pH-independent hydrolysis of 4-nitrophenyl 2,2-dichloropropionate in aqueous micellar solutions: Relative contributions of hydrophobic and electrostatic interactions

Article abstract of DOI:10.1002/poc.384

The pH-independent hydrolysis of 4-nitrophenyl 2,2-dichloropropionate (NPDCP) in the presence of aqueous micelles of sodium dodecyl sulfate, sodium dodecylbenzene sulfonate, alkyltrimethylammonium chlorides, alkyldimethylbenzylammonium chlorides (alkyl = cetyl and dodecyl) and polyoxyethylene(9) nonylphenyl ether was studied spectrophotometrically. The observed rate constants, k_obs, decrease in the following order: bulk water >cationic micelles >anionic micelles >non-ionic micelles. This order is different from that observed for pH-independent hydrolysis of 4-nitrophenyl chloroformate (NPCF), whose reaction is faster in cationic micelles than in bulk water. A proton NMR study on solubilization of a model ester, 4-nitrophenyl 2-chloropropionate, showed that the methylene groups in the middle of the surfactant hydrophobic chain are most affected by the solubilizate. Lower polarity and high ionic strength of interfacial water decrease the rates of hydrolysis of both NPCF and NPDCP, but t he fraction of the former ester that diffuses to the interface is probably higher than that of the latter. Therefore, whereas the (negatively charged) transition state of NPCF is stabilized by cationic interfaces and destabilized by anionic interfaces, that of NPDCP is negligibly affected by ionic interfaces, which explains the observed rate retardation by all ionic micelles. Calculated activation parameters corroborate our explanation.

Full text of DOI:10.1002/poc.384

JOURNAL OF PHYSICAL ORGANIC CHEMISTRY
J. Phys. Org. Chem. 2001; 14: 526–532
DOI:10.1002/poc.384
pH-independent hydrolysis of 4-nitrophenyl 2,2-dichloro-
propionate in aqueous micellar solutions: relative contri-
butions of hydrophobic and electrostatic interactions
Omar A. EI Seoud,1* Marie-FrancËoise Ruasse2 and Shirley Possidonio1
1Instituto de QuõÂmica, Universidade de SaÄ o Paulo, C.P. 26077, 05513-970 SaÄo Paulo, S.P., Brazil
2Institut de Topologie et de Dynamiques des System, Universite de Paris VII±CNRS, 1 Rue Guy de la Brosse, F-75005 Paris, France
Received 17 November 2000; Revised 19 January 2001; Accepted 23 January 2001
ABSTRACT: The pH-independent hydrolysis of 4-nitrophenyl 2,2-dichloropropionate (NPDCP) in the presence of
aqueous micelles of sodium dodecyl sulfate, sodium dodecylbenzene sulfonate, alkyltrimethylammonium chlorides,
alkyldimethylbenzylammonium chlorides (alkyl = cetyl and dodecyl) and polyoxyethylene(9) nonylphenyl ether was
studied spectrophotometrically. The observed rate constants, kobs, decrease in the following order: bulk water
>cationic micelles >anionic micelles >non-ionic micelles. This order is different from that observed for pH-
independent hydrolysis of 4-nitrophenyl chloroformate (NPCF), whose reaction is faster in cationic micelles than in
bulk water. A proton NMR study on solubilization of a model ester, 4-nitrophenyl 2-chloropropionate, showed that
the methylene groups in the middle of the surfactant hydrophobic chain are most affected by the solubilizate. Lower
polarity and high ionic strength of interfacial water decrease the rates of hydrolysis of both NPCF and NPDCP, but the
fraction of the former ester that diffuses to the interface is probably higher than that of the latter. Therefore, whereas
the (negatively charged) transition state of NPCF is stabilized by cationic interfaces and destabilized by anionic
interfaces, that of NPDCP is negligibly affected by ionic interfaces, which explains the observed rate retardation by
all ionic micelles. Calculated activation parameters corroborate our explanation. Copyright  2001 John Wiley &
Sons, Ltd.
KEYWORDS: 4-nitrophenyl 2,2-dichloropropionate; hydrolysis; aqueous micellar solutions; hydrophobic interac-
tions; electrostatic interactions
INTRODUCTION
anionic, sodium dodecyl sulfate (SDS), sodium dode-
cylbenzene sulfonate (SDBS); cationic, cetyltrimethy-
The effects of organized assemblies on chemical
reactivity have been rationalized in terms of differences
between the properties of interfacial and bulk water and,
for ionic micelles, local concentrations of reactants in the
Stern region and electrostatic interactions between the
charged interface and reactants and/or transition states.1–6
Micelle-mediated, pH-independent hydrolyses offer in-
sights into the subtle interactions that affect chemical
reactivity in organized assemblies because their mech-
anisms are simple and have been studied in sufficient
detail, and because they can be used to probe properties
(e.g. polarity and ionic strength) of interfacial water.7–9
Recently, we studied the pH-independent hydrolysis of
4-nitrophenyl chloroformate (NPCF), in the presence of
the aqueous micelles of the following surfactants:
lammonium
chloride
(CMe3ACl),
cetyldimethylbenzylammonium chloride (CMe2BzACl),
dodecyltrimethylamminium chloride (DMe3ACl), dode-
cyldimethylbenzylammonium chloride (DMe2BzACl);
non-ionic, polyoxyethylene(9) nonylphenyl ether, Arko-
pal N-090. The reaction was enhanced by cationic
micelles and retarded by anionic and non-ionic micelles.
These results were explained in terms of electrostatic
stabilization/destabilization of the reaction transition
state (TS), by charged interfaces and by differences
between the properties of interfacial and bulk water.9
We report here on the pH-independent hydrolysis of 4-
nitrophenyl 2,2-dichloropropionate (NPDCP) in the
presence of the above-mentioned aqueous micelles. We
were interested in determining how pH-independent
hydrolyses are affected by increasing reactant chain
length and hydrophobicity. In contrast to the hydrolysis
of NPCF, the corresponding reaction of NPDCP is
retarded by all micelles. Hydrophobic interactions of the
reactant state (RS) with the surfactant and micellar
*Correspondence to: O. A. EI Seoud, Instituto de Qu´ımica,
Universidade de Sa˜o Paulo, C.P. 26077, 05513-970 Sa˜o Paulo, S.P.,
Brazil.
Email: elseoud@iq.usp.br
Contract/grant sponsor: FAPESP.
Copyright  2001 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2001; 14: 526–532

HYDROLYSIS OF NPDCP IN AQUEOUS MICELLAR SOLUTIONS
527
‘medium’ effect cause rate retardation. Negligible
interactions of the (negatively charged) TS with the
ionic interface explain the small dependence of reaction
rates on the charge of the latter, and rate retardation by
cationic micelles.
monitoring the liberation of 4-nitrophenol at 320 nm as
a function of time. The relative standard deviation for
kobs, i.e. (standard deviation/kobs) Â 100 was ꢀ0.2%, and
when the experiment was run 3 times, the difference
between any two kobs was ꢀ2%.
1H NMR spectra. Proton NMR spectra were obtained
with a Bruker DRX-500 instrument at a digital resolution
of 0.05 Hz per data point. Chemical shifts were measured
at 30°C, relative to internal dioxane (5 Â 10À3 M), then
transformed into the TMS scale by using
ꢁdioxane = 3.53 ppm.12
EXPERIMENTAL
Materials. The reagents were obtained from Aldrich,
Fluka, Merck and Clariant (SDBS and Arkopal N-090)
and were purified by standard procedures.10
NPDCP was prepared by reacting the corresponding
acyl chloride with sodium 4-nitrophenoxide under phase-
transfer conditions. First, 2,2-dichloropropionic acid was
generated from its sodium salt by acidification with
sulfuric acid, followed by extraction of the aqueous
solution with dichloromethane. The solvent was removed
and the acid distilled, b.p. 80–81°C/10 mmHg (90–92°C/
14 mmHg).11 A mixture of 2,2-dichloropropionic acid
(13.16 g, 0.09 mol) and thionyl chloride (12.24 g,
0.1 mol) was refluxed for 3 h and the acyl chloride was
purified by distillation, b.p. 117–119°C (117.4 –117.8°C/
753 mmHg).11 Equal amounts (0.03 mol) of 4-nitrophe-
nol and NaOH were dissolved in 25 ml of water. After
agitation for 30 min, solid tetrabutylammonium hydro-
gensulfate (0.2g) was added, followed by a solution of
2,2-dichloropropionyl chloride (3.20 g, 0.02 mol) in
25 ml of dichloromethane. The mixture was vigorously
stirred for 30 min and the organic phase was separated,
washed with cold water, and dried over anhydrous
MgSO4. After solvent evaporation the ester was purified
with flash column chromatography, using hexane as
eluent; yield, 30%; IR (KBr, Perkin-Elmer FT-1750),
RESULTS AND DISCUSSION
We found that the reaction under consideration is
independent of solution pH in the range 1.0–4.0 (4%
acetonitrile in water). The pH-independent hydrolysis of
the structurally similar 4-nitrophenyl dichloroacetate is
associated with a sizeable solvent deuterium kinetic
isotope effect, kH O/kD O = 3.1.7a,b These results are
2
2
compatible with reaction Scheme 1 and the TS structure
depicted.7
In the TS, shown a second ‘general base’ water
molecule abstracts a proton from the attacking ‘nucleo-
philic’ water molecule, leaving the organic moiety with a
net negative charge.7 In the presence of 0.1 mol lÀ1 SDS
or CMe3ACl the micellar reaction was found to be
independent of [HCl] in the range 0.001–0.1 mol lÀ1 and
showed a sharp isosbestic point at 288 nm. Additionally,
the kinetics were rigorously first order; identical kobs were
obtained in the presence or absence of 4 Â 10À5 mol lÀ1
4-nitrophenol; an initial rapid release of 4-nitrophenol
was not observed. Hence the reactions in bulk aqueous
medium and in micellar pseudo-phases have similar
mechanisms, with water attack being the rate-limiting
step. This conclusion is similar to that reached for other
micellar, pH-independent hydrolysis reactions.8,9
In presence of the micelle, the reaction is represented
by Scheme 2, where E refers to NPDCP, kw and km are
pseudo-first-order rate constants in bulk water and in the
micelle, respectively, and the binding constant Ks is
1771 cmÀ1 ꢀCO; 1528 and 1348 cmÀ1, ꢀNO (asymmetric
2
and symmetric, respectively). The ester 4-nitrophenyl 2-
chloropropionate was prepared by a similar procedure
from the corresponding acyl halide and sodium 4-
nitrophenoxide; yield 38%; IR (KBr), 1774 cmÀ1, ꢀCO
;
1532 and 1347 cmÀ1, ꢀNO (asymmetric and symmetric,
2
respectively)
The purity of the esters and surfactants was established
by microanalysis (Microanalysis Laboratory, Instituto de
Qu´ımica, Universidade de Sa˜o Paulo) and, for surfac-
tants, by surface tension measurement (Lauda TE 1C
digital ring tensiometer). Their critical micelle concen-
trations (c.m.c.) were determined at 30°C in the presence
of 0.01 M HCl.
Kinetic measurements. The apparatus was that em-
ployed previously.9 All experiments were carried out in
triplicate, under pseudo-first-order conditions, in the
presence of 0.01 M HCL and 4% (v/v) acetonitrile.
Preliminary runs showed that the observed rate constant,
kobs, is independent of [NPDCP] in the range (0.5–
5) Â 10À5 M. In subsequent work, the final [NPDCP] was
0.6 to 2 Â 10À5 M. The reaction was followed by
Scheme 1
Copyright  2001 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2001; 14: 526–532

528
O. A. EL SEOUD, M.-F. RUASSE AND S. POSSIDONIO
splitting of protons D of the last two surfactants is the
presence of interfacial benzyl group, as discussed else-
where.9
Scheme 2
written in terms of the molarity of micellized surfactant,
i.e. (CD À c.m.c.), where CD is the analytical concentra-
tion of surfactant.
From scheme 2, the following equation was derived:8
kobs ˆ‰kw‡kmKsꢁCDÀc.m.c†Š=‰1‡KsꢁCDÀ c.m.c†Š ꢁ1†
Figures 1 and 2 show the dependence of kobs on
[surfactant], where the points are experimental and the
curves were calculated from Eqn. (1) by iteration, by
using experimentally determined kw and c.m.c. The
results for km and Ks are given in Table 1.
The dependence of kobs on temperature was studied at
[surfactant] in the plateau region of the rate constant–
surfactant plots (Figs 1 and 2). The results are given in
Table 2, and the corresponding activation parameters are
given in Table 3. Plots of log kobs versus 1/T were
rigorously linear, which shows that micellar structural
changes, if they do occur in the temperature range
employed, have no measurable effects, e.g. on the heat
capacity of the TS. Tables 2 and 3 also show data for
hydrolysis in electrolyte solutions in aqueous dioxane;
this will be addressed later.
PlotsofDꢁsurfactant = ꢁsurfactantinD2O À ꢁsurfactant ‡ model
ester 1 in D2O were found to be linear, and the slopes are
given in Table 4, which shows that model ester 1 is
solubilized (on average) in the region of surfactant
protons C and D.
Table 1 shows that the reaction is retarded by all
surfactants (km/kw from 0.02 to 0.11), is only slightly
affected by micellar charge (kDMe ACl/kSDS = 2.2) and,
3
for similarly charged surfactants, is almost insensitive to
variations of the structure of the hydrophilic group
(kDMe BzACl/kDMe ACl = 1.2). This is at variance with
2
3
the hydrolysis of NPCF in the presence of the same
surfactants, where the reaction shows catalysis by
cationic micelles (km/kw between 1.7 and 2.7) and a
We measured 1H NMR chemical shifts of the
surfactant discrete protons, ꢁsurfactant, as a function of
[ester] in order to determine its average solubilization site
in the micellar pseudo-phase. Model ester- 1, 4-
nitrophenyl 2-chloropropionate, was used instead of
NPDCP, because the latter would have undergone
extensive hydrolysis during sample preparation and
spectra acquisition. The structures presented show the
relevant surfactant protons of SDS, CMe3ACl, CMe2B-
zACl, and DMe2BzACl. The reason for the additional
stronger dependence on micellar charge (kDMe ACl/
3
kSDS = 13.5).
The question now arises of what the reasons are for the
different micellar effects on hydrolysis of NPCF and
NPDCP. At the outset, we emphasize that the rate-
limiting step of both reactions is the same, namely attack
on the ester carbonyl group by a ‘nucleophilic’ water
molecule catalyzed by a second ‘general base’ water
Figure 1. Dependence of observed rate constants, kobs, on [surfactant] for anionic and non-ionic micelles at 35°C. The points
are experimental and the curves were calculated using Eqn. =1)
Copyright  2001 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2001; 14: 526–532

HYDROLYSIS OF NPDCP IN AQUEOUS MICELLAR SOLUTIONS
529
Figure 2. Dependence of observed rate constants, kobs, on [surfactant] for cationic micelles at 35°C. The points are experimental
and the curves were calculated using Eqn. =1)
Table 1. Results of the application of Eqn. =1) to rate constants for the hydrolysis of 4-nitrophenyl 2,2-dichloropropionate in the
presence of ionic and non-ionic aqueous micelles at 35°C
d
Surfactant
104 c.m.c. (mol lÀ1 a
)
Ks, (mol lÀ1 b
)
103 km (sÀ1 b,c
)
km/kw
SDS
31
848 Æ 96
274 Æ 11
673 Æ 34
650 Æ 13
189 Æ 6
268 Æ 6
677 Æ 6
1.5
0.8
0.04
0.02
0.08
0.10
0.10
0.11
0.02
SDBS
2.6
CMe3ACl
CMe2BzACl
DMe3ACl
DMe2BzACl
Arkopal
N-090
2.3
0.024
19.0
28.7
0.62
2.7
3.4
3.3
3.9
0,0007
a
Experimental c.m.c., determined at 30°C in the presence of 0.01 M HCl by surface tension (see Experimental).
b Calculated from Eqn. (1) by iteration, by using experimental c.m.c.
c
Relative standard deviations in km are ꢀ1%.
d At 35°C, the rate constant in water, kw, is 0.0344 sÀ1
.
molecule.7,13 We address the question raised by con-
sidering the following: (i) strength of ester–micelle
association, (ii) average micellar solubilization site of
the ester, (iii) medium effects arising from the low
polarity and the high ionic strength of interfacial water
relative to bulk water and (iv) stabilization/destabiliza-
tion of RS and/or TS by different mechanisms, including
electrostatic and hydrophobic interactions.
NPCF. Therefore, part of the micellar effect on hydro-
lysis of both esters may arise from this large difference in
substrate–micelle association.
Concerning point (ii), our NMR data show that model
ester 1 is located, on average, in the region occupied by
surfactant protons C and D, i.e. the same solubilization
site of 4-nitrophenyl chloroacetate (model ester 2) that
has been employed (in a similar NMR experiment)
instead of NPCF. LogPoctanol [the partition coefficient of
a substrate between water and n-octanol, a measure of its
With regard to point (i), we note that NPDCP binds
efficiently to all micelles, and Ks for NPDCP ꢂ 10Ks for
Table 2. Dependence on temperature of observed rate constants, kobs, for hydrolysis of 4-nitrophenyl 2,2-dichloropropionate in
different reaction media
103 kobs (sÀ1
)
SDS
(0.2 M)
CMe3ACl CMe2BzACl DMe3ACl DMe2BzACl Arkopal N-090 CH3SO3Na (CH3)4NCl
T (°C) Water
(0.2 M)
(0.2 M)
(0.3 M)
(0.3 M)
(0.2 M)
(1.0 M)a
(0.1 M)a
15
25
35
45
13.4
21.6
34.4
51.8
0.31
0.58
1.09
1.85
0.95
1.76
3.11
5.14
1.18
2.04
3.64
5.88
1.18
2.15
3.79
5.97
1.39
2.47
4.18
6.4
0.29
0.52
0.85
1.36
0.39
0.67
1.10
1.72
0.50
0.89
1.46
2.30
a
In 50% (w/v) aqueous dioxane.
Copyright  2001 John Wiley & Sons, Ltd.
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530
O. A. EL SEOUD, M.-F. RUASSE AND S. POSSIDONIO
Table 3. Activation parameters for the pH-independent hydrolysis of 4-nitrophenyl 2,2-dichloropropionate in different reaction
mediaa,b
Reaction medium
D‡H (kcal molÀ1
)
D‡S (cal KÀ1molÀ1
)
D‡G (kcal molÀ1
)
Water
7.7
10.5
9.7
9.2
9.3
8.7
8.8
8.4
8.7
À40.4
À38.1
À38.6
À39.8
À39.5
À41.3
À44.2
À44.7
À43.5
19.7
21.9
21.2
21.1
21.0
21.0
21.9
21.8
21.6
SDS, 0.2 M
CMe3ACl, 0.2 M
CMe2BzACl, 0.2 M
DMe3ACl, 0.3 M
DMe2BzACl, 0.3 M
Arkopal N-90, 0.2 M
CH3SO3ÀNa
‡a
(CH3)4N ClÀb
‡
a
The errors are Æ0.1 kcal molÀ1 (D‡H and D‡G) and 0.5 cal KÀ1molÀ1 (D‡S).
b Salt concentration, 1.0 M in 50% (w/v) aqueous dioxane.
hydrophobicity,14 calculated using the LogP program
(ACD, Toronto, Canada)] are 1.66, 1.65, 2.17 and 3.24
for model ester 2, NPCF, model ester 1 and NPDCP,
respectively. That is, the first two esters have practically
the same hydrophobicity and are expected to be
solubilized (on average) in the same region of the
micellar pseudo-phase. NPDCP partitions into n-octanol
11.7 times more than model ester 1 and is expected,
therefore, either to be solubilized deeper within the
micelle or to associate more strongly with it. Hence part
of the micellar effect on both hydrolysis reactions may
arise from different solubilization sites of NPCF and
NPDCP within the micellar pseudo-phases.
In considering point (iii), we take into account the
dynamic nature of the micelle and the binding process
itself; both result in RS/TS sampling interfacial water
whose properties are akin to those of electrolyte solutions
in aqueous organic solvents.15–17 The low microscopic
polarity and high ionic strength of this water (relative to
bulk water) is expected to affect the present reactions
because their TS differ in polarity and solvation from
their RS. Aqueous dioxane has been proposed as a model
for interfacial water and we employed 1.0 M Me4NCl
and/or 1.0 M MeSO3Na in 50% (w/v) aqueous dioxane in
order to mimic the polarity and ionic strength of
interfacial water of cationic and anionic micelles,
respectively. Solubility constraints precluded the use of
more concentrated electrolyte solutions. Table 3 shows
that the reaction in these model media is slower than that
both in bulk water and the corresponding ionic micelles.
Although the ionic strength in the Stern layer is >1.0 M,16
rates of other pH-independent hydrolyses decrease as a
function of increasing [electrolyte].7a–c,8a,c That is,
hydrolysis of NPDCP in these models solvents would
have been even slower had we been able to use
[electrolyte] >1.0 M. Although the results in these media
and in micellar solutions show the same trend, i.e. the
reaction in both is slower than in bulk water, rate
retardation does not seem to have the same origin, as
shown by considering the contributions of DD‡H (= D‡H
in micelles or electrolyte solution ÀD‡H in bulk water)
and TDD‡S to DD‡G of the reaction in ionic micelles.
Whereas DD‡H contributes 1–2.8 kcal molÀ1, the TDD‡S
term makes
a minor contribution, from 0.3 to
0.7 kcal molÀ1 (1 kcal = 4.184 kJ). On the other hand,
for the reaction in model media both terms contribute
almost equally to DD‡G, and TDD‡S is negative. It is
interesting that the data for Arkopal-N090 are similar to
those for model media, i.e. DD‡H and (negative) TDD‡S
contribute equally to DD‡G.
Table 4. Dependence of chemical shift differences, Dꢁ = ꢁsurfactant À ꢁ=surfactant ‡ model ester1) of the surfactant discrete protons on
[4-nitrophenyl 2-chloropropionate] at 30°Ca,b
Slope (Hz / mol 4-nitrophenyl 2-chloropropionate)
‡
‡
Surfactant
Me3N
Me2N
CH2 (A)
CH2 (B)
CH2 (C) (CH2)5 (D) (CH2)n (D')
2025 Æ 41
CH3 (E)
C6H5CH2
SDS
576 Æ 34
870 Æ 83
1075 Æ 38
1131 Æ 59
543 Æ 70
CMe3ACl
CMe2BzACl
DMe2B-
zACl
355 Æ 72
1195 Æ 106 1757 Æ 142 1665 Æ 70
215 Æ 24 489 Æ 52
185 Æ 52 212 Æ 15
1528 Æ 193
1248 Æ 34
1759 Æ 34 À160 Æ 27 À319 Æ 34 952 Æ 29
1659 Æ 48
103 Æ 33
À64 Æ 31 694 Æ 47
a
Measurements were carried out at 500.13 MHz (see Results and Discussion for designation of the discrete surfactant protons).
b The following values of ꢁ (ppm) were observed for the surfactant discrete protons in the absence of solubilizate: SDS (0.1 M), 3.801, 1.459, 0.661 and 1.088
for protons (A), (B), (D) and (E), respectively; CMe3ACl (0.1 M), 2.944, 3.150, 1.556, 1.157, 1.089 and 0.671 for protons (Me3N ), (A), (B), (C), (D) and (E),
respectively; CMe2BzACl (0.1 M), 2.811, 2.761, 1.502, 1.002, 1.107, and 0.701 for protons (Me2N ), (A), (B), (D), (D') and (E), respectively; DMe2BzACl
(0.1 M), 2.820, 2.772, 1.531, 1.032, 1.064 and 0.681 for protons (Me2N ), (A), (B), (D), (D') and (E), respectively.
‡
‡
‡
Copyright  2001 John Wiley & Sons, Ltd.
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HYDROLYSIS OF NPDCP IN AQUEOUS MICELLAR SOLUTIONS
531
The large Ks and Dꢁsurfactant indicate that RS of NPDCP
is stabilized by hydrophobic interactions with the
surfactant tail [point (iv)]. On the other hand, the small
dependence of km on micellar charge [km(DMe3ACl)/
km(SDS) = 2 and 13.5 for NPDCP and NPCF, respec-
tively] indicates that TS does not interact significantly
with the ionic interface. It is tempting to use an analogy to
the ‘spatiotemporal hypothesis’ introduced by Menger.18
In order to rationalize the fast rates of certain intramol-
ecular reactions and the high catalytic efficiency of
enzymes, he argued that ‘the rate of reaction between
functionalities A and B is proportional to the time that A
and B reside within a critical distance.’ Evidence has
readily than NPCF to the micellar interface where such
activity is relatively high. Uneven hydration of the
oxyethylene units19 and uncertainty about the localiza-
tion of NPDCP in the non-ionic micelle preclude detailed
interpretation of the inhibition by Arkopal N-090.)
CONCLUSIONS
In contrast to micelle-mediated pH-independent hydro-
lysis of NPCF, the reaction of NPDCP is slower than that
in bulk water and shows little dependence on the structure
and charge of the surfactant. We explain our results in
terms of a combination of hydrophobic stabilization of
the RS, low water activity at the reaction site and
negligible contribution of electrostatic effects of the ionic
interface to the stability of TS. This explanation agrees
with the higher enthalpies and entropies of activation of
the micellar reaction relative to that in bulk water (Table
3). Experimental determination of the average solubiliza-
tion site of the substrate and of the activation parameters
is important for understanding the subtle interactions that
affect chemical reactivity in organized assemblies.
˚
been given to show that this distance should be <3 A. For
the reaction studied, the analogy to this idea, is as
follows: (a) because both micelle and substrate solubi-
lization are dynamic in nature, some RS/TS should
diffuse to the ionic interface and are affected by
electrostatic interactions; (b) the TSs of concern carry a
net negative charge, so that electrostatic interactions with
the micellar interface are much more important for TS
than for RS. These are stabilizing for cationic micelles
(rate increase) and destabilizing for anionic micelles (rate
decrease), provided that the TS comes within a certain
distance from the interface. The contribution of electro-
static interactions depends, therefore, on the fraction of
TS that comes within this distance. If this fraction is
insignificant, the micelle-mediated reaction will be
dominated by other (retarding) effects, e.g. stabilization
of the RS and medium effects. (Although the rate
variations that are being discussed here are very modest
compared with those discussed by Menger,18 the analogy
employed is useful, provided that the dynamic nature of
the micellar system is taken into account.)
Acknowledgements
We thank FAPESP for financial support, CAPES/
COFECUB for travel funds, CNPq for fellowships to S.
Possidonio (graduate) and O.A. El Seoud (research
productivity) and K. Greiner and U. Haller for their help
during the preparation of the manuscript.
The preceding discussion agrees with contributions of
DD‡H and TDD‡S to DD‡G of hydrolysis of both esters,
and we concentrate on cationic micelles. For example,
RS/TS of NPCF diffuse easily within the micelle, so that
the reaction is sensitive to charge of the interface
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(kDMe ACl/kSDS = 13.5) and km >kw. The reaction is
3
associated
with
negative
DD‡H
(À2.9
to
À3.4 kcal molÀ1) due to electrostatic stabilization of the
TS, and negative TDD‡S (À2.5 to À3.0 kcal molÀ1) due
to decrease in the number of degrees of freedom on going
from RS to TS (the latter is associated with the interface).
Hydrolysis of NPDCP is associated with a positive DD‡H
whereas TDD‡S makes a smaller contribution. Both
quantities agree with an RS that is stabilized by
hydrophobic interactions with the surfactant tail, a
reaction occurring in an aqueous medium of low water
activity, and a TS whose stability is little affected by the
ionic interface. (The question of water activity is
important because the reaction is second order in water,
i.e. km/kbulk water = k3m [interfacial water]2/k3w [bulk
water]2, where k3 refers to the third-order rate constant.
Consequently, the effect of decreased water activity is
expected to be larger for NPDCP because it diffuses less
Copyright  2001 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2001; 14: 526–532

532
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