Influence of solvent on the ortho substituent effect in the alkaline hydrolysis of phenyl esters of substituted benzoic acids

Article abstract of DOI:10.1002/poc.1628

The second-order rate constants k (dm³mol^-1 s ^-1) for the alkaline hydrolysis of phenyl esters of meta-, para-and ortho-substituted benzoic acids in aqueous 5.3 M NaClO₄ and 1.0 M Bu₄NBr were measured by UV/Vis spectrophotometry at 25 °C. The variations in the ortho inductive, ortho resonance, as well as meta and para polar effects with solvent parameters were studied using data for the alkaline hydrolysis of phenyl esters of substituted benzoic acids in various media. The dependence of the ortho substituent effect on solvent can be precisely described with the following equation: δlog k_ortho = log k _ortho - log k_H = 0.059 + 2.19σ₁ +0.304σ^°_R + 2.79E^B_s -0.016 δEσ₁-0.085δEσ^°_R, where δE is the solvent electrophilicity, δE = E_S-E _H2O, characterizing the hydrogen-bond donating power of the solvent. The increase in the meta and para polar substituent effects with decrease in the solvent hydrogen-bond donor capacity (electrophilicity) was approximately to the same extent (-0.068δEσ^°_{m, p}) as the resonance term for the ortho substituents. The steric term of ortho substituents was independent of the solvent parameters. The variations in the ortho inductive, ortho resonance, as well as meta and para polar substituent effects with the solvent electrophilicity were to the same extent as in phenyl benzoates containing the substituents in the phenyl part. The substituent effects in the alkaline hydrolysis of ethyl benzoates appeared to vary with the solvent electrophilicity nearly to the same extent as in the alkaline hydrolysis of substituted phenyl esters of benzoic acids. Copyright 2009 John Wiley Sons, Ltd.

Full text of DOI:10.1002/poc.1628

Research Article
Received: 13 July 2009,
Revised: 14 August 2009,
Accepted: 17 August 2009,
Published online in Wiley InterScience: 13 November 2009
(www.interscience.wiley.com) DOI 10.1002/poc.1628
Influence of solvent on the ortho substituent
effect in the alkaline hydrolysis of phenyl
esters of substituted benzoic acids
Vilve Nummerta, Mare Piirsalua and Ilmar A. Koppela
*
The second-order rate constants k (dm3 molS1 sS1) for the alkaline hydrolysis of phenyl esters of meta-, para- and
ortho-substituted benzoic acids in aqueous 5.3 M NaClO4 and 1.0 M Bu4NBr were measured by UV/Vis spectropho-
tometry at 25 -C. The variations in the ortho inductive, ortho resonance, as well as meta and para polar effects with
solvent parameters were studied using data for the alkaline hydrolysis of phenyl esters of substituted benzoic acids in
various media. The dependence of the ortho substituent effect on solvent can be precisely described with the
following equation: Dlog kortho ¼ log kortho S log kH ¼ 0.059 R 2.19sI R 0.304sR- R 2.79EBs S 0.016DEsI S 0.085DEsR-,
where DE is the solvent electrophilicity, DE ¼ ES S EH2O, characterizing the hydrogen-bond donating power of the
solvent. The increase in the meta and para polar substituent effects with decrease in the solvent hydrogen-bond
donor capacity (electrophilicity) was approximately to the same extent (S0.068DEsm- ,p) as the resonance term for the
ortho substituents. The steric term of ortho substituents was independent of the solvent parameters. The variations in
the ortho inductive, ortho resonance, as well as meta and para polar substituent effects with the solvent electro-
philicity were to the same extent as in phenyl benzoates containing the substituents in the phenyl part. The
substituent effects in the alkaline hydrolysis of ethyl benzoates appeared to vary with the solvent electrophilicity
nearly to the same extent as in the alkaline hydrolysis of substituted phenyl esters of benzoic acids. Copyright ß 2009
John Wiley & Sons, Ltd.
Keywords: ortho effect; phenyl esters of benzoic acids; solvent electrophilicity; substituent effects
resonance substituent effects with the solvent electrophilicity,
DES, was nearly the same in both, the alkaline hydrolysis of
INTRODUCTION
In our previous works,[1–6] we investigated the ortho, meta, para
and alkyl substituent effects dependent on the solvent
parameters and temperature for the alkaline hydrolysis of
substituted phenyl benzoates, C6H5CO2C6H4-X, phenyl tosylates,
CH3C6H4SO2OC6H4-X and alkyl benzoates, C6H5CO2R, containing
substituents at the phenyl or alkyl moiety. The significance of
different solvent parameters (i.e. the solvent electrophilicity,
polarity and polarizability) for the ortho, meta, para and alkyl
substituent effects was studied on the basis of kinetic data for the
alkaline hydrolysis of substituted phenyl benzoates, phenyl
tosylates and alkyl benzoates, determined in water, aqueous 80%
DMSO, aqueous 2.25 M Bu4NBr, 1.0 M Bu4NBr, 0.5 M Bu4NBr, 5.3 M
NaClO4 and 4.8 M NaCl (References[1–6] and the references cited
therein). The ortho, para and meta polar as well as the alkyl polar
substituent effects were found to be mainly dependent on the
solvent electrophilicity, DES,[7–13] which characterizes the hydro-
gen-bond donating power of the solvent. The variation in the
ortho inductive term with the solvent electrophilicity, ES, was
found to be ca. 3–2.5-fold smaller than that for the para
substituents, while the ortho resonance term appeared to vary
with the solvent electrophilicity nearly similarly to that for the
para substituents.[3,5,6] The steric term in the alkaline hydrolysis of
ortho-substituted phenyl benzoates and alkyl benzoates was
approximately independent of temperature and solvent.[1,3–5]
The variation in the meta and para polar, ortho inductive and
substituted phenyl benzoates and the alkaline hydrolysis of
substituted phenyl tosylates, though the susceptibilities to
the polar effect of substituents in these two reactions
series in water differed 2-fold,[1,3,6] i.e. [(r8)m,p Tos
]
/
[(r8)m,p Benz
]
ꢀ [(rI)ortho Tos
]
/[(rI)ortho Benz
]
ꢀ 2:
ðD log korthoÞBenz ¼ log kX ꢁ log kH ¼ 0:014 þ 1:428sI þ 0:838sꢂR
þ 1:228EsB ꢁ 0:0212DEsI ꢁ 0:0685DEsꢂR
(1)
ðR ¼ 0:988; s ¼ 0:081; s0 ¼ 0:153; n=n0 ¼ 41=46Þ
ðD log korthoÞTos ¼ log kX ꢁ log kH ¼ 0:015 þ 2:77sI þ 1:82sRꢂ
ꢁ 0:0298DEsI ꢁ 0:0887DEsRꢂ
ðR ¼ 0:996; s ¼ 0:096; s0 ¼ 0:094; n=n0 ¼ 42=47Þ
(2)
The purpose of the present work was to study the significance
of the solvent electrophilicity[7–9] and polarity[7–9] parameters on
the ortho inductive, ortho resonance, as well as meta and para
polar substituent effects in the alkaline hydrolysis of substituted
*
Correspondence to: I. A. Koppel, Institute of Chemistry, University of Tartu,
Jakobi 2, 51014 Tartu, Estonia.
E-mail: ilmar.koppel@ut.ee
a
V. Nummert, M. Piirsalu, I. A. Koppel
Institute of Chemistry, University of Tartu, Jakobi 2, 51014 Tartu, Estonia
J. Phys. Org. Chem. 2010, 23 497–504
Copyright ß 2009 John Wiley & Sons, Ltd.

V. NUMMERT, M. PIIRSALU AND I. A. KOPPEL
phenyl benzoates containing the substituents at the benzoyl
moiety. It was interesting to check whether the variation in the
substituent polar effects with solvent parameters in the alkaline
hydrolysis of esters depends on the magnitude of the r value for
a reaction series considered. The susceptibility for the meta and
para polar substituent effects, r, for the alkaline hydrolysis of
phenyl esters of substituted benzoic acids in water was 1.6 times
higher than that for the meta and para polar effects for the same
reaction series containing substituents at the phenyl moiety.[14]
Previously, the ortho, meta and para substituent effects in the
alkaline hydrolysis of phenyl esters of substituted benzoic acids,
X-C6H4CO2C6H5, in pure water,[14] in aqueous 0.5 M Bu4NBr,
2.25 M Bu4NBr,[15] and 50% DMSO[16,17] at 25 8C were studied. In
order to extend the study of the solvent-dependent ortho, meta
and para substituent effects, in the present work the secon-
d-order rate constants k for the base-catalysed hydrolysis of
phenyl esters of meta-, para- and ortho-substituted benzoic acids,
X-C6H4CO2C6H5 (X ¼ H, 3-Cl, 3-NO2, 3-CH3, 4-NO2, 4-Cl, 4-F, 4-CH3,
4-OCH3, 2-NO2, 2-CN, 2-F, 2-Cl, 2-Br, 2-I, 2-CF3, 2-CH3, 2-OCH3,
2-NH2), were determined in aqueous 1.0 M Bu4NBr and aqueous
5.3 M NaClO4. For comparison, the dependence of the substituent
effects on the solvent electrophilicity and polarity parameters in
the alkaline hydrolysis of ethyl esters of substituted benzoic acids,
X-C6H4CO2C2H5, was studied as well. For this purpose, additional
rate constants for the alkaline hydrolysis of ethyl esters of
substituted benzoic acids (X ¼ H, 2-NO2, 2-CN, 2-Cl) in aqueous
0.5 M Bu4NBr and 5.3 M NaClO4 were measured at 25 8C. In the
previous works,[2,3,6] the ortho inductive, ortho resonance, as well
as meta and para polar substituent effects in the alkaline
hydrolysis of substituted phenyl benzoates and tosylates,
containing substituents in the phenyl moiety, were found to
become weaker in aqueous 5.3 M NaClO4 solution and to grow in
aqueous Bu4NBr solutions as compared to the same effects in
pure water. The polar substituent effects were found to grow with
decrease in the electrophilic properties of the solvent, and to
decrease in solvents in which the electrophilic solvating power is
stronger as compared to that of pure water.[2–6] In aqueous
organic salt solutions of Bu4NBr, the electrophilic solvating power
is considered to be reduced, whereas in inorganic salt solutions, it
is considered to be increased, as compared to the hydrogen-
bond donating power of pure water.
Bu4NBr at 25 8C are given in Table 1. For the alkaline hydrolysis of
ethyl esters of benzoic acids, X-C6H4CO2C2H5, the second-order
rate constants, k (dm3 molꢁ1 sꢁ1), measured in water, in aqueous
0.5 M Bu4NBr, and in aqueous 5.3 M NaClO4 at 25 8C are listed in
Table 2.
DATA PROCESSING AND RESULTS
To study the influence of the ortho inductive, ortho resonance, as
well as meta and para polar substituent effects in the alkaline
hydrolysis of phenyl esters of substituted benzoic acids,
X-C6H4CO2C6H5, in aqueous 5.3 M NaClO4 and 1.0 M Bu4NBr at
258C, the corresponding log k values determined in the present
work (Table 1) were treated according to the equations of
Hammett [Eqn (3)][23] and Charton [Eqn (4)]:[24]
log km;p ¼ log k0 þ rm;ps
log kortho ¼ log k0 þ ðrIÞorthosI þ ðrꢂRÞorthosꢂR þ dorthoEsB
(3)
(4)
To check the significance of the solvent electrophilicity and
polarity parameters for the meta and para as well as the ortho
substituent effects, the corresponding values of Dlog km,p ¼ log
kX ꢁ log kH and Dlog kortho ¼ log kX ꢁ log kH for the alkaline
hydrolysis of phenyl and ethyl esters of substituted benzoic acids,
X-C6H4CO2C6H5 and X-C6H4CO2C2H5, at 25 8C in various media
were correlated with Eqn (5):
D log km;p;ortho ¼ co þ c1ðm;pÞs þ c2ðorthoÞsI þ c3ðorthoÞsRꢂ
þ c4ðorthoÞEsB þ c5DE þ c6DY þ c7ðm;pÞDEs
þ c8ðorthoÞDEsI þ c9ðorthoÞDEsRꢂ þ c10ðm;pÞDYs
þ c11ðorthoÞDYsIþc12ðorthoÞDYsRꢂ
(5)
The steric term of ortho substituents was considered to be
independent of the solvent parameters[3,5] and, therefore,
the corresponding cross-terms with the solvent parameters
were omitted. In the data processing including ortho-substituted
derivatives, the log k value for the unsubstituted derivative
(X ¼ H) as standard was included besides the ortho-substituted
derivatives. The values of the Hammett polar substituent
constants, s, reviewed by Hansch, Leo and Taft,[25] the Taft
inductive sI[26] and resonance s8 constants[27] [sR8 ¼ (s8)para ꢁ sI]
R
were used in the data processing. As steric constants for the ortho
X
EXPERIMENTAL
H
substituents, EsB constants[14] (EsB ¼ log kHþ ꢁ log kHþ , where
X
H
The syntheses of phenyl and ethyl esters of substituted benzoic
acids, X-C6H4CO2C6H5 and X-C6H4CO2C2H5, the technique of
kinetic measurements, as well as the purification of reagents were
described earlier.[14–20] Ethyl benzoates were obtained commer-
cially (Aldrich). The preparation of ortho-substituted ethyl
benzoates and ethyl 4-nitrobenzoate had been described
earlier.[21,22] For kinetic measurements with phenyl esters of
substituted benzoic acids, aqueous 0.0187 M Bu4NOH in aqueous
1.0 M Bu4NBr and 0.0386 M NaOH in aqueous 5.3 M NaClO4 were
used. To estimate the rate constants for ethyl esters of substituted
benzoic acids in pure water and aqueous 0.5 M Bu4NBr, 0.0936 M
Bu4NOH was applied. In aqueous 5.3 M NaClO4, 0.10 M NaOH was
used. For kinetic measurements, the UV/Vis spectrophotometric
method was applied.[19]
kHþ and kHþ are the rate constants for the acidic hydrolysis
of ortho-substituted and unsubstitued phenyl benzoates in
water at 50 8C[28]) were used. For comparison, Charton’s steric
y-scale,[14,24,29] calculated on the basis of the van der Waals radii,
rv, was used as well. In the data processing according to Eqn (5),
the log k values for the alkaline hydrolysis of phenyl esters of
ortho-, meta- and para-substituted benzoic acids determined at
25 8C in aqueous 1 M Bu4NBr and 5.3 M NaClO4 (the present
work), as well as in pure water, aqueous 0.5 M Bu4NBr, 2.25 M
Bu4NBr and 50% (v/v) DMSO given in papers[14–17] were included.
When the log k values for ethyl esters of substituted benzoic acids
were correlated with Eqn (5), log k values from the literature[30,31]
and, also shown in Table 2 were used. In Eqn (5) the values of DE
and DY are the differences in electrophilicities and polarities on
going from pure water to the corresponding aqueous binary
solution, DE ¼ ES ꢁ EH2O, DY ¼ YS ꢁ YH2O, respectively. The stan-
dard medium, where DE and DY are equal to zero, is pure water
The second-order rate constants, k (dm3 molꢁ1 sꢁ1), for the
alkaline hydrolysis of phenyl esters of substituted benzoic acids,
X-C6H4CO2C6H5, in aqueous 5.3 M NaClO4 and aqueous 1.0 M
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Copyright ß 2009 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2010, 23 497–504

SOLVENT EFFECTS ON HYDROLYSIS OF PHENYL ESTERS
Table 1. The second-order rate constants k (dm3 molꢁ1 sꢁ1) for the alkaline hydrolysis of phenyl esters of substituted benzoic acids,
X-C6H4CO2C6H5, in aqueous 5.3 M NaClO4 and 1.0 M Bu4NBr at 25 8C
X
l (nm)a
k (dm3 molꢁ1 sꢁ1 b
)
nc
k (dm3 molꢁ1 sꢁ1 b
)
nc
l (nm)d
e
5.3 M NaClO4
1.0 M Bu4NBrf
0.0743 ꢃ 0.0083
0.360 ꢃ 0.038
3.14 ꢃ 0.09
H
250
240
250
250
250
250
250
250
250
250
250
240
240
250
255
245
250
250
260
0.0897 ꢃ 0.0086
0.231 ꢃ 0.011
1.25 ꢃ 0.05
3
3
3
5
3
6
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
292
240
2-NO2
2-CN
2-F
2-Cl
2-Br
3
3
285
240
0.384 ꢃ 0.031
0.236 ꢃ 0.039
0.231 ꢃ 0.010
0.110 ꢃ 0.010
0.0178 ꢃ 0.005
0.0503 ꢃ 0.0010
0.0171 ꢃ 0.0009
0.0115 ꢃ 0.0003
1.46 ꢃ 0.10
0.224 ꢃ 0.002
0.0976 ꢃ 0.0043
0.0677 ꢃ 0.0036
0.0496 ꢃ 0.0026
0.0256 ꢃ 0.0015
0.0220 ꢃ 0.0030
0.00936 ꢃ 0.00102
—
4
4
285
290
2-I
3
3
235
240
2-CF3
2-OCH3
2-CH3
2-NH2
4-NO2
3-NO2
3-Cl
4-Cl
4-F
3-CH3
4-CH3
4-OCH3
3
3
240
250
—
3
—
292
8.51 ꢃ 0.18
1.11 ꢃ 0.18
4.03 ꢃ 0.10
3
292
0.320 ꢃ 0.034
0.294 ꢃ 0.020
0.0912 ꢃ 0.0038
0.0684 ꢃ 0.0008
0.0433 ꢃ 0.0013
0.0219 ꢃ 0.0017
0.536 ꢃ 0.082
0.227 ꢃ 0.022
0.141 ꢃ 0.012
0.0424 ꢃ 0.0010
0.0215 ꢃ 0.0007
—
3
3
3
3
250, 290
235, 292
245, 290
290
3
—
290
—
a l is the wavelength used in the kinetic measurements in 5.3 M NaClO4.
b The mean values of the second-order rate constants and their standard deviations.
c n is the number of remaining k values after exclusion of significantly deviating rate constants in the calculation of mean values.
d l is the wavelength used in the kinetic measurements in 1.0 M Bu4NBr.
e Aqueous 0.0386 M NaOH was used.
f Aqueous 0.0187 M Bu4NOH was used.
and the standard substituent is X ¼ H. The electrophilicity values
of Koppel and Palm,[7–13] E, and the polarity parameters, Y,
calculated as a function of the dielectric constant e in the form
(e ꢁ 1)/(e þ 2) were used. The dependence of zero-order partial
correlation coefficients of the independent variables on each
other for the substituent constants, sI, s8R, EBs , y and solvent
parameters, DE, DY, were calculated. No colinearity was detected
between the independent variables (very low values of R2). The
Table 2. The second-order rate constants k (dm3 molꢁ1 sꢁ1) for the alkaline hydrolysis of ethyl esters of benzoic acids,
X-C6H4CO2C2H5, in water, in aqueous 0.5 M Bu4NBr and 5.3 M NaClO4 at 25 8C
X
l (nm)a
k (dm3 molꢁ1 sꢁ1 b
)
nc
k (dm3 molꢁ1 sꢁ1 b
)
nc
k (dm3 molꢁ1 sꢁ1 b
)
nc
Waterd
0.0319 ꢃ 0.0002
0.0950 ꢃ 0.0122
0.530 ꢃ 0.020
0.0417 ꢃ 0.0021
—
0.5 M Bu4NBre
0.0131 ꢃ 0.0004g
0.0605 ꢃ 0.086
0.374 ꢃ 0.015
0.0157 ꢃ 0.0004
—
5.3 M NaClO4
f
H
244
240, 274
250
250
240
3
3
3
3
—
3
3
3
3
—
0.00828 ꢃ 0.00083
0.0183 ꢃ 0.0007
0.0926 ꢃ 0.0042
0.0118 ꢃ 0.0001
0.113 ꢃ 0.007
4
3
4
3
3
2-NO2
2-CN
2-Cl
4-NO2
a l is the wavelength used in the kinetic measurements.
b The mean values of the second-order rate constants and their standard deviations.
c n is the number of the remaining k values after exclusion of the significantly deviating rate constants in the calculation of mean
values.
d Aqueous 0.037 M and 0.0936 M Bu4NOH was used.
e Aqueous 0.0936 M Bu4NOH was used.
f Aqueous 0.10 M NaOH was used.
g Reference [4].
J. Phys. Org. Chem. 2010, 23 497–504
Copyright ß 2009 John Wiley & Sons, Ltd.
www.interscience.wiley.com/journal/poc

V. NUMMERT, M. PIIRSALU AND I. A. KOPPEL
Table 3. Correlation with Eqns (3) and (4) for the alkaline hydrolysis of phenyl esters of substituted benzoic acids, X-C6H4CO2C6H5, in
aqueous 5.3 M NaClO4 and 1.0 M Bu4NBr 25 8Ca
Eqn
log k0
rm,p or (rI)ortho
(rR)ortho
dortho
Rb
sc
nd
5.3 M NaClO4
—
3
4
4
ꢁ1.091 ꢃ 0.035
ꢁ1.022 ꢃ 0.054
ꢁ1.101 ꢃ 0.120
1.63 ꢃ 0.09
2.11 ꢃ 0.09
2.19 ꢃ 0.21
—
2.58 ꢃ 0.13
0.989
0.994
0.972
0.093
0.067
0.150
9
11
0
0
ꢁ1.16 ꢃ 0.13
11e
1.0 M Bu4NBr
—
1.18 ꢃ 0.26
3
4
4
ꢁ1.166 ꢃ 0.036
ꢁ1.073 ꢃ 0.146
ꢁ1.095 ꢃ 0.068
2.54 ꢃ 0.09
2.35 ꢃ 0.23
2.36 ꢃ 0.11
—
2.84 ꢃ 0.31
0.996
0.977
0.995
0.084
0.154
0.073
8
10
10e
1.19 ꢃ 0.12
ꢁ1.32 ꢃ 0.07
a Data analysis was performed at confidence level t ¼ 0.99.
b R is the correlation coefficient.
c s is the the standard deviation.
d n is the number of points remaining after exclusion of the significantly deviating points.
e The Charton steric constants, y,[29] were used.
data processing was carried out using a multiple parameter linear
least-squares (LLSQ) procedure.[32] Significantly deviating points
were excluded using a Student criterion. The exclusion of the
significantly deviating points was performed on the different
confidence levels of the t-test. The results of the data treatment
with Eqns (3)–(5) are presented in Tables 3 and 4.
In aqueous 1.0 M Bu4NBr we found:
log km;p ¼ ðꢁ1:166 ꢃ 0:036Þ þ ð2:54 ꢃ 0:09Þs
ðR ¼ 0:996; s ¼ 0:084; n ¼ 8Þ
(8)
log kortho ¼ ðꢁ1:073 ꢃ 0:146Þ þ ð2:35 ꢃ 0:23ÞsI þð1:18 ꢃ 0:26ÞsRꢂ
þ ð2:84 ꢃ 0:31ÞEsB ðR ¼ 0:977; s ¼ 0:154; n ¼ 10Þ
ð9Þ
DISCUSSION
In aqueous 5.3 M NaClO4 the polar effect of meta and para
substituents was 0.1 units of r lower, but in aqueous 1.0 M Bu4NBr
0.8 units of r higher as compared to pure water (r ¼ 1.72 in water
at 25 8C[14]). The susceptibility to the ortho inductive and the
resonance effects diminished by ca. 0.1 and 0.3 units of r on
moving from pure water to aqueous 5.3 M NaClO4 and increased
by 0.2 and 0.9 units of r in the transition from water to aqueous
1.0 M Bu4NBr ((rI)ortho ¼ 2.13 and (rR)ortho ¼ 0.31 in water at
25 8C[14]). The observed variation in the susceptibility to inductive
effect from the ortho position with solvent was less than three
times smaller as compared to that for the meta and para polar
effects, as well as the ortho resonance effect. In the alkaline
hydrolysis of phenyl esters of substituted benzoic acids,
X-C6H4CO2C6H5, containing substituents in the acyl part, we
found that the variation in the substituent effects in a transition
from water to aqueous 1.0 M Bu4NBr and 5.3 M NaClO4 is
approximately equal to the variation in the substituent effects
with solvent in the alkaline hydrolysis of benzoates containing
substituents in the phenyl part. In the alkaline hydrolysis of
ortho-substituted phenyl benzoates with substituents in the
phenyl part, C6H5CO2C6H4-X, the susceptibility to the ortho
inductive effect and resonance increased by 0.1 and 0.7 units of r
on moving from water to aqueous 1.0 M Bu4NBr,[20] and
diminished by 0.1 and 0.3 units of r on moving from water to
aqueous 5.3 M NaClO4.[2] Recently,[15] in the alkaline hydrolysis of
substituted phenyl benzoates, on moving from water to aqueous
0.5 M and 2.25 Bu4NBr, the r values for the meta and para polar,
the ortho inductive and resonance effects in the acyl part were
found to vary with the solvent to the same extent as compared to
the variation in the r values for the corresponding substituent
effects in the phenyl part.
Influence of substituents in aqueous 5.3 M NaClO4 and 1.0 M
Bu4NBr
On moving from water to aqueous 5.3 M NaClO4, and aqueous
1.0 M Bu4NBr, for all phenyl esters of substituted benzoic acids,
X-C6H4CO2C6H5, a decrease in the rate constants of the alkaline
hydrolysis was observed. For the unsubstituted derivative (X ¼ H),
approximately the same retardation (0.60 units of log k at 25 8C)
was detected in the transition from water to aqueous 5.3 M
NaClO4 as in going from water to aqueous 1.0 M Bu4NBr. On
moving from water to aqueous 5.3 M NaClO4, the decrease in the
rates was higher for esters with electron-withdrawing substituents
than that for the esters with electron-donating substituents. In the
transition from water to aqueous 1.0 M Bu4NBr, the decrease was
higher for esters with electron-donating substituents.
The log k values for the alkaline hydrolysis of phenyl esters of
meta and para-substituted benzoic acids, X-C6H4CO2C6H5, in
aqueous 5.3 M NaClO4 and 1.0 M Bu4NBr show excellent
correlations with the Hammett equation (Eqn (3)). The log k
values for ortho-substituted esters correlate well with the Charton
equation (Eqn (4)) (Table 3). For the alkaline hydrolysis in aqueous
5.3 M NaClO4, we obtained:
log km;p ¼ ðꢁ1:091 ꢃ 0:035Þ þ ð1:63 ꢃ 0:09Þs
(6)
ðR ¼ 0:989; s ¼ 0:093; n ¼ 9Þ
log kortho ¼ ðꢁ1:022 ꢃ 0:054Þ þ ð2:11 ꢃ 0:09ÞsI þ ð2:58 ꢃ 0:13ÞEsB
ðR ¼ 0:994; s ¼ 0:067; n ¼ 11Þ
(7)
www.interscience.wiley.com/journal/poc
Copyright ß 2009 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2010, 23 497–504

SOLVENT EFFECTS ON HYDROLYSIS OF PHENYL ESTERS
J. Phys. Org. Chem. 2010, 23 497–504
Copyright ß 2009 John Wiley & Sons, Ltd.
www.interscience.wiley.com/journal/poc

V. NUMMERT, M. PIIRSALU AND I. A. KOPPEL
Influence of solvent on substituent effects
D log kortho ¼ log kX ꢁ log kH ¼ ð0:059 ꢃ 0:055Þ
þ ð2:19 ꢃ 0:07ÞsI þ ð0:304 ꢃ 0:089ÞsRꢂ
þ ð2:79 ꢃ 0:09ÞEsB ꢁ ð0:0164 ꢃ 0:0064ÞDEsI
ꢁ ð0:0854 ꢃ 0:0114ÞDEsRꢂ
The variation in the susceptibilities to meta and para polar, ortho
inductive, ortho resonance and ortho steric effects with the
solvent parameters in the alkaline hydrolysis of phenyl and ethyl
esters of substituted benzoic acids, X-C6H4CO2C6H5 and
X-C6H4CO2C2H5, was studied in two different ways: (i) by
correlating the Dlog km,p,ortho ¼ log kX ꢁ log kH, values in various
media at 25 8C with Eqn (5), only the solvent electrophilicity, DE,
or (ii) by including parameters such as the solvent electrophilicity,
DE and the solvent polarity, DY. The data treatment was
performed separately for meta- and para-substituted derivatives
and for ortho-substituted derivatives (Table 4). If the scale is not
shown in Table 4, the corresponding argument scale was
excluded during the data treatment as insignificant (c11(ortho)DYsI
and c12(ortho)DYsR8).
ðR ¼ 0:986; s ¼ 0:114; s0 ¼ 0:167; n=n0 ¼ 63=63Þ
(11)
Almost the same results as shown by Eqn (11) were obtained
when the Charton steric y scale,[29] based on van der Waals radii,
was used for the ortho substituents. Only on using the Charton
steric y scale, the susceptibility to the steric influence of ortho
substituents was twice lower and has the opposite sign
D log kortho ¼ log kX ꢁ log kH ¼ ð0:026 ꢃ 0:054Þ
þ ð2:23 ꢃ 0:08ÞsI þ ð0:431 ꢃ 0:089ÞsRꢂ
ꢁ ð1:31 ꢃ 0:04Þy ꢁ ð0:0180 ꢃ 0:0064ÞDEsI (12)
ꢁ ð0:0789 ꢃ 0:0114ÞDEsRꢂ
When the Dlog k values for the alkaline hydrolysis of phenyl
and ethyl esters of substituted benzoic acids were treated
according to Eqn (5), the calculated values of reaction constants
c
1(m,p), c2(ortho), c3(ortho) and c4(ortho) as the susceptibilities to the
ðR ¼ 0:986; s ¼ 0:114; s0 ¼ 0:166; n=n0 ¼ 62=63Þ
meta and para polar effect, ortho inductive, ortho resonance and
ortho steric effects, in the standard solution (pure water) were
approximately the same as found earlier for pure water with Eqns
(3) and (4), respectively (Table 4). Earlier[14,15] the corresponding
values of c1(m,p), c2(ortho), c3(ortho) and c4(ortho) for the alkaline
hydrolysis of phenyl and ethyl esters of substituted benzoic acids
for pure water at 25 8C were found to be 1.72, 2.13, 0.31 and 2.67,
as well as 1.44, 1.95, 0.33 and 2.73, respectively.
We found the contribution of the cross-terms containing the
solvent electrophilicity parameter to be significant for the meta
and para polar effects as well as for ortho inductive and ortho
resonance effects (i.e. c7(m,p)DEs ¼ 0, c8(ortho)DEsI ¼ 0, c9(ortho)
DEs8R ¼ 0) in both the alkaline hydrolysis of phenyl esters as well
as ethyl esters of substituted benzoic acids. The cross-term
containing the solvent polarity was different from zero for the
meta and para polar effects in the alkaline hydrolysis of both
phenyl and ethyl esters of substituted benzoic acids
(c10(m,p)DYs ¼ 0, Table 4). However, the contribution of the
solvent polarity to the polar effect of meta and para substituents
can be considered almost insignificant due to its extremely low
relative weight (Table 4). In the case of ortho derivatives the
cross-terms containing the solvent polarity are insignificant for
both the inductive and resonance effects. The corresponding
cross-terms (c11(ortho)DYsI and c12(ortho)DYs8R) were excluded as
insignificant during the data processing.
In the alkaline hydrolysis of phenyl esters of substituted
benzoic acids, the meta and para polar effect was found to vary
with the solvent electrophilicity, DE, nearly three times higher
(c7(m,p) ¼ ꢁ0.0683 ꢃ 0.0055) than that for the ortho inductive
effect (c8(ortho) ¼ ꢁ0.0164 ꢃ 0.0064). In the case of ethyl esters of
ortho-substituted benzoic acids, the susceptibility of the ortho
inductive effect to the solvent electrophilicity was approximately
twice lower (c8(ortho) ¼ ꢁ0.0412 ꢃ 0.0086) than that for the meta
and para polar effects (c7(m,p) ¼ –0.0953 ꢃ 0.0049). The ortho
resonance effect was found to vary with the solvent electro-
philicity to nearly the same extent as the resonance effect from
the para position, though in pure water the role of the resonance
from the ortho position was negligible. When moving from water
to media in which the electrophilic solvating power, i.e. the
hydrogen-bond donor power, is reduced (DE<0), the resonance
effect from the ortho position increases essentially. In the
transition from pure water to aqueous 1.0 M Bu4NBr solution
(DE ¼ ꢁ7.97) the (rR)ortho value increased by 0.87 units of rR
(Table 3).
The present study proved that in the alkaline hydrolysis of
substituted phenyl benzoates, the meta, para polar effect, as well
as ortho inductive and ortho resonance effect for substituents in
the acyl moiety (X-C6H4CO2C6H5) vary with the solvent electro-
philicity, DE, nearly to the same extent as was found earlier for the
substituents in the phenyl moiety (C6H5CO2C6H4-X). The
corresponding values of the reaction constants were:
c7(m,p) ¼ ꢁ0.0646, c8(ortho) ¼ ꢁ0.0212 and c9(ortho) ¼ ꢁ0.0685.[3]
Consequently, in the alkaline hydrolysis of phenyl and ethyl
esters of substituted benzoic acids, X-C6H4CO2C6H5 and
X-C6H4CO2C2H5, the solvent electrophilicity parameter, DE, can
be considered as the main factor responsible for the solvent-
dependent variation in the meta and para polar, as well as ortho
inductive and ortho resonance effects.
In the medium considered, the meta, para and ortho
substituent effects in the alkaline hydrolysis of phenyl esters
of substituted benzoic acids can be expressed by Eqns (10)–(12)
(Table 4)
Approximately the same values for the reaction constants c7(m,p)
,
c8(ortho) and c9(ortho), characterizing the variation in the meta, para
polar effect, ortho inductive and ortho resonance effects with the
solvent electrophilicity, were found in the alkaline hydrolysis of
substituted phenyl tosylates (c7(m,p) ¼ ꢁ0.0656, c8(ortho)
¼
ꢁ0.0298 and c9(ortho) ¼ ꢁ0.0887).[6] Consequently, in the alkaline
hydrolysis of the phenyl and ethyl esters of substituted benzoic
acids, phenyl tosylates and phenyl esters of benzoic acid, the
same variation in the substituent effects with the solvent
electrophilicity was found, though the ratio of the susceptibilities
to polar effect of substituents in pure water differed by more than
1.7 times. The influence of the steric factor of ortho substituents
on the alkaline hydrolysis of both phenyl and ethyl esters of
substituted benzoic acids was found to be independent of the
D log km;p ¼ log kX ꢁ log kH ¼ ðꢁ0:010 ꢃ 0:023Þ
þ ð1:77 ꢃ 0:04Þs ꢁ ð0:0683 ꢃ 0:0055ÞDEs
ðR ¼ 0:995; s ¼ 0:087; s0 ¼ 0:108; n=n0 ¼ 55=55Þ
(10)
www.interscience.wiley.com/journal/poc
Copyright ß 2009 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2010, 23 497–504

SOLVENT EFFECTS ON HYDROLYSIS OF PHENYL ESTERS
solvent parameters as assumed in Eqn (5) (constants c4(ortho) in
Table 4).
parameters in the alkaline hydrolysis of phenyl and ethyl esters of
meta-, para- and ortho-substituted benzoic acids was studied. The
solvent electrophilicity, i.e. the hydrogen-bond donor power of
the solvent, was found to be the main solvent property
responsible for the changes in the ortho, meta and para polar
substituent effects with the solvent. The variation in the ortho
inductive, ortho resonance as well as meta and para polar
substituent effects with the solvent electrophilicity appeared to
be to the same extent as found previously in phenyl benzoates,
C6H5CO2C6H4-X, containing substituents in the phenyl part and in
the alkaline hydrolysis of substituted phenyl tosylates,
CH3C6H4SO2OC6H4-X, though the r values for meta and para
derivatives in water for the reaction series considered differ ca.
twice. The steric factor for the ortho derivatives was independent
of the solvent used.
The combined influence of the substituent effects and the
solvent electrophilicity, i.e. the H-bond donation power on the
alkaline hydrolysis of esters can be explained by the contribution
of the corresponding structure factors and the solvent to stabilize
the transition-state and the ground state in the alkaline
hydrolysis.[33] The alkaline hydrolysis of aryl benzoates is accepted
to follow the stepwise BAc2 mechanism[34–37] by forming a
negatively charged tetrahedral intermediate R1COꢁ(OH)OR2:
X
O–
O
+ HO–
X
X
X
C
O
O
– HO–
OH
O
X
X
O–
Acknowledgements
+
HO
This work was supported by the Estonian Science Foundation
(Grant No. 6701).
The negative charge localized on oxygen in the transition state
is a good hydrogen-bond acceptor.[38] In all the used solvents, the
polar effects of electron-withdrawing substituents were found to
enhance the rates in the alkaline hydrolysis of esters. The polar
effects of the electron-withdrawing substituents in the alkaline
hydrolysis are considered to stabilize the transition state by the
delocalization of the negative charge in the transition state and
destabilize the resonance forms in the esters’ ground state.[34–39]
According to Buncel,[40] in the hydrolysis of esters the
transition-state structure varies with the solvent composition.
In media in which the hydrogen-bond donating ability is reduced
as compared to pure water (DE<0 for aqueous DMSO and Bu4NBr
solutions), the negative charge of the transition state is enhanced.
Due to reduced solvation of the transition state, the contribution
of the electron-withdrawing substituents to delocalize and
electron-donating substituents to localize the negative charge in
the transition state is increased as compared to pure water.
In aqueous 5.3 M NaClO4 solution in which the electrophilic
solvating power (H-bond donating capability) is increased in
comparison to pure water (DE>0), the transition state becomes
less negative due to complexation of Naþ to the negatively
charged transition state[41] and due to the increased H-bond
donating capability of water in inorganic salt solutions. In
inorganic salt solutions with stronger electrophilic power
compared to water (DE>0) the role of the solvent is increased
and the polar substituent effects are reduced during stabilization
of the transition state. In solutions with stronger electrophilic
solvating power (DE>0), electron-withdrawing substituents do
not favour the complexation of Naþ or water molecules by
H-bond to the negatively charged transition state.
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J. Phys. Org. Chem. 2010, 23 497–504
Copyright ß 2009 John Wiley & Sons, Ltd.
www.interscience.wiley.com/journal/poc

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J. Phys. Org. Chem. 2010, 23 497–504