On the Mechanism of Ester Hydrolysis: Trifluoroacetate Derivatives

Article abstract of DOI:10.1021/jo990550j

The hydrolysis rate of trifluoacetates of ArX (X = p-CH₃, H, p-F, p-Cl, and m-Cl) was measured as function of pH and buffer concentration. All the reactions were catalyzed by the buffer bases, and the Bronsted plot showed a small upward curvature. The kinetic isotope effect is ca. 2.5-2.3 for the reactions of water. The Hammett plots as well as the plots of log k_B vs pK_lg are linear. The slopes of these plots are remarkably similar to those corresponding to the hydrolysis of aryl acetates and aryl formates. From these results, we conclude that the reaction corresponds to general base-catalyzed addition of water and break of the leaving group with no intermediate with finite lifetime despite the fact that the trifluoromethyl group stabilized the intermediate considerably.

Full text of DOI:10.1021/jo990550j

6000
J . Org. Chem. 1999, 64, 6000-6004
On th e Mech a n ism of Ester Hyd r olysis: Tr iflu or oa ceta te
Der iva tives
Mariana A. Fernandez and Rita H. de Rossi*
Instituto de Investigaciones en Fı´sico Quı´mica de Co´rdoba (INFIQC), Facultad de Ciencias Qu´ımicas,
Departamento de Quı´mica Orga´nica, Universidad Nacional de Co´rdoba, Ciudad Universitaria,
5000 Co´rdoba, Argentina
Received March 29, 1999
The hydrolysis rate of trifluoacetates of ArX (X ) p-CH3, H, p-F, p-Cl, and m-Cl) was measured as
function of pH and buffer concentration. All the reactions were catalyzed by the buffer bases, and
the Bronsted plot showed a small upward curvature. The kinetic isotope effect is ca. 2.5-2.3 for
the reactions of water. The Hammett plots as well as the plots of log kB vs pKlg are linear. The
slopes of these plots are remarkably similar to those corresponding to the hydrolysis of aryl acetates
and aryl formates. From these results, we conclude that the reaction corresponds to general base-
catalyzed addition of water and break of the leaving group with no intermediate with finite lifetime
despite the fact that the trifluoromethyl group stabilized the intermediate considerably.
The mechanism of hydrolysis of esters has been
is a determining factor for the transition-state structure
and that aromaticity and the accompanying delocalized
nature of the attacking charge do not result in significant
alteration in the transition-state structure.
considered to involve a two-step mechanism for a long
time. The formation of a tetrahedral intermediate in this
mechanism (eq 1) is largely rate determining.1
Recently, we reported on the effect of cyclodextrin on
the hydrolysis of trifluoracetate esters and included some
results regarding the hydrolysis of the substrates itself.8
Our results seemed to be more concordant with a mech-
anism involving a tetrahedral intermediate. Therefore,
we undertook the study of other substrates in order to
get insight into the mechanism for these compounds. Our
aim was to determine whether the higher stability of the
tetrahedral intermediate due to the trifluoromethyl
group9 could change the transition state from concerted
to stepwise. We report here the reactions of compounds
3-5 and some complementary results of those already
published8 for substrates 1 and 2.
On the other hand, William et al.2 have postulated a
concerted mechanism for the reactions of phenyl esters
with phenolate ions when the pKa of the leaving group
is between 2 and 11. Guthrie,3 on the basis of literature
results and thermodynamic calculations of the stability
of the intermediates involved, suggested that in general
the aryl acetate reactions occur without intermediates
of significant lifetime due to very small barriers for the
bond formation and rupture from the intermediate.
Further Bro¨nsted analysis has led J encks and co-workers
to conclude that substituted phenyl formates as well as
phenyl acetates react with phenolate anions through a
concerted mechanism.4 These conclusions have not been
universally accepted, and evidence from Bro¨nsted studies
favoring a stepwise reaction pathway for aryl acetates
has been presented.5,6
Comparison of the free energy relationships of these
compounds and those of related ones led us to conclude
that the transition state of these reactions have the same
degree of bond formation and rupture as that of acetates
and formates with the same leaving group, contrary to
our previous belief based on a limited number of data.10
From studies of isotopic effect, Hengge7 concluded that
the reaction of oxyanion nucleophiles with p-nitrophenyl
acetate occurs by a concerted mechanism. These authors
also suggested that the pKa of the oxyanion nucleophiles
Resu lts
The hydrolysis of substrates 1-5 was measured as a
function of pH in the range 5.00-9.91 (Tables
S1-S3).11,8 At each pH, at least five buffer concentra-
tions were used and the rate increased linearly with
buffer concentration in all cases. The observed pseudo-
(1) Mender, M. L. Chem. Rev. 1960, 60, 53. J ohnson, S. L. Adv. Phys.
Org. Chem. 1967, 5, 237. J encks, W. P. Chem. Rev. 1972, 72, 705.
(2) Ba-Saif, S.; Luthra, A. K; Williams, A. J . Am. Chem. Soc. 1989,
111, 2647.
(3) Guthrie, J . P. J . Am. Chem. Soc. 1991, 113, 3941.
(4) Stefanidis, D.; Cho, S.; Dhe-Pagamon, S.; J encks, W. P. J . Am.
Chem. Soc. 1993, 115, 1650-1656.
(8) Ferna´ndez, M. A.; de Rossi, R. H. J . Org. Chem. 1997, 62, 7554.
(9) Manion Schilling, M. L.; Roth, H. D.; Herndon, W. C. J . Am.
Chem. Soc. 1980, 102, 4272.
(10) Fernandez, M. A.; de Rossi, R. H. 14th International Conference
on Physical Organic Chemistry, 4th Latin American Conference on
Physical Organic Chemistry, Florianopolis, Brazil, 1998, p 105.
(11) Supporting Information.
(5) Buncel, E.; Um, I. H.; Hoz, S. J . Am. Chem. Soc. 1989, 111, 971-
975.
(6) Kwon, D.-S.; Lee, G.-J .; Um, I. H. Bull Korean Chem. Soc. 1990,
11, 262-265.
(7) Hengge, A. C.; Hess, R. A. J . Am. Chem. Soc. 1994, 116, 11256.
10.1021/jo990550j CCC: $18.00 © 1999 American Chemical Society
Published on Web 07/14/1999

The Mechanism of Ester Hydrolysis
J . Org. Chem., Vol. 64, No. 16, 1999 6001
Ta ble 1. Ca ta lytic Ra te Con sta n ts (kB, M-1, s-1) for
Com p ou n d s 1-5 a t 25 °C
2-
2-
watera
(-0.49,c
1.25d)
CH3COO- HPO4
CO3
HO-b
(-0.45,c
(-0.79,c (-1.15,c (-0.30,c
substrate
1.26d)
1.98d)
2.96)
0.69d)
p-CH3
H
p-F
p-Cl
m-Cl
0.0273
0.0367
0.0596
0.0837
0.111
8.85
12.7
27.9
45.5
83.7
101
290
891
2158
2667
6.14
8.51
7.26
11.8
14.7
3.12
4.31
7.49
8.51
a
b
d
kw/53.42. kB × 10-5
.
c âlg value. F value.
Ta ble 2. Kin etic Solven t Isotop e Effect for Differ en t
Ba sesa
D
kBH/kB
2-
2-
substrate
water
HPO4
CO3
HO-
p-CH3 (1)
H (2)
p-Cl (4)
m-Cl (5)
2.78 ( 0.13
2.59 ( 0.09
2.38 ( 0.16
2.49 ( 0.10
2.1 ( 1.5
2.2 ( 1.1
2.2 ( 0.7
1.0 ( 0.3
0.3 ( 0.1
0.2 ( 0.1
0.7 ( 0.3
0.5 ( 0.1
0.8 ( 0.2
0.4 ( 0.1
0.8 ( 0.2
0.7 ( 0.3
F igu r e 1. Buffer-independent observed rate constants vs pH
for the hydrolysis of compounds 4 (b, left and bottom axes)
and 5 (9, right and top axes).
a
The errors have been calculated using the average standard
deviation of each rate constant and the following equation.
compounds 4 and 5. A similar plot (not shown) is obtained
for 3. The plots fit very well to an equation of the form of
eq 3 where kw is the water reaction and kHO is the HO--
catalyzed reaction for the three compounds:
kH
kD
∆kH ∆kD kH
∆
)
+
H
D
kD
k
[
]
k
ko ) kw + kHO[HO]
(3)
Ta ble 3. Dep en d en ce of th e Ca ta lytic Ra te Con sta n t kB
for th e Differ en t Ba ses on th e p KBH
The kinetic isotope effect was determined for sub-
strates 1, 2, 4, and 5 in neutral and in basic solution
(Table S4, Supporting Information).11 At each pH two or
three buffer concentrations were used so the value of the
isotope effect was calculated for kw, kB, and kHO. The value
a
b
d
f
g
X
pKLg
âHO
R2c
âw
R2e
â1
â2
p-CH3 10.26 0.549 0.9781 0.304 0.9990 0.297 0.55
H
10.00 0.502 0.9865 0.328 0.9914 0.300 0.50
9.81 0.458 0.9990 0.350 0.9811 0.308 0.48
9.38 0.478 0.9991 0.368 0.9775 0.318 0.48
9.02 0.474 0.9998 0.370 0.9789 0.328 0.47
p-F
p-Cl
m-Cl
D
of kwH/kw was obtained from the intercept of the plot of
a
kobs vs buffer concentration in water and in deuterated
water at pH 6.2 for 1 and 5 and at pH 7.07 and 7.14 for
2 and 4, respectively. The values of kOH and kOD are the
slopes of the plots of ko vs OH and OD, respectively. The
isotope effects (Table 2) for the buffer bases have large
errors because they have been calculated from data at
only two or three buffer concentrations. The errors are
particularly large for HPO42- in the reactions of 1 and 2
because the observed catalysis for this substrates is
barely outside experimental errors.
Values taken from Stretwieser, A.; Heathcock, C. H. Quı´mica
Orga´nica, 3rd ed.; Macmillan Publishing Co. Inc.: Madrid, Spain,
1986; p 888. b Calculated without the point for water. c Correlation
d
coefficient for âHO
.
Calculated excluding the data for HO-.
e Correlation coefficient for âw. f Calculated with the data for water,
g
acetate, and phosphate. Calculated with the data for phosphate,
carbonate, and hydroxide ion.
first-order rate constant can be represented by eq 2
kobs ) ko + kcat[BT]
(2)
where ko represents the buffer-independent rate constant,
kcat is the rate constant for catalysis by the buffer, and B
represents the total buffer concentration. To determine
whether the acid or base component of the buffer was
the catalyst, the values of kcat were plotted vs the fraction
of free base. In all cases, the intercept at zero free base
fraction was zero within experimental error, indicating
that the base was the catalyst. In Table 1 are collected
the catalytic coefficients of the various bases.
In the case of compounds 4 and 5 we attempted to
determine the catalysis by chloroacetate, but the ob-
served rate constants were the same within experimental
error in the range of buffer concentration used (0.01-
0.10 M, pH 2.9, 3.3, and 3.75). These results were
Discu ssion
Ba se Ca ta lysis. The values of the isotope effect
obtained for the water reaction (Table 2) indicate that
proton transfer occurs in the rate-determining step. They
are similar to the values reported for the neutral hydro-
lysis of 4-nitrophenyl and 2,4-dinitrophenyl trifluoro-
acetate in acetonitrile containing 1% water, i.e., 2.3,12 and
also for the general base-catalyzed hydrolysis of aryl and
alkyl formates.13 The kinetic isotopic effect for the buffer
bases is lower than that of water and may indicate that
in the mechanism of catalysis there is some contribution
from nucleophilic as well as general base mechanism of
catalysis. The inverse isotope effect for HO- is typical
for this type of reactions14 and has been interpreted as
arising from desolvation of hydroxide.
expected since from the Bro¨nsted â value, i.e., 0.47 (âHO
,
Table 3), for 5, kB is calculated as 1.16 M-1 s-1, and then
at pH 3.75 the observed rate constant should vary within
6.06 and 6.15 s-1. The measured values are 6.4 ( 0.5 and
(12) Neuvonen, H J . Chem. Soc., Perkin Trans. 2 1986, 1141.
(13) Stefanidis, D.; J encks, W. P. J . Am. Chem. Soc. 1993, 115,
6045-6050.
(14) (a) Bruice, T. C.; Fife, T. H. J . Am. Chem. Soc. 1962, 84, 1973.
(b) Bunton, C. A.; Shiner, V. J . J . Am. Chem. Soc. 1961, 83, 3207.
6.7 ( 0.6 s-1
.
In Figure 1 are plotted the observed rate constants
extrapolated to zero buffer concentration vs pH for

6002 J . Org. Chem., Vol. 64, No. 16, 1999
Fernandez and de Rossi
F igu r e 3. Hammett plots of the catalytic rate constant for
the bases (kB). The values of σ were taken from March, J .
Advanced Organic Chemistry, Reactions, Mechanism and
Structure, 4th ed.; J ohn Wiley and Sons: New York, 1992;
p 280: CO32-, F ) 2.96 (r2 ) 0.967), 1 (left ordinate); HO-,
F ) 0.692 (r2 ) 0.797), [ (right ordinate) HPO42-, F ) 1.98
F igu r e 2. Dependence of the catalytic rate constant for the
2-
bases (kB) on the pK of the leaving group (pKlg): (A) CO3
âlg ) -1.15 (r2 ) 0.897) 1 (right ordinate); HO-, âlg) -0.30
(r2 ) 0.914), [ (left ordinate); (B) water, âlg ) -0.49 (r2
)
0.957) 2; CH3COO- âlg ) -0.45 (0.989), b (left ordinate);
HPO42-, âlg ) -0.79 (r2 ) 0.967), 9 (right ordinate).
(r2 ) 0.987), 9 (right ordinate); CH3COO-, F ) 1.26 (r2
0.92%), b water, F ) 1.25 (0.991), (left ordinate).
)
We suggest that water is the nucleophile for the
general-base-catalyzed pathway and the base acts as
general base, removing a proton from the nucleophile in
the transition state. A similar mechanism was suggested
by Marlier for the alkaline hydrolysis of methyl formate
based on isotope effects on oxygen15 and in several other
reactions of water with different electrophiles.16,17
tively, 11.99, 22.3, 68.1, 106, and 116 for Z ) p-CH3, H,
p-F, p-Cl, and m-Cl.
The data for all the bases give linear Hammett plots
(Figure 3) with a slope 1.24 and 0.69 for water and OH,
respectively. This is in accordance with the lower âLg for
the latter base and with the Hammond postulate. The
stronger base is the one less sensitive to the substituent
probably because of an earlier transition state. Similarly,
as observed with the change in âlg with pKBH also there
is a linear relationship between F and pKBH for acetate,
phosphate, and carbonate with slope 0.325.
A better Hammett correlation is obtained by using the
values of σ rather than those of σ -, which is an indication
that the correlation does not arise from the rate-limiting
ArO‚‚‚C bond rupture from the tetrahedral interme-
diate.19 In contrast, the reactions of HO- give a better
correlation with σ- although the F value is lower than
the corresponding to the other bases (see Table 1 and
Figure 3 caption). This result is somehow inconsistent
with the fact that F for HO- is lower than that for the
other bases as mentioned above. The fact that we are
considering only four substituents and that only p-F has
a σ- value significantly different from σ makes it difficult
to draw a sound conclusion; for this reason, we will not
discuss this point further. Experiments with substrates
having groups with higher differences between σ and σ-
(for instance p-NO2) are precluded because of their high
reactivity under our reaction conditions.
Str u ctu r e-Rea ctivity Cor r ela tion s. There is a
linear relationship between the log kB and the pKa of the
leaving group (Figure 2). The value of âLg increases in
absolute number as the base becomes stronger, for
acetate, phosphate, and carbonate. The slope of the line
(not shown) of âLg vs pKBH is -0.11, which is the same
as that calculated from data reported for the general
base-catalyzed hydrolysis of phenyl formates.13 The âLg
for the water reaction, i.e., -0.49, is also remarkably
similar to literature values for the formates, namely
-0.46.13 The value of âLg for HO- (-0.30) is lower than
expected considering those of the other bases; however,
it is in good agreement with the value reported for the
hydrolysis of aryl acetates, i.e., -0.32.18 The low value
for this base may be explained considering that the OH
reacts through its solvation shell so the negative charge
is more diffuse and needs less stabilization for the
electron-withdrawing groups of the phenyl substituent.
An experimental consequence of this low value of âLg for
HO- as compared with the other bases is that the relative
contribution of the catalyzed pathway increases as the
pKa of the leaving group decreases. For instance, for
CO32-/HCO3- buffer the ratio kB/ko at pH 9.02 is, respec-
The dependence of kB with pKBH (Figure 4) can be
analyzed in different ways. One is to draw a line through
(15) Marlier, J . F. J . Am. Chem. Soc. 1993, 115, 5953.
(16) J encks, W. P. Acc. Chem. Res. 1980, 13, 161.
(17) de Rossi, R. H.; Veglia, A. J . Org. Chem. 1983, 48, 1879.
(18) Kirsch, J . F.; J encks, W. P. J . Am. Chem. Soc. 1964, 86, 837.
(19) DeTar, D. F. J . Am. Chem. Soc. 1982, 104, 7205.

The Mechanism of Ester Hydrolysis
J . Org. Chem., Vol. 64, No. 16, 1999 6003
F igu r e 5. Schematic representation of the different reaction
pathways for the hydrolysis of esters.
The possibility of catalysis by hydrogen bonding can
be analyzed considering the equilibrium constant shown
in eq 4 for acetate as base.
F igu r e 4. Bro¨nsted plots for substrates 1, 2, 4, and 5. (A)
Z ) H [ (right ordenate); Z ) CH3, 2 (left ordinate); (B) Z )
m-Cl 9 (right ordinate); Z ) p-Cl b (left ordinate).
all the anionic bases leaving the point for the water
reaction out of the line. Positive deviation of the water
reaction has been observed in other cases.4 Another
possibility is to draw a line through all the points leaving
the HO- out of the correlation since this base seems to
behave anomaly in other types of linear-free relationship
correlations (see above). Finally, we can consider all the
points together. In this case the Bro¨nsted plot have a
slightly upward curvature. We calculated two Bro¨nsted
values as follows. We estimated the value of â1 drawing
a line throughout the points for water, acetate and
phosphate and another one using carbonate, phosphate
and HO-. The data are listed in Table 3, and it can be
seen that the value changes from approximately 0.3 to
0.5.20 The reaction of trifluoroethanol with [4-(trifluoro-
methyl)benzyl]methyl(4-nitrophenyl)sulfonium tetrafluoro-
borate has a Bro¨nsted value of 0.26 for anionic catalysts.21
In the hydrolysis of aryl formates a pronounced increase
in â (from 0.1 to 0.3 to 1.2-1.9) was observed when the
pKa of the base increased. This increment has been
attributed to a change in mechanism from general base
catalyzed to nucleophilic. A similar interpretation can
account for our results. However, in our case the plots
are only slightly curved upward, and then it may be that
there is only a small contribution from the nucleophilic
mechanism. The general base-catalyzed addition of water
to triaryl methyl carbocation ranges from 0.5 to 0.33.22
The â1 value corresponds to the general base-catalyzed
reaction and â2 to reactions where there is nucleophilic
as well as general base catalysis.
Using eq 5 as described by Hine,23,24 we calculate pKAH
) -12.8 for the transition state, which is far bellow any
reasonable estimate of the acidity of the transition state;
therefore, this mechanism can be disregarded.
log KAB ) 0.013(pKAH - pHw)(pKH - PKBH
) (5)
Mech a n ism of t h e R ea ct ion . The formation of
tetrahedral intermediates in acyl transfer reactions is a
matter of controversy, and concerted vs stepwise mech-
anisms have been discussed in terms of isotopic effects,7,15
linear free relationships,4,5 and theoretical calculations.3
To analyze the possible mechanism we can consider a
J encks-More O’Ferral diagram25,26 (Figure 5). In the
horizontal coordinates the C-O bond formation to the
nucleophile is represented, whereas in the vertical coor-
dinates the bond rupture of the leaving group is repre-
sented.
Assuming a diagonal reaction path for RdCH3 or H, a
change of these groups by CF3, which is strongly electron-
withdrawing, should destabilize the upper left corner by
at least 6 kcal/mol.27 On the other hand, the substitution
of the CH3 group of acetate by CF3 is expected to stabilize
the tetrahedral intermediate. For instance, the equilib-
rium constant for the aldol condensation reaction shown
in eq 6 is 2.66 × 103 M-1 for RdCF3 and 1.89 × 10-3 M-1
for RdCH3; i.e., the CF3 leads to a stabilization of 8.4
kcal/mol.28
(23) Hine, J . J . Am. Chem. Soc. 1972, 94, 5766.
(24) Dietze, P. E.; J encks, W. P. J . Am. Chem. Soc. 1989, 111, 340.
(25) More O’Ferral, R. A. J . Chem. Soc. B 1970, 274.
(26) J encks, W. P. Chem. Rev. 1972, 72, 705.
(27) This value is calculated considering that the pKa of CF3COOH
is 0.23 and that of CH3COOH is 4.75. See: Oakenfull, D. G.; J encks,
W. P. J . Am. Chem. Soc. 1971, 93, 78.
(20) The kB values for 1 and 2 have large errors because the increase
in rate with buffer concentration is very small.
(21) Dietze, P. E., J encks, W. P. J . Am. Chem. Soc. 1989, 111, 340.
(22) Gandler, J . R. J . Am. Chem. Soc. 1985, 107, 8218.

6004 J . Org. Chem., Vol. 64, No. 16, 1999
Fernandez and de Rossi
Kin etic Solven t Isotop ic Effect. The buffer was prepared
in D2O (95%), and the pD was measured. To this value 0.432
was added to calculate the DO- concentration, which is defined
as KD O/antilog pD (pKD ) 14.914.33) The pKBD for D2PO4
-
O
2
2
was calculated according to Bell.34,35
This increase in stability is in part due to a decrease
in the dissociation rate constant but mainly to a high
increase in the association rate constant.
∆pK ) 0.017pKBH + 0.44
For carbonate, it was determined using D2O as solvent. The
measured value is in good agreement with the one calculated
as indicated above.
Kin etic P r oced u r es. The reactions were carried out in an
Applied Photophysics SF 17MV stopped-flow apparatus with
unequal mixing. The substrate was dissolved in dry ACN and
placed in the small syringe (0.1 mL). The large syringe (2.5
mL) was filled with water solution containing all the other
ingredients. The total acetonitrile concentration was less than
4%.
All reactions were run at 25.1 ( 0.1 °C and at constant ionic
strength (0.2 M) using NaCl as compensating electrolyte. The
observed rate constants were determined by following the
appearance of the corresponding phenols (3, 280 nm; 4, 282
nm; 5, 279 nm). The rate was measured also at other
wavelengths, to see if the tetrahedral intermediate complex
could be detected. Under all conditions the same value of the
rate was observed and a faster process that could be attributed
to the accumulation of the tetrahedral intermediate was not
found, although we carefully looked for it.
These two effects together should drive the transition
state toward the tetrahedral intermediate. However, the
similarities between the free energy relationship for the
reactions reported here and those of formates and
acetates led us to conclude that the reactions are con-
certed, although the position of the transition state may
be closer to the tetrahedral intermediate. We conclude
that the mechanisms of hydrolysis of the substrates 1-5
do not involve an intermediate and are concerted just as
the hydrolysis of other aryl esters. It appears that the
stabilization provided by the strongly electron-withdraw-
ing CF3 group is not enough to drive the reaction toward
the lower right-hand corner of the diagram (Figure 4),
and this is because the expulsion of the phenolate from
the intermediate has no barrier.3
Exp er im en ta l Section
Ack n ow led gm en t. This research was supported in
part by the Consejo Nacional de Investigaciones Cien-
tificas y Te´cnicas (CONICET), the Consejo de Investi-
gaciones Cient´ıficas y Tecnolo´gicas de la Provincia de
Co´rdoba, (CONICOR), Agencia Nacional de Ciencia y
Tecnolog´ıa (FONCYT), and the Universidad Nacional
de Co´rdoba, Argentina. M.A.F. is a grateful recipient
of a fellowship from CONICET
Aqueous solutions were made up from water purified in a
Millipore apparatus. Acetonitrile Merck HPLC grade was used
as received.
The pH measurements were done in a pH meter Orion 720A
at controlled temperature and calibrated with buffers prepared
in our laboratory according to the literature.29
The substrates 3-5 were prepared by the reaction of the
appropriate phenol with trifluoroacetic anhydride following
literature methods.30 The product was obtained after distilla-
tion of the remaining trifluoroacetic anhydride, and trifluoro-
acetic acid was distilled in a Bu¨chi GKR-51 Kugelrohr appara-
tus. The purity was controlled by comparison of the spectrum
of a completely hydrolyzed solution with a mock solution of
the corresponding phenol and by IR comparison with litera-
ture.31
Su p p or tin g In for m a tion Ava ila ble: Tables S1-S4 con-
taining the observed rate constant for the hydrolysis of 3-5
as a function of pH and buffer concentration and the observed
rate constants for the reactions in D2O. This material is
available free of charge via the Internet at http://pubs.acs.org.
J O990550J
(28) Guthrie, P. J .; Barker, J . A. J . Am. Chem. Soc. 1998, 120, 6698.
(29) Analytical Chemisty. The Working Tools; Strouts, C. R. N.,
Gilgilan, J . H., Wilson, H. N., Eds.; Oxford University Press: London,
1958; pp 228-233.
(30) Clark, R. F; Simons, J . H. J . Am. Chem. Soc. 1953, 75, 6305.
(31) Crower, G. A. J . Fluorine Chem. 1971, 1, 219.
(32) Glasoe, P. K., Lons, F. A. J . Phys. Chem. 1960, 64, 188
(33) Shoesmith, D. W.; Lee, W. Can. J . Chem. 1976, 54, 3553.
(34) Bell, R. P. The Proton in Chemistry, 1st ed.; Cornell University
Press: Ithaca, NY, 1959
(35) Murray, C. J .; J encks, W. P. J . Am. Chem. Soc. 1990, 112,
1880-1889.