Kinetics and mechanism of alkaline hydrolysis of 4-nitrophthalimide in the absence and presence of cationic micelles

Article abstract of DOI:10.1002/kin.1036

Pseudo-first-order rate constants (k_obs) for alkaline hydrolysis of 4-nitrophthalimide (NPTH) decreased by nearly 8- and 6-fold with the increase in the total concentration of cetyltrimethyl-ammonium bromide ([CTABr]_τ) from 0 to 0.02

Full text of DOI:10.1002/kin.1036

Kinetics and Mechanism of
Alkaline Hydrolysis of 4-
Nitrophthalimide in the
Absence and Presence of
Cationic Micelles
M. NIYAZ KHAN, ZUNOLIZA ABDULLAH
Department of Chemistry, Faculty of Science, University of Malaya, 50603 Kuala Lumpur, Malaysia
Received 26 June 2000; accepted 21 March 2001
ABSTRACT: Pseudo-first-order rate constants (kobs) for alkaline hydrolysis of 4-nitrophthalim-
ide (NPTH) decreased by nearly 8- and 6-fold with the increase in the total concentration of
cetyltrimethyl-ammonium bromide ([CTABr]T) from 0 to 0.02 M at 0.01 and 0.05 M NaOH,
respectively. These observations are explained in terms of the pseudophase model and pseu-
dophase ion-exchange model of micelle. The increase in the contents of CH3CN from 1 to 70%
v/v and CH3OH from 0 to 80% v/v in mixed aqueous solvents decreases kobs by nearly 12- and
11-fold, respectively. The values of kobs increase by nearly 27% with the increase in the ionic
strength from 0.03 to 3.0 M. The mechanism of alkaline hydrolysis of NPTH involves the re-
actions between HOϪ and nonionized NPTH as well as between HOϪ and ionized NPTH. The
micellar inhibition of the rate of alkaline hydrolysis of NPTH is attributed to medium polarity
effect. ᭧ 2001 John Wiley & Sons, Inc. Int J Chem Kinet 33: 407–414, 2001
INTRODUCTION
action was negligible in the cationic micellar pseudo-
phase compared to that in aqueous pseudophase at
[NaOH] Յ 0.05 M [8]. Because of the insignificant
rate of alkaline hydrolysis in cationic micellar pseu-
dophase, the rate data were adequately explained in
terms of the pseudophase model (PM) of micelle [9]
rather than the pseudophase ion-exchange (PIE) model
of micelle [10]. But the rate of hydrolysis of N-hy-
droxyphthalimide, within [NaOH] range of 0.006 to
0.018 M, was explained in terms of the PIE model
[11]. The values of Ka of phthalimide and N-hydrox-
Phthalimide and its derivatives constitute an important
class of organic compounds with numerous uses in
biology and syntheticas well as polymer chemistry
[1]. Micelles provide a microreaction medium that
seems to be similar to one for many biological reac-
tions [2,3]. The kinetics and mechanism of alkaline
hydrolysis of phthalimide in the absence [4,5] and
presence of micelles [6] have been studied in some
detail. Cationicmicelles inhibited the rate of alkaline
hydrolysis of phthalimide [7], and the rate of this re-
10
yphthalimide are 3 ϫ 10Ϫ [12] and 1 ϫ 10Ϫ6 M
[13], respectively, and the rate of hydrolysis of phthal-
imide was found to be independent of [NaOH] within
its range 0.005–0.050 M, while the rate of hydrolysis
of N-hydroxyphthalimide was dependent upon
[NaOH] at [HOϪ] Ն 0.006 M.
Correspondence to: M. Niyaz Khan (niyaz@kiniia.wm.equ.my)
Contract grant sponsor: National Scientific Research and De-
velopment Council of Malaysia
Contract grant number: 09-02-03-0785
᭧ 2001 John Wiley & Sons, Inc.

408
KHAN AND ABDULLAH
Table I Pseudo-First-Order Rate Constants (kobs) for
phthalimide and whether the PIE model is applicable,
the rate of alkaline hydrolysis of NPTH has been stud-
ied in the absence and presence of cetyltrimethylam-
monium bromide (CTABr) micelles. The effects of
mixed aqueous–organicsolvents and ionicstrength on
the rate of alkaline hydrolysis of NPTH have been also
studied in the absence of micelles. The observed re-
sults and their probable explanation(s) are described
in this manuscript.
Hydrolysis of NPTH at Different [ϪOH]a
Ϫ
Ϫ
1
Ϫ
1
Ϫ1
[ OH]/M
104 kobs/s
104 kcalcdb/s
104 kcalcdc/s
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
5 d
1.7 ϫ 10
2.2 ϫ 10
3.4 ϫ 10
2.7 ϫ 10
6.8 ϫ 10
0.0005f
0.001
0.005
0.01
0.01
0.01
0.03
0.05
13.0 Ϯ 0.2e
14.8 Ϯ 0.3
15.5 Ϯ 0.4
19.3 Ϯ 0.3
20.5 Ϯ 0.3
19.9 Ϯ 0.2
18.8 Ϯ 0.3
19.9 Ϯ 0.3
23.9 Ϯ 0.5
24.7 Ϯ 0.3
22.6 Ϯ 0.3
32.4 Ϯ 0.3
33.9 Ϯ 0.6
68.2 Ϯ 1.0
62.6 Ϯ 1.0
117 Ϯ 2
13.3
14.7
15.7
17.7
18.1
17.9
18.3
20.2
22.5
22.5
22.5
31.8
41.0
64.2
64.2
111
12.7
14.6
16.1
19.3
19.7
19.6
19.7
19.9
5
5
4
4
EXPERIMENTAL
Materials
All the chemicals used were supplied by Fluka, Sigma,
or Aldrich and were of the highest commercially avail-
able purity. Buffer solutions of desired pH were
freshly prepared. The stock solutions of NPTH were
prepared in acetonitrile.
0.1
0.1
0.2
0.3
155 Ϯ 6
157
0.4
214 Ϯ 3
203
0.5
248 Ϯ 1
249
0.5
0.6
227 Ϯ 1
301 Ϯ 3
249
206
Kinetic Measurements
0.8
1.0
1.0
358 Ϯ 5
476 Ϯ 7
514 Ϯ 5
388
481
481
The rate of alkaline hydrolysis of NPTH was studied
spectrophotometrically by monitoring the disappear-
ance of the reactant NPTH at 260 nm. The observed
data, absorbance (Aobs) versus reaction time (t), obeyed
a pseudo-first-order rate law under the experimental
conditions imposed. The details of the kinetic proce-
dure and data analysis have been described elsewhere
[7,14].
4
a [NPTH]0 ϭ 1 ϫ 10Ϫ M, ␭ ϭ 260 nm, ionicstrength 1 M,
35ЊC and reaction mixture for each measurement contains 1% v/v
CH3CN.
b Calculated from eq. (1) as described in the text.
c Calculated from the relationship kcalcd ϭ (ab [ϪOH])/(1 ϫ
b[ϪOH]) with 104a ϭ 19.9 sϪ1 and 10Ϫ5b ϭ 1.27 MϪ1 (see text).
d [ϪOH] ϭ (10w
pK))/␥ where desired pH was attained by car-
(pH
Ϫ
bonate buffers, pKw ϭ 13.62, and activity coefficient ␥ ϭ 0.7 at 1
M ionicstrength.
RESULTS
e Error limits are standard deviations.
f Hydroxide ion concentrations were maintained by using the
stock solutions of NaOH.
Effect of [NaOH] on kobs (Pseudo-First-
Order Rate Constant for Hydrolysis of
NPTH)
The pKa of 4-nitrophthalimide (NPTH) is expected
to be significantly lower than that of phthalimide and
consequently pH-independent hydrolysis of NPTH
compared to that of phthalimide must occur at rela-
tively lower [HOϪ]. The mechanism of a reaction re-
mains essentially unchanged with the change in the
reaction medium from pure aqueous to micelles, and
the effects of ionic micelles on the rate of a reaction
can be explained, at least in terms of polarity and ionic
strength of the micellar surface. Thus, it becomes es-
sential to study the reaction mechanism and the effects
of mixed aqueous–organicsolvents as well as ionic
strength on the reaction rate before studying the effects
of micelles on the same reaction rate. Thus, in order
to find out the effect of 49NO2 group on the rate of
cationic micellar-mediated alkaline hydrolysis of
A series of kineticruns were carried out at different
[NaOH] within its range 1.7 ϫ 10Ϫ5 to 1.0 M at a con-
stant ionicstrength (1.0 M) and 35 ЊC. Pseudo-first-
order rate constants (kobs) as summarized in Table I
obeyed eq. (1):
ab[HOϪ] ϩ bc[HOϪ]2
kobs
ϭ
(1)
1 ϩ b[HOϪ])
where a, b, and c are unknown kineticparameters. The
nonlinear least-squares calculated values of a, b, and
1
c
are (17.9 Ϯ 3.3) ϫ 10Ϫ4 sϪ , (2.08 Ϯ 5.54) ϫ
105 MϪ , and (46.3 Ϯ 0.8) ϫ 10Ϫ MϪ sϪ , respec-
tively. The fitting of observed data to eq. (1) is evident
from the values of rate constants (kcalcd , Table I) cal-
1
3
1
1

ALKALINE HYDROLYSIS OF 4-NITROPHTHALIMIDE
409
Table II Pseudo-First-Order Rate Constants (kobs) for
Hydrolysis of NPTH at Different Contents (X) of
Organic Cosolvents in Mixed Aqueous Solventsa
crease with the increase in CH3CN content from 1 to
70% v/v and CH3OH content from 0 to 80% v/v, re-
spectively. These results may be qualitatively ex-
plained in terms of simple theories, such as that of
Hughes and Ingold, and solvent polarity effect.
It should be noted that the presence of 0.01 M
NaOH in CH3OH solvent will generate CH3OϪ be-
cause the Ka values of CH3OH and H2O are nearly the
same. Thus, the increase in the content of CH3OH in
mixed aqueous solvents will increase [CH3OϪ] and de-
crease [HOϪ] because [CH3OϪ] ϩ [HOϪ] is constant
(ϭ0.01 M). Although the nucleophilic reactivity of
CH3OϪ toward electrophilic carbonyl carbon is nearly
60-fold larger than that of HOϪ, it has been shown
elsewhere [14] that the rate constants kobs are for hy-
drolysis and not for methanolysis of NPTH under the
present experimental conditions.
Cosolvent (X)
CH3OH
CH3CN
Ϫ
1
Ϫ1
% v/v
104 kobs/s
104 kobs/s
5
10
20
30
40
50
60
70
80
21.2 Ϯ 0.2b
18.5 Ϯ 0.2
13.0 Ϯ 0.5
10.9 Ϯ 0.1
8.33 Ϯ 0.3
6.24 Ϯ 0.03
4.52 Ϯ 0.03
3.29 Ϯ 0.03
2.12 Ϯ 0.02
18.2 Ϯ 0.1b
14.0 Ϯ 0.2
8.49 Ϯ 0.07
5.38 Ϯ 0.4
3.69 Ϯ 0.03
2.67 Ϯ 0.02
2.38 Ϯ 0.03
2.05 Ϯ 0.04
4
a [NPTH]0 ϭ 1 ϫ 10Ϫ M, [NaOH] ϭ 0.01 M ␭ ϭ 260 nm,
35ЊC and reaction mixture for each measurement for X ϭ CH3OH
contains 1% v/v CH3CN.
b Error limits are standard deviations.
Effect of [NaCl] on kobs at 0.03 M NaOH
and 35ЊC
culated from eq. (1) using the calculated values of ki-
neticparameters a, b, and c. The calculated value of
b is statistically unreliable because its standard devi-
ation is Ͼ200%. A relatively more reliable value of b
is expected to result by treating the observed data ob-
In order to determine the effect of ionic strength on
the rate of alkaline hydrolysis of NPTH, a few kinetic
runs were carried out within the [NaCl] range 0.1–3.0
M. The values of kobs, as summarized in Table III,
showed a mild increase (ϳ27%) with the increase in
the ionicstrength from 0.03 to 3.0 M.
tained within [HOϪ] range 1.7 ϫ 10Ϫ to 5 ϫ 10Ϫ ,
where the contribution of bc[HOϪ]2 is negligible com-
pared with ab[ϪOH] in eq. (1), with the equation
kobs ϭ ab[HOϪ]/(1 ϩ b[HOϪ]). Such a data treatment
5
3
1
5
gave 104a ϭ 19.9 Ϯ 0.3 sϪ and 10Ϫ b ϭ 1.27 Ϯ
1
0.14 MϪ .
Effect of [CTABr]T on kobs at Two Different
[NaOH] and 35ЊC
The [HOϪ] range of 1.7 ϫ 10Ϫ to 6.8 ϫ 10Ϫ
M
5
4
was achieved by the use of carbonate buffers. The val-
ues of kobs remained unchanged with the change in the
total concentration of carbonate buffer from 0.1 to 0.7
M at pH 10.2 Ϯ 0.1. These results show that the pH-
independent rate of hydrolysis of NPTH is insensitive
to general base catalysis. The rate of alkaline hydrol-
ysis of phthalimide [15] and N-hydroxyphthalimide
[13] were found to be insensitive and sensitive, re-
spectively, to general base catalysis.
A series of kineticruns was carried out under varying
total concentrations of cetyltrimethyl-ammonium bro-
mide ([CTABr]T) at 0.01 M NaOH in aqueous solvent
containing 1% v/v CH3CN. Several kineticruns were
also carried out at different [CTABr]T and 0.05 M
Table III Pseudo-First-Order Rate Constants (kobs) for
Hydrolysis of NPTH at Different [NaCl]a
Ϫ
1
[NaCl]/M
104 kobs/s
32.4 Ϯ 0.3b
34.3 Ϯ 0.5
35.7 Ϯ 0.6
36.8 Ϯ 0.5
37.9 Ϯ 0.5
38.6 Ϯ 0.5
37.4 Ϯ 0.6
41.1 Ϯ 0.6
Effect of Mixed CH3CN–H2O and CH3OH–
H2O Solvents on kobs at 0.01 M NaOH and
35ЊC
0.1
0.4
0.8
1.2
1.6
2.0
2.5
3.0
The rate of alkaline hydrolysis of NPTH was studied
within the respective acetonitrile and methanol content
range of 1 to 70 and 0 to 80% v/v in mixed aqueous
solvents. Mixed aqueous–organicsolvents were pre-
pared by adding the appropriate amounts of water and
organic cosolvent to the reaction mixtures. Pseudo-
first-order rate constants (kobs), as shown in Table II,
revealed an approximately 11.6- and 11.3-fold de-
a [NPTH]0 ϭ 1 ϫ 10Ϫ M, [NaOH] ϭ 0.03 M ␭ ϭ 260 nm,
35ЊC and reaction mixture for each measurement contains 1% v/v
CH3CN.
4
b Error limits are standard deviations.

410
KHAN AND ABDULLAH
Table IV Pseudo-First-Order Rate Constants (kobs) for Hydrolysis of NPTH at Different [CTABr]T and [NaOH]a
0.01
0.05
[NaOH]/M ϭ
104[CTABr]T M
104 kobs
s
104 kcalcdb s
104 kcalcdc s
104 kobs
s
104 kcalcdd s
104 kcalcde s
Ϫ
1
Ϫ
1
Ϫ
1
Ϫ
1
Ϫ
1
Ϫ1
0.2
0.4
0.6
1.0
2.0
3.0
4.0
5.0
7.0
22.6 Ϯ 0.3f
22.6 Ϯ 0.4
22.6 Ϯ 0.3
22.8 Ϯ 0.3
22.0 Ϯ 0.5
18.8 Ϯ 0.6
13.0 Ϯ 0.3
9.60 Ϯ 0.2
6.83 Ϯ 0.05
5.32 Ϯ 0.06
4.19 Ϯ 0.05
3.77 Ϯ 0.07
3.51 Ϯ 0.05
2.80 Ϯ 0.03
ϭ
33.9 Ϯ 0.6f
37.2 Ϯ 0.5
36.6 Ϯ 0.6
37.9 Ϯ 0.4
34.2 Ϯ 1.0
24.1 Ϯ 0.9
19.8 Ϯ 0.5
17.4 Ϯ 0.4
13.5 Ϯ 0.3
12.9 Ϯ 0.3
10.7 Ϯ 0.1
8.57 Ϯ 0.08
7.57 Ϯ 0.08
6.23 Ϯ 0.05
34.7
24.5
19.8
17.1
14.2
12.1
9.67
8.10
7.79
7.56
4.713
35.3
23.5
19.3
17.1
14.7
13.0
10.6
8.22
7.41
6.53
3.829
17.6
12.6
10.1
7.62
5.98
4.26
3.20
3.00
2.85
3.586
17.5
12.6
10.1
7.63
5.99
4.26
3.20
3.00
2.85
3.697
10.0
20.0
60.0
100.0
200.0
108 ͚di2
104 cmc
ϭ 2.5 Mg (2.7 M)h
1.8 Mg (1.9 M)h
a [NPTH]0 ϭ 1 ϫ 10Ϫ4 M, ␭ ϭ 260 nm, 35ЊC, and reaction mixture for each measurement contains 1% v/v CH3CN.
h
h
1
b Calculated from eq. (3) with 104 cmc ϭ 2.5 M, 104 kW ϭ 22.6 sϪ1, 104 kM ϭ 2.71 sϪ1, and KS ϭ 6769 MϪ
.
OH
h
h
1
h
c Calculated from eq. (5) with 104 cmc ϭ 2.5 M KBr ϭ 20, ␤ ϭ 0.8, 104 k0,
ϭ 17.9 sϪ1, 104 kOH,
ϭ 463 MϪ sϪ1, 104 k0,M
ϭ
ϭ
W
W
msh
1
2.70 sϪ1, 104 kOH.M ϭ 0, and KS ϭ 6710 MϪ
.
h
h
1
d Calculated from eq. (3) with 104 cmc ϭ 1.8 M, 104 kW ϭ 36.4 sϪ1, 104 kM ϭ 7.33 sϪ1, and KS ϭ 6333 MϪ
.
OH
h
h
1
h
e Calculated from eq. (5) with 104 cmc ϭ 1.8 M, KBr ϭ 20, ␤ ϭ 0.8, 104 k0,W ϭ 17.9 sϪ1, 104 kOH,W ϭ 463 MϪ sϪ1, 104 k0,M
msh
1
4.75 sϪ1, 104 kOH.M ϭ 8.90 sϪ1, and KS ϭ 11888 MϪ
.
f Error limits are standard deviations.
g The values of cmc were obtained from graphical technique [23].
h Parenthesized values were obtained from iterative technique [22] using eq. (3).
NaOH. The observed data (kobs vs. [CTABr]T) are
shown in Table IV.
shown in Scheme I. A similar mechanism has been
found to occur in several related reactions under es-
sentially similar experimental conditions. The ob-
served rate law (rate ϭ kobs [NPTH]T , where
[NPTH]T ϭ [NPTH] ϩ [NPTϪ]), and Scheme I can
lead to eq. (2):
DISCUSSION
The change in kobs with the change in [NaOH] (Table
I) may be explained through a reaction mechanism as
k0Ki[HOϪ] ϩ kOHKi[HOϪ]2
kobs
ϭ
(2)
1 ϩ Ki[HOϪ])
where k0 ϭ kЈOH Kw/Ka , with Ka ϭ [NPTϪ][Hϩ]/
[NPTH] and Kw ϭ [Hϩ][HOϪ] and Ki ϭ KiЈ/[H2O] ϭ
Ka/Kw. The occurrence of a reaction step for rate law
kw[H2O][NPTϪ], which is kinetically indistinguishable
from the reaction step for rate law kЈ [HOϪ
OH
][NPTH], may be ruled out based on evidence de-
scribed elsewhere [16]. Similarly, the notion that the
rate of hydrolysis of NPTH at considerably high val-
ues of [NaOH] does not involve NPTϪ as reactive spe-
cies may be shown to be incorrect based on kinetic
evidence discussed elsewhere [17].
Scheme I
Equation (2) is similar to eq. (1), with k0 ϭ a,

ALKALINE HYDROLYSIS OF 4-NITROPHTHALIMIDE
411
Ki ϭ b, and kOH ϭ c. The value of kOH (ϭ0.0463
proximation, in terms of pseudophase model (PM) of
micelle [9]. The PM model can lead to eq. (3):
1
1
MϪ sϪ ) is nearly 12-fold larger than kOH (ϭ3.84 ϫ
1
1
10Ϫ3 MϪ sϪ ) for phthalimide, which may be attrib-
uted to the favorable substituent effect exerted by the
nitro group in NPTH. It is interesting to note that the
kW ϩ kMh KS[Dn]
h
kobs
ϭ
(3)
1 ϩ KS[Dn])
4
1
values of kobs (Ϸ20 ϫ 10Ϫ sϪ , Table I) in the pH-
independent rate region are even slightly smaller than
h
h
where kW and kM represent pseudo-first-order rate
constants for hydrolysis of NPTH in the aqueous pseu-
dophase and micellar pseudophase, respectively; KS is
the micellar binding constant of NPTH; and [Dn] ϭ
[CTABr]T Ϫ cmc, with cmc representing critical mi-
celle concentration. The value of cmc was obtained by
using both iterative [22] and graphical [23] techniques.
The values of cmc at different [NaOH] are summa-
4
1
the corresponding kobs (ϭ24 ϫ 10Ϫ sϪ ) for phthal-
imide [18]. These results can be explained if the pH-
independent rate of hydrolysis is governed by the rate
law kЈ [OHϪ][NPTH] only. Under such conditions,
OH
kobs ϭ kOЈHKw/Ka, and hence it is possible that the ratio
kЈ /Ka for NPTH may not be significantly different
OH
from kOЈH/Ka for phthalimide if the substituent (NO2)
effects on both kЈOH and Ka are nearly the same in mag-
nitude. The calculated values of kЈOH(ϭk0Ki ϭ
h
rized in Table IV. The value of kW was taken as the
average value of kobs obtained at [CTABr]T Ͻ cmc.
The unknown kineticparameters, kMh and KS were cal-
culated from eq. (3) using the nonlinear least-squares
1
1
253 MϪ sϪ ) and Ka (ϭKiKw ϭ 3.1 ϫ 10Ϫ9 M with
Kw ϭ 2.4 ϭ 10Ϫ14 M2 at 35ЊC [19]) are nearly 5- and
7-fold, respectively, larger than the corresponding val-
h
technique. The calculated respective values of kM
1
1
ues of kOЈH(ϭ53 MϪ sϪ ) and Ka(ϭ4.3 ϫ 10Ϫ10 M)
for phthalimide. These results are conceivable from
the fact that the NO2 group is a powerful electron-
withdrawing group compared with H, and hence the
NO2 group must increase Ka. The electron-withdraw-
ing NO2 group is expected to increase both the elec-
trophilicity of carbonyl carbon and the leaving ability
of the leaving group in NPTH, and consequently it
increases the rate of reaction of HOϪ with NPTH
(Scheme I) compared with phthalimide.
and KS are (2.71 Ϯ 0.38) ϫ 10Ϫ4 sϪ and 6770 Ϯ
680 MϪ at 0.01 M NaOH and (7.33 Ϯ 0.41) ϫ
10Ϫ4 sϪ and 6330 Ϯ 460 MϪ at 0.05 M NaOH. The
fitting of the observed data to eq. (3) appears to be
satisfactory, as evident from the standard deviations
associated with the calculated kinetic parameters and
from the kcalcd values listed in Table IV.
1
1
1
1
The occurrence of ion exchange in ionic micellar-
mediated ionicor semi-ionicreactions has been un-
equivocally established [24–28]. The possible ion-ex-
change processes in the present reacting system are
BrϪ/HOϪ, BrϪ/NPTϪ, and NPTϪ/HOϪ. But the ion-ex-
change processes BrϪ/NPTϪ and NPTϪ/HOϪ may be
ignored compared with BrϪ/HOϪ due to the fact that
the maximum concentration of NPTϪ was kept at a
The rate constant kЈ is not expected to be affected
OH
by the ionic strength because kЈOH is the second-order
rate constant for a reaction involving a neutral and an
ionic reactant [20]. But the rate constant kOH (for a
bimolecular reaction involving two ionic reactants)
should be affected by the ionic strength. The nearly
26% increase in kobs due to the increase in ionic
strength from 0.03 to 3.0 M is possibly due to the ionic
strength effect on kOH , because at 0.03 M NaOH, the
contribution of kOHKi[HOϪ]2 compared to k0Ki[HOϪ]
in eq. (2) is ϳ41%.
The effects of mixed aqueous–organic solvents
were studied at 0.01 M NaOH, and under such con-
ditions, the contribution of kOHKi[HOϪ]2 in eq. (2) is
only 19%. Thus, 81% of the contribution to kobs comes
from k0Ki[HOϪ], and the reactions that obey such a
rate law are known to be inhibited by the increase in
the content of organic cosolvent in mixed aqueous sol-
vents. Pseudo-first-order rate constants for alkaline hy-
drolysis of phenyl benzoate decreased by slightly more
than 10-fold with the increase in CH3CN content from
2 to 70% v/v in mixed aqueous solvent [21].
4
relatively very small level (1 ϫ 10Ϫ M), and the dif-
ference in hydrophobicity of NPTϪ and HOϪ or BrϪ
should be very large. Thus, the most effective ion ex-
change, in the present experimental conditions, is
BrϪ/HOϪ. The kobs versus [CTABr]T profile for such
reactions is generally explained in terms of the pseu-
dophase ion-exchange (PIE) model [2,3,10]. In view
of the PIE model, the concentrations of HOϪ (ionic
reactant) and BrϪ (counterion) in the micellar and
aqueous pseudophase are generally governed by an
ion-exchange equilibrium, eq. (4):
OH
KBr ϭ ([HOWϪ][BrMϪ])/([HOMϪ][BrWϪ])
ϭ ([HOWϪ]mBr)/(mOH[BrWϪ]) (4)
where the subscripts W and M represent aqueous
The increase in [CTABr]T from ϳ3 ϫ 10Ϫ4 to 0.02
M resulted in an approximately monotonicdecrease in
kobs at both 0.01 and 0.05 M NaOH (Table IV). Such
results have been generally explained, to a first ap-
and micellar pseudophase, respectively, mBr ϭ
[BrMϪ]/[Dn] and mOH ϭ [HOMϪ]/[Dn].
The reaction scheme, in terms of PM model, can
lead to eq. (5):

412
KHAN AND ABDULLAH
h
h
msh
k0,W ϩ kOH,Wh[HOWϪ] ϩ (k0,M ϩ kOH,M
m )KS[Dn]
OH
kobs
ϭ
(5)
1 ϩ KS(Dn)
msh
1
where kOH,M ϭ kOH,Mh VM , with VM representing mi-
11897 MϪ . Thus, based on the criterion that the best
data fit is the one for which the value of ͚Di2 is the
least, it is not possible to ascertain the most appropriate
values of KBrOH and ␤. Such uncertainty in the use of
the PIE model is not due to ignoring the other possible
ion-exchange processes (NPTϪ/BrϪ and NPTϪ/HOϪ)
because similar uncertainty was encountered in the
data treatment of alkaline hydrolysis of securinine,
which involved only one ion-exchange BrϪ/HOϪ [29].
This is an apparent weakness in the PIE model, as
reported by other workers [30,31]. The value of
h
h
cellar molar volume [2,3], k0,W and k0,M represent
first-order rate constants for pH-independent hydrol-
ysis of NPTH and kOH,Wh and kOH,Mh are second-order
rate constants for the reactions of HOϪ with ionized
NPTH in aqueous and micellar pseudophase, respec-
tively.
The values of mOH at different [Dn] were obtained
from a kineticequation derived from PIE formalism
[2,3,10]. The value of ␤ (ϭfraction of counterions
bound to micelle) and KBrOH were considered to be 0.8
and 20, respectively, and k0,W (ϭ17.9 ϫ 10Ϫ sϪ )
kOH,W (ϭ0.04 MϪ sϪ ) for securinine is not signifi-
h
4
1
h
1
1
and kOH,Wh (ϭ0.0463 MϪ sϪ ) were obtained by car-
rying out the experiments in the absence of micelles.
The unknown parameters, k0,Mh, kOH,Mmsh, and KS were
cantly different from kOH,W (ϭ0.046 MϪ sϪ ) for
1
1
h
1
1
NPTϪ. However, nonkineticexperimental methods
OH
gave the values of KBr and ␤ as 7–31 and 0.8, re-
calculated from eq. (5) using the nonlinear least-
spectively [10]. The value of ͚di2 obtained for the data
h
OH
squares technique. The respective values of k0,M
,
treatment with eq. (5) at KBr ϭ 20 and ␤ ϭ 0.8 is
msh
1
kOH,M
,
and KS are (3.80 Ϯ 0.64) ϫ 10Ϫ4 sϪ ,
nearly 20% lower than that with eq. (3) at 0.05 M
NaOH (Table IV). This shows that the ion-exchange
BrϪ/HOϪ is kinetically significant at 0.05 M NaOH
(i.e., kOH,Mmsh does not seem to be negligible). But the
derived values of k0,Mh, kOH,Mmsh, and KS from eq. (5)
are not very reliable because of the probable uncer-
(Ϫ8.9 Ϯ 4.8) ϫ 10Ϫ4 sϪ , and 4900 Ϯ 860 MϪ at
1
1
1
0.01
M
NaOH and (4.75 Ϯ 0.91) ϫ 10Ϫ4 sϪ ,
(8.90 Ϯ 2.42) ϫ 10Ϫ4 sϪ , and 11888 Ϯ 1494 MϪ1 at
0.05 M NaOH. The negative value of kOH,Mmsh at 0.01
M NaOH is physically and chemically meaningless.
1
h
OH
Thus, relatively more reliable values of k0,M and KS
tainties in the chosen values of KBr (ϭ20) and ␤
at 0.01 M NaOH were calculated from eq. (5) by set-
ting kOH,M ϭ 0. The calculated values of k0,M and
(ϭ0.8).
msh
h
1
The value of KS (ϭ6600 MϪ ) is nearly 2.5-fold
msh
1
1
KS at kOH,M ϭ 0 are (2.70 Ϯ 0.39) ϫ 10Ϫ4 sϪ and
larger than KS (ϭ2600 MϪ ) for phthalimide [7]. The
6710 Ϯ 683 MϪ , respectively. The contribution of
1
CTABr micellar surface affinity of the m-nitroben-
zoate ion is nearly 3-fold larger than that of the o- or
p-nitrobenzoate ion [28]. The reason for the low cat-
ionic micellar surface affinity of the p-nitrobenzoate
ion is attributed to the significant polarity of the nitro
group. The relative cationic micellar surface affinity
of o- m-, and p-nitrobenzoate ions is not yet fully un-
derstood. The structural location of the nitro group in
NPTH roughly corresponds to in between the p- and
m-nitrobenzoate ions. Kineticdata revealed a nearly
1.4-fold larger affinity of the benzoate ion [32] com-
pared to the o- and p-nitrobenzoate ion [28] toward
cationic micelles. Thus, a ϳ2.5-fold larger value of
KS for NPTH compared to KS for phthalimide is
conceivable in view of the effect of the nitro group
on KS .
A skeptic might think that cationic micelles are of-
ten used to increase the rate of hydrolysis reactions,
whereas the reverse is observed here. Ionicmicelles
can increase the rate of a bimolecular reaction involv-
ing one or both reactant molecules as ions with charge
similar to the charge of counterions under one, or more
than one, of the following circumstances. (a) Micelles
cause an increase in the concentration of reactants into
kOH,W [HOWϪ] compared with k0,W is nearly 19% at
0.01 M NaOH. The least-squares (͚di2 where di ϭ
kobs i Ϫ kcalcd i) value obtained from eq. (3) is similar
h
h
msh
to one obtained from eq. (5) with kOH,M ϭ 0. It may
be noted that the change in KBr from 15 to 100 at
OH
OH
␤ ϭ 0.8 and in ␤ from 0.8 to 0.2 KBr ϭ 20 did not
reveal an appreciable change in ͚di2 values. Thus, the
msh
characteristic that kOH,M ϭ 0 indicates that ion-ex-
change BrϪ/HOϪ could not produce enough [HOM
]
Ϫ
msh
h
to make kOH,M
m
OH
significant compared with k0,M
in eq. (5).
The contribution of kOH,Wh [HOWϪ] compared with
h
k0,W is nearly 56% at 0.05 M NaOH. The data treat-
ment with eq. (5) resulted in a change in ͚di2 from
8
8
h
3.823 ϫ 10Ϫ to 3.737 ϫ 10Ϫ , in k0,M from 4.00 ϫ
10Ϫ to 5.68 ϫ 10Ϫ4 sϪ , in kOH,M
from 8.72 ϫ
4
1
msh
10Ϫ to 13.6 ϫ 10Ϫ4 sϪ , and in KS from 11313 to
4
1
13850 MϪ with the change in KBr from 10 to 100
1
OH
at ␤ ϭ 0.8. Similarly, the change in ␤ from 0.8 to 0.2
OH
at KBr ϭ 20 revealed the change in ͚di2 from
8
8
h
3.829 ϫ 10Ϫ to 4.024 ϫ 10Ϫ , in k0,M from 4.75 ϫ
10Ϫ to 5.71 ϫ 10Ϫ4 sϪ , in kOH,M
10Ϫ to 30.3 ϫ 10Ϫ4 sϪ , and in KS from 11888 to
from 8.72 ϫ
4
1
msh
4
1

ALKALINE HYDROLYSIS OF 4-NITROPHTHALIMIDE
413
the micellar phase by strongly binding the reactant(s).
(b) The rate of a reaction is either insensitive to the
polarity change or increases with the decrease in the
polarity of the reaction medium. (c) Both reactants
have the same site of average location into the micellar
phase. (d) The magnitude of the second-order rate con-
stant for a fully or semi-ionic bimolecular reaction into
the micellar phase should be sufficiently large to cause
a rate-increasing effect due to the increase in the con-
centration of micellized ionic reactant(s) through the
usual ion-exchange process. (e) Micelles stabilize the
transition state more strongly than the reactant state in
a critical rate-determining step of the reaction. In the
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h
present reaction system, the rate constants k0,M and
h
k0,W should not be affected by the occurrence of
HOϪ/BrϪ
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stand for pH-independent hydrolysis of NPTH. Al-
h
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its
,
kOH,M
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KOH,W is 0.0463
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HOϪ
h
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[21] (where
and with phenyl benzoate
kOH,W ϭ 0.68 MϪ sϪ ),
maxima were not obtained in the reactions of
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HOϪ
kOH,W ϭ 0.04
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.
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4
1
32 ϫ 10Ϫ sϪ
3.0 M increased kobs from
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41 ϫ
4
10Ϫ
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414
KHAN AND ABDULLAH
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