The hydrolysis of epoxides catalyzed by inorganic ammonium salts in water: Kinetic evidence for hydrogen bond catalysis

Article abstract of DOI:10.1039/c7cp06790a

Naturally-occurring inorganic ammonium ions have been recently reported as efficient catalysts for some organic reactions in water, which contributes to the understanding of the chemistry in some natural environments (soils, seawater, atmospheric aerosols, .) and biological systems, and is also potentially interesting for green chemistry as many of their salts are cheap and non-toxic. In this work, the effect of NH₄⁺ ions on the hydrolysis of small epoxides in water was studied kinetically. The presence of NH₄⁺ increased the hydrolysis rate by a factor of 6 to 25 compared to pure water and these catalytic effects were shown not to result from other ions, counter-ions or from acid or base catalysis, general or specific. The small amounts of amino alcohols produced in the reactions were identified as the actual catalysts by obtaining a strong acceleration of the reactions when adding these compounds directly to the epoxides in water. Replacing the amino alcohols by other strong hydrogen-bond donors, such as trifluoroethanol (TFE) or hexafluoroisopropanol (HFIP) gave the same results, demonstrating that the kinetics of these reactions was driven by hydrogen-bond catalysis. Because of the presence of many hydrogen-bond donors in natural environments (for instance amines and hydroxy-containing compounds), hydrogen-bond catalysis is likely to contribute to many reaction rates in these environments.

Full text of DOI:10.1039/c7cp06790a

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The hydrolysis of epoxides catalyzed by inorganic
ammonium salts in water: kinetic evidence for
hydrogen bond catalysis†
Cite this:DOI: 10.1039/c7cp06790a
B. Noziere, *a F. Fache,b A. Maxut,a B. Fenet,c A. Baudouin,c L. Finea and
`
C. Ferronatoa
Naturally-occurring inorganic ammonium ions have been recently reported as efficient catalysts for
some organic reactions in water, which contributes to the understanding of the chemistry in some
natural environments (soils, seawater, atmospheric aerosols, . . .) and biological systems, and is also
potentially interesting for green chemistry as many of their salts are cheap and non-toxic. In this work,
+
the effect of NH4 ions on the hydrolysis of small epoxides in water was studied kinetically. The
+
presence of NH4 increased the hydrolysis rate by a factor of 6 to 25 compared to pure water and these
catalytic effects were shown not to result from other ions, counter-ions or from acid or base catalysis,
general or specific. The small amounts of amino alcohols produced in the reactions were identified
as the actual catalysts by obtaining
a strong acceleration of the reactions when adding these
compounds directly to the epoxides in water. Replacing the amino alcohols by other strong hydrogen-
bond donors, such as trifluoroethanol (TFE) or hexafluoroisopropanol (HFIP) gave the same results,
demonstrating that the kinetics of these reactions was driven by hydrogen-bond catalysis. Because of
the presence of many hydrogen-bond donors in natural environments (for instance amines and
hydroxy-containing compounds), hydrogen-bond catalysis is likely to contribute to many reaction rates
in these environments.
Received 5th October 2017,
Accepted 8th December 2017
DOI: 10.1039/c7cp06790a
rsc.li/pccp
effect does not seem to be as impacted by water as with amines
1. Introduction
or amino acids.14 In addition, NH4 salts are generally cheaper
+
Most chemical reactions in natural environments near or at the than organocatalysts and many of them are non-toxic: NH4Cl
Earth surface and in biological systems take place in aqueous and (NH4)2SO4 are food additives (‘‘salmiak’’ or E510 and E517,
media containing inorganic ions. The inorganic ammonium ion, respectively), and NH4I is a pharmaceutical and cosmetic additive.
+
NH4 , is one of the most common in these media and is known Thus, inorganic ammonium salts could be interesting catalysts
to interfere with organic or biochemical reactions in soils,1 for greener organic synthesis or chemical manufacturing in water.
+
seawater,2,3 atmospheric aerosols,4–8 and biological systems.9,10
The present work investigates the role of NH4 ions on
+
Understanding the role of NH4 in organic reactions in water reactions that do not proceed by iminium or enamine inter-
thus improves the understanding of the chemistry in these mediates: the hydrolysis of epoxides in water. For this, the
environments. In some reactions NH4 has been shown to act hydrolysis of 1,2-epoxybutane (1,2-EB), cis-2,3-epoxybutane
+
as a catalyst, by a mechanism thought to proceed through (2,3-EB), 2,3-dimethyl-2,3-epoxybutane (DM-2,3-EB), and cis-
iminium and/or enamine intermediates,5,8 as in organocatalysis and trans-(2-methyloxirane-2,3-diyl)dimethanol (IEPOX 3 and
with amines and amino acids.11–13 In these cases, the catalytic IEPOX 4), chosen for their environmental relevance,15,16 were
studied in water at room temperature (T = 303 ꢀ 3 K) and in the
presence of various NH4+ salts. The products and kinetics of the
a Institut de Recherches sur la Catalyse et l’Environnement de Lyon (IRCELYON),
reactions were studied in the presence of ammonium- and non-
ammonium salts. The dependence of the hydrolysis rates on
NH4+ and the potential contribution of Brønsted acid or base
catalysis to these rates were then examined by varying both the
´
CNRS & Universite Claude Bernard Lyon 1, Villeurbanne, France.
E-mail: barbara.noziere@ircelyon.univ-lyon1.fr
b
c
´
´
Institut de Chimie et Biochimie Moleculaires et Supramoleculaires (ICBMS),
´
CNRS & Universite Claude Bernard Lyon 1, Villeurbanne, France
´
Centre Commun de RMN (CCRMN), Universite Claude Bernard Lyon 1 & Ecole
NH4+ activity, aNH
+, and pH of the solutions. Finally, hydrogen
4
´
Superieure de Chimie, Physique, Electronique de Lyon (CPE), Villeurbanne, France
bond catalysis was identified as the kinetically-controlling
mechanism by examining the hydrolysis rate obtained when
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c7cp06790a
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adding directly in water the amino alcohols produced in the the hydrolysis of epoxides was established by studying the
+
presence of NH4 or other strong hydrogen-bond donors.
reactions of 1,2-EB and 2,3-EB in pure water and in aqueous
solutions of ammonium and non-ammonium salts: NH4F,
(NH4)2SO4, NaCl, MgSO4, and Na2SO4. In all cases, the formation
of diols (structures in Table 1) confirmed that hydrolysis took
place and the reaction rates were measured from the formation
of these compounds (Fig. 1 and Experimental section for details
2. Results and discussion
2.1 Specific effect of NH4 on the hydrolysis rate
+
A complete list of the kinetic experiments and their conditions on the kinetic analysis). As the hydrolysis of epoxides in pure
+
is given in Table S1 of the ESI.† The kinetic effect of NH4 on water is known to be of the first order17 and our study will show
Table 1 Reaction products observed, with their retention times (‘‘RT’’), main mass fragments in GC/MS analysis, and chemical shifts in 1H-NMR (in bold =
shifts used in the kinetic and yield analysis)
Main chemical shifts,
dH (ppm)
Main MS
fragments (amu)
Main chemical
shifts, dH (ppm)
Epoxide
Main products
RT (min)
9.45
0.96–0.99
1.37–1.59
3.45–3.51
3.58–3.68
31, 41, 59
3.75–3.82
1.0–1.1
2.69–2.72
2.9–3.0
3.1–3.2
3.87–3.89
3.93–4.0
Organosulfates (in (NH4)2SO4 only)
1.12–1.17
3.59–3.62
3.71–3.77
9.76
43, 45, 57
1.27–1.32
3.21–3.33
3.15–3.22
9.87
43, 59, 85
1.22–1.25
1.34–1.36
1.37–1.39
1.12
3.35–3.85
—
—
1.39–1.40
??
??
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and (NH4)2SO4 solutions were significantly larger than those in
pure water, while those in NaCl, MgSO4, or Na2SO4 solutions
were lower or comparable, even at large salt concentrations
+
(1 M). These results demonstrated the specific effect of NH4
on the reaction kinetics, and the lack of the effect of the other
ions (Na+, Mg2+) and counter-ions (Fꢁ, Clꢁ, SO42ꢁ). This, in
turn, ruled out a catalytic mechanism driven by electrostatic
effects (for instance, ionic force).
The order of the reaction in 1,2-EB, 2,3-EB, and DM-2,3-EB in
the ammonium solutions was investigated by comparing the
initial formation rate for the diols with the initial concentration
of epoxide for solutions with aNH = 0.12 and 0.5 M. The slopes
+
4
obtained in the ln–ln scale were all close to unity, within
uncertainties (1.1, 0.9, 1.0 and 1.3, 1.0, 1.2, respectively),
indicating that the reactions were of the first order in epoxide
and validating the first-order kinetic analysis. For each of the
epoxides studied, the hydrolysis rate measured in ammonium
solutions, kI, was then corrected for the corresponding rate
in pure water,
Fig. 1 Experimental time-profiles and kinetic analysis for the hydrolysis of
1,2-EB in an aqueous solution of (NH4)2SO4 0.5 M: top: real-time evolution
of the concentrations of 1,2-EB (blue), 1,2-butanediol (red) and 1-amino-
2butanol (black) observed by NMR. Bottom: First-order analysis for 1,2-
butanediol giving the hydrolysis rate, kI (sꢁ1).
I
k0 = kI ꢁ kIN,
(1)
I
and the variations of k0 with aNH
+
were studied for all the
4
epoxides. The results are shown in Fig. 2.
I
In all cases, k0 increased strongly with aNH
+
, and exceeded
4
the rate in pure water by a factor of 6 to 25 in the most
concentrated ammonium solutions. The slopes of the lines
in Fig. 2 (in ln–ln scale) were 1.2, 1.0, 0.7, and 1.1 for 1,2-EB,
that this is also the case in salt solutions (below in this section),
first-order analyses were applied to the concentration profiles
(Fig. 1) to obtain first-order rates, kI (sꢁ1) (Table S1, ESI†). The
uncertainties of these rates were estimated to ꢀ15%, which is
typical for absolute reaction rates determined experimentally.
The hydrolysis rates measured in pure water for 1,2-EB and
2,3-EB, kIN (sꢁ1), were in agreement with those reported in the
literature (Table 2).17 For DM-2,3-EB and IEPOX, the kIN was
estimated as the lower limits of the rates measured in this work
2,3-EB, DM-2,3-EB, and IEPOX, respectively, thus implying that
I
the hydrolysis rate k0 was of the first order in aNH
+
. Note that
4
the rates measured for the lowest concentration of ammonium
(aNH = 0.12 M) for DM-2,3-EB were not taken into account as,
+
4
after subtraction of kIN, they contained large uncertainties. As kI
had been shown above to be of the first order in epoxide, the
for the respective compounds. The hydrolysis rates in NH4F first order in aNH indicated that the rate-limiting step of the
+
4
), H+ (kHII ), and H2O (kIN) and comparisons with
+
II
NH
Table 2 Rate coefficients measured in this work for the hydrolysis of epoxides catalyzed by NH4 (k
þ
þ
4
II
OH
the literature for acid, neutral, and base catalysis (k
)
II
OH
(Mꢁ1 sꢁ1
)
kIHI (Mꢁ1 sꢁ1
)
k
(Mꢁ1 sꢁ1
)
kIN (sꢁ1
)
II
NH
Epoxide
k
þ
þ
4
(5.2 ꢀ 1.6) ꢂ 10ꢁ2
7.4 ꢂ 10ꢁ2 (ref. 18)
(3.0 ꢀ 0.5) ꢂ 10ꢁ6
(3.9 ꢀ 0.9) ꢂ 10ꢁ6
1 ꢂ 10ꢁ4 (ref. 17)
1 ꢂ 10ꢁ4 (ref. 17)
5.5 ꢂ 10ꢁ7 ( ref. 17)
1 ꢂ 10ꢁ6 (ref. 17)
(2.0 ꢀ 0.5) ꢂ 10ꢁ2
1.5 ꢂ 10ꢁ2 (ref. 17)
0.218
(55 ꢀ 11) ꢂ 10ꢁ6
1.5 ꢀ 0.3 15 (ref. 18)
1 ꢂ 10ꢁ4 (ref. 17)
5 ꢂ 10ꢁ6
(2.5 ꢀ 1.5) 10ꢁ2
(0.16 ꢀ 0.02) ꢂ 10ꢁ6
3.6 ꢂ 10ꢁ2 (ref. 19 and 20)
5 ꢂ 10ꢁ2 (ref. 21 and 22)
1 ꢂ 10ꢁ4 (ref. 17)
1 ꢂ 10ꢁ8
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I
Fig. 3 Variations of the corrected hydrolysis rates, k0 (sꢁ1), with pH
in ammonium solutions with aNH
+
4
= 0.51 M (full symbols) and in sulfuric
acid solutions (open symbols) and expected variations due to strong
base catalysis (black dashed line). The dashed lines represent the best fits
to linear regressions. Green = 1,2-EB; red = 2,3-EB, blue = DM-2,3-EB,
yellow = IEPOX.
I
Fig. 2 Variations of the corrected hydrolysis rates, k0 , with aNH
+
4
in aqueous
solutions of NH4F (open symbols) and (NH4)2SO4 (full symbols) at 303 K for
1,2-EB (green), 2,3-EB (red), DM-2,3-EB (blue), and IEPOX (yellow).
I
for the rates in pure water and the resulting k0 (sꢁ1) are shown in
hydrolysis mechanism was the first reaction between the epoxide Fig. 3 (open symbols) and listed in Table S6, ESI.† The corres-
+
and NH4 . Second-order rate coefficients, kII
(Mꢁ1
I
s
ꢁ1), were ponding second-order rate coefficients, kHIIþ (Mꢁ1 sꢁ1), obtained
þ
NH4
by dividing k0 by the activity of H+, aH+, are given in Table 2. They
were generally in good agreement with those reported in the
literature, especially those reported in ref. 17, thus confirming
the validity of our measurements and analyses.
I
then obtained for each epoxide by dividing k0 by aNH
+. The
4
results are given in Table 2 and show that these rate coefficients
vary widely with the structure of the epoxide, increasing by over
a factor of 10 from a single alkyl- substituted to a four-time
substituted epoxide (from 1,2-EB to DM-2,3-EB), and decreasing
by over a factor of 20 with the presence of two hydroxyl groups
on the epoxide (from 1,2-EB to the IEPOX).
The potential contribution of strong base catalysis to the
measured hydrolysis rates was also determined from the corres-
ponding rate coefficients reported in the literature (see Table 2).
The results are reported in Fig. 3 (black dashed line). Fig. 3 thus
I
clearly shows that, in ammonium solutions (full symbols), k0
2.2 Potential contributions of general and specific acid or
base catalysis to the hydrolysis rate
does not display any of the strong variations with pH that are
typical for strong acid or strong base catalysis. In Table S6, ESI,†
the contribution of the strong acid to k0I is shown not to exceed
10% in most cases and that of the strong base catalysis to be
The potential contribution of acid or base catalysis to the
corrected reaction rate, k0 , was investigated by measuring this
I
rate in ammonium solutions with a pH value between 5.7 and
10. This allowed checking both for the potential contribution of
strong (or ‘‘specific’’) acid or base catalysis, indicated by the
negligible over the entire range of aNH studied.
+
4
I
2.3 Product identification and yields
variations of k0 over a wide range of pH, and for the contribu-
tion of weak (or ‘‘general’’) acid catalysis by various species As indicated in the Experimental section, the products of all the
present in the reaction mixtures, indicated by a local variation reactions studied were analyzed by NMR and GC-FID. For all
I
of k0 near pH = pKa.23 In particular, the range of pH studied the reactions, the NMR spectra at a large conversion (450%)
here allowed examining the potential role as weak acid catalyst displayed characteristic shifts identical to those of the authentic
+
of NH4 /NH3 (pKa = 9.25) and of the amino alcohols produced standards of the expected diol or tetrol products (Fig. 4 for the
I
in the reactions (cf. Section 2.3, pKa = 8–10). The values of k0
reaction of 1,2-EB and Section S2, ESI† for the others), confirming
obtained at these different pH values are shown in Fig. 3 for that these compounds were the main products. For the reactions
aNH = 0.51 M (full symbols) and given in Table S1, ESI† for all of 1,2-EB, 2,3-EB, and DM-2,3-EB this was also confirmed by
+
4
the other values of aNH
+
studied. These results clearly show GC/MS analyses, where the chromatograms at large conversion
4
I
that k0 did not vary with pH over the range studied, which ruled displayed one prominent peak (Section S3, ESI†), with a retention
out the contribution of both strong and weak acid or base time and mass spectrum identical to those of the standards of
catalysis to the measured hydrolysis rate in this work, including 1,2-butanediol, 2,3-butanediol, and 2,3-dimethyl-2,3-butanediol,
+
the potential roles of NH4 or amino alcohols.
respectively (Table 1). The NMR spectra also indicated the for-
For comparison, the hydrolysis rates resulting from specific mation of amino alcohols in the reactions (Fig. 4 and Section S2,
(strong) acid catalysis were also measured for each epoxide in ESI†). For 1,2-EB the formation of 1-amino-2-butanol and
sulfuric acid solutions. The reaction rates obtained by first- 2-amino-1-butanol was unambiguously confirmed by compar-
order analyses are listed in Table S1, ESI.† They were corrected ison with authentic standards (Fig. 4). In the reaction of 2,3-EB,
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Fig. 4 Evolution of the 1H-NMR spectrum during the reaction of 1,2-EB in aqueous solutions (NH4)2SO4 0.5 M, evidencing the formation of
1,2-butanediol (red region), 1-amino-2-butanol and 2-amino-1-butanol (grey regions) and possibly organosulfates (green region).
the formation of amino alcohol was indicated by a triplet at
3.15 ppm, typical of protons adjacent to –NH2 groups.
For the reaction of DM-2,3-EB the formation of amino
alcohol was only indicated by the formation of a singlet at
1.34 ppm, corresponding to the methyl groups on the quatern-
ary carbon bearing the ꢁNH2 substituent, as expected from the
structure in Table 1. For the IEPOX, the formation of amino
alcohol (expected structures in Table 1) could not be confirmed
because of the complexity of the spectra. Finally, in the reaction
of 1,2-EB in (NH4)2SO4 solutions the formation of triplets near
4 ppm, that did not form in NH4F solutions, suggested the
formation of small amounts of organosulfates. The product
yields were quantified by NMR and, for the alcohols, also by
GC/FID (Table S1, ESI† and Experimental section for details
Fig. 5 Variations of the yields for alcohols (full symbols) and amino
on the quantifications). In the reaction of 1,2-EB, the yield
alcohols (open symbols) with aNH
2,3-EB (blue), DM-2,3-EB (red), IEPOX (yellow).
+
in the hydrolysis of 1,2-EB (green),
4
of 1,2-butanediol decreased with aNH , from about 85% with
+
4
aNH
+
= 0.1 M to 25% with aNH = 1.5 M. Correspondingly, the
+
4
4
amino alcohol yield increased from 30% with aNH = 0.3 M to
+
4
with aNH . In H2SO4 solutions, the alcohol yields measured
+
65% with aNH
+
= 1 M (Fig. 5), where 1-amino-2-butanol
4
4
were all close to 100%, within uncertainties (102 ꢀ 1, 99 ꢀ 2,
97 ꢀ 6, 92 ꢀ 3 for 1,2-EB, 2,3-EB, DM-2,3-EB and IEPOX,
respectively) and independent of pH, in agreement with the
literature.18
represented about 2/3 and 2-amino-1-butanol about 1/3, which
was consistent with previous studies.24 The diol and amino
alcohols thus accounted, together, for 85 to 90% of the pro-
ducts of 1,2-EB in all the solutions. In the (NH4)2SO4 solutions,
the organosulfate yield was estimated to be 15% or less. In the
reactions of 2,3-EB, and DM-2,3-EB the diol yields were larger
2.4 Identification of the catalytic mechanism
and decreased less with aNH than for 1,2-EB (093 to 70% and Monitoring the reactions in real time showed that the amino
+
4
100 to 79%, respectively. Fig. 5). For 2,3-EB the amino alcohol alcohols produced were not further consumed and that their
yield measured was between 8 and 15% and increased with concentrations remained constant (Fig. 1). As these compounds are
aNH
+
so that the diol and amino alcohol together accounted for known to be efficient catalysts for the hydrolysis of epoxides25,26
4
about 100% of the products (Fig. 5). For DM-2,3-EB, the amino and no other type of intermediates resulting from the epoxy and
alcohol yield measured was about 2%. For the hydroxy- NH4+ could be proposed, their potential role as the actual catalysts
substituted epoxides, IEPOX3 and 4, the tetrol yields were 78 for the reactions in ammonium solutions was investigated. The
and 67%, respectively. Thus, all the epoxides studied produced hydrolysis of 1,2-EB and 2,3-EB were thus studied in water and
some amounts of amino-alcohols, and in quantities increasing in the presence of 1-amino-2-butanol only (Table S1, ESI†).
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In both cases hydrolysis took place and the expected diols were was between 0 and 1.5 M, and the range of epoxide concentra-
produced with large hydrolysis rates, kI, comparable or larger tions was 20–130 mM. To examine the potential contribution
than those measured in the most concentrated ammonium of acid or base catalysis, the pH of the solutions was varied
solutions (Table S1, ESI†), while the amino alcohol was not from about 5.7 to 10.0 by adding small amounts of NaOH. The
consumed. This confirmed the role of the amino alcohols as mixtures were continuously stirred over the duration of the
the actual catalysts in the ammonium solutions. Based on experiments.
similar reactions in the literature, the mechanism proposed
for the observed catalytic effect is hydrogen bond catalysis,25–36
3.1 Product investigations
1
in which the opening of each epoxide is activated by hydrogen The reaction products were identified by H-NMR and GC/MS.
bonds between the oxygen atom and the protons from two For 1H-NMR a Bruker AV 500 or Av 400 Avance III spectrometer
H-bonding catalysts (Scheme 1).27–30,33,34 The reason for the (respectively 500.13 and 400.13 MHz) was used. The spectra
smaller rate obtained with 2,3-EB than with 1,2-EB, while this were recorded using the basic pulse zg sequence with a spectral
was the other way around in ammonium solutions, was thus width of 8 kHz, and a relaxation delay of 60 s. The excitation
likely due to the fact that 1-amino-2-butanol is not the product pulse width was calibrated for each solution. Tetradeuterated
of 2,3-EB in ammonium solutions (Table 1) and that its actual sodium trimethylsilylpropionate (TSPD4) was added to the reaction
product, 2-amino-3-butanol, is more substituted and thus a mixtures as an internal standard for the absolute quantification
more efficient H-bond donor.25,26,33 To confirm the hydrogen- of the reactants and products. All spectra were processed with
bonding mechanism, the hydrolysis of 1,2-EB was studied MestReNova software (Mestrelab Research). In these experi-
in pure water again, but adding other stronger hydrogen bond ments, the epoxides and their products were unambiguously
donors than the amino alcohols: trifluoroethanol (TFE) or identified by comparison with the spectra of the reference
hexafluoroisopropanol (HFIP)37 (Table S1, ESI†). The obtained compounds (characteristic shifts, dH, in Table 1). The product
hydrolysis rates, once corrected for the single hydrogen avail- yields were determined by dividing the product concentration
able in TFE and HFIP instead of the two available in the amino at each reaction time by the amount of epoxide reacted.
alcohols, were identical to those obtained with the same
The reactions of 1,2-EB, 2,3-EB, and DM-2,3-EB were also
concentration of amino alcohol. This confirmed that hydrogen studied by GC/MS and GC/FID using an Agilent Hewlett-Packard
bond catalysis was the kinetically-driving mechanism. Note that 6890 chromatograph coupled both to a 5975 quadrupole Mass
it cannot be excluded that at a later stage of the reaction the diols Spectrometer using Electron Impact ionization (70 eV) and to a
(or tetrols) produced by the reactions may also contribute to the Flame Ionization Detector. The chromatograph was equipped
catalysis by becoming the most abundant hydrogen-bond donors. with an Agilent DB-VRX capillary column (60 m ꢂ 0.25 mm i.d.;
In that case, hydrogen bonding by the diols or tetrols might also 1.4 mm thickness). These analyses required extracting the aqueous
enhance the hydrolysis rates in acid-catalyzed mechanisms.
reaction mixtures in an organic solvent. This was performed by
adding ethyl acetate, stirring for 5 min, and letting the solutions
settle for 10 min. The organic phase was then extracted three
times (1 mL each time), and the combined extracts dried under
a stream of nitrogen and re-dissolved in 1 mL of methanol. 1 mL
3. Experimental
All the reactions studied were performed in small glass vials, in of these extracts was injected in the capillary in split mode
15 mL of solution in H2O for the GC analyses, and in 2–6 mL of (ratio 50 : 1) at 250 1C. The carrier gas was helium, used at a
D2O solution for the NMR analyses. The salt concentration used flow-rate of 1.0 mL/min. The temperature program consisted of
Scheme 1 Proposed mechanism for the hydrolysis of epoxides in aqueous ammonium solutions.
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an initial temperature at 70 1C followed by a gradient of AIM model II (http://www.aim.env.uea.ac.uk/aim/model2/model2a.
5 1C minꢁ1 up to 260 1C. For the reactions studied, all the php).38 The results are given in Table S1, ESI,† where it can be seen
+
products eluted within 12.67 min. These GC analyses allowed that the activity coefficient for NH4 in the solutions studied was
monitoring only the diol or tetrol products, which were identi- significantly different from 1 (closer to 0.5). No significant variation
fied by their retention times on the chromatograms and by of aNH with pH was predicted by the model over pH values = 5–10.
+
4
their mass spectra by comparison with authentic standards and For the few experiments performed in H2SO4, aH+ was assumed
with the NIST library (National Institute of Standards and identical to the concentration of proton, determined from the pH.
Technology) (Section S3, ESI†).
Product yields, in particular for the diols and tetrols, Yol, and
The reactions kinetics were monitored in GC/FID by taking for the amino alcohols, YAA, could be obtained for all the
1 mL of reaction mixtures at regular intervals and performing reactions monitored by NMR by extrapolating to t = 0 the
the extraction described above. 1,3-Dibromopropane (2 mM) was time-evolution of the ratio:
used as an internal standard. Calibration curves (Section S4, ESI†)
Ct
Yol
¼
;
(5)
were established for known solutions of reference compounds in
methanol, which were directly injected in the GC column. The
quantification of the products extracted from the reaction mixtures
also took into account extraction efficiencies, E, defined as:
D½epoxideꢃ
where, as above, Ct is the concentration of the diol or tetrol
produced at time t and D[epoxide] is the difference between the
initial concentration of the epoxide and its concentration at
time t, all monitored directly by NMR. Extrapolating these
ratios to t = 0 allowed compensating for the potential consump-
tion of the products in the reactions.
E = Cextr/Cinit
,
(2)
where Cinit and Cextr are the concentrations before and after
extraction, respectively. These efficiencies (given in Section S5,
ESI†) were specific to compounds and solution concentrations,
and established by extracting known solutions of reference
compounds. In GC/FID the product yields were determined
from the product concentration at a large degree of conversion,
assuming complete conversion of the epoxide. The uncertainties
of the yields were estimated at ꢀ15%.
3.3 Chemicals
cis- and trans-(2-methyloxirane-2,3-diyl)dimethanol (IEPOX3 and 4),
2C-methyl-threitol and 2C-Methyl-erythritol were custom-
synthesized by SICHEM Gmbh, Germany. Their identity and
purity were confirmed by 1H NMR: cis-(2-methyloxirane-2,3-
diyl)dimethanol Z99.9%; trans-(2-methyloxirane-2,3-diyl)-
dimethanol Z97.5%; 2C-methyl-threitol Z98%; 2C-methyl-
erythritol Z98%. The following reagents were purchased from
Sigma Aldrich: 1,2-epoxybutane (99%); 2,3-dimethyl-2,3-epoxybutane
(99%); 1,2-butanediol (98%); 2,3-butanediol (98%); 2,3-dimethyl-
2,3-butanediol (99,9%); 1-amino-2-butanol; 2-amino-1butanol,
97%; ammonium sulfate (99%); methanol Chromasolv (HPLC);
1.3-dibromopropane 99.9%; sulfuric acid (99%); ethyl acetate
Chromasolv Plus (HPLC); tetradeuterated sodium trimethylsilyl-
propionate (TSPD4) 98 atom% D; D2SO4 96–98 wt% D2O; D2O
99.9 atom% D. cis-2,3-Epoxybutane (98%) was purchased from
Alfa Aesar.
3.2 Kinetic analyses
The reaction kinetics were studied at 303 ꢀ 3 K by 1H-NMR and
GC/FID. To avoid taking into account potential side-reactions,
the kinetics were all determined from the formation of the
main diol or tetrol product. Typical time-dependent profiles
for the concentration of 1,2-EB and 1,2-butanediol during a
reaction in (NH4)2SO4 solution obtained by MR are shown in
Fig. 1. Before applying a kinetic analysis, the order of reaction
in epoxide was determined from the variations of the initial
rate of product formation with the initial concentration of
epoxide. As explained in the Results section, this order was
unity in this work, thus the time profiles were analysed with the
following equation:
4. Conclusions
I
Ct = CN ꢂ (1 ꢁ eꢁk t),
(3)
A systematic study of the hydrolysis of 1,2-EB, 2,3-EB, DM-2,3-
EB, and IEPOX in aqueous solutions of ammonium salts has
shown that the specific presence of NH4 ions increases the
hydrolysis rate of all epoxides by up to a factor of 25 compared
to that of pure water. Contributions of the counter ions, of
electrostatically-driven catalysis (ionic force. . .), and of acid
or base catalysis to these hydrolysis rates were all ruled out.
The catalytic effects were shown to result from the hydrogen
bonding mechanism driven by the amino alcohols produced in
where Ct and CN are the product concentrations at time t and
complete conversion, respectively. The value of CN was deter-
mined experimentally in each experiment by fitting eqn (2) to the
time profiles (Fig. 1), and first-order reaction rates, kI (sꢁ1) were
obtained from the slopes of the following expression (Fig. 1):
+
ln(1 ꢁ Ct/CN) = ꢁkI t.
(4)
The combination of the uncertainties in the measured the first step of the reaction, and possibly also by the diols or
concentrations, the determination of CN, and the linear regres- tetrols produced by the hydrolysis.
sions led to overall uncertainties of ꢀ15% in the obtained first-
order rates, kI. Second-order rate coefficients in the catalyst, NH4 concentrations that are relevant in atmospheric aerosols
kII (Mꢁ1 ꢁ1), were then obtained by dividing kI by the activity (aNH
up to 3 or 5 M) shows, by a comparison similar to Fig. 3,
of the catalyst, generally aNH
Extrapolating the hydrolysis rates obtained in this study to
+
s
+
4
+
+. aNH
+ was calculated with the that NH4 /hydrogen-bonding catalysis should dominate over
4
4
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¨
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