
amine addition products to diols at later times. The potential
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hydrolysis of other systems and at lower pH values was not
experimentally investigated.
Amine Structure−Reactivity Effects. To explore the
effect of amine structure on the kinetics of the amine
nucleophilic addition reaction, other amines were selected for
study based on their differing carbon substitution character-
istics. Table 2 contains the results of these experiments for the
nucleophilic addition reactions with methacrylic acid epoxide
indicated that methanol was a slightly better nucleophile on a
per-mole basis than was water.18 However, only primary amine
addition products were observed at the high pH values where
the unprotonated amine form was dominant, and only
hydrolysis products were observed at the low pH values
where the protonated amine form was dominant, thus
indicating that the high-concentration solvent water was the
better absolute nucleophile than the low-concentration alcohol
group on MEA. Nonetheless, under lower water content
conditions and low pH conditions, it is possible that MEA
could use its alcohol group for nucleophilic addition reactions.
In the case of glycine, there are several ionization states
possible as a function of pH. The amino group has a pKa of
9.78, and the carboxylic group has a pKa of 2.35.31 Therefore, at
high pH values, the carboxylic acid group is deprotonated and
the amino group is neutral, at intermediate pH values, both
functional groups are charged (carboxylic acid group:
deprotonated; amino acid group: protonated), and at low pH
values, the carboxylic acid group is neutral and the amino group
is protonated. Again, although the deprotonated carboxylic acid
group might be expected to be a reasonably strong nucleophile
as a negatively charged entity, no products were observed
wherein glycine used its carboxylic group to perform
nucleophilic addition on 1,2-epoxybutane. At high pH, only
amine addition products were observed, at intermediate pH,
both amine addition and hydrolysis products were observed,
and at low pH, only hydrolysis products were observed. As for
MEA, this result does not necessarily rule out that under lower
water conditions and low pH conditions glycine could use its
carboxylic acid group for nucleophilic addition reactions.
Therefore, it appears that these multifunctional amines can
be thought of as possessing extremely strong amino
nucleophilic strength for the pH ranges for which the amino
group remains unprotonated, but that relatively low concen-
tration alcohol and deprotonated carboxyl groups are not
strong enough nucleophiles to compete with solvent water. In
our previous work on the relative nucleophilic strengths of
various reactants with methacrylic acid epoxide, we determined
that methanol and acetic acid had similar nucleophilic strengths
as compared to water.18 These previous results are consistent
with the present finding that solvent water can outcompete
alcohol and carboxyl groups in the nucleophilic addition
reactions of the epoxides investigated in this work.
Atmospheric Implications. The mechanistic findings that
the amine−epoxide nucleophilic addition reactions are water-
catalyzed and that amines are extremely strong nucleophiles
seems to suggest that these reactions may be more ubiquitous
under atmospheric conditions than processes that are kineti-
cally limited to acid-catalyzed conditions and utilize weaker
nucleophiles (such as the acid-catalyzed epoxide reactions with
water and sulfate that have previously been investigated as an
explanation for the presence of isoprene-derived polyol and
organosulfate species in SOA). While most of the amines
studied in the present work are too volatile to partition
significantly to atmospheric SOA, here we consider the
potential reactivity of a hypothetical semivolatile amine that is
present at 1 M SOA concentration. Using an average rate
constant from Table 2 of 3 × 10−4 M−1 s−1 this amine would be
expected to have a lifetime of ∼1 h in its reaction with 1,2-
epoxybutane, which is quite fast enough to be an atmospheri-
cally significant SOA phase reaction. Since the rate constants
were not found to be very sensitive to the structure of the
Table 2. Amine +1,2-Epoxybutane Nucleophilic Addition
Rate Constants
reactions of 1,2-epoxybutane with various amines. While the
effect of the amine structure on the nucleophilic addition rate
constants is significantly less than the effect of the epoxide
structure, it is clear that the more sterically hindered amines
react somewhat more slowly.
Multifunctional Amine Structure−Reactivity Effects.
To investigate whether multifunctional amines have different
structure−reactivity properties than the monofunctional
amines, the carbon dioxide capture fluid monoethanolamine
(MEA) and the amino acid glycine were qualitatively
investigated in their reactions with 1,2-epoxybutane (addition-
ally, the nucleophilic addition rate constant was also measured
for MEA and is reported in Table 2).
In the case of MEA, it is possible that the alcohol functional
group could act as the nucleophile at low pH where the amino
group’s nucleophilic character has been deactivated by
protonation. It is important to note that both of these
functional groups are also in competition with solvent water as
a nucleophile under the experimental conditions. A previous
nucleophilic strength study of methanol and water in their
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The Journal of Physical Chemistry A
Article
amine, actual semi- or nonvolatile amines present in the
atmosphere would likely have epoxide reaction rate constants
similar to the amines studied in this work. However, the most
important atmospherically relevant epoxide, the isoprene-
derived species 2-methyl-2,3-epoxy-1,4-butanediol (IEPOX),
resembles 2-methyl-2,3-epoxybutane in its carbon substitution
environment. Table 1 indicates that this change in carbon
substitution would be expected to decrease the amine
nucleophilic addition rate constant by a factor of 200; therefore,
a better estimate for the lifetime of an amine nucleophilic
addition reaction with IEPOX is on the order of one week,
which is on the order of the lifetime of an SOA particle.
Therefore, it appears that amine−epoxide reactions are
kinetically feasible on atmospheric SOA.
However, there is a more general situation that will likely
circumvent the importance of amine reactions with epoxides on
SOA. While very weakly basic amines like aniline can have
significant unprotonated concentrations at acidic pH values
(see Figure 4), most atmospherically relevant amines are
aliphatic amines and thus stronger bases (like t-butylamine) and
are not likely to have significant unprotonated concentrations
below pH = 7. Therefore, most atmospheric amines are
expected to exist in protonated form on the mostly acidic
SOA39,40 thought to be prevalent in the atmosphere; in the
protonated form, the amines are extremely weak nucleophiles
and may not compete well with the other nucleophilic species
commonly present on SOA (such as water and sulfate).
Therefore, it seems unlikely that amine nucleophilic reactions
with epoxides are important on acidic SOA.
AUTHOR INFORMATION
Corresponding Author
*E-mail: mjelrod@oberlin.edu.
Notes
■
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This material is based upon work supported by the National
Science Foundation under Grant No. 1153861.
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ASSOCIATED CONTENT
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S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.jpca.5b07852.
1
Full H NMR spectral assignments of the reactants and
products and computational chemistry results for
geometries and energies. (PDF)
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