Effects of urea, Na+ and Li+ ions on the kinetics and mechanism of intramolecular general base-catalyzed glycolysm of ionized phenyl salicylate in ethane-1,2-diol-acetonitrile solvents at a constant water concentration

Article abstract of DOI:10.1002/(SICI)1099-1395(199802)11:2<109::AID-POC974>3.0.CO;2-4

Pseudo-first-order rate constants (k₁) for the reaction of ethane-1,2-diol (DOL) with ionized phenyl salicylate (PS^-), obtained in mixed DOL-CH₃CN solvent at constant [H₂O] and [NaOH], obey the relationship k₁ = α[DOL]_T/(1 + 2K_A[DOL]_T), where α is the apparent second-order rate constant, K_A is the association constant for the dimerization of DOL and [DOL]_T is the total concentration of DOL. The values of K_A, in the presence of Na⁺ ions, decrease with increase in [H₂O]. Lithium ions cause almost complete depolymerization of polymeric DOL (i.e. K_A ≈ 0) under the experimental conditions imposed. The effect of 0.5 M urea on the structural behavior of the mixed solvent is kinetically insignificant.

Full text of DOI:10.1002/(SICI)1099-1395(199802)11:2<109::AID-POC974>3.0.CO;2-4

JOURNAL OF PHYSICAL ORGANIC CHEMISTRY, VOL. 11, 109–114 (1998)
‡
‡
Effects of urea, Na and Li ions on the kinetics and
mechanism of intramolecular general base-catalyzed
glycolysm of ionized phenyl salicylate in ethane-1,2-diol±
acetonitrile solvents at a constant water concentration
M. Niyaz Khan
Department of Chemistry, Faculty of Science, Universiti Malaya, 50603 Kuala Lumpur, Malaysia
Received 19 May 1997; revised 30 June 1997; accepted 22 July 1997
ABSTRACT: Pseudo-first-order rate constants (k ) for the reaction of ethane-1,2-diol (DOL) with ionized phenyl
1
�
salicylate (PS ), obtained in mixed DOL–CH CN solvent at constant [H O] and [NaOH], obey the relationship
3
2
k = a[DOL] /(1 ‡2K [DOL] ), where a is the apparent second-order rate constant, K is the association constant
1
T
A
T
A
‡
for the dimerization of DOL and [DOL] is the total concentration of DOL. The values of K , in the presence of Na
T
A
ions, decrease with increase in [H O]. Lithium ions cause almost complete depolymerization of polymeric DOL (i.e.
2
K ꢀ0) under the experimental conditions imposed. The effect of 0.5 M urea on the structural behavior of the mixed
A
solvent is kinetically insignificant.  1998 John Wiley & Sons, Ltd.
KEYWORDS: phenyl salicylate; ethane-1,2-diol; urea; sodium salt; lithium salt; transesterification; kinetics;
intramolecular general base catalysis
INTRODUCTION
ꢁ2% (v/v) CH CN,0.01 M NaOH and 0.01 M KOH] and
3
DOL–CH CN [containing 5% (v/v) H O and 0.01 M
3
2
Intramolecular general acid (GA) and general base (GB)
catalysis appear to be the ubiquitous feature of many
enzymatic catalyses. It is widely believed that the
NaOH] revealed characteristically different [DOL]–rate
profiles. Lithium ions as compared with other alkali
4
1
metal cations produced a different effect on [CH OH]–
3
�
dielectric constant of the microenvironment of the active
site where enzyme-catalyzed reactions occur is signifi-
rate profiles for the methanolysis of PS5 in a reaction
medium of nearly isodielectric constant. Many solution
properties of CH OH–H O solvent are different from
2
cantly lower than that of a pure aqueous solvent. It is
3
2
6
therefore of interest to study the effects of solvents with
low dielectric constants on intramolecular GA and GB
catalysis.
those of DOL–H O solvent. It was therefore decided to
study the effects of Li and Na ions on the rate of
ethane-1,2-diolysis of PS in DOL–CH CN solvent
2
‡
‡
�
3
Intramolecular GB catalysis is considered to involve
containing a constant content of H O. The results and
the probable explanations are presented in this paper.
2
�
3
the alkanolysis of ionized phenyl salicylate (PS ). The
structural behavior of mixed aqueous–alkanol solvents
�
has been elucidated by studies on the alkanolysis of PS
where the change in the content of the alkanol changes
both the concentration of alkanol and the dielectric
constant of the reaction medium. Ethane-1,2-diolysis of
EXPERIMENTAL
�
PS in mixed HOCH CH OH (DOL)–H O [containing
Materials. Reagent-grade chemicals obtained commer-
cially were used throughout the kinetic study. A stock
solution of phenyl salicylate was prepared in acetonitrile.
2
2
2
Kinetic measurements. The rate of reaction of-ethane-
*
Correspondence to: M. N. Khan, Department of Chemistry, Faculty
�
1
,2-diol (DOL) with ionized phenyl salicylate (PS ) in
of Science, Universiti Malaya, 50603 Kuala Lumpur, Malaysia.
E-mail: niyaz@kimia.um.edu.my.
Contract/grant sponsor: Universiti Malaya Research Vote; Contract
grant number: F408/96.
Contract/grant sponsor: National Science Council for R&D, IRPA;
Contract grant number: 09-02-03-0003.
mixed DOL–CH CN–H O solvent was studied spectro-
3
2
photometrically by monitoring the appearance of product
phenolate ion) at 290nm. Details of the experimental
(
7
procedure are described elsewhere.

1998 John Wiley & Sons, Ltd.
CCC 0894–3230/98/020109–06 $17.50

1
10
M. N. KHAN
�
The general reaction scheme for the cleavage of PS in
DOL–CH CN–H O solvent is as follows:
3
2
k
k
k3
2
1
�
�
�
�
�
�!
�!
Sal ‡PhOHH PS
ES ‡PhOH
Sal
2
O
DOL
H O
2
‡DOL ‡PhOH
�
�
where Sal , ES and PhOH represent salicylate ion,
ionized 2-hydroxyethylsalicylate and phenol, respec-
tively, and k , k2 and k3 are pseudo-first-order rate
1
�
constants for the reaction of DOL with PS , hydrolysis of
PS and hydrolysis of ES , respectively. Under the
�
�
present experimental conditions, k /k > 10 (Ref. 7) and
1
2
(
k ‡k )/k > 50 (Ref. 8), hence these results show that
1
2
3
k and k are negligible compared with k .
2
3
1
All the kinetic runs were carried out for a period of
more than three half-lives of the reactions and the
observed data (observed absorbance versus time) obeyed
a first-order rate raw. Details of the data analysis are
9
described elsewhere.
A product analysis study has been described in an
1
0
earlier paper.
Figure 1. Plots showing the dependence of pseudo-®rst-
�
order rate constants (k ) for the reaction of DOL with PS on
1
the content of DOL in the reaction mixtures containing mixed
RESULTS
�
4
�
DOL±CH CN solvents, 2.0 Â10 M PS , 0.01 M NaOH and
3
4
% (v/v) H O ("), 8% (v/v) H O ‡0.5 M urea (◊), 8% (v/v)
2
2
Effects of varying the content of DOL at constant
H O] and [urea]
H O (!), 18% (v/v) H O ‡0.5 M urea (&), 18% (v/v) H O
2 2 2
[
2
(
~) and 28% (v/v) H O (*). The solid lines are drawn
2
through the least-squares calculated points
�
The rate of reaction of PS with DOL was studied at
5 °C with 4% (v/v) H O and 0.01 M NaOH and within
3
2
the DOL concentration range 5–90% (v/v) in mixed
DOL–CH CN solvents. The reaction rates were also
3
studied at 8 and 18% (v/v) H O and within the DOL
DISCUSSION
2
concentration range 5–70% (v/v) in the absence and
presence of 0.5 M urea. A few kinetic runs were carried
out within the DOL concentration range 5–70% (v/v) at
Pseudo-first-order rate constants (k ) for the reaction of
1
DOL with phenyl salicylate obtained within the DOL
concentration range 10–90% (v/v) were found to be
2
8% (v/v) H O. The results are shown graphically in
2
�
10
Figure 1.
independent of [HO ] within the range 0.01–0.05 M.
‡
In order to establish the effect of Li ions on the rate of
reaction of DOL with PS , several kinetic runs were
Hence the rate of ethane-1,2-diolysis of phenyl salicylate
under the present experimental conditions should be
�
1
1–13
carried out at 8, 18 and 28% (v/v) H O and in the absence
independent of pH. The pH-independent hydrolysis
2
3
and presence of 0.5 M urea with different concentrations
and alkanolysis of phenyl salicylate have been shown to
involve intramolecular GB catalysis. The occurrence of
kinetically indistinguishable intramolecular GA catalysis
in these reactions has been ruled out. The mechanism of
of DOL in DOL–CH CN solvents containing 0.01 M
3
LiOH. The results are shown as the plots of pseudo-first-
order rate constants (k ) versus [DOL] in Figure 2.
1
�
It may be noted that the reaction mixtures at 8% (v/v)
alkanolysis of ionized phenyl salicylate (PS ) has been
3
H O with ꢁ25% (v/v) DOL and 18% (v/v) H O with
discussed elsewhere.
2
2
ꢁ
15% (v/v) DOL were initially colourless but became
The dielectric constant (e) of CH CN (e = 37.0 at
3
14
15
turbid as the reaction progressed. The reaction half-life at
which this turbidity appeared decreased with decrease in
the content of DOL. The presence of 0.5 M urea
decreased this turbidity. For example, in the absence of
25 °C ) is similar to that of DOL (e = 37.7 at 25 °C ).
Therefore, e of mixed DOL–CH CN solvents containing
3
a constant [H O] and different ratios of [DOL] and
2
[CH CN] may be considered to be constant. Hence the
3
urea at 18% (v/v) H O and 15% (v/v) DOL the turbidity
change in k values with the change in the content of
2
1
appeared at 2.2 half-lives, whereas such turbidity did not
appear until 3.4 half-lives in the presence of 0.5 M urea.
DOL in DOL–CH CN solvents containing a constant
3
[H O] cannot be attributed to the effect of e of the
2

1998 John Wiley & Sons, Ltd.
JOURNAL OF PHYSICAL ORGANIC CHEMISTRY, VOL. 11, 109–114 (1998)

GLYCOLYSIS OF IONIZED PHENYL SALICYLATE
111
may provide the relationship between [(DOL)] and
18
[
DOL] as given by the equation
T
[
DOL]T
[(DOL)] ˆ1
ꢂ2†
‡2K [DOL]
A
T
2
where K = [(DOL) ]/[(DOL)] = [(DOL) ]/[(DOL) ]-
A
2
3
2
1
9
[
(DOL)] = …[(DOL) ]/[(DOL) ][(DOL)]
and
n
n�1
[
(DOL)] is the concentration of monomeric DOL.
In mixed DOL–CH CN–H O solvents with H O
3
2
2
contents of ꢁ28% (v/v), the following ion-pair formation
between cations and anions:
K
1
�
‡
�
‡
PS ‡M „PS ÁM
K
2
�
‡
‡
OH ‡M „�OH ÁM
cannot be completely ruled out. These reactions can lead
‡
�
to the equation provided that 1 << K [M ] or K [HO ],
2
2
Figure 2. Plots showing the dependence of pseudo-®rst-
�
[PS]T
‡ꢀ‰M‡Š
�
order rate constants (k ) for the reaction of DOL with PS on
1
the content of DOL in the reaction mixtures containing mixed
[PS Šˆ
ꢂ3†
1=2
T
1
�
4
�
DOL±CH CN solvents, 2.0 Â10 M PS , 0.01 M LiOH and
3
8
% (v/v) H O (~), 8% (v/v) H O ‡0.5 M urea (*) 18% (v/v)
2
2
‡
H O (!), 18% (v/v) H O ‡0.5 M urea (&) and 28% (v/v)
2
2
where [PS] and [M ] represent the total concentration
of phenyl salicylate and cation, respectively, and
T
T
H O (◊)
2
�
1=2
b = K K
.
1
2
�
The enhanced reactivity of PS towards a nucleophile
with hydrogen attached to the nucleophilic site is the
consequence of intramolecular GB catalysis due to the
1
1–13
�
‡
reaction medium. The rate law for the overall reaction of
ionized o-phenolic group.
The ion-pair (PS .M )
�
DOL with PS may be given as
formation is expected to decrease the efficiency of
intramolecular GB catalysis. The rate of reaction of DOL
with PS .M is therefore ignored compared with that
�
�
‡
rate = k[DOL][PS ]
(1)
�
with the free form (PS ) of ionized phenyl salicylate.
where k is the apparent second-order rate constant for
ethane-1,2-diolysis of PS . Equation (1) predicts that the
The observed rate law, rate = k [PS] , and equation (1)
with [(DOL)] and [PS ] obtained from equations (2) and
(3) can yield the equation
1
T
�
�
plot of pseudo-first-order rate constants (k , where
1
k = k[DOL] ) versus total concentration of DOL
1
T
(
[DOL] ) should be linear with zero intercept provided
k[DOL]T
T
k1 ˆ
ꢂ4†
ꢂ1 ‡ꢀ‰M‡Š †ꢂ1 ‡2K [DOL] †
1=2
that [DOL] = [DOL] , i.e. all the DOL molecules exist in
T
T
A
T
monomeric form, (DOL). This appears to be true only at
very low contents of DOL in the presence of 0.01 M
NaOH (Figure 1). Pseudo-first-order rate constants show
negative deviations from linearity at high contents of
‡
At constant [M ] , equation (4) is reduced to equation
5) provided that b remains constant with changes in the
ratio of mixed solvent components, where a = k/
T
(
�
DOL (Figure 1). Causes such as self-association of PS
‡
1=2
�
(
1 ‡b[M ]T ).
molecules only and association of DOL and PS
molecules, of the non-linear nature of such a plot have
been ruled out. The most probable cause for the non-
1
0
ꢁ[DOL]T
k1 ˆ1
ꢂ5†
‡2K [DOL]
linear variation of k against [DOL] may be ascribed to
A
T
1
the self-association of DOL molecules.
‡
1=2
T
The concept of self-association of alkanol molecules in
mixed aqueous solvents has been used to explain the
where ꢁ ˆk=ꢂ1 ‡ꢀ‰M Š †:
Pseudo-first-order rate constants (k
) obtained in the
1
�
3–5,7,16,17
rate–[alkanol] profiles in the alkanolysis of PS .
The characteristics of self-association of DOL molecules
presence of 0.01 M NaOH obeyed equation (5). The
unknown parameters, a and K , were calculated from
A

1998 John Wiley & Sons, Ltd.
JOURNAL OF PHYSICAL ORGANIC CHEMISTRY, VOL. 11, 109–114 (1998)

1
12
M. N. KHAN
a
Table 1. Values of a and K calculated from equation (5)
A
3
�1 �1
3
�1
[MOH] (M)
[H O] (%, v/v)
2
[Urea] (M)
10 a (M
s
)
10 K (M
)
[DOL]range (%, v/v) No. of runs
A
b
c
c
0
.01
4
8
8
0.0
0.0
0.5
0.0
0.5
0.0
10.9 Æ0.6
9.09 Æ0.76
9.72 Æ0.90
5.36 Æ0.42
4.89 Æ0.29
3.06 Æ0.18
266 Æ20
223 Æ25
245 Æ30
119 Æ15
112 Æ11
51.8 Æ6.7
5–90
5–70
5–70
5–70
5–64
11
10
10
10
10
10
8
6
9
8
9
1
1
2
8
8
8
8
8
8
8
8
5–70
d
e
0.01
0.0
0.885 Æ0.032
0.979 Æ0.041
1.05 Æ0.02
1.01 Æ0.04
1.10 Æ0.02
25–90
25–70
15–80
15–64
10–70
0.5
0.0
0.5
0.0
1
1
2
a
b
c
d
e
�4
[
PS]
0
= 2 Â10 M, 35°C, ꢂ = 290 nm, organic co-solvent CH
3
CN.
MOH = NaOH.
Error limits are standard deviations.
MOH = LiOH.
The values of a were calculated from the equation: k
1
= É ‡a [DOL]
T
.
�
equation (5) using the non-linear least-squares technique.
(DOL) and free PS because water molecules are much
better solvating molecule than acetonitrile molecules.
The decrease in the solvation energy of (DOL) and free
The calculated values of a and K at different [H O] and
A
2
in the absence and presence (0.5 M) of urea are
summarized in Table 1. The fitting of the observed data
to equation (5) is evident from the standard deviations
�
PS should decrease the apparent activation energy for
the reaction of DOL with PS . (ii) The mechanism of the
reaction of DOL with PS involves a transient intramo-
�
�
associated with a and K (Table 1) and from the plots in
A
Figure 1 where the solid lines are drawn through the
least-squares calculated points.
lecular intimate ion pair (I ). The stability of such an
1
intermediate should increase with decrease in the
dielectric constant of the reaction medium. Hence the
decrease in the content of H O in DOL–CH CN–H O
An increase in water content from 4 to 28% (v/v)
�
�
3
�3 �1 �1
M
decreased a from 10.9 Â10 to 3.06 Â10
s
2
3
2
1
and K from 0.266 to 0.052 M . The decrease in K due
solvent should stabilize the apparent transition state and
A
A
to the increase in [H O] is due to the water molecules
consequently increase the rate constant, k. Significantly
2
acting as the depolymerizing agents for the DOL
larger values of a for DOL (Table 1) compared with the
5
structure in DOL–CH CN solvent. Nearly 1.5- and 2.5-
corresponding values of a for CH OH may be partially
3
3
fold decreases in a and K , respectively, were obtained in
the methanolysis of PS under essentially similar
attributed to the stabilization of the intramolecular
intimate ion pair due to internal hydrogen bonding as
A
�
5
�3
experimental conditions. The value of k (2.10 Â10
shown by I in a solvent of low dielectric constant and
2
�
1
�1 17
�
M
s ) for the methanolysis of PS in CH OH–H O
containing low content of water.
3
2
�1 �1 4
s )
�
3
solvent is slightly larger than k (1.52 Â10
M
�
for ethane-1,2-diolysis of PS in DOL–H O solvent.
2
�
3
�1 �1
However, the value of a (10.9 Â10
is nearly three times larger than a (3.15 Â10
M
s ) for DOL
�
3
�1 �1
M
s )
5
for methanol at 4% (v/v) H O in mixed alkanol–
2
�
1=2
acetonitrile solvents. It seems to be unlikely that K K
1
2
in DOL–CH CN is smaller than that in CH OH–CH CN
3
3
3
at 4% (v/v) H O and therefore k should be significantly
2
larger at 4% (v/v) H O than k at >28% (v/v) H O in
2
2
DOL–CH CN solvent. The value of k has been shown to
3
be nearly 30 times larger at 10% (v/v) H O than at 30%
2
‡
(
v/v) H O in the presence of Na ion for methanolysis of
2
�
5
PS .
The increase in k with decrease in the content of H O
2
in mixed DOL–CH CN solvents may be attributed to the
3
following reasons. (i) The decrease in the content of H O
2
in mixed DOL–CH CN–H O solvents is expected to
3
2
decrease the number of H O molecules in the solvation
2
�
shells of monomeric DOL and free PS molecules. This
characteristic, in turn, decreases the solvation energy of

1998 John Wiley & Sons, Ltd.
JOURNAL OF PHYSICAL ORGANIC CHEMISTRY, VOL. 11, 109–114 (1998)

GLYCOLYSIS OF IONIZED PHENYL SALICYLATE
113
‡
�3 �1 �1
�1=2
�1
The plots of k versus [DOL] obtained in mixed DOL–
K , 2.42 Â10
M
s , 2.70 M
and 19.2 M for
1
‡
�3 �1 �1
�1=2
�1
CH CN solvents containing 0.01 M LiOH and a constant
Na and 1.80 Â10
M
s , 11.5 M
and 0.0 M
3
‡
5
content of H O turned out to be linear (Figure 2). Similar
for Li at 30% (v/v) H O and 30 °C.
2
2
�
results were obtained in the methanolysis of PS under
similar experimental conditions. The linearity of these
Urea molecules are known to reduce hydrophobic
interactions.
5
20
Urea molecules are expected to be
plots indicates that K ꢀ0 under the experimental
preferentially solvated by water molecules in DOL–
CH CN–H O solvent. Hence the presence of urea should
A
conditions of these obervations. Lithium ions break the
alkanol structure owing to their high surface charge
density. Hence for the mixed DOL–CH CN–H O
3
2
increase both a and K owing to entrapment of water
A
molecules by the solvation shells of the urea molecules.
However, the presence of 0.5 M urea at 8 and 18% (v/v)
H O did not produce a detectable effect on a and K at
3
2
solvents containing ꢁ28% (v/v) H O, 0.01 M LiOH and
2
�
4
2
Â10 M phenyl salicylate, K ꢀ0. Equation (4) is
A
2
A
reduced to
0.01 M NaOH and on a at 0.01 M LiOH (Table 1). This
shows that under such conditions, the concentration of
urea was not sufficient to cause a significant decrease in
the number of water molecules in the solvation shells of
DOL and CH CN molecules. The values of a and K are
k[DOL]T
k1 ˆ
ꢂ6†
‡ꢀ‰Li‡Š
1=2
1
T
3
A
if K = 0.
not appreciably different at 4 and 8% (v/v) H O (Table
A
2
Equation (6) predicts that the plot of k versus [DOL]
1). The use of higher concentrations of urea (>0.5 M) was
1
T
should be linear with essentially zero intercept if b
restricted owing to relatively low solubility of urea in
remains constant with change in the content of CH CN in
CH CN.
3
3
DOL–CH CN solvents with a constant [H O]. This
Many solution properties of polyhydric alcohols and
mixed aqueous polyhydric alcohols are very different
from those of monohydric alcohols and mixed aqueous
3
2
seems to be true at ꢃ18% (v/v) H O. The linear plots
2
at 8% (v/v) H O revealed negative intercepts. The large
2
6,21
‡
negative intercepts reveal the change in b with the change
monohydric alcohols.
and [Na ] on rates of intramolecular general base-
catalyzed methanolysis and ethane-1,2-diolysis of PS
However, the effects of [Li ]
‡
in the content of CH CN at low content of DOL in DOL–
3
‡
5
�
CH CN solvents containing a constant [Li ] and
3
T
5
[
H O].
2
in mixed alkanol–acetonitrile solvents containing a
‡
1=2
T
The slopes {a = k/(1 ‡b[Li ] )} of the linear plots
constant [H O] appeared to be insensitive to the different
2
of k versus [DOL] were calculated by the linear least-
solution properties of these mixed solvents.
1
T
squares technique and the results obtained are summar-
ized in Table 1. The values of a slightly increased (ca
2
0%) with increase in the content of H O from 8 to 28%
2
Acknowledgments
(v/v). Similar observations were made in the methano-
�
5
lysis of PS at 0.01 M LiOH. The value of a at a constant
The work was supported by the Universiti Malaya
Research Vote F408/96 and by the National Science
Council for R & D, IRPA, Grant No. 09-02-03-0003.
‡
[H O] is significantly larger at 0.01 M Na than at 0.01 M
2
‡
Li (Table 1). The value of k may not be expected to
change with change in cation from Li to Na ion.
Hence the lower a values in the presence of Li ions are
the consequence of the larger b values in the presence of
Li ions compared with Na ions.
‡
‡
17
‡
REFERENCES
‡
‡
‡
The proposal that K ꢀ0 in the presence of Li does
A
1
. (a) W. P. Jencks. Catalysis in Chemistry and Enzymology.
McGraw Hill. New York (1969); and (b) A. R. Fersht. Enzymes
Structure and Mechanism. Freeman, San Franscisco (1977); and
not necessarily mean that k should be larger in the
presence of Li than Na where K ≠ 0. The rate of
ethane-1,2-diolysis of ionized phenyl salicylate is
proportional to the concentration of both monomeric
DOL, [(DOL)], and free ionized phenyl salicylate, [PS ]
1
‡
‡
A
(
c) F. H. Westeimer. Adv. Phys. Org. Chem. 21, 1 (1985).
2
. (a) A. B. Maude and A. Williams. J. Chem. Soc. Perkin Trans. 2
179 (1997), and references cited therein; (b) M.-K. Leung and M. J.
Frechet. J. Chem. Soc., Perkin Trans. 2 2329 (1993).
. M. N. Khan. J. Phys. Chem. 92, 6273 (1988).
�
3
4
[equation (1)]. In terms of equation (4), the decrease in
. M. N. Khan. Int. J. Chem. Kinet. 20, 443 (1988).
K should increase k provided that b remains unchanged
with the change from Li to Na . Significantly lower
values of a in the presence of Li compared with Na
indicate that b values are larger in the presence of Li
5. M. N. Khan. Indian J. Chem. 35B, 1047 (1996).
A
1
‡
‡
6
. J. B. F. N. Engberts. Water, a Comprehensive Treatise, edited by F.
Pranks, Vol. 6, Chapt. 4. Plenum Press, New York (1979), and
references cited therein.
‡
‡
‡
7. M. N. Khan. Int. J. Chem. Kinet. 19, 757 (1987).
‡
‡
8
. W. J. Irwin, Q. N. Masuda and A.-I. Wan Po. Int. J. Pharm. 21, 35
1984).
than Na . Hence although K ꢀ0 in the presence of Li ,
A
‡
‡
the values of k are smaller for Li than for Na simply
1
9
. M. N. Khan. J. Chem. Soc., Perkin Trans. 2 199 (1989).
‡
‡
because the a values are smaller for Li than for Na
Table 1). Similar results were obtained in the methano-
10. M. N. Khan. Int. J. Chem. Kinet. 23, 837 (1991).
1
1. M. L. Bender, F. J. Kezdy and B. Zerner. J. Am. Chem. Soc. 85,
3017 (1963).
(
�
lysis of PS where the respective values of k, b and K
are 2.18 Â10
A
1
2. B. Capon and B. C. Ghosh. J. Chem. Soc B 472 (1966).
�
3
�1 �1
�1=2
�1
M
s , 0.94 M
and 20.5 M for
13. M. N. Khan. J. Mol. Catal. 40, 195 (1987).

1998 John Wiley & Sons, Ltd.
JOURNAL OF PHYSICAL ORGANIC CHEMISTRY, VOL. 11, 109–114 (1998)

1
14
M. N. KHAN
19. E. Grunwald, K.-C. Pan, A. Effio. J. Phys. Chem. 80, 2937 (1976).
1
4. S. V. Anantakrishnan. J. Sci. Ind. Res. 30, 319 (1971).
1
5. R. C. Weast. Handbook of Chemistry and Physics, 63rd ed. p. E-
2. CRC Press, Boca Raton, FI. (1982).
2
0. T. Asakawa, M. Hashikawa, K. Amada, S. Miyagishi. Langmuir
1, 2376 (1995), and references cited therein.
5
1
16. M. N. Khan, A. A. Audu. Int. J. Chem. Kinet. 22, 37 (1990).
17. M. N. Khan, A. A. Audu. J. Phys. Org. Chem. 5, 129 (1992).
18. M. N. Khan and Z. Arifin. Langmuir 12, 261 (1996).
2
1. C. A. Bunton, L.-H. Gan, F. H. Hamed, J. R. Moffatt. J. Phys.
Chem. 87, 336 (1983).

1998 John Wiley & Sons, Ltd.
JOURNAL OF PHYSICAL ORGANIC CHEMISTRY, VOL. 11, 109–114 (1998)