Specific influence of salts on the hydrolysis reaction rate of p-nitrophenyl anthranilate in binary acetonitrile-water solvents

Article abstract of DOI:10.1016/j.molliq.2014.09.004

The hydrolysis reaction rates of p-nitrophenyl anthranilate (p-NPA) have been surveyed in aqueous buffer solutions mixed with acetonitrile (MeCN) containing various salts at 50 ± 0.1 °C. Increase in pH of the buffer solution from 8.50 to 10.0 results in an increase of the hydrolysis rate constant [log (k/s^{- 1})] in all solutions mixed with 0-75% (v/v) MeCN. The log (k/s^{- 1}) values significantly decelerate as MeCN contents increase to about 50% (v/v). With further increment of MeCN contents, however, the reversal increases in log (k/s^{- 1}) are observed. All the added salts significantly influence the hydrolysis rate in solutions containing the borate buffer of pH = 9.18. Alkali metal perchlorates (LiClO₄and NaClO₄) cause the deceleration with increasing salt concentration. The presence of as low as 0.1 mol dm^{- 3}of NaN₃causes a three-fold acceleration, compared to the rate without the salt in no MeCN media. The rate acceleration by NaN₃becomes gradually weaker as the MeCN contents increase. Contrastingly, the rate acceleration by Et₄NBr is enhanced with increasing MeCN contents. In 50% (v/v) MeCN solution, the acceleration in log (k/s^{- 1}) caused by added salts is in the order of NaN₃> (n-Bu)₄NBr ~ Et₄NBr > Et₄NCl. The Arrhenius plots in the 50% (v/v) MeCN media without salts and with 0.20 mol dm^{- 3}of LiClO₄, NaN₃and Et₄NBr salts give the good linearity of high activation energy values in the temperature range of 35 to 60 ± 0.1°C, suggesting that the hydrolysis reactions are just of temperature dependence. The whole results have been discussed in terms of changes in the water structure and/or activities of H₂O and OH^-in the presence of both the added organic solvent and salts, and also in terms of the nucleophilicity of anions from the added salts in the modified media.

Full text of DOI:10.1016/j.molliq.2014.09.004

Journal of Molecular Liquids 199 (2014) 294–300
Contents lists available at ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
Specific influence of salts on the hydrolysis reaction rate of p-nitrophenyl
anthranilate in binary acetonitrile–water solvents
Leta Danno Bayissa, Yoshihito Ohmae, Masashi Hojo ⁎
Department of Chemistry, Faculty of Science, Kochi University, Akebono-cho, Kochi 780-8520, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 18 July 2014
Received in revised form 27 August 2014
Accepted 1 September 2014
Available online 11 September 2014
The hydrolysis reaction rates of p-nitrophenyl anthranilate (p-NPA) have been surveyed in aqueous buffer
solutions mixed with acetonitrile (MeCN) containing various salts at 50 ± 0.1 °C. Increase in pH of the buffer
−1
solution from 8.50 to 10.0 results in an increase of the hydrolysis rate constant [log (k/s )] in all solutions
−1
mixed with 0–75% (v/v) MeCN. The log (k/s ) values significantly decelerate as MeCN contents increase to
−
1
about 50% (v/v). With further increment of MeCN contents, however, the reversal increases in log (k/s ) are
observed. All the added salts significantly influence the hydrolysis rate in solutions containing the borate
Keywords:
buffer of pH = 9.18. Alkali metal perchlorates (LiClO
concentration. The presence of as low as 0.1 mol dm of NaN
the rate without the salt in no MeCN media. The rate acceleration by NaN
4
and NaClO
4
) cause the deceleration with increasing salt
causes a three-fold acceleration, compared to
becomes gradually weaker as the
Buffer solution
Bulk water structure
Nucleophilicity
Raman spectrum
Gutmann's donor number
Kinetics
−
3
3
3
MeCN contents increase. Contrastingly, the rate acceleration by Et NBr is enhanced with increasing MeCN
4
contents. In 50% (v/v) MeCN solution, the acceleration in log (k/s− ) caused by added salts is in the order of
1
NaN
3
N (n-Bu)
4
NBr ~ Et
4
NBr N Et
, NaN
4
NCl. The Arrhenius plots in the 50% (v/v) MeCN media without salts and
and Et NBr salts give the good linearity of high activation energy values in
−
3
with 0.20 mol dm of LiClO
4
3
4
the temperature range of 35 to 60 ± 0.1 °C, suggesting that the hydrolysis reactions are just of temperature
dependence. The whole results have been discussed in terms of changes in the water structure and/or activities
of H
of anions from the added salts in the “modified” media.
O and OH− in the presence of both the added organic solvent and salts, and also in terms of the nucleophilicity
2
©
2014 Elsevier B.V. All rights reserved.
1
. Introduction
the electrostatic interaction of the solvent molecules with the ionic sub-
stances, whereas specific solvent effects denote the chemical interaction
due to the presence of electron-donating and/or electron-accepting
centers in the solvent molecules.
There is an increased interest in understanding the effects of reaction
medium, of both pure solvents and binary mixtures on the rate
and equilibrium constants of chemical and biochemical reactions [5–9].
Solvent effects on the kinetic studies of the hydrolysis of various
esters have been reported [10–15]. The effects of acetonitrile–
water mixtures on the hydrolysis of 4-nitrophenyl chloroformate
and heptafluorobutyrate [16], bis(p-nitrophenyl) phosphate [17],
p-nitrophenyl acetate, benzoate [18,19] and anthranilate [20] and
oxazolinone [21,22] have been reported.
The p-nitrophenyl ester is known to be an active ester due to the
strong electronegativity of the nitro group. This active ester, however,
is well-tolerated by the aromatic amine group, which is a nucleophile,
in p-nitrophenyl anthranilate (p-NPA). p-Nitrophenyl anthranilate has
two different functional groups: an aryl amine that can react with free
reducing glycans by reductive amination and an active p-nitrophenyl
ester that can react with nucleophiles. This bifunctional linker has
been used for preparing fluorescent neoglycoproteins from naturally
occurring complex glycans [1]. It is worth noting that the linking
property of this active ester, p-nitrophenyl anthranilate, under different
conditions could be affected by its interaction with water.
Hydrolysis of an ester may occur through three pathways: base
catalyzed, pH independent, and acid catalyzed reaction pathways [2].
These pathways are independent of each other as demonstrated by
Hine [3]. El-Mallah et al. [4] have indicated that the rate of chemical
reactions in solution can be influenced by two types of the solvent
effects, i.e., nonspecific and specific. Nonspecific solvent effects denote
In addition to solvent effects, the presence of various salts also has
been examined regarding their significant impact on reactions of organ-
ic compounds. In their report on the hydrolysis of organic compounds,
Mabey and Mill [23] have indicated the effects of both solvent
composition and added salts. The added salts can lead either to rate
acceleration or deceleration, depending on the substrate, the specific
salts, and their concentration.
There have been many experimental and computational approaches
addressing the issue of water structure in the presence of added salts
⁎
Corresponding author. Tel.: +81 88 844 8306; fax: +81 88 844 8359.
E-mail address: mhojo@kochi-u.ac.jp (M. Hojo).
http://dx.doi.org/10.1016/j.molliq.2014.09.004
167-7322/© 2014 Elsevier B.V. All rights reserved.
0

L.D. Bayissa et al. / Journal of Molecular Liquids 199 (2014) 294–300
295
and organic solvent mixtures, since water is a common solvent and
many chemical reactions take place in water [24]. Disruption of the
tetrahedral water structure in the presence of added salt and organic
solvents has been reported [25,26]. We have proposed an innovative
idea [27] that the properties of bulk water originate from the intermo-
2
MeCN–H O solvents of 0–70% (v/v) MeCN containing the buffer solu-
tion of pH = 9.18 at 50 °C. In the absence of salts, the pH effects from
8.50 to 10.0 were examined in the mixed solvents, 0–75% (v/v) MeCN.
The primary mechanism of the hydrolysis reaction of p-nitrophenyl
anthranilate is depicted in Scheme 1. Both the molecule of H
2
O and
−
lecular hydrogen-bonded network among a huge number of H
molecules (e.g., n N ~10 ). Water may lose its properties as bulk
w
2
O
the hydroxide ion (OH ), which is a prominent nucleophilic anion,
can attack the carbonyl carbon of the substrate p-NPA, and liberate the
p-nitrophenoxide ion (which is the leaving group). We would not like
to discuss the changes in the reaction rate as just the “medium effects”
but would like to try differentiating each factor in the influences on
the reaction rate.
7
water to get that of a non-aqueous solvent, such as an alcohol (R–O–H)
or even an ether (R–O–R) if the highly “self-assembled structure” of
bulk water is disturbed in the following causes: (a) the residual water
−
3
−3
(
2
c(H O) ~10 mol dm ) in organic solvents; (b) aqueous solutions
or organic solvent–water mixtures containing salts at high concentra-
tions; (c) nanoscale water droplets in nano-tubes or reversed micellar
systems; (d) water on metal electrodes, ion-exchange resins, proteins,
organic solvents in the solvent extractions, and glass vessels; (e) water
at higher temperatures or under supercritical conditions. Such
2. Experimental
2.1. Materials and equipment
water can be “reduced” to authentic singular H
“dihydrogen ether,” (H)–O–(H)] [28]. Reichardt et al. [29] have
concisely interpreted “dihydrogen ether” that, at high salt concen-
2
O molecules
All chemicals utilized were commercially available and used as re-
ceived. p-Nitrophenyl anthranilate (4-nitrophenyl 2-aminobenzoate,
≥98.0%) and benzoate (N95%) were from TCI and Wako, respectively.
[
−
3
trations [c(salt) N 5 mol dm ], region C, according to the solvation
model of Frank and Wen [30], can be abolished and only regions A
and B survive, resulting in an aqueous solvent called “dihydrogen
ether.” Very recently, we [31] have dissolved precious metals, especial-
ly, pure gold in the mixed media between dilute nitric acid and seawa-
ter successfully, after discovering [32] the fact that dilute nitric acid
indeed possesses the strong oxidation ability when dilute nitric acid
contains concentrated salts.
Salts of LiClO
(≥98), (n-Bu)
co-solvent, acetonitrile (of the GR grade), were from Wako. Other salts
of NaClO (≥98.0%), Et NTsO (tosylate or p-toluenesulfonate, ≥97%),
and Pr
4
(≥98.0%), Et
4
NBr (≥98.0%), Et
4
NCl (≥98.0%), NaN
3
4
NBr (≥98.0%), and Na
2
B
4
O
7
·10H
2
O (≥99.6) and the
4
4
4
NBr (≥98.0%) were from Aldrich. Instead of the commercially
available (from Wako) buffer solution, we used the carefully prepared
(in laboratory) borate buffer solutions of pH ranging from 8.50 to 10.0
3
in order to avoid the influences of NaN contained as the stabilizer in
In binary solvents between water and many organic solvents,
we [28,33–37] have examined the effects of added salts on the solvolysis
the commercial borate buffer. Pure water purified by MilliQ System
was used in all the experiments. A Horiba F-51 digital pH meter was
used for measuring pH of the buffered solutions. Raman spectra of
(
hydrolysis) reaction rates of various organic compounds. Then, for the
first time, we have been able to explain successfully the concentrated
salt effects on solvolysis reactions of haloalkanes and related com-
pounds without resorting to different types of ion pairs. The exponential
2
D O containing MeCN were recorded with a Horiba-Jobin Yvon LabRam
Spectrograph HR-800 at room temperature (~23 °C). Excitation for the
Raman Spectroscopy was provided by a 514.5 nm argon laser.
N
increases in solvolysis rates of S 1 substrates in the presence of concen-
+
2+
trated alkali metal (M ) and alkaline earth metal (M ) perchlorates
2.2. Kinetic procedure
+
have been attributed to the favorable carbocation (R ) formation
through the association (direct “chemical” interaction) between metal
Kinetic measurements were performed using a Shimadzu UV–Vis
spectrophotometer (Model UV-2550) equipped with a thermostated
cell holder whose temperature was controlled within 25 ± 0.1 °C, in a
1.0 cm quartz cuvette. Reaction solutions were prepared by combining
the appropriate amounts of water and acetonitrile, the borate buffer of
+
2+
−
cations (M or M ) and the leaving group anion (X ) of a substrate
R–X) in the “modified” solvent. However, the solvolysis (hydrolysis)
reaction rate of a typical S 2 substrate is decelerated by the addition
(
N
of alkali metal or alkaline earth metal perchlorates. The reasons are as
follows: (1) the metal cations have no way to give more of the
active species for the typical S 2 substrate; (2) the activity of water,
N
which should attack the substrate, decreases in the presence of the
metal perchlorates.
In the present study, as part of our interest in studying medium
effects on reactions of toxic organic compounds, we report the effects
of an organic solvent, i.e., acetonitrile (MeCN), and various salts on
the hydrolysis reaction rate of p-nitrophenyl anthranilate in binary
−
3
pH = 9.18 (the final concentration of 5.0 mmol dm ) and salts and
then were left to stand for about 30 min in a Taitec constant tempera-
ture water bath at 50 ± 0.1 °C in order to reach the thermal equilibrium.
Prior to the beginning of the reaction, a stock solution of the substrate
−
3
−3
(p-NPA) (5.0 × 10 mol dm ) in MeCN was prepared since the sub-
strate is sparingly soluble in pure water. Reactions were initiated by
transferring 1.0 mL of the stock solution into a reaction vessel (50 mL)
−
4
−3
to reach the final substrate concentration of 1.0 × 10
mol dm
.
O-
NH2
NH2
O
O
NO2
+
O
OH
H O / OH-
2
-
H+
NO2
p-Nitrophenyl Anthranilate
NH2
p-Nitrophenoxide ion
O
O-
Scheme 1. Schematic representation of hydrolysis reaction mechanism of p-NPA.

296
L.D. Bayissa et al. / Journal of Molecular Liquids 199 (2014) 294–300
The samplings of 3–4 mL were carried out from reaction solutions at cer-
tain time intervals and the sampled solutions were immediately dipped
into an ice water bath. The reaction progress was then followed spectro-
photometrically by monitoring the liberation of p-nitrophenoxide ion at
60, 70, and 75% (v/v) MeCN, respectively. By the way, the hydrolysis
rate constants of p-nitrophenyl benzoate also give a similar minimum
at about 50% (v/v) MeCN with pH = 8.5 and 9.18 at 40 or 50 °C. The
appearance of these minima can be attributed mainly to an increase
in the OH activity due to the increase in dehydration of OH− by pro-
gressive addition of MeCN. Similar increases in hydrolysis rate constants
with increasing contents of organic solvents have been reported by
Gomez-Tagle et al. [17] for bis(p-nitrophenyl) phosphate.
−
λ
max = ca. 400 nm as a function of time. The apparent first order rate
constants were obtained from the slopes of plot of ln (A − A ) vs. t,
where A and A are the absorbance values at the reaction final and at
∞
t
∞
t
time t, respectively. All rate constants were obtained from linear plots
2
with correlation coefficients (R ) of normally 0.999 or more.
a
We may mention that the pK value of boric acid increases monoto-
nously as 9.24, 9.57, 10.40, 10.81, 11.25, and 11.84 for 0, 10, 30, 40, 50,
and 70% (w/w) MeCN in water at 25 °C [38]. The apparent pH values,
by the pH meter measurements after the reaction completion, were
found to increase just linearly from 9.23 to 11.23 with increasing
MeCN contents up to 75% (v/v).
3
. Results and discussion
3
.1. Effects of pH and MeCN contents
At the constant temperature of 50 ± 0.1 °C, the hydrolysis of
−
4
−3
p-nitrophenyl anthranilate (1.0 × 10 mol dm ) has been examined
in solutions of 0–75% (v/v) MeCN as co-solvent with water containing
the pH buffer ranging from 8.50 to 10.0. The evaluated first order rate
3.2. Influences of metal perchlorates on the hydrolysis rate
−
1
The changes in the hydrolysis reaction rate constant, log (k/s−1),
of p-nitrophenyl anthranilate were examined in the presence of
constants [log (k/s )] increase linearly as −4.55, −3.86, −3.56 and
−
3.18 with increasing pH of the buffer solutions as 8.50, 9.18, 9.50
and 10.0, respectively, in sole aqueous solutions. The increase in the
rate acceleration is even about 23 times for pH increment from 8.50 to
−
3
0.0–1.0 mol dm
NaClO ) in buffer solutions of pH = 9.18 mixed with 0–70% (v/v)
MeCN at 50 °C. Fig. 2 shows that the log (k/s ) value decreases linearly
with increasing LiClO concentration in solutions containing no MeCN.
However, the linear curve is altered to concave curves as MeCN contents
4
added alkali metal perchlorate salts (LiClO and
4
−
1
1
0.0. In all the solutions containing MeCN up to 75% (v/v), similar in-
creases in the rate constants are evidenced with increasing pH of the
4
−
1
buffer, as shown in Fig. 1. For instance, the log (k/s ) value in solution
containing 20% (v/v) MeCN increase from −4.76 to −3.44 with the pH
increase from 8.50 to 10.0.
−
1
increase. We have to just note that the log (k/s ) value without salt in
the 70% (v/v) MeCN solvent is larger than that in the 50% (v/v) MeCN
solvent (cf. Fig. 1 and Table 1). The hydrolysis rate of p-NPA decelerates
Increasing contents of the organic solvent (MeCN) cause significant
changes in the hydrolysis reaction rate of p-nitrophenyl anthranilate.
as the concentration of LiClO
the above exception).
4
and/or contents of MeCN increase (with
−
1
With the constant pH buffer of 9.18, the log (k/s ) value decrease
linearly as −3.87, −3.96, −4.11, −4.21, and −4.41 when the MeCN
contents increase as 0, 10, 20, 30 and 50% (v/v), respectively (Fig. 1).
The decrease in hydrolysis reaction rate with increasing MeCN contents
is mainly ascribed to an alteration of water properties (as discussed
in the final section of the present paper) and hence deterioration
of water activities in the presence of the organic solvent, MeCN.
El-Mallah et al. [4] have noted that MeCN affects the water structure
by intercomponent hydrogen bonding due to the basicity difference in
the solvent structure. Similarly, Al-Jallal et al. [19] have reported a
possible disruption of water tetrahedral structure with increasing
MeCN contents.
+
The rate deceleration caused by the Na salt is slightly smaller
+
than that of the Li salt (cf. Table 1). This is in agreement with
their respective crystal ionic sizes [39] and the ability to be hydrated
by water molecules. As the hydrolysis of p-NPA must be an S 2 type
N
reaction, the addition of these neutral metal perchlorate salts may
cause the hydrolysis rate to decelerate, which may be due to two
interdependent reasons: strong solvation of Li and Na+ ions with
+
2
H O molecules (the prominent molecules responsible to attack the
substrate to initiate the hydrolysis reaction) and/or distortion of
water structure (which leads to distraction of the water activity) in
the presence of these added salts and MeCN. In the presence of
salts in high concentrations, bulk water molecules are consumed
for hydrating the ions, therefore, the bulk water region in solution
should be reduced [27].
As the MeCN contents increase to larger than about 50% (v/v),
however, the completely reversal trends are observed in the reaction
rates, i.e., the hydrolysis rate of p-NPA increased again as shown
−
1
in Fig. 1. For the pH = 9.18 buffer, the log (k/s ) value varies
as −4.41, −4.42, −4.34, and −4.26 in solutions mixed with 50,
-
-
-
3
4
5
-
4
-
-
4.5
-
5
5.5
0
0.2
0.4
0.6
0.8
1
c (LiClO ) / mol dm-
3
0
20
40
60
4
MeCN contents/ % (v/v)
Fig. 2. Changes in log (k/s−1) of p-NPA hydrolysis with LiClO
concentrations in aqueous
4
borate buffer solutions of pH = 9.18 containing MeCN at 50 ± 0.1 °C: (○) 0; (●) 10;
(Δ) 20; (□) 50; (▼) 70% (v/v) MeCN.
Fig. 1. Changes in log (k/s−1) of p-NPA hydrolysis with MeCN contents in aqueous borate
buffer solutions of various pH values at 50 ± 0.1 °C: (○) 8.50; (●) 9.18; (Δ) 9.50; (▲) 10.0.

L.D. Bayissa et al. / Journal of Molecular Liquids 199 (2014) 294–300
297
Table 1
for bulk water is ca. 40 [42] and not the original value of 18, this value
has been evaluated for isolated water molecules dissolved in 1,2-dichlo-
roethane [41].
Log (k/s−1) values for the hydrolysis rate of 1.0 × 10−4 mol dm−3 of p-NPA in the
presence of added metal perchlorate salts in solutions containing the pH buffer of 9.18
and MeCN at 50 ± 0.1 °C.
4
Fig. 4 shows the influences of a non-metallic salt, Et NBr, on log
MeCN contents/% (v/v) c(salt)/mol dm−
0.10
3
Salts
−1
(
k/s ) in buffer solutions containing 0–70% (v/v) MeCN, cf. Table 2.
0
0.20
0.50
1.0
Generally speaking, the hydrolysis rate of p-NPA increases with increas-
ing Et NBr concentration. The influence of Et NBr is rather weak in
LiClO
4
0
0
0
0
0
0
0
0
0
−3.87
−3.93 −3.97 −4.11 −4.32
−4.04 −4.11 −4.27 −4.49
−4.21 −4.30 −4.49 −4.72
−4.63 −4.72 −4.93 −5.19
4
4
1
2
5
7
−3.96
−4.11
−4.41
−4.34
solutions containing smaller proportions of MeCN. However, its effect
prominently becomes stronger as the MeCN contents increase. The
−1
4
slope of Δlog (k/s ) vs. Δc (Et NBr) is 0.17, 0.30, 0.71, and 1.35 for
−4.94 −5.09 −5.39
–
solutions containing 0, 20, 50, and 70% (v/v) MeCN, respectively.
NaClO
4
(−3.87) −3.88 −3.95 −4.08 −4.24
(−4.11) −4.17 −4.24 −4.40 −4.60
(−4.41) −4.58 −4.69 −4.88 −5.11
(−4.34) −4.76 −4.91 −5.13 −5.41
−
Even though the Br ion is a good nucleophile, its ability to attack the
2
5
7
carbonyl carbon of the substrate may be hindered with the strong
hydration by water in solutions containing lower MeCN contents.
However, with increasing MeCN contents, particularly ≥50% (v/v)
−
MeCN, the Br ion may be more free from the solvation (hydration)
3
.3. Influences of NaN
3
and non-metallic salts on the hydrolysis rate
since the water activities should decrease remarkably in the presence
of both large proportions of MeCN and the Et
steep rate acceleration by Et NBr can be observed in solutions of higher
MeCN contents. The salt Et NCl also gives the rate acceleration but much
weaker than Et NBr. The weaker effect of Cl , compared to that of Br ,
in solutions of lower MeCN contents is reasonable because of the
4
NBr salt. Hence, the
On the hydrolysis rate of p-NPA, the influence of various kinds
4
−
3
and concentrations of salts from 0.0 to 1.0 mol dm was examined in
.0–70% (v/v) MeCN solutions containing the pH = 9.18 buffer. The
4
−
−
0
4
−
1
log (k/s ) value is greatly affected by the added salts of all kinds
used in this study: the salts of NaN , Et NCl, Et NBr, Et NTsO, Pr NBr,
and (n-Bu) NBr. Fig. 3 shows that NaN causes the remarkable acceler-
−
−
3
4
4
4
4
weaker nucleophilicity of Cl than that of Br in protic media [40].
We may mention that the phase separation kept us from evaluating
the rate constants in solutions containing both ≥50% (v/v) MeCN
4
3
−
3
ation even at lower concentrations of the salt (≤0.30 mol dm ),
especially in the solution without MeCN. Compared to the system
containing no added salt, the addition of 0.10 mol dm NaN
−
3
and ≥0.50 mol dm
phenomena of the binary MeCN–H
chloride salts have been reported [43].
Another salt, Et NTsO, gives somehow a strange effect on the
Et
4
NCl (cf. Table 2). The phase separation
−
3
3
causes
2
O mixtures by the addition of
an acceleration by three times. The hydrolysis rate is too fast to be
−
3
evaluated in the presence of ≥0.50 mol dm NaN
3
.
4
The remarkable acceleration in the presence of NaN
agreement with strong nucleophilicity of the N
the OH ion (the prominent nucleophile), the N
3
is in good
hydrolysis of p-NPA, especially, in solutions of higher MeCN contents.
−
3
−
3
ion. In addition to
ion, being a strong
The nucleophilicity of the tosylate ion should be very weak. In lower
−
−1
MeCN contents, as expected, the log (k/s ) is not significantly
nucleophile, can attack the carbonyl carbon of the p-NPA substrate
and liberate the p-nitrophenoxide ion significantly. Edwards and
Pearson [40] have reported the following overall nucleophilicity order
affected (cf. Table 2), and even slightly decreases. However, unexpected
increases in reaction rates are observed with increasing Et NTsO
4
concentration in ≥50% (v/v) MeCN media. Now that the nucleophilicity
−
−
−
−
−
for S
N
2 mechanisms in protic media as RS N ArS N I N CN NOH
of the tosylate ion is small, we may attribute the unexpected accelera-
−
−
−
−
−
−
N N
3
2
N Br N ArO N Cl N pyridine N AcO N H O. However, the rate
tion to an activity increase of OH brought by the distortion of water
acceleration observed is not uniform in each solution: the slope of
4
structure with the addition of the very bulky salt, Et NTsO.
−
1
−3
Δlog (k/s ) vs. Δc(NaN
.20, 2.30 and 1.80 in solutions containing 0, 20, 50, and 70% (v/v)
MeCN contents, respectively (cf., Figs. 3 and 7). The influence of NaN
becomes gradually weaker with increasing MeCN contents probably be-
3
) [0–0.1 or 0.2 mol dm ] decrease as 4.70,
3
3.4. Cooperative effects of salts and MeCN
3
Fig. 5 is a graphical illustration for the changes in the hydrolysis
rate of p-NPA hydrolysis with the added tetraethylammonium (Et NCl
in no MeCN media at
−
+ −
cause the activity of N
3
is reduced by the association of Na N
3
in the
4
media of which permittivity [4] and solvation ability [41,42] are certain-
ly decreased. We have to stress that true Gutmann's donor number, DN
and Et NBr) salts, (n-Bu) NBr, NaN , and LiClO
4 4 3 4
-
3.5
-
4
-
4.5
0
0.2
0.4
0.6
0.8
1
c (Et NBr)/ mol dm-
3
4
Fig. 3. Changes in log (k/s−1) of p-NPA hydrolysis with NaN
borate buffer solutions of pH = 9.18 containing MeCN at 50 ± 0.1 °C: (○) 0; (Δ) 20;
▲) 30; (□) 50; (▼) 70% (v/v) MeCN.
concentrations in aqueous
Fig. 4. Changes in log (k/s− ) of p-NPA hydrolysis with Et
1
NBr concentrations in pH =
3
4
9.18 aqueous buffer solutions containing MeCN at 50 ± 0.1 °C: (○) 0; (●) 10; (Δ) 20;
(□) 50; (▼) 70% (v/v) MeCN.
(

298
L.D. Bayissa et al. / Journal of Molecular Liquids 199 (2014) 294–300
Table 2
breakers” and their effects on the possible water disruption should
Log (k/s−1) values for the hydrolysis rate of 1.0 × 10−4 mol dm−3 of p-NPA in the
+
+
+
increase in the order of Et
In 20% (v/v) MeCN media, the rate acceleration by NaN
to be weaker than that in no MeCN. The Et NBr salt shows a stronger
effect than Et NCl. Generally, the rate acceleration decreased in the
order of NaN N Et NBr N Et NCl in solutions of 20% (v/v) MeCN. The
salt of (n-Bu) NBr remains still in the salt group of rate deceleration,
not so distinct as LiClO
4 4 4
N b Pr N b (n-Bu) N .
presence of various salts in solutions containing the pH buffer of 9.18 and MeCN at
3
is found
50 ± 0.1 °C.
4
c(salt)/mol dm−
0.10
3
Salts
MeCN
contents/%
4
0
0.20
0.30
0.50
1.0
3
4
4
(v/v)
4
NaN
3
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
(−3.87) −3.40 −3.15 −2.99
–
–
–
–
–
–
4
4
and NaClO .
2
3
5
7
(−4.11) −3.79 −3.55 −3.45 −3.30
(−4.28) −3.97 −3.75 −3.66 −3.47
(−4.41) −4.18 −3.97 −3.86 −3.73
Fig. 6 shows the changes of log (k/s−1) in solutions of 50% (v/v)
MeCN in the presence of various salts. The most remarkable event in
the 50% (v/v) MeCN media is the fact that the hydrolysis rate constant
(−4.34) −4.16 −3.95
(−3.87) −3.84 −3.83
(−4.11) −4.09 −4.08
(−4.41) −4.39 −4.33
(−4.34) −4.32 −4.28
(−3.87) −3.84 −3.81
(−3.96) −3.89 −3.87
(−4.11) −4.02 −3.99
(−4.41) −4.33 −4.26
(−4.34) −4.16 −4.03
(−3.87) −3.92 −3.95
(−4.11) −4.15 −4.14
(−4.28) −4.29 −4.31
(−4.41) −4.39 −4.33
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
Et
Et
4
NCl
NBr
−3.75 −3.63
−4.01 −3.95
is accelerated by (n-Bu)
ation effects in zero and 20% (v/v) MeCN media. The rate acceleration
caused by the increasing concentration of Pr NBr or (n-Bu) NBr is
significantly enhanced with increasing MeCN contents. For instance,
4
NBr. Whereas, this bulky salt causes the deceler-
2
5
7
–
–
–
–
4
4
4
−3.78 −3.73
−3.83 −3.74
−3.94 −3.83
−4.06 −3.70
−
1
−3
1
2
5
7
the log (k/s ) value in the presence of 1.0 mol dm
4
of (n-Bu) NBr
in 50% (v/v) MeCN media is as high as −3.41, which is much larger
−
3
than −3.70 for 1.0 mol dm Et
as Pr NBr and (n-Bu) NBr, in the solution containing the larger volume
of an organic solvent can destroy the bulk water structure largely, cf. ref.
34]. Hence, the hydrolysis reactions in such solutions should be quite
different from the corresponding reactions in dilute aqueous media.
4
NBr. The presence of bulky salts, such
−3.65
–
Et
4
NTsO
−4.01 −4.04
−4.20 −4.15
−4.29 −4.13
−4.16 −3.57
4
4
2
3
5
7
[
(−4.34) −4.20 −4.04 −3.91
–
–
−
3
For the solutions containing salts up to 0.50 mol dm , the rate accel-
erations reduce in the order of NaN N (n-Bu) NBr ~ Et NBr N Et NCl
in 50% (v/v) MeCN media. At 1.0 mol dm , however, the influence of
n-Bu) NBr is superior to that of Et NBr, which is ascribed to mainly the
destruction of water properties to a greater extent and/or the increase
Pr
4
NBr
(−3.87) −3.82 −3.80
(−4.11) −4.00 −4.00
(−4.41) −4.23 −4.16
–
–
–
−3.80 −3.81
−3.99 −3.88
−3.95 −3.44
2
5
7
3
4
4
4
−
3
(−4.34) −4.10 −3.93 −3.82
–
–
(
4
4
(
n-Bu)
4
NBr
(−3.87) −3.80 −3.85
(−4.11) −4.10 −4.13
(−4.28) −4.20 −4.20
(−4.41) −4.33 −4.27
–
–
–
–
−3.98 −4.07
−4.19 −4.11
−4.19 −3.92
−4.06 −3.41
2
3
5
7
−
−
in nucleophilicity of the Br ion (and also the OH ion).
Fig. 7 shows the changes in the slopes of Δlog (k/s−1) / Δc (salt) vs.
MeCN contents [% (v/v)] in the presence of various salts. Here we can
(−4.34) −4.08 −3.91 −3.84
–
–
3
clearly notice that the acceleration by NaN (of lower concentrations,
−
3
5
Et
0 °C. Here, we can clearly notice that all the NaN
3
, Et
4
NCl, and
≤0.20 mol dm ) is extremely larger than any other salts used, even
4
NBr salts accelerate the hydrolysis rate with their respective
gives the influence signif-
though, its effect become weaker as the MeCN contents increase. On
concentrations. As mentioned above, NaN
icantly stronger than Et
to 1.0 mol dm
log (k/s ) value decreases from −3.87 to −4.07. The addition of
Pr
NBr causes the log (k/s−1) value to increase slightly from −3.87
to −3.81 for 0.0 to 1.0 mol dm
these tetraalkylammonium cation sizes [39] and their distortion effects
on water properties/activities. The (n-Bu)
size than Pr
3
the other hand, the effects caused by Et
enhanced with increasing MeCN contents.
We may decide that the small acceleration effect by Et
4
significantly altered, i.e., a rather uniform effect, although its effect at
higher concentrations (≥0.50 mol dm ) in ≥50% (v/v) MeCN media
cannot be evaluated because of the phase separation (vide supra). In
4 4
NBr and (n-Bu) NBr salts are
4
NBr or Et
4
NCl. In the presence of 0.0
−
3
−3
(n-Bu)
4
NBr (except for 0.10 mol dm ), the
NCl is not
−
1
−3
4
−
3
Pr
4
NBr. This is in agreement with
70% (v/v) MeCN media, as the general rule, the rate acceleration caused
+
4
N
cation is larger in
by added salts is in the order of NaN
and the deceleration by LiClO (or NaClO
nucleophilicity, therefore, LiClO
hydrolysis of this S
3
~ (n-Bu)
4
NBr N Et
4
NBr N Et
ion has no
4
NCl
+
−
4
N , and hence is expected to cause more remarkable
4
4
). The ClO
4
distraction in water property. We [32] have discussed that cations
such as Et
4
4
(or NaClO ) cannot assist the
+
+
+
4
N , Pr
4
N , and (n-Bu)
4
N
are, potentially, water “structure
N
2 substrate. The rate deceleration becomes
-
-
-
3
4
5
-
3
-
-
3.5
-
4
4.5
0
0.2
0.4
0.6
0.8
1
0
0.2
0.4
0.6
0.8
1
c (salts)/ mol dm-3
c (salts)/ mol dm-3
Fig. 5. Changes in log (k/s−1) of p-NPA hydrolysis with salt concentrations in pH = 9.18
aqueous buffer solutions of containing no MeCN at 50 ± 0.1 °C: (Δ) NaN ; (○) Et NCl;
●) Et NBr; (◊) (n-Bu) NBr; (▼) LiClO ; (∇) NaClO
Fig. 6. Changes in log (k/s−1) of p-NPA hydrolysis with salt concentrations in pH = 9.18
aqueous buffer solutions containing 50% (v/v) MeCN at 50 ± 0.1 °C: (Δ) NaN ; (○)
Et NCl; (●) Et NBr; (◊) (n-Bu) NBr; (▼) LiClO ; (∇) NaClO
3
4
3
(
4
4
4
4
.
4
4
4
4
4
.

L.D. Bayissa et al. / Journal of Molecular Liquids 199 (2014) 294–300
299
-
-
-
-
3
4
5
6
3
3.1
3.2
3.3
T /10 K-1
-1
-3
Fig. 7. Changes in slopes of Δlog (k/s−1) / Δc (salt) vs. MeCN contents for hydrolysis of
p-NPA in aqueous buffer solutions of pH = 9.18 at 50 ± 0.1 °C containing salts: (Δ)
Fig. 9. Arrhenius plots of log (k/s−1) in 50% (v/v) MeCN–H
O buffered solution
2
−
3
(
with pH = 9.18) in the presence of various salts: (○) no salt; (▼) 0.20 mol dm
3 4 4 4 4
NaN ; (○) Et NCl; (●) Et NBr; (◊) (n-Bu) NBr; (▼) LiClO .
; (Δ) 0.20 mol dm− NaN
3
; (◊) 0.20 mol dm
−3
LiClO
4
3
4
Et NBr.
remarkably as both its salt concentration and the MeCN contents
increase because more water molecules are involved in solvating
Li (or Na ) ions and the bulk water properties should be reduced.
good linearity from 35 to 60 °C, resulting in the activation energy (E
a
)
−
1
of 107.0, 98.8, 84.2, and 104.2 kJ mol
for no salt, LiClO
4 3
, NaN ,
+
+
−
3
and Et
4
NBr of 0.2 mol dm , respectively. These large values of the
activation energy in the systems with added salts suggest that the
hydrolysis reaction of the substrate is just of temperature dependence.
3
.5. Temperature dependence
Fig. 8 shows the Arrhenius plots of log (k/s−1) in binary MeCN-H
2
O
mixed solvents (buffered with pH = 9.18) of 0 - 30% (v/v) MeCN. The
2
3.6. Changes in Raman spectra of water (D O) with the addition of MeCN
−
1
−1
relationship between log (k/s ) and T is linear over the temperature
range of 35 – 60 °C (or 65 °C) for 10, 20, and 30% (v/v) MeCN, whereas
the relation gives convex curves at lower temperatures, i.e., b 50
and b 40 °C for 0.0 and 5.0% (v/v) MeCN, respectively. The activation
Raman spectroscopy has been often employed to investigate the
dynamical structure of water for a long time [44–46]. It is a technique
preferably used for studies of water structure because the proportions
of intensities of main OH stretching modes (around 3200 and
a
energy (E ) values are evaluated from the straight part to be 91.5,
−
1
−1
9
3
3.8, 94.6, 99.6, and 102.1 kJ mol
for the 0.0, 5.0, 10, 20, and
3400 cm ) are thought to reflect well the network of hydrogen
0% (v/v) MeCN media. We can notice that the log (k/s−1) values
bonds as well as its disturbance by the presence of other substances
like solutes and organic solvents [34]. Distortion of water structure in
the presence of added solvents and salts, or with increasing tempera-
ture, has been extensively investigated and reported [33,34,37,47].
of p-NPA give a maximum at 5.0 or 10% (v/v) MeCN for 40 or 35 °C,
respectively, while the rate constant decreases just monotonously
with increasing MeCN contents (in 0.0, 5.0, 10, 20, and 30% media)
at the higher temperatures, e.g. 60 °C. In an additional study, we
have examined the hydrolysis rate of p-nitrophenyl benzoate in the
2
Fig. 10 shows the change in Raman spectra of D O with increasing
MeCN contents [0–90% (v/v)] at room temperature. Pure D
OD stretching band with two peaks at around 2380 and 2500 cm
2
O gives an
−
1
−1
binary MeCN-H
2
O media. The log (k/s ) values of p-nitrophenyl
,
benzoate give a maximum at about 10% (v/v) MeCN (buffered with
pH = 8.50 and 9.18) at 40 or 50 °C.
which are assigned to strongly hydrogen bonded D O and weakly
2
hydrogen-bonded D
2
O, respectively [48]. Upon addition of MeCN to
Fig. 9 shows the Arrhenius plots of log (k/s−1) in 50% (v/v)
D
O, the peak intensity at around 2500–2600 cm
−1
increases while
2
−
1
MeCN–H
2
O buffered solution (with pH = 9.18) in the presence of
O solution systems give the
the intensity at around 2380 cm
decreases. The increase in MeCN
various salts. All the 50% (v/v) MeCN–H
2
-
-
-
3
4
5
3
3.1
3.2
3.3
T /10 K-1
-1
-3
Fig. 8. Arrhenius plots of log (k/s−1) in binary MeCN–H
pH = 9.18) : (○) 0.0; (●) 5.0; (Δ) 10; (▴) 20; (◊) 30% (v/v) MeCN.
O mixed solvents (buffered with
2
2
Fig. 10. Changes in Raman spectra of D O with increasing MeCN contents: (1) 0; (2) 50;
(3) 75; (4) 90% (v/v) MeCN.

300
L.D. Bayissa et al. / Journal of Molecular Liquids 199 (2014) 294–300
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.23 with increasing contents of MeCN as 0, 50, 75, and 90% (v/v),
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2
) at around 2400 and 2500–2600 cm , respectively. The ratios
[
−
1
1 2
/I ) around 2400 and 2500–2600 cm
[
[
1
2
[
0
2
[
[
−
1
and 2500 cm
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2
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1
(
υ
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(
[
[
[
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4
. Conclusion
The hydrolysis reaction rate of p-nitrophenyl anthranilate (p-NPA)
is greatly influenced by pH, added salts of various kinds and MeCN
contents in binary MeCN–H O mixed solvents. Increase in MeCN con-
tents up to 50% (v/v) retards the hydrolysis reaction of p-NPA, whereas,
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[
[
[
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−
1
further increments of MeCN cause the reversal increases in log (k/s ).
Metal perchlorate salts, such as LiClO and NaClO , decelerate the
[
[
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4
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−
only the activities of H
2
O and OH but also the nucleophilicities (toward
[
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−
−
the carbonyl carbon atom in p-NPA) of the anions (N
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the salts.
[
[
[
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The present work is partially supported by a Grant-in-Aid for
the commission project of “Research Program on Climate Change
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