Transition State Structure for the Water-Catalyzed Hydrolysis of p-Nitrophenyl Trifluoroacetate in Acetonitrile

Article abstract of DOI:10.1021/jo972027f

The neutral hydrolysis of p-nitrophenyl trifluoroacetate in acetonitrile solvent has been studied by varying the molarities of water from 1.0 to 5.0 at 25°C. The reaction is found to be third order in water. The reaction is also third order in D₂

Full text of DOI:10.1021/jo972027f

J . Org. Chem. 1998, 63, 6115-6118
6115
Tr a n sition Sta te Str u ctu r e for th e Wa ter -Ca ta lyzed Hyd r olysis of
p-Nitr op h en yl Tr iflu or oa ceta te in Aceton itr ile
K. S. Venkatasubban,* Melissa Bush, Eileen Ross, Michael Schultz, and Oscar Garza
Chemistry Division, Department of Natural Sciences, University of North Florida,
J acksonville, Florida 32224
Received November 4, 1997
The neutral hydrolysis of p-nitrophenyl trifluoroacetate in acetonitrile solvent has been studied by
varying the molarities of water from 1.0 to 5.0 at 25 °C. The reaction is found to be third order in
water. The reaction is also third order in D2O and in methanol when D2O and methanol replace
water as the solvent. The kinetic solvent isotope effect is (kH2O/kD2O) ) 2.90 ( 0.12. Proton
inventories at each molarity of water studied are consistent with an eight-membered cyclic transition
state model. In this model, three protons undergo bonding changes. Such an eight-membered
transition state model can easily accommodate linear hydrogen bonds for the three transferred
protons. These results are consistent with the experimental findings of Bell and Critchlow1 on the
reversible addition of water to 1,3-dichloroacetone in dioxane and the theoretical findings of Wolfe
and co-workers2 on the hydration of formaldehyde.
Sch em e 1
The water-catalyzed hydrolysis of esters and related
carboxylic acid derivatives in aqueous solutions is thought
to proceed via a three-proton transition state involving
two water molecules as shown below.3
Resu lts
A simple kinetic pathway for the neutral hydrolysis of
1 in acetonitrile is shown in Scheme 1. The decomposi-
tion of the tetrahedral intermediate 2 to products should
be considerably faster than the reversal of 2 to reactants
because p-nitrophenoxide is a better “leaving group” than
hydroxide ion. Thus, the kinetic information obtained
in this study pertains to the addition step rather than
the elimination step.
Proton transfers through a set of ordered water mol-
ecules between two active site residues seem to play a
critical role at the active site of the enzyme carbonic
anhydrase.4 Do such ordered molecules of water play a
role in proton transfers in a nonaqueous medium for
simple organic reactions? There is considerable interest
in the role of hydrogen bonds in such proton transfer
mechanisms in view of the recent suggestions that low
barrier hydrogen bonds (LBHB) may play an important
role in enzyme catalysis.5 We have studied the neutral
hydrolysis of p-nitrophenyl trifluoroacetate, 1, in aceto-
nitrile with water concentrations ranging from 1.0 to 5.0
M in order to characterize the transition state structure
for water reactions in a nonaqueous medium (Table 1).
We have also conducted proton inventories at different
molarities of water to test whether there is any variation
in the transition state structure with increasing water
content. Interest in the mechanism of water-catalyzed
reactions of organic substrates at low concentrations of
water in a predominantly nonaqueous medium was
spurred by the pioneering work of Hammett and his
co-workers.6
For Scheme 1:
rate ) k1[1][H2O]x
) kobs[1], since [H2O] . [1]
Hence, kobs ) k1[H2O]x and log kobs ) log k1 + x log[H2O].
A plot of log kobs versus log[H2O] should be linear, and
the slope of such a plot should give x, an apparent order
with respect to water. Figure 1 shows such a plot.
Figure 1 also includes plots of log kobs versus log [D2O]
and the plot of log kobs versus log [CH3OH]. All these
plots are linear and parallel suggesting that the apparent
order in H2O, D2O and methanol are the same. The
apparent orders are 3.22 for both H2O and D2O, and 2.87
for CH3OH.
The water reaction is 2.96 times slower in D2O than
in H2O, and the measured kinetic solvent isotope effect
is the same, within experimental error, for the different
molarities of water studied. The rates were also mea-
sured in mixtures of H2O and D2O for each molarity of
water employed. The plot of kn, the measured rate
constant at an atom fraction of deuterium n of the
solvent, is nonlinear against n and gives a bowl-shaped
(1) Bell, R. P.; Critchlow, J . E. Proc. R. Soc. London, Ser. A. 1971,
325, 35-55.
(2) Wolfe, S.; Kim, C. K.; Yang, K.; Weinberg, N.; Shi, Z. J . Am.
Chem. Soc. 1995, 117, 4240-4260.
(3) Venkatasubban, K. S.; Davis, K. R.; Hogg, J . L. J . Am. Chem.
Soc. 1978, 100, 6125.
(4) Venkatasubban, K. S.; Silverman, D. N. Biochemistry 1980, 19,
4984.
(5) Cleland, W. W.; Kreevoy, M. M. Science 1994, 264, 1887.
(6) (a) Steigman; Hammett. J . Am. Chem. Soc. 1937, 59, 2536. (b)
Farinacci; Hammett. J . Am. Chem. Soc. 1937, 59, 2542.
S0022-3263(97)02027-6 CCC: $15.00 © 1998 American Chemical Society
Published on Web 08/11/1998

6116 J . Org. Chem., Vol. 63, No. 18, 1998
Venkatasubban et al.
Ta ble 1. Obser ved F ir st-Or d er Ra te Con sta n ts for th e Neu tr a l Hyd r olysis of p-Nitr op h en yl Tr iflu or oa ceta te in
Aceton itr ile a t Va r iou s Mola r ities of Wa ter , D2O, a n d Mixtu r es of H2O/D2O a t 25 ( 0.10 °C
104kobs, s-1
atom fraction
of deuterium
1.0 M L2Oa
2.0 M L2O
3.0 M L2O
4.0 M L2O
5.0 M L2O
0
3.03 ( 0.62
2.32 ( 0.10
1.83 ( 0.16
1.33 ( 0.14
1.06 ( 0.14
2.86
2.81 ( 0.50
23.5 ( 0.32
18.4 ( 0.50
14.2 ( 0.22
10.4 ( 0.41
2.70
107.7 ( 0.66
308.4 ( 2.0
221.9 ( 1.6
178.6 ( 0.87
134.5 ( 1.54
101.9 ( 1.00
3.03
545.1 ( 2.0
414.3 ( 4.1
316.5 ( 2.4
253.1 ( 2.3
184.1 ( 0.87
2.96
0.249
0.499
0.748
0.998
68.1 ( 0.58
36.8 ( 0.87
2.92
(kH2O/kD2O)
a
L ) H or D.
the exact values of these orders are not critical they
suggest that three molecules of H2O, (D2O or CH3OH)
are involved in the rate-limiting transition state. The
fact that the reaction is approximately three times slower
in D2O compared to water suggests that proton transfers
occur as part of the rate-determining step.
In proton inventory experiments rates are measured
in H2O and D2O and mixtures of H2O and D2O. The
measured rate constants (kn) are plotted against the atom
fractions of deuterium (n) of the solvent. The relationship
between kn and n was first derived for special cases by
Gross and Butler8 and later put into the currently used
general form of eq 1 by Kresge.9 Equation 1 relates the
rate constant kn for a given n value to the rate constant
k0 in pure H2O as shown below.
TS
(1 - n + nφ/)
∏
i
i
F igu r e 1. Plots of log kobs vs log[H2O], log[D2O], and log[CH3-
OH].
kn ) k0
(1)
RS
(1 - n + nφ )
∏
j
j
where φ/ and φj are the isotopic fractionation factors for
i
each exchangeable hydrogenic site present in the transi-
tion state and the reactant state, respectively. The
isotopic-fractionation factor φ for any given site is the
ratio of the preference of that site for protium compared
to deuterium relative to similar preference of a solvent
site. For sites that show a greater preference for protium
than deuterium relative to the solvent, the fractionation
factor will be less than unity. The application of the
Gross-Butler equation to characterize proton transfers
has been amply illustrated by Gold,10 Kresge,11 and
Schowen.12 Schowen’s group has been mainly instru-
mental in utilizing this technique to its full advantage.
For the system under study the only exchangeable
protons in the reactant state are those of solvent water,
and the isotopic fractionation factor for water protons is
1.0. Therefore, eq 1 reduces to eq 2, which contains terms
from the transition state only.
F igu r e 2. Plots of log kn vs n at [H2O] ) 5.0. The solid line is
based on kn ) k0(1 - n + 0.697n)3. The broken line is drawn
to emphasize the nonlinearity of the solid line.
proton inventory for each molarity of water studied.7
Figure 2 shows the proton inventory plot at a water
molarity of 5.0. No solvent isotope effect was measured
for the methanol reaction because the reaction is too slow
to make any accurate rate measurements at 25 °C.
TS
k ) k
(1 - n + nφ/)
(2)
∏
n
0
i
i
If more than one transition state site contributes to the
isotope effect, then the plot of kn versus n will be
Discu ssion
(8) (a) Gross, P. Z. Electrochem. 1938, 44, 299-301. (b) Orr, W. J .
C.; Butler, J . A. V. J . Chem. Soc. 1937, 330-335.
(9) Kresge, A. J . Pure Appl. Chem. 1964, 8, 243-258.
(10) Gold, V. Adv. Phys. Org. Chem. 1969, 7, 259-331.
(11) Kresge, A. J .; More O’Ferall, R. A.; Powell, M. S. In Isotopes in
Organic Chemistry; Buncel, E., Lee, C. C., Eds.; Elsevier: 1987; Vol.
1, p 7.
The kinetic orders in H2O, D2O, and CH3OH, are 3.22
( 0.25, 3.22 ( 0.25, and 2.87 ( 0.18, respectively. While
(7) Venkatasubban, K. S.; Schowen, R. L. CRC Crit. Rev. Biochem.
1984, 17, 1-44.

Hydrolysis of p-Nitrophenyl Trifluoroacetate in Acetonitrile
J . Org. Chem., Vol. 63, No. 18, 1998 6117
Ta ble 2. List of P a r tia l Solven t Isotop e Effect (kn /k0) Va lu es a t Differ en t Mola r ities of Wa ter
(kn/k0)
n, atom fraction
of deuterium
1.0 M H2O
2.0 M H2O
3.0 M H2O
1.0
4.0 M H2O
5.0 M H2O
(kn/k0) (mean)
0
1.0
1.0
1.0
1.0
1.0
0.249
0.499
0.748
0.998
(kH2O/kD2O)
γ
0.766
0.605
0.439
0.350
2.86
0.837
0.655
0.504
0.370
2.70
0.719
0.579
0.436
0.330
3.03
0.760
0.598
0.464
0.338
2.96
0.771 ( 0.049
0.614 ( 0.030
0.461 ( 0.031
0.346 ( 0.015
2.90 ( 0.124
0.314 ( 0.249
0.632
0.342
2.92
0.547
0.169
0.610
0.052
0.190
Albery has suggested the evaluation of a quantity
called γ, which is a measure of solvent isotope effect
curvature.13 Kresge has illustrated with specific ex-
amples the usefulness of the γ method in delineating
reaction mechanisms.11 The reciprocal of γ serves to
specify the minimum number of protons that are respon-
sible for a given isotope effect curvature. The γ values
for each of the five proton inventories we conducted at
different molar concentrations of water were calculated
using the (kn/k0) values at n ) 0.998 and 0.499 (Table 2).
The γ values calculated vary quite a bit, but not in a
monotonic way. Hence, we calculated a mean γ value
from the five values obtained, which is 0.31 ( 0.25. This
implies that there are a minimum of three protons (1/
0.31 ≈ 3) that contribute to the isotope effect.
A three-proton model for the transition state is shown
below. In this model, protons Hb, Hd, and Hf undergo
F igu r e 3. Plot of (kn/k0) vs n. The plot includes the data for
five different molarities of water ranging from 1.0 to 5.0 M.
The error bars are standard deviations based on five different
(kn/k0) values at each value of n.
nonlinear and bowl shaped. Such a bowl-shaped plot
implies a rate-determining transition state where mul-
tiple proton transfers are occurring. Figure 2 is a plot
of kn vs n for the hydrolysis of 1 in water molarity of 5.0
M. The solid line in Figure 2 is based on a three-proton
model with each proton having an identical fractionation
factor of 0.697 ( 0.005. Thus, the experimental proton
inventory data can be described by eq 3:
major bonding changes and therefore contribute to the
overall isotope effect. For these protons the fractionation
factor changes from a reactant state value of 1.0 to a
transition state value of 0.697. Protons Ha, Hc, and He
undergo minimal bonding changes, and their fraction-
ation factor remains unaltered at a value of 1. This
model is essentially the same as the one suggested for
kn ) k0(1 - n + 0.697n)3
(3)
the hydration of 1,3-dichloroacetone in dioxane.1
A
similar model is suggested by Wolfe and co-workers for
the hydration of formaldehyde based on theoretical
calculations.2 Eigen was the first one to suggest such a
cyclic cooperative mechanism for the hydration of a
carbonyl group.14 A similar cyclic transition state involv-
ing a cooperative mechanism has been proposed for the
water-catalyzed decomposition of nitramide by Cox.15
The advantage of this eight-membered transition state
over a six-membered transition state involving two water
molecules is that it accommodates linear hydrogen bonds
as suggested by Gandour.16 Wolfe and co-workers have
argued that the linearity of hydrogen bonds formed in
the transition state balances the entropic disadvantage
of bringing four molecules together.2 A reviewer sug-
gested that a plot like the one in Figure 1 be done, but
using the activity (partial pressure) of water in acetoni-
trile. Such a plot (not shown) yields a slope of about 11.17
Equation 3 can be rearranged to eq 4:
kn
) (1 - n + 0.697n)3
(4)
k0
The advantage of eq 4 is that all the proton inventory
data for different molarities of water can be included in
a single plot. Figure 3 shows the plot of (kn/k0) against
n. The solid line is based on the right-hand side of eq 4.
Thus, all the proton inventory data for all the molarities
of water are adequately described by the three-proton
model. The transition state structure is independent of
the molarity of water in the acetonitrile. Hence, a
transition state model involving three water molecules
wherein only three protons contribute to the isotope effect
is favored.
(12) (a) Schowen, R. L. In Isotope Effects on Enzyme-Catalyzed
Reactions; Cleland, W. W., O’Leary, M. H., Northrop, D. B., Eds.;
University Park Press: Baltimore, 1972; p 64. (b) Schowen, K. B.;
Schowen, R. L. Methods Enzymol. 1982, 87C, 551. (c) Schowen, K. B.
In Transition States of Biochemical Process; Gandour, R. B., Schowen,
R. L., Eds.; Plenum Press: New York, 1978; p 255.
(13) Albery, W. J .; Davies, M. H. J . Chem. Soc. Faraday Trans. 1972,
167.
(14) Eigen, M. Discuss. Faraday Soc. 1965, 39, 7-15.
(15) Cox, R. A. Can. J . Chem. 1996, 74, 1779-83.
(16) Gandour, R. D. Tetrahedron Lett. 1974, 295-298.
(17) We thank one of the reviewers for suggesting this plot.

6118 J . Org. Chem., Vol. 63, No. 18, 1998
Venkatasubban et al.
This suggests that a large number of water molecules
are involved in the rate-determining step. A mechanism
involving such a large number of water molecules is
presumably a general solvation effect. Such a mecha-
nism may be operating, but we believe the simplest
mechanism consistent with the experimental data is the
cyclic cooperative mechanism involving three water
molecules. The reaction in methanol apparently involves
three methanol molecules. Thus, the transition state
model is the same as that of the water reaction except
that protons Ha, Hc, and He are replaced by the CH3
groups of methanol.
There is yet another model for the water reaction thaat
is consistent with the experimental data. In this model
the substrate is rapidly hydrated by a single molecule of
water, followed by a rate-limiting decomposition of the
hydrate involving two additional water molecules. The
hydrates of trifluoromethyl derivatives of carbonyl com-
pounds are known to be formed rather readily. The
transition state again involves a cyclic eight-membered
ring with three protons undergoing bonding changes as
shown below.
The cyclic transition state model for water reaction in
acetonitrile is in contrast to an acyclic three-proton
transition state model proposed for water reactions of
esters and related compounds in aqueous solutions as
shown earlier.3 We believe that this difference is due to
the stabilization of the developing charges on the oxygens
by hydrogen bonds to bulk water in aqueous solution.
This kind of stabilization by bulk water is difficult to
achieve in a predominantly nonaqueous medium, which
forces a cyclic transition state with minimal development
of charges. In order to achieve the cyclic transition state
a third water molecule is necessary to accommodate
linear hydrogen bonds.
Exp er im en ta l Section
p-Nitrophenyl trifluoroacetate, deuterium oxide, HPLC
grade water, and HPLC grade acetonitrile were all purchased
from Aldrich and were used as obtained. A stock solution of
the substrate (∼1.3 × 10-2 M) in acetonitrile was prepared
fresh for every set of runs. All the solutions prepared were
bubbled with nitrogen for 15 min and kept in a 20 mL serum
tube in a desiccator until use. All D2O solutions were prepared
in a glovebox flushed with nitrogen to minimize any contami-
nation with H2O. H2O (3 mL) (or D2O) in acetonitrile was
thermostatted in a cuvette placed in a constant temperature
cell holder of a Varian DMS 90 spectrophotometer at 25 ( 0.1
°C. Then 20 µL of a 1.3 × 10-2 M stock solution of the
substrate was mixed with the H2O/acetonitrile solution to
initiate the reaction. The reaction was followed by monitoring
the appearance of the product p-nitrophenol that was released
at 320 nm for about 3-5 half-lives. First-order rate constants
were calculated using a kinetics calculation program designed
for the DMS-90 spectrophotometer and the Varian DS-15 data
station. Each rate constant reported is an average of five
kinetic runs.
In order to test the validity of the three-proton model for
the transition state, we analyzed our data using a general
nonlinear least squares program. In this analysis we
used all the individual (kn/k0) values instead of using only
the mean (kn/k0) value for each n value. Statistically,
using all the (kn/k0) values is more representative of the
true situation than using the mean values. This analysis
revealed that there is improvement at the 99.7% confi-
dence level (F-test) in going from a single fractionation
factor model to two fractionation factor model and there
is improvement at the 90% confidence level in going from
a two fractionation factor model to a three fractionation
factor model. In the three fractionation factor model the
errors in the φ values are more reasonable when the
fractionation factors are identical (φ/i ) 0.697 ( 0.005)
than when the fractionation factors are different. For
the later situation the errors become ridiculously large.
Thus, both statistical analysis of our data and the γ
method seem to validate the three-proton model with
identical fractionation factors for the transition state.
Ack n ow led gm en t. Support for this research from
the Petroleum Research Fund administered by The
American Chemical Society, Research Corporation, and
the TSI Grants administered by the University of North
Florida is gratefully acknowledged. It is a pleasure to
thank professor A. J . Kresge for his thoughtful com-
ments on this paper and his help in the statistical
analysis of the data. We thank professor William J .
Wilsons for his help in the statistical analysis. Our
grateful acknowledgment also goes to Professor J ulio
Mata for replotting Figure 1 using activity of water.
J O972027F