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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,
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.
Bayissa, Leta Danno
