Kinetic study of hydrolysis of benzoates. Part XXIII - Influence of the substituent and temperature on the kinetics of the alkaline hydrolysis of alkyl benzoates in aqueous 2.25 M Bu4NBr and 80% DMSO

Article abstract of DOI:10.1002/poc.508

The second-order rate constants k₂ (M^-1 s^-1) for the alkaline hydrolysis of substituted alkyl benzoates, C₆H₅CO(O)R (R = CH₃, CH₂Cl, CH₂CN, CH₂C≡CH, CH₂C₆H₅, CH₂CH₂Cl, CH₂CH₂OCH₃), were measured in aqueous 2.25 M n-Bu₄NBr and in 80% (v/v) DMSO solution at several temperatures. The log k values were analyzed using the equation log k = log k₀ + ρσ + δE_s^B. The E_s^B scale has been proposed for the steric effect of alkyl substituents in the alkyl part of esters: E_s^B = (log k_R - log k_CH(3))_H+, where k is the rate constant for the acidic hydrolysis of substituted alkyl benzoates or acetates in water. As polar substituent parameters, both Taft σ* and σ_I constants were used. The dual parameter treatments of the log k values with σ and E_s^B constants gave excellent correlations (R = 0.997). For 2.25 M n-Bu₄NBr, 80% (v/v) DMSO and pure water at 25 °C, calculated susceptibilities to the inductive effect of alkyl substituents ρ* were found to be 2.07, 2.21 and 1.64, respectively. The corresponding ρ_I values were 4.64, 4.94 and 3.64. The dependence of ρ_I on solvent and temperature in the alkaline hydrolysis of substituted alkyl benzoates was similar to that observed earlier for meta- and para-substituents in the alkaline hydrolysis of substituted phenyl benzoates and tosylates. The substituent dependence of the activation energy, E, was found to be completely caused by the polar effect. Susceptibility to steric effect in the alkaline hydrolysis of alkyl benzoates (δ ≈ 1) appeared to be independent of the solvent and temperature. Copyright

Full text of DOI:10.1002/poc.508

JOURNAL OF PHYSICAL ORGANIC CHEMISTRY
J. Phys. Org. Chem. 2002; 15: 353–361
Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/poc.508
Kinetic study of hydrolysis of benzoates. Part XXIIIÐ
In¯uence of the substituent and temperature on the
kinetics of the alkaline hydrolysis of alkyl benzoates in
aqueous 2.25 M Bu NBr and 80% DMSO
4
Vilve Nummert* and Mare Piirsalu
Institute of Chemical Physics, Tartu University, Jakobi 2, EE2400 Tartu, Estonia
Received 30 April 2001; revised 20 February 2002; accepted 26 February 2002
À1 À1
ABSTRACT: The second-order rate constants k (M
s ) for the alkaline hydrolysis of substituted alkyl benzoates,
2
epoc
C H CO(O)R (R = CH , CH Cl, CH CN, CH CꢀCH, CH C H , CH CH Cl, CH CH OCH ), were measured in
6
5
3
2
2
2
2
6
5
2
2
2
2
3
aqueous 2.25 M n-Bu NBr and in 80% (v/v) DMSO solution at several temperatures. The log k values were analyzed
4
B
s
B
s
using the equation log k = log k ‡ ꢀꢁ ‡ ꢂE . The E scale has been proposed for the steric effect of alkyl
0
B
substituents in the alkyl part of esters: E = (log k À log kCH3)H
‡, where k is the rate constant for the acidic
s
R
hydrolysis of substituted alkyl benzoates or acetates in water. As polar substituent parameters, both Taft s* and s
I
B
constants were used. The dual parameter treatments of the log k values with s and Es constants gave excellent
correlations (R = 0.997). For 2.25 M n-Bu NBr, 80% (v/v) DMSO and pure water at 25°C, calculated susceptibilities
4
to the inductive effect of alkyl substituents r* were found to be 2.07, 2.21 and 1.64, respectively. The corresponding
r values were 4.64, 4.94 and 3.64. The dependence of r on solvent and temperature in the alkaline hydrolysis of
I
I
substituted alkyl benzoates was similar to that observed earlier for meta- and para-substituents in the alkaline
hydrolysis of substituted phenyl benzoates and tosylates. The substituent dependence of the activation energy, E, was
found to be completely caused by the polar effect. Susceptibility to steric effect in the alkaline hydrolysis of alkyl
Lb et dn .z oates (ꢂ ꢁ 1) appeared to be independent of the solvent and temperature. Copyright  2002 John Wiley & Sons,
Additional material for this paper is available from the epoc website at http://www.wiley.com/epoc
KEYWORDS: alkyl benzoates; alkaline hydrolysis; substituent effects
INTRODUCTION
influence of the polar and steric factors and to establish
how these factors vary with the solvent and temperature
when the electronegativity of the substituent in the alkyl
component of ester is varied.
In previous papers, the temperature dependence of
substituent effects on the alkaline hydrolysis of ortho-,
meta- and para-substituted phenyl benzoates, C H
6
5
The kinetics of the alkaline hydrolysis of esters
1–7
8–13
CO C H X, and tosylates, CH C H SO OC H X,
RCO R' involving a variable substituent in the alkyl part
2
6
4
3
6
4
2
6
4
2
in water, in aqueous 2.25 M Bu NBr and in 80% (v/v)
have been studied mainly in the case of saturated
4
DMSO, and also in the alkaline hydrolysis of substituted
hydrocarbon substituents with a rather narrow range of
s* variation. We could find only a few publications
14,15
14–22
alkyl benzoates, C H CO R, in water
were studied.
6
5
2
As a further extension of our studies on substituent
effects, the kinetic effect of alkyl substituents (R = CH3,
CH Cl, CH CN, CH CꢀCH, CH C H , CH CH Cl,
providing data on the kinetics of the alkaline hydrolysis
of esters with electronegative or electron-accepting
substituents (ꢁ* > 0) in the alkyl part of esters. The
kinetics of the alkaline hydrolysis of alkyl benzoates in
DMSO–water mixtures has been studied only in the case
2
2
2
2
6
5
2
2
CH CH OCH ) on the alkaline hydrolysis of substituted
2
2
3
alkyl benzoates, C H CO R, in aqueous 2.25 M Bu NBr
6
5
2
4
23–27
and in 80% (v/v) DMSO at various temperatures was
studied. The aim of this work was to separate the
of saturated hydrocarbon substituents.
To our
knowledge, there are also no kinetic data in the literature
on the alkaline hydrolysis of substituted alkyl benzoates
in 2.25 M aqueous n-Bu NBr over a wide temperature
range.
4
*
Correspondence to: V. Nummert, Institute of Chemical Physics,
Tartu University, Jakobi 2, EE2400 Tartu, Estonia.
Contract/grant sponsor: Estonian Science Foundation; Contract/grant
number: 3978.
The structure–reactivity relationship in the alkaline
hydrolysis of esters RCO R' involving a variable alkyl
2
Copyright  2002 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2002; 15: 353–361

3
54
V. NUMMERT AND M. PIIRSALU
34
substituent R (a,b-saturated) in the acyl component, was
i.e. ꢀ*/ꢀ° ≠ 1 and ꢀ*/ꢀ° ≠ constant. Equality of the r*
and r° values under conditions different from the
standard values should be observed only in the case of
a proportional change in both r* and r° values with
change in conditions.
28,29
introduced by Taft
and steric factors:
as the sum of independent polar
Ã
Ã
log k ˆ log k ‡ ꢀ ꢁ ‡ ꢂE
ꢂ1†
0
s
30,31
Some authors
have maintained that the Taft steric
Es scale does not represent a complete separation of steric
EXPERIMENTAL
and polar effects. Therefore, several scales of steric
constants as true measures of the steric effects have been
proposed.
The preparation procedure and characteristics of sub-
stituted alkyl benzoates, C H CO R, and the technique
6
5
2
16,32,33
Hancock and co-workers
and Palm and co-
for kinetic measurements have been described pre-
34,35
c
7
workers
calculated ‘corrected’ steric parameters Es
viously. Cyanomethyl (R = CH CN) and propargyl
2
o
and E , respectively, by removing the contribution of
s
(R = CH CꢀCH) benzoates were provided by Bogatkov
2
hyperconjugation from the steric substituent constants E .
s
et al. (Moscow M. V. Lomonosov Institute of Fine
36–41
Charton
defined steric parameters ꢃ based on the
X
Chemical Technology). As alkali, tetrabutylammonium
van der Waals radii r (n = r À r ). The steric constants
x
x
x
H
hydroxide (n-Bu NOH) was used for the kinetic measure-
4
e
referenced to hydrogen, E , were calculated by Unger
and Hansch from the Taft E values by subtracting 1.24.
s
ments in aqueous 2.25 M Bu NBr and in 80% (v/v)
4
4
2
s
dimethyl sulfoxide (DMSO). The preparation of n-
Bu NOH solution and the purification of n-Bu NBr have
43,44
MacPhee et al.
recalculated the Taft E scale based
s
4
4
1
on a single defining reaction (the acid-catalyzed esteri-
fication of carboxylic acids in MeOH) and termed the
been described earlier. In water, NaOH was used as
alkali. NaOH was purified from carbonates by ion
exchange by Albert and Serjeant as described. DMSO
45,46
5
5
scale E '. Kramer
proposed the calculated EB
s
constants for 96 alkyl C H
-substituents. For aliphatic
n
2n‡1
(pure grade) was dried over BaO and distilled over CaH
under vacuum. To avoid the influence of salt effects,
2
4
6,47
56
substituents C H
it has been shown
that the Taft
n
2n‡1
E values are linearly related to the s* constants. Some
s
the kinetic measurements in 2.25 M Bu NBr, were
4
22,48
authors
have considered the E values for the b-
s
performed at a constant n-Bu NOH concentration:
4
substituted alkyl groups to be too negative and those
groups have been excluded from the analysis as
cOH = 0.0184 M. The measurements in aqueous 80%
À
DMSO and in water were performed in the alkali
concentration range 0.002–0.06 M.
essentially deviating points.
In the present work, the steric EsB scale
49,50
defined as
For kinetic measurements the spectrophotometric
1
the Taft E values for aliphatic substituents in the alkyl
s
method was applied. Wavelengths of l = 282 nm in
part of esters was used in the data analysis. We preferred
alkyl groups containing electronegative substituents
aqueous 2.25 M Bu NBr and l = 280 nm in 80% DMSO
4
were used for all the substituted alkyl benzoates. In water
the following wavelengths were used: l = 275 nm for
X = CH CN, 274 nm for X = CH CꢀCH, 240 nm for
—
CH X and —CH CH X. Saturated hydrocarbon sub-
2 2 2
stituents were avoided to prevent complications in the
data analysis. In order to compare the inductive effect of
2
2
X = CH Cl and 240 nm for X = CH C H . The kinetic
2
2 6 5
alkyl substituents (R = XCH ) with the same effect of
2
measurements were carried out under pseudo-first-order
conditions with excess alkali. The pseudo-first-order rate
constants k1 were determined using a least-squares
51
substituents in a benzene ring, the inductive scale sI
was used:
computer program. In aqueous 2.25 M Bu NBr, the
second-order rate constants were calculated by dividing
Ã
4
ꢁ
=ꢁ
ˆ 0:45
ꢂ2†
IꢂX†
ꢂXCH2†
the pseudo-first-order rate constants k by the alkali
concentration. The measurements were repeated and the
arithmetic means of the corresponding second-order rate
1
Considering that the polar effect of aliphatic sub-
stituents depends on the solvent, two main problems
arise. The susceptibility of the polar effects of aliphatic
substituents, r*, to the change of the solvent composition
has been observed to be weak in comparison with the
Hammett r values in the case of meta- and para-
constants k were calculated. In 80% DMSO and in
2
water, the rate constants for each alkyl benzoate were
determined at 3–5 hydroxide concentrations. At each
hydroxide concentration 3–6 runs were performed and
the arithmetic means of the corresponding pseudo-first-
52–54
substituted phenyls.
The second problem is con-
nected with the relationship between the Taft r* values
and the Hammett reaction constants r. Although the
susceptibility to polar effects of aliphatic substituents, r*,
and meta- and para-substituted phenyls, r, has been
equalized in case of the alkaline hydrolysis of ethyl and
order rate constants k were calculated. The second-order
rate constants k2 for 80% DMSO and water were
calculated according to the equation
1
k ˆ k c
À
‡ constant
ꢂ3†
1
2 OH
28
benzyl esters in aqueous acetone, no universal propor-
tionality between the r* and r° values has been detected,
The data analysis was carried out on a PC/XT 486
Copyright  2002 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2002; 15: 353–361

ALKALINE HYDROLYSIS OF ALKYL BENZOATES
355
Table 1. Second-order rate constants k for the alkaline hydrolysis of alkyl benzoates C H CO -R in aqueous 2.25 M Bu NBr,
2
6
5
2
4
a
8
0% (v/v) DMSO and water at 25°C
2
À1 À1
1
0 k (M
s )
2
R
2.25 M Bu NBr
80% DMSO
Water
4
CH3
1.42 Æ 0.12
146 Æ 7
32.0 Æ 1.1
CH CN
–
213 Æ 4
167 Æ 2
22.2 Æ 1.8
–
2
CH Cl
92.9 Æ 4.3
5.71 Æ 0.42
3.72 Æ 0.22
1.74 Æ 0.10
1.51 Æ 0.09
3370 Æ 180
194 Æ 1
106 Æ 5
51.0 Æ 3.2
31.1 Æ 1.3
2
CH CꢀCH
2
(
CH ) Cl
2
2 2
CH C H
6.44 Æ 0.41
6
5
(CH ) OCH
–
2 2 3
a
2
A table containing the second-order rate constants k at several temperatures is available as supplementary material (Table S1) at the epoc website at http://
www.wiley.com/epoc.
computer, using the multiple parameter linear least-
for the alkyl component of the esters, were compared
57
squares (LLSQ) procedure. In this paper the results of
the data treatment are given mainly at a confidence level
of 0.99.
with the Taft steric scale E , proposed for the acyl
s
2
component. No correlation (R = 0.364, see Table 4) was
B
observed between the Taft Es values and the Es
constants when all alkyl substituents were taken into
account, including those with electronegative groups at
DISCUSSION
the b-position. At the same time, a fairly good correlation
B
(
R = 0.986) between the Taft E values and the E
s s
Data analysis
constants was obtained when substituents with electro-
negative groups at the a-position (R = CH CN, CH Cl,
2
2
The log k2 values for the alkaline hydrolysis of
CH
2
OCH
3
, CH
2
COCH
3
, CH
2
C
6
H , CH ) were included
5 3
substituted alkyl benzoates in aqueous 2.25 M Bu NBr,
(see Fig. 1):
4
8
0% DMSO and pure water at various temperatures
B
(
Tables 1 and S1) were analyzed with the following
Es ˆ ꢂ0:038 Æ 0:032† ‡ 1:52ꢂÆ0:11†Es
ꢂ7†
equations:
R ˆ 0:986; s ˆ 0:059; n=n ˆ 6=6
0
T = constant, X ≠ constant:
Ã
Ã
B
log k ˆ log k ‡ ꢀ ꢁ ‡ ꢂE
ꢂ4†
ꢂ5†
R
0
s
For a chloromethyl substituent, CH Cl, two alternative
2
B
B
X
B
values for Es were used, Es = À0.17 and À0.49,
obtained from the data in water at 25°C for the acidic
hydrolysis of substituted alkyl formates and acetates,
log k ˆ log k ‡ ꢀ ꢁ ‡ ꢂEs
R
0
I I
T ≠ constant, X = constant:
log k ˆ log A À E=2:3RT
B
respectively. When Es = À0.49 was used, the point for
the chloromethyl substituent deviated significantly (Fig.
ꢂ6†
B
1
). Therefore, in further data analysis Es = À0.17 for the
chloromethyl substituent was applied.
Steric constants for the variable substituent in the
alcohol component of the ester, E , were calculated
For comparison, the E values for R = CH CH OCH ,
B
s
s
2
2
3
CH CH OH and CH CH OEt were calculated according
B
s
R
CH
3
2
2
2
2
using the equation E = (log kH
‡
À log k
‡
), where
are the rate constants for the acidic
hydrolysis of R-substituted and CH -substitued alkyl
H
to the equation
R
CH
3
kH
‡
and kH
‡
3
Ã
log k ˆ ꢂ0:915 Æ 0:137† ‡ ꢂ2:56 Æ 0:14†ꢁ
4
9,50
benzoate or acetate, respectively, in water.
substituent constant Es for chloromethyl group,
The steric
‡
ꢂ0:94 Æ 0:17†Es
ꢂ8†
B
R = CH Cl, was calculated as the difference in the log k
2
values for the acidic hydrolysis of chloromethyl formate
for the alkaline hydrolysis of substituted ethyl acetates,
50
and methyl formate in water at 25°C. The substituent
RCO Et, in water at 25°C
when mainly alkyl
substituents with electronegative groups at the a-position
were included: R = CH CN, CH Cl, CH Br, CH OCH ,
2
B
parameters s*, s and E and the Taft E steric constants
I
s
s
are listed in Table 2.
2
2
2
2
3
The cross-correlations among independent variables,
CH C H , CH CH , CH . The magnitudes of E found
2 6 5 2 3 3 s
2
B
2
characterized by values R (s*E ), R (s*E ) and
according to Eqn. (8) for b-alkyl substituents appeared to
be considerably smaller than the Taft Es values,
determined from the acidic hydrolysis: for R =
CH CH OCH , CH CH OH and CH CH OC H , the
s
s
2
B
2
B
s
R (E E ), appeared to be insignificant [R (ꢁ*E ) =
.066, R (ꢁ*E ) = 0.126, R (E E ) = 0.364].
s
s
2
2
s s s
B
0
B
s
The magnitudes of the steric constants E , determined
2
2
3
2
2
2
2
2 5
Copyright  2002 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2002; 15: 353–361

3
56
V. NUMMERT AND M. PIIRSALU
E
A
Copyright  2002 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2002; 15: 353–361

ALKALINE HYDROLYSIS OF ALKYL BENZOATES
357
benzoates were compared. The log kcal values at 25°C
were calculated using Eqn. (4). For the relation log
kcalc = a ‡ b log k the following values were found: in
obs
2
.25 M Bu NBr, a = 0.0051 Æ 0.0443, b = 1.002 Æ 0.031,
4
s = 0.066; in 80% DMSO, a = À0.0029 Æ 0.034,
b = 1.010 Æ 0.048, s = 0.082; in water, a = 0.019 Æ
0
0
.022, b = 1.008 Æ 0.023, s = 0.048. R was in range
.995–0.998. The average deviation of (log k À log
obs
kcalc) was 0.05 log k units.
In¯uence of solvent and substituents
The alkaline hydrolysis of substituted alkyl benzoates
6
5,66
proceeds by a B 2 mechanism
via a tetrahedral
Ac
À
intermediate [RCO(OH)OR'] , which involves the attack
of a hydroxide ion on the carbonyl carbon as the rate-
determining step. When the ester contains electron-
withdrawing electronegative substituents the electron
Figure 1. Dependence of the Taft steric substituent
B
constants, E , on the E
values. The Es values for
2 5
s
R = (CH ) OCH , (CH ) OH and (CH ) OC H were calcu-
s
À
2
2
3
2 2
2 2
lated according to Eqn. (8). For R = CH Cl two alternative
density on the carbonyl carbon decreases, the OH attack
2
on that carbon is facilitated, the rate increases and the
activation energy decreases. The reaction site could be
considered as trigonal in the reactant state but tetrahedral
in the transition state and the steric factor should lead to a
decrease in the rate constant.
B
values for Es were used: (&) À 0.17 and (D) À 0.49
(
E ) and E values were À0.48 and À0.77, À0.39 and
s calc
s
À0.85, and À0.58 and À0.97, respectively.
For all alkyl benzoates the rate constants decrease on
In the data analysis according to Eqns ((4)–(6)), the
going from water to 2.25 M aqueous Bu NBr and increase
4
second-order rate constants k reported in Tables 1 and
2
considerably on transfer from pure water to 80% aqueous
DMSO. Although the rate for the unsubstituted deriva-
tive, i.e. methyl benzoate, changes in the opposite
direction, the polar effect of substituents was found to
increase by nearly the same magnitude in the considered
S1 and in Ref. 7 were used. The results of the statistical
data treatment with Eqns ((4)–(6)) for the alkaline
hydrolysis of substituted alkyl benzoates in aqueous
2
.25 M Bu NBr, 80% DMSO and water are given in
4
1
Tables 2 and 3. In Fig. 2, the relationships between the
media (see Table 3). It has been assumed that in alkaline
B
s
(
log k À 1.2E ) values and the s* constants in the three
hydrolysis of benzoates in aqueous 2.25 M Bu NBr
4
investigated media at 25°C are shown. Figure 3
illustrates the variation of the activation energies E with
the substituent constants s*.
solution, hydrophobic solvation (with the organic part
of quaternary ammonium ions) stabilizes ester molecules
in the ground state and the tetrahedral intermediate is less
solvated than in water. The decrease in the rate of the
alkaline hydrolysis of methyl benzoate in aqueous 2.25 M
The experimental log kobs values and the predicted log
kcalc values for the alkaline hydrolysis of substituted alkyl
Table 3. Results of the analyses of log k values for the alkaline hydrolysis of alkyl benzoates C H CO R in aqueous 2.25 M
6
5
2
a
Bu NBr, 80% (v/v) DMSO and water in terms of Eqns (4) and (5) at several temperatures
b
4
2
.25 M Bu NBr
80% DMSO
Water
4
t (°C)
Eqn
rI
ꢂ
rI
ꢂ
rI
ꢂ
1
2
3
4
4
5
5
5
5
0
5
0
5
5
5
5
5
5
4.74 Æ 0.19
4.64 Æ 0.19
4.37 Æ 0.17
–
1.19 Æ 0.22
1.24 Æ 0.24
1.26 Æ 0.21
–
5.04 Æ 0.23
1.05 Æ 0.19
1.32 Æ 0.18
–
3.68 Æ 0.17
3.64 Æ 0.15
–
0.99 Æ 0.21
0.98 Æ 0.19
–
4.94 Æ 0.21
–
4.57 Æ 0.20
1.05 Æ 0.26
3.50 Æ 0.22
0.95 Æ 0.27
4.26 Æ 0.20c 1.20 Æ 0.24
–
–
–
–
d
4.15 Æ 0.18 1.09 Æ 0.22
4.37 Æ 0.22
0.99 Æ 0.27
3.26 Æ 0.11
1.00 Æ 0.14
a
The r* values calculated with Eqn. (4) for 2.25 M Bu
4
NBr, 80% DMSO and water are as follows: at 15°C, 2.12 Æ 0.08, 2.26 Æ 0.11, 1.65 Æ 0.11; at 25°C,
2
.07 Æ 0.08, 2.21 Æ 0.11, 1.64 Æ 0.07; at 35°C, 1.95 Æ 0.09; at 40°C, 2.05 Æ 0.11, 1.57 Æ 0.09; at 45°C, 1.90 Æ 0.26; at 50°C, 1.85 Æ 0.07, 1.94 Æ 0.11,
1.47 Æ 0.08, respectively. The ꢂ values were in the range 0.91–1.29.
b
4
Correlation coefficient = 0.993–0.998. Number of data: n = 7 for 2.25 M Bu NBr, n = 6 for 80% DMSO and n = 8 for water.
log k = 0.639 for R = CH
log k = 1.923 for R = CH
c
2
CN–benzoate at 50°C calculated according to Eqn. (6) was included.
Cl–benzoate at 50°C calculated according to Eqn. (6) was included.
d
2
Copyright  2002 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2002; 15: 353–361

3
58
V. NUMMERT AND M. PIIRSALU
B
À1
)
Figure 2. Relationship between the log k À 1.2E values
Figure 3. Dependence of the activation energies E (kJ mol
s
and the substituent constants s* for the alkaline hydrolysis
of substituted alkyl benzoates C H CO R in aqueous 2.25 M
Bu NBr, 80% DMSO and pure water at 25°C. ꢂ = 1.2 was
used, as the average value of ꢂ for 2.25 M Bu NBr, 80%
on the substituent constants s* for the alkaline hydrolysis of
substituted alkyl benzoates C H CO R in aqueous 2.25 M
4
6
5
2
6
Bu NBr, 80% DMSO and pure water
5
2
4
4
DMSO and pure water
similar electrophilicitie’s of these two media, which are
6
9
Bu NBr and the increase in the activation energy E
considerably smaller than that of water. The r* value at
25°C is increased by 0.43 units of r* on going from pure
4
compared with water are in accord with the enhanced
stabilization of the ground state by the solvation.
The observed increase in the alkaline hydrolysis rates
when pure water is replaced by 80% DMSO has been
water to 2.25 M Bu NBr and by 0.57 units of r* from
4
pure water to 80% DMSO (Table 3). The change in the r*
value with the solvent for the alkaline hydrolysis of alkyl
benzoates proved to be nearly the same as that found for
the acidic dissociation of substituted acetic acids in these
67,68
attributed to the cumulative effect of two factors:
the
capacity of DMSO to solvate effectively the transition
state and the presence of a highly desolvated and hence
solvents. The r* values for the acidic dissociation of a-
À
52,70
active OH ion in aprotic solvents.
substituted acetic acids for water,
aqueous DMSO
80% (w/w)
70,71
54,72,73
The variation of the reaction rate with the substituent
in the alkaline hydrolysis of alkyl benzoates appeared to
be nicely described by a linear combination of polar and
steric terms [see Eqns (4) and (5), Table 3 and Fig. 2] in
and 7.75 M Bu NBr,
were
4
found to be 1.71, 2.21 and 2.25, respectively. The change
in the r* value with the solvent appeared to be
comparable to the variation of r with the solvent in the
I
the case of the three media considered: 2.25 M Bu NBr
case of ortho-substituents in the alkaline hydrolysis of
4
solution, 80% DMSO and pure water. Dual parameter
analysis of the log k values with s and Es constants gave
phenyl benzoates.
B
On going from pure water to aqueous 2.25 M Bu NBr
4
an excellent correlation with a correlation coefficient
R = 0.997.
and 80% DMSO, the changes in the r value for the
alkaline hydrolysis of alkyl benzoates was about 1.0 and
I
The magnitudes of r* at various temperatures
appeared to be in ranges 1.8–2.1, 1.9–2.26 and 1.47–
1.3 units of r (Table 3). A similar change in the (r°)
I
m,p
value on going from water to 2.25 M Bu NBr and 80%
4
1
.65 for 2.25 M Bu NBr, 80% DMSO and pure water,
DMSO has been found for the alkaline hydrolysis of
meta- and para-substituted phenyl benzoates and phenyl
4
1
respectively (Table 3). The corresponding magnitudes of
r at 25°C are 4.64, 4.94 and 3.64.
tosylates and also for the acidic dissociation of meta- and
I
74
The susceptibility to the steric effect of the substituents
ꢂ, appeared to be independent of the solvent and
temperature. The ꢂ values in the range 0.9–1.3 (Table
para-substituted benzoic acids. The change in the
(r ) value with the solvent in the acidic dissociation
I CH X
2
of acetic acids appeared to be nearly the same as the
change in the r° values for the acidic dissociation of
benzoic acids on going from pure water to the gas phase:
3
), approximately of the same magnitude as in 2.25 M
Bu NBr, 80% DMSO and pure water, support the
4
assumption made by Taft that the steric effect is the
same in the corresponding basic and acidic ester
hydrolyses (ꢂ = 1.0 for the acidic hydrolysis of substi-
ꢂꢀ †
ˆ 3:167ꢂÆ0:151† ‡ 0:957ꢂÆ0:031†ꢀꢃ ꢂ9†
I CH X
2
49
tuted alkyl benzoates at 49.5°C ).
Equation (9) includes the (r )
stituted acetic acids and the r° values for the meta- and
para-substituted benzoic acids in H O, aqueous 80%
values for a-sub-
I CH X
74
2
On going from pure water to 2.25 M Bu NBr and to
4
8
0% DMSO, the polar effect of aliphatic substituents
2
70
increases considerably. This is in accordance with the
DMSO, aqueous 7.75 M Bu NBr, DMSO and in the gas
4
Copyright  2002 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2002; 15: 353–361

ALKALINE HYDROLYSIS OF ALKYL BENZOATES
359
polar effect a similar conclusion was proposed in the case
of and meta- and para-substituted phenyls.
The magnitudes of (Es)calc for b-substituents, like the
Es values for a-substituents in the acyl component of
esters, turned out to be ꢄ1.5 times larger than the
B
corresponding Es values for the alkyl part of the esters
(Fig. 1). Probably, in the alkaline hydrolysis, b-
substituents in the acyl component of esters exert a
considerably less pronounced steric hindrance than in
their acidic hydrolysis.
Dependence on temperature
The activation energies E for the alkaline hydrolysis of
alkyl benzoates (Table 2, Fig. 3) appeared to increase in
the case of most alkyl benzoates on going from water to
Figure 4. Relationship between the (rI)CH2X values for alkyl
2
.25 M Bu NBr solution, and to decrease in the case of all
4
substituents CH X and the r° constants for meta-and para-
2
alkyl benzoates when the medium is changed from water
to 80% DMSO. The dependence of E on substituent
effects is completely caused by the polar effect of the
alkyl substituents (see Fig. 3). The slope of the relation-
ship between the activation energy E and s* constants is,
substituted phenyls in various media. Acidic dissociation of
carboxylic acids (points 1±7); alkaline hydrolysis of benzoates
(
(
points 8±10). Water (points 1 and 8), aqueous 80% DMSO
points 2 and 9), aqueous 7.75 M Bu NBr (point 3), pure
4
DMSO (points 4±6), aqueous 2.25 M Bu NBr (point 10) and
4
gas phase (point 7)
like the r* values, greater in 2.25 M Bu NBr and in 80%
4
DMSO than in pure water. In the basic hydrolysis of
esters, the electron-withdrawing substituents stabilize the
translation state by charge delocalization and, therefore,
decrease the activation energy E. In contrast, the
electron-repelling substituents localize the negative
charge of the activated complex and so increase E. The
substituent effect on the activation energy is greater in
7
5
phase [for the gas phase (ꢀ )
(
= 12.7, ꢀ° = 10.0 ]
I CH X
2
Fig. 4). From pure water to the gas phase the change in
the r value for a-substituted acetic acids was 8.7 units of
r and the corresponding change in the r° value for
I
I
benzoic acids was 9.1 units of r°.
In water at 25°C, the log k values for the alkaline
50
2
.25 M Bu NBr and 80% DMSO than in water owing to
hydrolysis of substituted ethyl acetates, RCO C H ,
4
2
2 5
the reduced electrophilic solvation of the reaction center
in 2.25 M Bu NBr and 80% DMSO. In aqueous 2.25 M
were found to be related to the log k values of the alkaline
hydrolysis of substituted alkyl benzoates C H CO R as
4
6
5
2
Bu NBr, the polar effect of electron-withdrawing sub-
follows:
4
stituents and the salting-in effect of the medium influence
the activation energy in two opposite directions. For
chloromethyl benzoate the salting-in effect of the solvent
is compensated for by the increased polar effect of the
substituent (see Fig. 3) and the activation energy for
log kRCOOEt ˆ 0:925ꢂÆ0:053†
1:53ꢂÆ0:059† log k
‡
ꢂ10†
PhCOOR
R ˆ 0:994; s ˆ 0:12; n ˆ 9
R = CH3, CH Cl, CH CN,
chloromethyl benzoate in 2.25 M Bu NBr equals that
4
À1
found for water (40.2 kJ mol ). In 80% DMSO solution
where
CH C H ,
2 6 5
2
2
a similar situation arises for electron-repelling substitu-
ents. In 80% DMSO, the increased electron-repelling
effect of substituents (compared with water) destabilizes
the activated complex as much as 80% DMSO destabi-
lizes the ground state in comparison with water.
CH CH OCH , CH Br, CH OCH , CH CH OH,
2
2
3
2
2
3
2
2
CH CH OCH CH . The log k values for the alkaline
2
2
2
3
hydrolysis of alkyl benzoates, R = CH Br, CH OCH ,
2
2
3
CH CH OH, CH CH OCH CH , were calculated using
2
2
2
2
2
3
the following relationship in water at 25°C:
The steric factor, independent of temperature, ap-
peared to influence only the log A value, the entropy of
log kCH3COOR ˆ 0:559ꢂÆ0:034†
B
the reaction. The magnitudes of (log A À 1.2 E ) were
s
‡
1:045ꢂÆ0:033† log k
ꢂ11†
PhCOOR
found to be virtually independent of the s* constants in
water and only very slightly dependent on s* in 2.25 M
A linear relationship between the log k values for ethyl
acetates and alkyl benzoates [Eqn. (10)] could be
observed owing to the 1.5-fold stronger influence of
both polar and steric effects of the acyl component of
Bu NBr and 80% DMSO. Therefore, the alkaline
hydrolysis of alkyl benzoates in water could be
considered as an isoentropic reaction series whereas in
4
2.25 M Bu NBr and 80% DMSO the isoentropic and
4
76
esters than of their alkyl component. Previously for the
isokinetic relationships are indistinguishable. It is
Copyright  2002 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2002; 15: 353–361

3
60
V. NUMMERT AND M. PIIRSALU
3
3
3
Table 4. Values of const and D  10 = (const  10 ) À(const  10 )
calculated according to Eqns (12) and (13)
1
1
S
1
H O
2
3
3
Compounds
Solvent S
const  10
D Â 10
1
Substituted alkyl benzoates
2.25 M Bu NBr
1.77
0.70
4
a
a
0
.79
0.32
8
0% DMSO
1.86
0.79
0.40
a
a
0
.87
Water
1.07
a
b
c
c
b
c
0
.47
b
m- and p-substituted phenyl benzoates
o-Substituted phenyl benzoates
2.25 M Bu NBr
1.34
0.93
0.33
0.89
0.95
1.01
0.60
4
c
8
0% DMSO
Water
b
2.25 M Bu NBr
0.29
0.35
4
c
8
0% DMSO
b
Water
0.6
a
r* was used.
Ref. 1.
Ref. 4.
b
c
surprising that the log A value or the entropic factor
substituents, XCH , appeared to be nearly 3.5 times
2
remains nearly unchanged in the case of alkyl substi-
larger than that of meta- and para-substituents when the
data at a single temperature were considered (Table 3).
Similarly, in the case of alkyl substituents the dependence
of the inductive term on temperature was observed to be
nearly 3.2 times stronger than the same dependence for
tuents on going from water to 2.25 M Bu NBr and 80%
4
DMSO. In contrast, for phenyl benzoates a considerable
change in the log A value (2.7 units) has been observed on
1
going from water to 2.25 M Bu NBr.
4
In Table 4 are listed the values characterizing the
meta- and para-substituents (see const values in Table
1
variation of the (r )
temperature (Fig. 5) and with the solvent. The values in
Table 4 were obtained from the following relationships:
, (r°) and (rI)ortho values with
4). The variation of the r* values with the solvent and
temperature was found to be identical with the corre-
I CH X
2
m,p
sponding change in r values in the case of ortho-
I
1
X
H
substituents. From the relationship
4.1868ꢁ = 2.3Rconst , it follows that in the alkaline
(E À E )/
ꢀ
ˆ const ‡ const ꢂ1=T†
ꢂ12†
ꢂ13†
1
1
hydrolysis of alkyl benzoates, the substituent-dependent
Á ˆ ꢂconst † À ꢂconst †
X
H
1
S
1 H O
2
activation energy (E À E ) varies with the solvent in a
similar manner to the variation in the case of meta- and
In water, the influence of the inductive effect of alkyl
para-substituted phenyl benzoates when the s scales for
I
alkyl benzoates and the s° scale for substituted phenyl
benzoates were used and in a similar manner to that of
ortho-substituted phenyl benzoates when the dependence
of the activation energy on the polar effect is described in
terms of s* constants.
Supplementary material
The second-order rate constants k2 for the alkaline
hydrolysis of substituted alkyl benzoates C H CO R in
6
5
2
aqueous 2.25 M Bu NBr, 80% (v/v) DMSO and water at
4
various temperatures (Table S1) are available as
supplementary material at the epoc website at http://
www.wiley.com/epoc.
Figure 5. Dependence of the (r )
values for the alkaline
I CH X
2
hydrolysis of alkyl benzoates (open symbols) and r°
constants for the alkaline hydrolysis of meta-and para-
substituted phenyl benzoates (closed symbols) on 1/T in
Acknowledgement
water (squares), 80% DMSO (triangles) and 2.25 M Bu NBr
(
We acknowledge financial support from the Estonian
Science Foundation (grant No. 3978).
4
diamonds)
Copyright  2002 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2002; 15: 353–361

ALKALINE HYDROLYSIS OF ALKYL BENZOATES
361
3
3
3
3
4
4
4
4
6. Charton M. J. Am. Chem. Soc. 1969; 91: 615–618.
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Copyright  2002 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2002; 15: 353–361