Effects of mixed CH3CN - H2O solvents on rate of intramolecular general base-catalyzed cleavage of ionized phenyl salicylate in the presence of 3-aminopropan-1 -ol, 1,2-diaminoethane, and glycine

Article abstract of DOI:10.1002/(SICI)1097-4601(1998)30:4<301::AID-KIN9>3.0.CO;2-W

The effects of mixed CH₃CN - H₂O solvents on rates of aminolysis of ionized phenyl salicylate, PS^-, reveal a nonlinear decrease in the nucleophilic second-order rate constants, k_n^ms. (for aminolysis) with increase in the content of CH₃CN until it becomes ≈ 50%. v/v. The values of k_n^ms remain almost unchanged with change in the CH₃CN content within 50 to 70 or 80%, v/v. The effects of mixed CH₃CN - H₂O solvents on pK_a of leaving group, phenol, and protonated amine nucleophile have been concluded to be the major source for the ob- served mixed solvent effects on k_n^ms.

Full text of DOI:10.1002/(SICI)1097-4601(1998)30:4<301::AID-KIN9>3.0.CO;2-W

Effects of Mixed CH3CN9
H2O Solvents on Rate of
Intramolecular General
Base-Catalyzed Cleavage
of Ionized Phenyl
Salicylate in the Presence
of 3-Aminopropan-1-ol,
1,2-Diaminoethane, and
Glycine
M. NIYAZ KHAN
Department of Chemistry, Universiti Malaya, 50603 Kuala Lumpur, Malaysia
Received 6 February 1997; accepted 5 May 1997
ABSTRACT: The effects of mixed CH3CN9H2O solvents on rates of aminolysis of ionized
phenyl salicylate, PSϪ, reveal a nonlinear decrease in the nucleophilic second-order rate con-
stants, knms, (for aminolysis) with increase in the content of CH3CN until it becomes Ϸ 50%,
ms
v/v. The values of kn remain almost unchanged with change in the CH3CN content within 50
to 70 or 80%, v/v. The effects of mixed CH3CN9H2O solvents on pKa of leaving group, phenol,
and protonated amine nucleophile have been concluded to be the major source for the ob-
served mixed solvent effects on knms. ᭧ 1998 John Wiley & Sons, Inc. Int J Chem Kinet: 30: 301–307,
1998.
INTRODUCTION
plexity. A huge amount of work has been carried out
on the effects of solvents on neutral, acid-, and base-
catalyzed solvolysis of various types of substrates. But
the effects of the mixed aqueous-organic solvents on
rates of aminolysis of esters and other related reactions
appear to be rare. Only recently, the effects of
CH3CN9H2O solvents on rates of reactions of p-ni-
trophenyl acetate with morpholine, piperazine, and pi-
peridine have been reported [1]. Such studies, how-
The mechanism of the effects of solvents on reaction
rates seems to be extremely complex and there appears
to be no perfect single theory to deal with such com-
Correspondence to: M. N. Khan
Contract grant Sponsor: University of Malaya
Contract grant number: F vote (F 408/96)
᭧ 1998 John Wiley & Sons, Inc.
CCC 0538-8066/98/040301-07

302
KHAN
ever, may be considered of some importance because
many enzyme-catalyzed and micellar-mediatedrelated
reactions are believed to occur in a micro reaction en-
vironment where water concentration, [H2O], is sig-
nificantly lower compared to [H2O] in pure water sol-
vent [2]. The most likely reason for the lack of kinetic
studies on the effects of mixed aqueous-organic sol-
vents on rates of aminolysis of esters and related re-
actions may be attributed to the fact that these reac-
tions require the use of amine buffers and the use of
buffers in mixed aqueous-organic solvents creates sev-
eral complications for the kinetic study. We chose the
present study on aminolysis of ionized phenyl salicyl-
ate, PSϪ, for the reason that this reaction system does
not require the use of amine buffers because the rate
of hydrolysis of PSϪ is independent of [ϪOH] within
its range of 0.005 to 0.060 mol dmϪ3 [3].
We have studied the effects of mixed aqueous-or-
ganic solvents on intramolecular general base, IGB,-
catalyzed hydrolysis of ionized phenyl and methyl sa-
licylates [3]. Recently, the effects of temperature, salt
and mixed aqueous-acetonitrile solvents on IGB-cat-
alyzed methanolysis of PSϪ have been reported [4].
The present study was initiated with an aim to discover
the effects of mixed aqueous-acetonitrile solvents on
rates of reactions of PSϪ with 1,2-diaminoethane, 3-
aminopropan-1-ol, and glycine. The results and their
probable explanation(s) are described in this article.
The details of the kinetic procedure and the data anal-
ysis have been described elsewhere [5].
Determination of pKa
The values of concentration ionization constants, Ka,
of conjugate acids of amines at 35ЊC and in mixed
CH3CN9H2O solvents were determined by the po-
tentiometric titration technique. The pH values of the
solutions were determined using a Witeg model W500
digital-pH meter. The pH values of CH3CN9H2O
solvent mixtures were corrected using the equation of
Douheret [6]
pH* ϭ pH(R) Ϫ ␦
(1)
where pH* is the corrected reading and pH(R) is the
meter reading obtained in mixed CH3CN9H2O sol-
vents. The values of ␦ at different contents of CH3CN
in mixed CH3CN9H2O solvents were determined by
Douheret [6].
The corrected pH values were used to calculate
pKa from eq. (2)
ϩ
pKa ϭ pH* ϩ log{[RNH3 ]/[RNH2]}
(2)
ϩ
where [RNH3 ] and [RNH2] represent the concentra-
tion of protonated and free amine, respectively. The
calculated values of pKa for 3-aminopropan-1-ol, 1,2-
diaminoethane and glycine are shown in Table I.
EXPERIMENTAL
Materials
Product Characterization
The detailed procedure as described elsewhere [5,7]
was used to affirm the formation of N-substituted sal-
icylamide, phenol, and salicylic acid in the reactions
of phenyl salicylate with 3-aminopropan-1-ol, 1,2-dia-
minoethane and glycine in mixed CH3CN9H2O sol-
All the chemicals used were of reagent grade and were
obtained from Fluka and Merck. Distilled water was
used throughout and the standard solutions of free
amines (3-aminopropan-1-ol and 1,2-diaminoethane)
were freshly prepared. The standard solution
vents containing 0.01 mol dmϪ NaOH. Transesteri-
3
3
(0.5 mol dmϪ ) of glycine was freshly prepared in
fication was not detected in the reaction of
3-aminopropan-1-ol with PSϪ in aqueous solvent con-
taining 0.8% v/v CH3CN [5]. The increase in the con-
tent of CH3CN from 2 to 80% v/v did not reveal de-
tectable transesterification in the cleavage of PSϪ
under the presence of 3-aminopropan-1-ol.
3
0.52 mol dmϪ NaOH aqueous solution. Stock solu-
tions of phenyl salicylate were prepared in acetonitrile.
Kinetic Measurements
The rates of aminolysis of phenyl salicylate were stud-
ied spectrophotometrically by monitoring the disap-
pearance of phenyl salicylate as a function of time at
350 nm and 35ЊC using a Shimadzu UV-VIS-NIR or
a Diode-array UV-VIS spectrophotometer. The con-
centrations of nucleophilic amines were larger than the
concentration of phenyl salicylate by the factor of
Ն100-fold. Under such conditions, the rate of ami-
nolysis of phenyl salicylate obeyed first-order rate law.
RESULTS
A series of kinetic runs was carried out within the total
amine concentration, [Am]T, range of 0.02 or 0.04 to
0.08 or 0.2 mol dmϪ at different contents of CH3CN
(ranging from 2 to 70 or 80 % v/v) in mixed aqueous
solvents containing 0.01 mol dmϪ NaOH at 35ЊC.
3
3

EFFECTS OF MIXED CH3CN9H2O SOLVENTS
303
Table I Ionization Constant (Ka) of Cationic Amines in CH3CN9H2O Solvents at 35ЊC
ϩ
ϩ
Ϫ
ϩ
CH3CN
% v/v
HOCH2CH2CH2NH3
H2NCH2CH2NH3
O2CCH2NH3
a
b
b
b
␦
pKa
pKa
pKa
2
10
20
30
40
50
60
70
Ϫ0.007
Ϫ0.052
Ϫ0.103
Ϫ0.153
Ϫ0.203
Ϫ0.255
Ϫ0.403
Ϫ0.950
10.19
10.16
10.16
10.15
10.10
10.06
10.09
10.57
10.04
10.02
9.99
10.04
9.98
9.94
9.98
10.46
9.73
9.73
9.77
9.82
9.87
9.96
10.11
10.62
a ␦ is the pH correction factor obtained from ref. [6].
b pKa ϭ pKa Ϫ ␦ where pKa is the apparent pKa calculated from eq. (2) using the uncorrected pH {i.e., pH(R).}.
app
app
The pseudo-first-order rate constants, kobs, obtained at
a constant content of CH3CN obeyed eq. (3)
PSH, the observed initial absorbance values (i.e., ab-
sorbance at reaction time t ϭ 0) at different total
amine concentrations and different contents of aceton-
itrile in mixed aqueous solvents revealed the absence
of PSH under the experimental conditions of the
present study. The concentrations of the protonated
amines were considered to be negligible under the ex-
perimental conditions imposed. This conclusion is
based upon the fact that the increase in the content of
CH3CN in mixed aqueous solvents is expected to in-
crease pKw more effectively than the pKa of protonated
amine nucleophile and the kinetic runs were carried
kobs ϭ k0 ϩ knms[Am]T
(3)
where k0 and knms represent first- and second-order rate
constants for hydrolysis and aminolysis of phenyl sal-
ms
icylate, respectively. The rate constants, k0 and kn
,
were calculated from eq. (3) using the linear least-
squares technique. The calculated values of k0 and
ms
kn at different contents of CH3CN are summarized
in Table II. The values of k0 are not significantly dif-
ferent from the corresponding k0 values obtained in
the absence of amine under essentially similar exper-
imental conditions (Table II). The fitting of the ob-
served data to eq. (3) is evident from a few represent-
ative plots of kobs vs. [Am]T as shown in Figure 1 and
from the standard deviations associated with the cal-
culated values of k0 and knms (Table II).
Ϫ3
out under the presence of
The
0.01 mol dm NaOH.
hydrolysis [8,9], alkanolysis [10], and aminolysis
[5,7,11] {reactions with primary and secondary
amines only} of PSϪ involve intramolecular general
base, IGB, catalysis where ionized phenolic group,
Ϫ
acts as IGB catalyst. The simplest mechanism
o - O ,
of these reactions is shown in Scheme I which involves
the formation of intramolecular intimate ion-pair, IIP,
and internally hydrogen bonded complex, HP. The na-
ture of the rate-determining steps in hydrolysis [12],
alkanolysis [10], and aminolysis [5,11] of PSϪ has
been discussed in previous reports. The presence of
70% v/v, ethanol in mixed aqueous solvents did not
appear to change the mechanism of the reactions of
PSϪ with primary and secondary amines [13].
The rate of a reaction is generally influenced by the
polarity, permittivity, polarizability, basicity, acidity,
specific solute-solvent interaction, and structure of sol-
vent. It is almost impossible to dissect these factors
from the overall solvent effect on a reaction rate. Gen-
erally all these factors are not equally influential to-
wards the rate of a reaction. Very often some of these
factors oppose each other and, thus, overall solvent
effect appears to be small. In the present reaction sys-
tem, the decrease in the relative permittivity of solvent
must increase [14] while the decrease in the solvent
Although the change in [Am]T from 0.02 or 0.04 to
3
0.08 or 0.2 mol dmϪ at a constant CH3CN content
and [NaOH] is bound to increase the pH of the reaction
medium, such an increase in pH is not sufficient to
cause a nonlinear increase in kobs with increase in
[Am]T. This may be attributed to the fact that the rate
of hydrolysis of phenyl salicylate has been found to
be independent of [ϪOH] within its range of 0.005 to
0.060 mol dmϪ3 [3].
DISCUSSION
Ionized phenyl salicylate, PSϪ, molecule absorbs
strongly (molar extinction coefficient, ⑀, ϭ
1
1
5950 dm3 molϪ cmϪ ) while nonionized phenyl sali-
cylate, PSH, molecule does not absorb to a detectable
1
1
extent (⑀ Ϸ 0 dm3 molϪ cmϪ ) [7] at 350 nm. In
terms of these molecular characteristics of PSϪ and

304
KHAN
Table II First- and Second-Order Rate Constants, k0 and knms, Calculated from Eq. (3) for Hydrolysis and Aminolysis
of PSϪ
ms
CH3CN
%, v/v
104k0
s
103kn
[Am]T Range
mol dm
Ϫ
1
Ϫ
Ϫ1
Ϫ3
Amine
dm3 mol 1 s
No. of Runs
5
3-Aminopropan-1-ol
2
10
20
30
40
50
60
70
9.42 Ϯ 0.49b
(8.14)c
6.60 Ϯ 0.83
(6.43)
6.12 Ϯ 0.59
(5.19)
5.24 Ϯ 0.55
(4.39)
4.32 Ϯ 0.26
(3.73)
3.71 Ϯ 0.40
(3.32)
3.38 Ϯ 0.16
(3.18)
3.39 Ϯ 0.29
(3.09)
3.10 Ϯ 0.39
8.45 Ϯ 1.06
5.41 Ϯ 2.76
5.71 Ϯ 0.52
4.48 Ϯ 1.44
3.21 Ϯ 0.57
3.40 Ϯ 0.74
2.36 Ϯ 0.61
3.16 Ϯ 0.84
5.88 Ϯ 0.86
5.20 Ϯ 0.59
4.12 Ϯ 0.39
3.81 Ϯ 0.32
3.21 Ϯ 0.22
3.40 Ϯ 0.08
3.19 Ϯ 0.35
(3.09)
52.3 Ϯ 0.4b
(2.59)d
37.9 Ϯ 0.8
(2.43)
24.4 Ϯ 0.4
(2.23)
17.0 Ϯ 0.4
(2.26)
13.2 Ϯ 0.2
(2.29)
11.5 Ϯ 0.3
(2.55)
11.6 Ϯ 0.2
(2.91)
11.8 Ϯ 0.4
(3.48)
10.6 Ϯ 0.7
81.5 Ϯ 0.8
59.3 Ϯ 2.1
37.5 Ϯ 0.4
26.6 Ϯ 1.1
20.9 Ϯ 0.4
18.0 Ϯ 0.6
18.5 Ϯ 0.7
17.4 Ϯ 1.0
20.4 Ϯ 0.6
15.2 Ϯ 0.4
9.81 Ϯ 0.30
6.92 Ϯ 0.24
5.34 Ϯ 0.16
4.69 Ϯ 0.06
4.35 Ϯ 0.41
4.79f
0.04–0.20
0.04–0.16
0.04–0.20
0.04–0.20
0.04–0.20
0.04–0.20
0.04–0.12
0.04–0.12
4
5
5
5
5
5
4
80e
2
0.02–0.08
0.04–0.20
0.04–0.20
0.04–0.20
0.04–0.20
0.04–0.20
0.04–0.20
0.04–0.12
0.04–0.12
0.4–0.20
0.04–0.20
0.04–0.20
0.04–0.20
0.04–0.20
0.04–0.20
0.04–0.12
0.04–0.06
3
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
2
1,2-Diaminoethane
10
20
30
40
50
60
70
2
10
20
30
40
50
60
70
Glycine
3
a Conditions: [phenyl salicylate]0 ϭ 2 ϫ 10Ϫ4 mol dmϪ3; Temp. ϭ 35ЊC; ␭ ϭ 350 nm; and [NaOH] ϭ 0.01 mol dmϪ
.
b Error limits are standard deviations.
c Parenthesized values are in the absence of amine and are obtained from refs. [15] and [23].
ms
d Parenthesized values are for k
obtained from ref. [4].
MeOH
e The reaction mixtures became turbid at the end of the kinetic runs due to presumably low products solubility in reaction solvent. Therefore,
pseudo-first-order rate constants, kobs, were calculated from the initial phase of the kinetic runs.
3
f This value is not reliable because it is obtained from two data points only. The reaction mixtures at [Am]T Ͼ 0.06 mol dmϪ became
turbid on shaking which could be due to the saltingout effect.
polarity should decrease the rate of aminolysis of PSϪ
because the reaction involves a neutral and an anionic
molecules as the reactants and the transition state is
apparently more polar than the reactant state (Scheme
I). The amine molecules are solvated predominantly
by water molecules at low CH3CN content. However,
the solvation shell of an amine molecule (RNH2) is
expected to involve more CH3CN molecules at high
CH3CN content. Since the solvation energy of (RNH2)
in water is presumably higher than in CH3CN, the in-
crease in the acetonitrile content should increase the
rate of aminolysis of PSϪ.
The possible explanation for ca. 2.5-fold decrease
in k0 with the increase in acetonitrile content from 2
to 80% v/v has been described in detail elsewhere [15].
The decrease in knms with partial loss of the efficiency
of IGB catalysis due to ion-pair, IP (Scheme II), for-
mation at relatively low relative permittivity of the re-

EFFECTS OF MIXED CH3CN9H2O SOLVENTS
305
Scheme I
specific solute-solvent interaction, structure of solvent,
and ion-pair, IP, formation on reactant state (PSϪ, XH,
and IIP, Scheme I) are expected to be essentially same
for aminolysis and methanolysis of PSϪ. The k2-step
involves internal proton transfer while k4-step involves
the cleavage of C9O bond coupled with internal pro-
Figure 1 Plots showing the dependence of pseudo-first-
order rate constants, kobs, upon the total concentration of 3-
aminopropan-1-ol, [Am]T, for the cleavage of PS at 2 (᭟),
10 (᭛), 20 (᭺), 30 (ٗ), 40 (᭝), 50 (᭹), 60 (᭞), 70 (x), and
80%, v/v, CH3CN (᭡). The solid lines are drawn through
the least-squares calculated points using eq. (3) and the pa-
rameters listed in Table II.
Ϫ
ms
ms
ton transfer. Thus, the ratio knms/kMeOH (where kn
ms
and kMeOH represent the nucleophilic second-order
rate constants for the reactions of PSϪ with RNH2 and
CH3OH, respectively, in mixed aqueous-acetonitrile
solvents) should be measure of the effect of mixed
H2O9CH3CN solvents on pKa of leaving group in k4-
step (i.e., phenol) and the differential effects of mixed
action solvent containing a predominantly aprotic co-
solvent, CH3CN, cannot be completely ruled out. The
ϩ
pKa of HOCH2CH2CH2NH3 is slightly larger than
ϩ
that of H2NCH2CH2NH3 under similar experimental
ϩ
H2O—CH3CN solvents on pKa of RNH3 and
conditions (Table I). But the apparent nucleophilic re-
activity toward PSϪ of H2NCH2CH2NH2 is ca. 1.6-fold
larger than that of HOCH2CH2CH2NH2 (Table II).
These results may be explained in terms of statisti-
cal factor (2 for H2NCH2CH2NH2 and 1 for
HOCH2CH2NH2) [5].
The general mechanism for hydrolysis, methano-
lysis, and aminolysis of PSϪ is shown by Scheme I.
As argued elsewhere, k4-step is the rate-determining
step for aminolysis [11] while k2-step is the rate-de-
termining step in the hydrolysis and alkanolysis
[10,12] of PSϪ. The present data are not sufficient to
ascertain whether k4-step or k2-step is rate-determining
step. In view of the previous reports [5,10,11], k2-step
and k4-step should be rate-determining step for highly
weak and basic nucleophiles, respectively. Thus,
ms
kn ϭ k1k2/k1 for highly weak nucleophiles and
ms
kn ϭ k1k4/k1 for highly basic nucleophiles.
The effects of polarity, permittivity, polarizability,
Scheme II

306
KHAN
not decrease nucleophilicity of amine nucleophile to a
significant extent. Similar effects of CH3CN9H2O
solvents are expected on the nucleophilicity of
ms
CH3OH. Thus, the ratio knms/k
may be considered
MeOH
to be largely dependent on pKa of leaving group
(phenol) within the CH3CN content range of ca. 2–
50% v/v.
The nonlinear change in pKa of amine with change
in the contents of CH3CN in mixed aqueous solvents
(Table I) may be attributed to the fact that the structure
of both water and organic cosolvent cannot remain the
same within the entire range (1–100% v/v) of the con-
tent of organic cosolvent [18,19]. The study on the
effects of mixed aqueous-methanol solvents on pKa of
phenol revealed the increase in pKa from 9.99 to 14.36
with increase in methanol content from 0.0 to 100%
w/w [20]. These results indicate a linear increase in
pKa of phenol with increase in methanol content from
10 to 70% w/w. The increase in the CH3CN content
from 2 to 70% v/v in mixed aqueous solvents in-
creased the pKa of phenol from 10.17 to 13.38 at 35ЊC
[21]. These results show a linear increase in pKa with
the increase in CH3CN content from 2 to 50% v/v.
Since the change in pKa of amine nucleophile with
increase in CH3CN content from 2 to 50% v/v is small
(Table I), the linear decrease in ln(knms/kMeOHms) with
increase in CH3CN content as shown in Figure 2 is
most likely the consequence of the effects of
CH3CN—H2O solvents on pKa of leaving group phe-
nol. The observed points at Ͼ50% v/v CH3CN devi-
ated from linearity of the plots (Fig. 2). One of the
various possible reasons for such deviation is the in-
crease in pKa of amine nucleophile with increase in
CH3CN content at Ͼ50% v/v CH3CN (Table I). The
pKa values of several cationic amines are larger by
several pK units in pure CH3CN solvent compared to
those in pure water solvent [22]. Similarly, pKa values
of several amidines in mixed aqueous-ethanol show
that pKa at 95.6%, w/w Ͼ pKa at 80% w/w Ͻ pKa at
50% w/w Ͻ pKa at 30%, w/w, ethanol [23].
Figure 2 Plots of ln Y (where Y ϭ kmn s/kMeOHms) vs. X
(where X ϭ %, v/v, content of CH3CN in mixed aqueous
solvent) for 1,2-diaminoethane (᭺), 3-aminopropan-1-ol
(᭝); and glycine (ٗ).
ϩ
ms
CH3OH2 . The observed values of knms/kMeOH as
shown graphically by Figure 2 reveal a decrease with
increase in the content of acetonitrile in mixed H2O—
CH3CN solvents. This may be explained as follows.
The mixed aqueous-organic solvents should have
little effect on the ionization constants of isoelectric
ionization reactions (i.e., reactions of the type BHϩ
4 B ϩ Hϩ). This may be true only if the reactant and
product molecules are highly hydrophilic. The effects
of mixed aqueous-alkanol solvents on ionization con-
stants, Ka, of several protonated amines have been re-
ported where the increase in the contents of organic
cosolvents increases the Ka [13,16,17]. However, such
an increase in Ka is limited to a certain range of the
content of organic cosolvent. The increase in Ka of
ϩ
RNH3 with increase in the content of organic cosol-
vent in mixed aqueous solvents is expected to be di-
rectly proportional to the size of the hydrophobic moi-
ϩ
ety of RNH3 . Although we could not find any report
on the effects of mixed aqueous-organic solvents on
ϩ
ϩ
Ka of CH3OH2 , the values of Ka of CH3OH2 may
not be significantly affected by the change in the con-
tent of CH3CN in mixed H2O9CH3CN solvents be-
cause CH3OH molecule is highly hydrophilic.
I would like to thank the University of Malaya for financial
support through F Vote (F408/96) and A. George for assist-
ing in the determination of pKa of cationic amine.
ϩ
The values of pKa, of HOCH2CH2CH2NH3 , and
H2NCH2CH2NH3ϩ were found to be slightly decreased
ϩ
(ca. 0.1 pK units), while pKa, of O2CCH2NH3 , re-
vealed a slight increase (ca. 0.2 pK units) with increase
in the CH3CN content from 2 to 50% v/v in mixed
CH3CN—H2O solvents. The values of pKa at 70% v/
v were found to be significantly higher than those at
Յ60% v/v CH3CN (Table 1). Thus, it appears that the
increase in CH3CN content from 2 to 50% v/v does
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