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Mechanism of the Hydrolysis of Endosulfan Isomers Swatantra Pratap Singh, Saumyen Guha, Purnendu Bose, and Sooraj Kunnikuruvan J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b02012 • Publication Date (Web): 19 Jun 2017 Downloaded from http://pubs.acs.org on June 19, 2017
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The Journal of Physical Chemistry
Mechanism of the Hydrolysis of Endosulfan Isomers
1 2 3 4
by
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Swatantra Pratap Singh†#, Saumyen Guha†*, Purnendu Bose†* and Sooraj Kunnikuruvan‡ †
6 7 8 9
‡
Department of Civil Engineering, Indian Institute of Technology Kanpur
Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur – 208016, INDIA
#Present Address
Department of Desalination and Water Treatment, Zuckerberg Institute of Water
Research, Ben-Gurion University of Negev, Sede-Boqer-89990, Israel
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Revised Manuscript
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for
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Possible Publication
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To
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Journal of Physical Chemistry A
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May 2017
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*Corresponding Authors: Purnendu Bose Professor & Head Department of Civil Engineering Indian Institute of Technology, Kanpur Kanpur-208016 Email:
[email protected] Saumyen Guha Professor Department of Civil Engineering Indian Institute of Technology, Kanpur Kanpur-208016 Email:
[email protected] 20
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ABSTRACT:
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The objective of this study was to elucidate the mechanism of abiotic hydrolysis of ES-
23
isomers, i.e., Endosulfan-1 (ES-1) and Endosulfan-2 (ES-2) using a combination of
24
experiments and Density Functional Theory (DFT) calculations. Hydrolysis of both ES-1
25
and ES-2 resulted in the formation of Endosulfan Alcohol (ES-A). The rate of hydrolysis
26
was first order in all cases and increased with both pH and temperature. Rate expressions
27
describing the hydrolysis rates of ES-1 and ES-2 as a function of pH and temperature were
28
obtained and validated with independent data sets. Density Functional Theory (DFT)
29
calculations were performed using three functionals (M06-2X, B3LYP and MPW1K) and
30
both IEFPCM-UFF and SMD to introduce solvent effects. The geometry optimization of
31
molecules ES-1 and ES-2 showed that the free energy of ES-1 was larger and therefore, ES-2
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was more thermodynamically stable isomer. DFT calculations also supported a hydrolysis
33
mechanism involving two successive attacks by OH- ions on C-O bonds resulting in the
34
attachment of OH- and the elimination of
35
was rate limiting.
36
supported the experimentally observed result of faster hydrolysis of ES-2 compared to ES-1.
37
The MPW1K functional along with IEFPCM-UFF for solvent effect simulated the free
38
energy of activation the closest for both ES-1 and ES-2 with less than 3% error with respect
39
to the values computed from the experimental observations. Kinetic rate expression for ES
40
hydrolysis derived based on the proposed mechanism was identical to the rate expression
41
derived from experiments. It was deduced that the hydrolysis rates of both ES isomers may
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vary over three orders of magnitude depending on the prevalent pH and temperature.
S O 3−
from the ES molecule but only the first attack
Calculations with all functionals and solvent effect combinations
43
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INTRODUCTION
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The organochlorine pesticide Endosulfan (ES) was introduced in 1950s as a replacement of
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DDT and has since been widely used all over the world. 1 1 1 1 1 1 1 The largest users of this
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pesticide were India and USA.1
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pollutants (POPs) in the Stockholm convention in 2011,2 all the countries are phasing out ES
49
usage. ES is highly toxic to fishes and other aquatic species.3-5 Toxicity to mammals include
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reported cases of gonadal toxicity, genotoxicity, and neurotoxicity.6-7 It is an endocrine
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disrupter in humans with primary effect on the central nervous system. Amongst the reported
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cases of ES toxicity, the Kerala tragedy in India is well-known.8-10 ES was categorized as
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‘extremely toxic’ by USEPA and EU, and ‘moderately toxic’ by WHO.
Since it’s induction into the list of persistent organic
54 55
ES is commercially available as a mixture of two diasteromers, α-Endosulfan (ES-1) and β-
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Endosulfan (ES-2), in approximately 7:3 ratio. While ES-2 has a symmetric structure, ES-1
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can have any one of the two possible asymmetrical enantiomers which may readily
58
interchange.11 This difference in molecular structure leads to large differences in physical
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properties such as, melting points, Henry’s constants, partition coefficients and activation
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energies for reaction.12-15 As a result, fate of these isomers differ in different environmental
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compartments.16 Air-water partitioning studies suggested faster volatilization of ES-1.12-13
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Long-range transport studies have also concluded more widespread transport of ES-1 in
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atmosphere.17-18 Irreversible conversion of ES-2 to ES-1 have been reported from symmetric
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ES-2 to asymmetric ES-1.11, 19
65 66
Abiotic hydrolysis of both ES-1 and ES-2 was reported in soil-water matrix in neutral to
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alkaline pH range.15,
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Endosulfan Alcohol (ES-A) was the primary abiotic hydrolysis product in both cases. In
20-24
The half-lives of ES isomers decreased with increasing pH.15
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most environmental systems, ES-2 hydrolysis rate was reported to be faster compared to ES-
70
115, 20-21, 23 although, the later has more entropy due to asymmetrical structure.11
71 72
Expectedly, faster abiotic hydrolysis at higher pH was reported20-21, 23 for both ES isomers.
73
Suspensions of sea sand, TiO2, R-Fe2O3, R-FeOOH, Laponite, and SiO2 were reported to
74
enhance the hydrolysis of both the ES isomers, whereas suspended creek sediment inhibited
75
hydrolysis.23 Microbially mediated conversion of ES in soil-water matrix was reported in
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aerobic environment through either hydrolytic or oxidative pathway, resulting in the
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formation of ES-A or Endosulfan Sulphate (ES-S) as the first intermediate.21, 25 In anaerobic
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environment, the oxidative pathway was absent and hydrolysis was the only first step of ES
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biodegradation pathway.26
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metabolites of ES isomers further down the pathway were Endosulfan Lactone (ES-L) and
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Endosulfan Ether (ES-E).
In both the environment, other reported biodegradation
82 83
Therefore, abiotic hydrolysis is an important first step of all natural attenuation processes for
84
ES.16, 21 Degree of partitioning of ES to various environmental components and its transport,
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bio-accumulation and bio-magnification, all depend to a large extent on the rate and extent of
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abiotic hydrolysis because the hydrolysis product ES-A is less hydrophobic and relatively
87
non-toxic compared to the parent ES-isomers.
88 89
A quantum-chemical computational approach can be used for the investigation of reaction
90
pathways and reaction intermediates in chemical systems.27-28 Density Functional Theory
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(DFT) is one such well-established quantum mechanical modeling approach in computational
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chemistry.29-30 Mechanism and pathway of acidic, neutral and alkaline hydrolysis of many
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organic compounds have been explored in recent years using DFT modeling.31-33
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objective of the present study was to elucidate the mechanism of abiotic hydrolysis of ES-
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isomers using a combination of experimental investigations and DFT modeling, and explain
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faster hydrolysis of ES-2 compared to ES-1.
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MATERIALS & METHODS
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Chemicals. Technical grade mixture of ES-isomers was kindly provided by United
100
Phosphorous Ltd., Ankaleshwar, India and pure isomers of ES were separated from it by thin
101
layer chromatography (TLC) in 7:3 hexane-acetone medium. Purity of ES-1 and ES-2 thus
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obtained was estimated using GC-ECD to be 98% and 97% respectively. Market grade ES
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(Endotaf 35) manufactured by Rallis India was purchased from the local market at Kanpur,
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India.
105
standard 2,4,5,6- tetrachloro-m-xylene (>99% purity) were purchased from Sigma-Aldrich,
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India. All solvents used were of HPLC grade (>99% purity) and chemicals were of analytical
107
reagent grade (AR Grade).
Pesticide standards (ES-1, ES-2, ES-S, ES-A, ES-L and ES-E) and the internal
108 109
Hydrolysis Experiments. Experiments were conducted in borosilicate glass vials of 40 mL
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volume with screw caps and Teflon-faced septa. Each vial contained de-ionized water and
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measured amount of 0.02M phosphate buffer required to set the initial pH to the desired level.
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Total volume of water and buffer was 40 mL, such that no headspace existed in the vials.
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Experiments were conducted at the following pH values: 6.0, 7.0, 7.5, 8.0, 8.5 and 9.0. Stock
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solutions (50.0 mg/L) of ES-1 and ES-2 in acetone were used and desired volume was
115
introduced to each vial using a micro syringe. The vials were sealed and kept in a temperature
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controlled (within ±2ºC) water-bath equipped with a shaker table for mixing.
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predetermined time intervals, vials were removed in duplicate and processed immediately for
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extraction and analysis. 5 ACS Paragon Plus Environment
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Endosulfan Extraction. ES extraction procedure was adopted from Tiwari and Guha,34. A
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measured volume of the sample was transferred to a vial containing ethyl acetate and 250 mg
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NaCl. The ratio of sample to ethyl acetate volume was 1:4. The vial was tightly capped and
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put on a vortex mixer (Vertex Genie-2, Scientific Industries, USA) for 10 minutes. The
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contents were centrifuged (BiofugeStratos, Heraeus, Germany) at 3000×g for phase
124
separation. The ethyl acetate fraction was collected and passed through anhydrous sodium
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sulfate to remove moisture. The percent recoveries of ES-1, ES-2 and ES-A were 98.6 ± 11.3,
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97.4 ± 6.6 and 92.01 ± 7.3 respectively.
127 128
Analytical Methods. ES-1, ES-2 and ES-A concentrations were measured using a gas
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chromatograph (Model Clarus 500, Perkin-Elmer, USA) equipped with an electron capture
130
detector and a capillary column (MXT-5) of size 30 m × 0.28 mm × 0.25 µm. Nitrogen was
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used as the carrier gas at 68.95 kPa (10 psi) and as the makeup gas at 30 mL/min. Injector and
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detector temperatures were 250 and 375°C, respectively. The oven temperature program was
133
as follows; start at 120ºC hold for 1 minute, increase to 170ºC @ 10ºC/min hold for 8
134
minutes, increases to 220ºC @ 18ºC/min hold for 8 minutes, increase to 240ºC @ 12ºC/min
135
hold for 5 minutes.
136 137
Computational Methods. Density Functional Theory (DFT) computations were carried out
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using Gaussian 09 program package.35 Calculations were done using M06-2X, B3LYP and
139
MPW1K functionals36-40 with 6-311++G (d, p) basis set. Geometry optimization of all the
140
chemical species involved in the hydrolysis reaction mechanism were performed in the
141
presence of water as solvent. In order to introduce solvent effects the integral equation
142
formalism of polarizable continuum model with UFF radii (IEFPCM-UFF) and SMD solvent
143
models were used.41-44 All free energy computations were carried out at 298.15 K and 1 atm.
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More details of the computations of free energies, free energy barrier and rate constants
145
(using Eyring equation) are given in the supporting information (Section S2).
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RESULTS & DISCUSSION
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Experiments carried out at pH 7 and 32 ± 3ºC
148
ES-isomers Hydrolysis Kinetics.
149
demonstrated that abiotic hydrolysis of both ES-1 (Figure 1a) and ES-2 (Figure 1b) produced
150
stoichiometric, i.e., equi-molar quantities of ES-A. For experiments with ES-1, no ES-2 was
151
detected at any time during the experiment and vice versa. This showed that in the water
152
phase, there was neither spontaneous conversion of ES-2 to ES-1, nor ES-2 first converted to
153
ES-1 prior to hydrolysis as postulated by some.11 Rice et al. (1997) showed that temperature
154
dependent conversion of ES-2 to ES-1 was possible in the air phase. Since, there was no air
155
phase in these experiments, the results show, such spontaneous abiotic conversion is not
156
possible in water phase alone.12-13 (a) ES-1
(b) ES-2
0.8
Concentratiom, µmol L-1
Concentration, µmol L-1
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0.6 0.4 0.2 0.0
0.6 0.4 0.2 0.0
0
40 80 Time, Hours
0
120
40 80 Time, Hours
120
ES-A
ES-1 or ES-2
Total (Sum of ES-1, ES-2, and ES-A)
157 158
0.8
Figure 1.
Hydrolysis of ES-isomers at pH 7; (a) ES-1, and (b) ES-2.
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Formation of ES-A by abiotic hydrolysis of ES isomers was consistent with the earlier
161
studies.20, 23 In order to determine the rates of hydrolysis at various pH (6.0, 7.0, 7.5, 8.0, 8.5,
162
and 9.0), additional hydrolysis experiments were carried out at a constant temperature of 35ºC
163
for both the isomers, ES-1 (Figure S.1a and Figure S.1b) and ES-2 (Figure S.1c and Figure
164
S.1d). In all cases, the hydrolysis rate was first order and increased with pH. Hydrolysis rate
165
of ES-2 was faster than ES-1 at all pH. This is also reported in earlier studies,15, 23 although
166
the exact relation between OH- ion concentration and hydrolysis rate constant was not
167
explored earlier. The hydrolysis rate was expressed using the first-order expression,
168
dC = −k H [OH − ]a .C = −k .C dt
(Eq. 1)
169
Where C is the concentration of ES at time t, k (h-1) is the first order hydrolysis rate constant,
170
kH (h-1 mol-1 L) is the intrinsic ES hydrolysis rate constant, [OH-] is the k concentration (in
171
M) and ‘a’ is the empirical order of [OH-] interaction. The value of ‘a’ would show moles of
172
hydroxide required for the hydrolysis of one mole of ES. It was expected that the value of ‘a’
173
would be either one or two. The ‘k’ values for ES-1 and ES-2 are summarized in Table S1.
174 175
Effect of pH. Effect of pH on the hydrolysis rate was described by the Eq. 3. The values of
176
kH were computed using following logarithmic transformation,
177
Log (k) = Log (kH) + a.Log [OH-]
(Eq. 2)
178
Plots of Log [k] vs.Log [OH-] for ES-1 and ES-2 are shown in Figure 2a and Figure 2b,
179
respectively. These plots show that one mole of hydroxide is required for hydrolysis of one
180
mole of ES (i.e., a = 1). The coefficients of regression were high in both cases and the kH
181
values for ES-1 and ES-2 were 7.211×103 and 9.958×103 h-1mol-1 L, respectively.
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(a) ES-1 0
0
-1
-1 Log (k)
Log (k)
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-2
(b) ES-2
-2
-3
-3
R2=0.964 -4 -8.0 -7.5 -7.0 -6.5 -6.0 -5.5 -5.0
R2=0.962 -4 -8.0 -7.5 -7.0 -6.5 -6.0 -5.5 -5.0
Log [OH-]
Log [OH-]
182 183 184
Figure 2.
Relation Between ES Hydrolysis Rate Constant (kH) and [OH-] Concentration; (a) ES-1, and (b) ES-2
185 186
Effect of Temperature. In order to determine the effect of temperature on kinetics of
187
hydrolysis, experiments were conducted at temperatures of 10, 20, 30 and 40ºC at a constant
188
pH of 9.0. The data is summarized in Figure S2a (for ES-1) and Figure S2b (for ES-2). The
189
hydrolysis rates fitted well with the first order model in all cases, with the intrinsic rate of
190
hydrolysis increasing with temperature. Modified Arrhenius equation45 was used to simulate
191
the effect of temperature on the intrinsic rate of hydrolysis as follows,
192
k H = k H 2 0 .θ
(Eq. 3)
(T − 20 )
193
Where, kH20 (h-1 mol-1 L) is the intrinsic ES hydrolysis rate constant at 20ºC, θ is the
194
temperature coefficient and T is the temperature in ºC. The Eq. 1 becomes,
195
dC = −k H 20 .θ (T −20) .[OH − ].C dt
(Eq. 4)
196
The rate parameter kH20 can be computed using the values of kH computed at different
197
temperature using the logarithmic transformation of Eq. 3 as follows,
198
Log ( k H ) = Log ( k H 20 ) + Log (θ ).[T − 20]
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(Eq. 5)
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199
Plots of Log [kH] vs [T – 20]ºC for ES-1 and ES-2 are shown in Figure 3a and Figure 3b,
200
respectively. The coefficient of regression was very high in both the cases. The values of kH20
201
and θ for ES-1 were 7.35 × 103 h-1mol-1 L and 1.099, respectively. The same values for ES-2
202
were 1.48 × 104 h-1mol-1 L and 1.107, respectively. The difference in the values of the
203
temperature constant ( θ ) for ES-1 and ES-2 were not statistically significant at 95%
204
confidence level. Therefore, an average value of the constant θ = 1.103 can be used in Eq. 4
205
for both the isomers of ES.
206 207
The value of kH20 for ES-2 was nearly double compared to that of ES-1. The values of kH20
208
and θ were used to compute the k values for ES-1 and ES-2 at 298.15 K and the free energy
209
barriers were computed using Eyring equation (Table 1).
(a) ES-1
5.0
5.5
Log (kH)
4.0 3.5 R2=0.987
4.0
3.0
-15 -10 -5 0
R2=0.979 -15 -10 -5 0
5 10 15 20 25
Figure 3.
5 10 15 20 25
(T - 20)ºC
(T - 20)ºC
210
212
4.5
3.5
3.0
211
(b) ES-2
5.0
4.5 Log (kH)
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Relation Between ES Intrinsic Hydrolysis Rate Constant (kH) and Temperature; (a) ES-1, and (b) ES-2.
213 214
Thermodynamic Stability of ES-isomers. Geometry optimization of both ES-1 and ES-2
215
molecules were carried out using DFT calculations. These calculations indicate that the
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free energy of the ES-1 molecule is about 3.26 kcal mol-1 larger as compared to the ES-2
217
molecule. DFT calculations indicates that the enthalpy of ES-2 is ~3.69 kcal mol-1 smaller as
218
compared to ES-1. Whereas the entropy of asymmetrical ES-1 (0.135 kcal mol-1 K-1) is
219
larger than symmetrical ES-2 (0.133 kcal mol-1 K-1), which is consistent with earlier studies11.
220
This indicates that the enthalpy is the major factor for the thermodynamic stability of ES2.
221
The higher thermodynamic stability of ES-2 molecule could be attributed to the interaction
222
between the orbitals of two oxygen atoms (O1 and O2 in Figure S4b and S4d) with the
223
double bonded carbon atoms (Cx and Cy in Figure S4b) which lie in the same plane. In case
224
of ES-1 molecule the orbital of one oxygen atom is away from the double bonded carbon
225
(Figure S4a and S4c). The higher melting and lower Henry’s law constant of the ES-2
226
molecule is attributable to the higher thermodynamic stability of ES-2 compared to ES-1.
227 228
Table 1. Comparison of free energy barriers (kcal mol-1) for the first step of hydrolysis of ES
229
isomers (formation of Intermediate-1) using various functionals with 6-311++G(d,p) basis set
230
and IEFPCM-UFF or SMD solvent model (298.15 K and 1 M) with the experimental free
231
energy of activation at 298.15 K. The relative error for the computed barriers with the
232
experimental valve is shown in parenthesis.
233
Elucidation of Reaction Mechanism. Based on experimentally obtained rate expression for
234
ES hydrolysis (Eq. 4), it was presumed that the rate-limiting step in ES hydrolysis involves a Experimental Free Energy of Activation
B3LYP IEFPCMUFF
SMD
MPW1K IEFPCMUFF
SMD
M06-2X IEFPCMUFF
SMD
ES-1→I-1
26.29
23.83 (-9.5%)
29.34 (11.6%)
27.03 (2.8%)
34.01 (29.4%)
23.97 (-8.8%)
31.25 (18.9%)
ES-2→I-1
25.87
21.3 (-17.7%)
27.99 (8.2%)
25.29 (-2.2%)
33.02 (27.6%)
22.42 (-13.3%)
29.73 (14.9%)
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single hydroxide attack on the ES molecule. First, the possibility of nucleophilic attack by
236
hydroxide on the sulphur atom of ES molecule was investigated through DFT calculations.
237
The nucleophile was repelled away from the sulphur centre due to the high steric interactions,
238
probably due to the lone electron pair of sulphur atom (Figure S3 and S6).46 The other
239
reasons, why nucleophilic attack at sulphur was ruled out are discussed in detail in supporting
240
information (Section S3). Finally, an SN2-type attack by hydroxide is proposed on either C1 or
241
C2 atom (Figure 4) for the ES-2 molecule and formation of intermediate-1 or 2, where both
242
nucleophile and the leaving-group are present. In case of the ES-1 molecule, an initial step
243
involving a SN2-type attack on the C1 atom resulted in the cleavage of C1-O bond and
244
formation of the intermediate-1 and similarly, an attack on the C2 atom resulted in the
245
formation of the intermediate-2 (Figure 4). Then the intermediate-1 or 2 undergo further
246
reaction with another hydroxide to form ES-A. The possibility of H2O (instead of OH-)
247
acting as a nucleophile was investigated by specifying an attack by H2O molecule on the C1
248
atom of ES-2 (Figure 4). The free energy barrier for such an attack is very high at 55.72 kcal
249
mol-1. This is also consistent with experimental results15 that show little or no hydrolysis of
250
ES below pH 5.
251 252
The optimized structures of ES-1, ES-2, ES-A, and possible intermediates were obtained
253
through DFT calculations. These structures and the corresponding free energy profile are
254
presented in Figure 5. Free energy barrier for the rate determining step (OH- attack on ES-1
255
and ES-2) was computed by employing M06-2X, B3LYP and MPW1K functionals with two
256
solvent models, IEFPCM-UFF and SMD 36, 38, 44 and results are shown in Table 1 along with
257
the same computed from the experimental observations. The table also shows the relative
258
error (in %) of the computed values with respect to the experimental values. Free energy
259
barrier obtained by using B3LYP functional was comparable with that obtained using M06-
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2X functional but MPW1K functional with IEFPCM-UFF solvent model produced the most
261
accurate simulation of the experimental activation energy for both ES-1 and ES-2 with
262
relative error of less than 3%. Free energies of activation obtained with SMD solvent model
263
was much larger for all three functionals in comparison to that of IEFPCM-UFF solvent
264
model. Therefore, the free energy of activation reported here are obtained using MPW1K
265
functional and IEFPCM-UFF solvent model unless otherwise specified. One may note, in all
266
models, irrespective of the absolute values, the free energy barrier for ES-1 was always larger
267
than ES-2. This is consistent with widespread reports15, 20-21, 23 and our experimental results
268
of faster hydrolysis rate of ES-2 compared to ES-1.
269
Cl Cl
OH
Cl
C1 C O S Cl Cl 2O O Cl
Cl Cl
OH
Cl
Cl Cl Cl
C C2 1 Cl Cl O O O S Cl
ES-1 (C-1)
ES-2 (C-1)
k1a3b
k1a3f
Cl Cl Cl Cl Cl Cl
k2a3b
OH C1 C2 O S Cl Cl O O Cl ES-1 (C-2)
k2a3f
k1b3b
k1b3f
Cl Cl Cl Cl
O
Cl
Cl Cl Cl Cl
OH C1 C2 O O O S
ES-2 (C-2)
ClCl
OH C1 OH C2 OO S
Cl
k2b3b
k2b3f
OH C1 O S O C2 OH O
Intermediate-2 (I-2)
Intermediate-1(I-1) k3a4b k3a4f
k3b4b
k3b4f
ClCl Cl
C1 OH C2 OH
+
SO32-
270
Cl Cl Cl ES-A
271
Figure 4. Proposed Mechanisms for Hydrolysis of ES-1 and ES-2 (C1 (CH2) and C2 (CH2)
272
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In case of ES-1, an initial attack on the C1 atom was found to have a free energy barrier of
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27.03 kcal mol-1. In contrast, an initial attack on the C2 atom of ES-1 (Figure 5) was found to
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have a free energy barrier of 34.01 kcal mol-1, which is attributed to steric hindrance
277
associated with such an attack. In case of ES-2 molecule, the initial attack by hydroxide can
278
occur either at C1 or C2 atoms with identical free energy barrier of 25.29 kcal mol-1. The
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presence of two identical reaction centers C1 and C2 increases the probability of nucleophilic
280
attack on ES-2. Whereas for ES-1 nucleophilic attack at C2 was not favored as indicated by
281
the free energy barrier (34.01 kcal mol-1), this increased probability of
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on ES-2 is also contributes to the faster hydrolysis of ES-2 compared to ES-1. The
283
enhancement in rate here is equivalent to lowering the free energy barrier for the reaction by
284
about RT ln2 (0.4 kcal mol-1 at 298.15 K). It is to be mentioned here that while studying the
285
kinetics this has been taken care.
nucleophilic attack
286 287
The free energy barriers for ES-1 and ES-2 hydrolysis computed from the experimental
288
observations were 26.29 kcal mol-1 and 25.87 kcal mol-1, respectively. The apparent free
289
energy barriers for the rate-limiting step determined from DFT calculations described above
290
were 27.03 kcal mol-1and 25.29 kcal mol-1for ES-1 and ES-2, respectively. The agreement
291
between experimental and DFT-calculated values were within 3% leading to the conclusion
292
that the rate-limiting step during hydrolysis involves a hydroxide attack on the C1 carbon of
293
ES-1 and on either C1 or C2 carbon of ES-2. The initial attack is followed by a second
294
hydroxide attack at the other carbon atom, leading to the formation of the hydrolyzed product
295
ES-A by elimination of
296
hydroxide, this also explains the linear dependence (a=1 in Eq. 1) of first order rate
297
expressing on [OH-] although two hydroxides are required to form the product ES-A.
SO
2− 3
(Figure 4). Since, the rate limiting step is the attack of the first
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298 299
The hydrolysis mechanism proposed above has some literature support; cyclic sulphites are
300
considered to be similar to epoxides,47 which undergo ring-opening reaction with
301
nucleophilic attack on the β-carbon.46-48 Further, the nucleophilic attack in cyclic sulphites is
302
known to occur at C-centre due to the good leaving group character of the sulphur moiety.47
303 304
Theoretical Kinetics. Kinetic rate expression (full derivation given in SI) for the hydrolysis
305
of ES-1 and ES-2 can be derived based on the mechanism identified from the DFT studies,
306
For C1 attack ES − 1 + OH −
307
k
1a 3 f → ← k 1a 3 b
For C2 attack ES − 1 + OH −
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k
1b 3 f → ← k 1b 3 b
I1 + OH − I 2 + OH −
k
3a 4 f → ← k 3a 4b
k
3b 4 f → ← k 3b 4 b
ES − A + SO32− ES − A + SO32−
(Eq. 6) (Eq. 7)
309 310
Where, k1a3f, k1b3f are the forward rate constants and k1a3b, k1b3b are the backward rate constants
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for the conversion of ES-1 to I1 and I2 through OH- attack at C1 and C2 carbons respectively
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(Figure 4). Thus,
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d [ ES -1]
314
dt
= {k1a3b [ I1 ] + k1b3b .[ I 2 ] − k1a3f .[ ES − 1] − k1b3f .[ ES − 1]}.[OH −]
(Eq. 8)
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Applying steady state for concentration of I1 and I2 and substituting value their values in Eq. 8 (Rate
316
constants defined in Figure 4),
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d [ ES-1] dt
( k + k1b3f )( k1a3b + k1b3b ) k1b3b ( k 3a4b + k 3b4b ) = 1a3f − ( k1a3f + k1b3f ) [ ES-1] OH − + [ ES-A ] SO 32− ( k1a3b + k1b3b + k 3a4f + k3b4f ) ( k1a3b + k1b3b + k 3a4f + k3b4f )
(Eq. 9)
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Since k1a3b