The reactivity of phosphonic esters in aqueous micellar solutions of cationic surfactants

Article abstract of DOI:10.1023/A:1014034118296

The micellar effect of cationic surfactants in alkaline hydrolysis of O-alkyl-O-aryl-chloromethyl phosphonates involves a positive contribution of concentrating the reagents and a negative effect of the micellar environment due to a loss in the activation entropy. The reactivity of substrates in micelles depends on both electronic and hydrophobic characteristics of substituents in the aryl group.

Full text of DOI:10.1023/A:1014034118296

Russian Chemical Bulletin, International Edition, Vol. 50, No. 7, pp. 1181—1185, July, 2001
1181
The reactivity of phosphonic esters in aqueous micellar solutions
of cationic surfactants
R. A. Shagidullina, L. Ya. Zakharova,« F. G. Valeeva, and L. A. Kudryavtseva
A. E. Arbuzov Institute of Organic and Physical Chemistry, Kazan Research Center of the Russian Academy of Sciences,
8 ul. Akad. Arbuzova, 420088 Kazan, Russian Federation.
Fax: +7 (843 2) 75 2253. E-mail: arbuzov@iopc.kcn.ru
The micellar effect of cationic surfactants in alkaline hydrolysis of O-alkyl-O-aryl-
chloromethyl phosphonates involves a positive contribution of concentrating the reagents and
a negative effect of the micellar environment due to a loss in the activation entropy. The
reactivity of substrates in micelles depends on both electronic and hydrophobic characteristics
of substituents in the aryl group.
Key words: micelles, cetyl-trimethylammonium bromide, kinetics, hydrolysis, substrate
specificity, phosphonic esters.
Scheme 1
The direct and reverse micelles, microemulsions, and
liquid crystals are the simplest analogs of biological
systems1 in which the most efficient, purposeful, and
selective mechanism of catalysis operates under mild
conditions. Among numerous recent studies of micellar
catalysis,2—5 one can distinguish two main directions:
first, a design of novel catalytic nanosystems operating by
the "guest—host" mechanism, and second, studies of the
effect of highly ordered media on the reaction rates. This
study concerns the second aspect. A qualitative analysis
of various components of the catalytic effect using true
values of the activation enthalpy and entropy in the
aqueous and micellar pseudo-phases was carried out to
interpret the regularity described in many reports, namely
a decrease in the reactivity of compounds upon transfer
of the reaction from the aqueous to micellar phase.
Earlier, we have studied the reactivity of a series of
phosphonic esters in the reverse micelles of sodium
dodecylsulfate and sodium bis(2-ethylhexyl)sulfo-
succinates as well as in the direct micelles of cetylpyri-
dinium bromide (CPB).6—8 A substantial change in the
nature of catalytic effect that was found on going from
aqueous to non-aqueous media stimulated further stud-
ies in this area. In this work, we studied the kinetics of
alkaline hydrolysis of a wide series of O-alkyl-Î-aryl-
chloromethyl phosphonates (1—9) in a micellar solu-
tion of cetyltrimethylammonium bromide (CTAB)
(Scheme 1). Both hydrophobicity and electronic prop-
erties of a substituent in an eliminatied group were
varied in the series of compounds 1—7, and the hydro-
phobicity of alkoxyl group at the phosphorus atom was
varied in substrates 1, 8 and 1, 9.
O
ClH2C
–
2 OH
P O
X
+
RO
ꢀ—'
O
ClH2C
–
–
O
X
P O
+
+ H2O
RO
R = Et, X = NO2 (ꢀ), Br ( ), H (!), Et ("), Bun (#),
n-C8H17 ($), i-C12H25 (%);
R = C6H13, X = H (&), NO2 (')
dodecylpyridinium bromide (DPB) of reagent purity grade were
twice crystallized from EtOH. The hydrolysis kinetics was
monitored spectrophotometrically on a Specord Ì-400 instru-
ment at 25 °C by a change in the absorption of the eliminated
group. The initial substrate concentration was 5•10–5 mol L
.
–1
The apparent rate constants (kapp) were calculated by the
formula ln(D∞ — D) = –kappt + const, where D and D∞ are the
optical densities of the solution at the moments t and after the
reaction was over. The kapp values were calculated using the
weighed least squares method.
The kinetic data were processed, similarly to works,6,7 in
the framework of a pseudo-phase model using the equation10
k2,w + k2,m/(VKSKOHC)
k′app
=
,
(1)
(1+ KSC)(1 + KOHC)
where k´app is the apparent second-order rate constant obtained
by division of kapp by the total concentration of nucleophile;
k2,w and k2,m (L mol–1 s–1) are the second-order rate constants
in the aqueous and micellar pseudo-phases, respectively; KS
and ÊOH (L mol–1) are the binding constants for a substrate
and hydroxide ion; V is the molar volume of a surfactant; C is
the concentration of a surfactant minus the critical concentra-
tion of micelle formation (CCM).
Experimental
Compounds 1—9 were synthesized according to a previ-
ously described procedure.9 CTAB («Sigma»), CPB, and
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1125—1129, July, 2001.
1066-5285/01/5007-1181 $25.00 ©ꢀ2001 Plenum Publishing Corporation

1182
Russ.Chem.Bull., Int.Ed., Vol. 50, No. 7, July, 2001
Shagidullina et al.
logk2,m
logk´app (max)
Results and Discussion
The reactivity of substrates 1—9 in water in the
absence of surfactants has been studied earlier.6 In the
CTAB micellar solution, the reactions of compounds
1—7 are accelerated up to 25 times due to solubilization
2
1
1
–
of the substrate and electrostatic attraction of OH ions
0
0
–1
–2
to the positively charged micelle surface (Fig. 1, Table 1).
The apparent rate constant decreases with weakening
the electron-acceptor effect of the substituent X in the
series 1—3. The order of the reactivity of the substrates
3—7 is opposite to the tendency found in water: the
maximal kapp was found for substrate 6 (Õ = n-Ñ8Í17)
and the minimal kapp was determined for 3 and 4
(Õ = Í, Et). As a whole, the effect of substituents
–1
–2
1
changes as follows: NO2 > Br > n-C8H17 > i-C12H25
>
B ≈ Et ≈ H. The findings of quantitative analysis of the
kinetic data are presented in Table 1.
–1.0
–0.5
0
0.5
lgk2,w
The pseudo-phase model (Eq. (1)) makes it possible
to calculate the second-order rate constants in the
micellar pseudo-phase for a wide series of substrates.
Fig. 2. Correlations between the second-order rate constants
for alkaline hydrolysis of substrates 1—7 in the micellar and
aqueous pseudo-phases (1) and those between the apparent rate
constant and rate constant in water (2), CTAB, 25 °C.
–1
kapp•103/s
The data on reactions in micellar solutions are not
available. Fig. 2 shows a linear correlation between the
second-order rate constants in the aqueous and micellar
pseudo-phases for the substrates 1—7. This fact supports
a suggestion that the reaction mechanism does not
change when the reaction is transferred from the aque-
ous into micellar phase. Meanwhile, the correlation
between the apparent rate constant k´app and reaction
rate constant in water k2,w holds only for substrates 1—5
and violates for 6 and 7. This fact can be explained by
0strengthening the concentrating effect (Fc ∼600—650)
with increasing the hydrophobicity of a substituent in
the phosphonic esters 6 and 7 compared to 3—5
(Fc = 150—190) (see Table 1).
3.0
a
2.5
6
2
2.0
7
1.5
3
1.0
4
5
0.5
0
–1
0.005
0.010
0.015 CCTAB/mol L
As can be seen in Table 1, the main reason for
acceleration of the reactions of substrates 1—7 is a local
increase in the concentration of compounds in the
CTAB micelles. The concentrating factor (Fc) decreases
initially in the series 1—3 in the range of 430—140 and
then increases with increasing in the chain length of the
substituent in the range of 150—640. This means that
the favorable effect of the micellar microenvironment
could increase the reaction rate by more than two orders
of magnitude. However, a significant decrease in the
reactivity upon the transfer of the reaction from the
aqueous to micellar phase (Fm < 1) results in a decrease
of the micellar effect by 6—25 times (see Table 1).
The reactivity decrease in the micellar pseudo-phase
compared to water is typical of most reactions of
nucleophillic substitution mediated by a ionic reactant
(see reviews12,13). The unfavorable effect of the micellar
microenvironment contradicts the Ingold—Hughes rule,14
which predicts benefit of ion-molecular reactions from a
decrease in the medium polarity due to charge delocal-
–1
kapp•103/s
30
b
20
10
1
8
–1
0.005
0.010
0.015 CCTAB/mol L
Fig. 1. The apparent rate constants for alkaline hydrolysis of
substrates 2—7 (a) and 1, 8 (b) in micellar CTAB solutions as a
function of the surfactant concentration (0.001 mol L–1 NaOH,
25 °C); the curve numbers correspond to numeration of sub-
strates in Scheme 1.

Russ.Chem.Bull., Int.Ed., Vol. 50, No. 7, July, 2001
1183
Reactivity of phosphorus acid esters in micelles
Table 1. The results of the quantitative analysis of the kinetic data (see Figs. 1, 4) according to Eq. (1)
a
a
Substrate
T/°C
k2,w
/L mol–1 s
(kapp/kw)max
KS
KOH
k2,m
/L mol–1 s
Fc
Fm
FcFm
–1
–1
–1
/L mol
b
1
1
25
25
25
25
15
35
45
25
25
25
25
25
25
25
25
25
15
35
45
4.0
4.0
4.0
4.0
2.0
7.5
7.0
35
9.5
7.0
11.1
7.8
5.5
5
1775
1190
2020
219
215
500
402
945
400
800
1675
2350
1490
2800
7600
527
430
250
260
240
145
80
4.2
2.0
4.0
3.0
135
90
0.055
0.10
0.78
4.0
2.5
7.0
13.5
0.016
0.01
0.0098
0.0082
0.0043
0.0031
0.014
0.38
0.90
0.60
1.7
2.0
432
266
187
0.014
0.026
0.19
1.0
1.2
1.1
0.99
0.028
0.042
0.053
0.051
0.037
0.039
0.117
0.14
0.36
0.35
0.38
0.2
6
6.5
36
c
d
1
e
1
10.8
10.8
7.2
11.9
8.2
6.3
6.0
7.7
9.8
22.5
24.9
77
6.0
10.6
8.2
6.9
14.0
0.55
0.24
0.19
0.16
0.12
0.08
0.12
3.0
3.0
1.7
4.5
9.9
b
2
240
b
3
144
146
190
604
640
657
766
55
b
4
5
7
9
75
87
b
b
6
22
24
74
95
20
21
24
15
350
470
365
337
24
30
29
31
b
7
8
b
d
9
109
e
9
19.7
20.0
20.0
15.0
57
52
57
k2,m
k2,w
KSKOH
a Calculated using the equation (kapp/kw)max
=
. In the right, the value of the maximal
2
V ( KS
+ KOH )
acceleration of the reaction is the ratio between the apparent rate constant (kapp) and the pseudo-first-order rate
constant in water (kw). The first term in the right expresses the effect of the micellar microenvironment (Fm) on
reactivity and the second term expresses the concentrating effect in micelles (Fc).
–1
b CTAB, 0.001 mol L NaOH.
–1
c CTAB, 0.0005 mol L NaOH.
–1
d CPB, 0.0005 mol L NaOH (see Ref 11).
–1
e DPB, 0.001 mol L NaOH.
ization in the transition state compared to the initial
state.
maximum. A positive effect of the micellar microenvi-
ronment (Fm ≥ 1) can occur in the systems with a low
extent of solubilization of reactants having a plateau on
the kinetic curves .
Earlier,8 we suggested that the violation of the
Ingold—Hughes rule in micellar systems is apparent. It
seems that the reactivity decrease in micelles is due to a
loss in the activation entropy because of a change in the
mutual arrangement and the mobility for the molecules
of reactants upon a sharp increase in their concentration
in the micelles rather than to a decrease in the local
polarity.
To verify the suggestion on the entropy control of
the reactivity, we studied the reaction kinetics for sub-
strates 1 and 9 in DPB micellar solutions at various
temperatures. The activation parameters were calculated
for the reaction occurring in the absence of the surfac-
tant (∆Íw, ∆Sw) using the k2,w rate constants and those
for the reaction in the micellar pseudo-phase (∆Ím,
∆Sm) calculated using the k2,m constants (Table 2). For
the hydrolysis of compound 1, the reactivity changes
according to the IngoldHugues concept. A micellar
effect is due to the positive contribution of both factors
(Fm ≥ 1 and Fc changes in the range of 6—11). When
the reaction is transferred into micelles, a decrease in
Examination of both our and reported kinetic
data8,11—13,15 shows that a trend to lowering the reac-
tivity (Fm < 1) is typical of systems with efficient
binding of reactants, i.e., of the reactions between hydro-
phobic compounds in aqueous solutions of surfactants
possessing high micelle-forming abilities. The kinetic
curves kapp—Csur for these reactions pass through a
Table 2. The activation parameters (∆H and ∆S) and rate constants (k/L mol–1 s–1) for the alkaline hydrolysis of
substrates 1 and 9 in the DPB micellar solutions at different temperatures (T/°C)
Sub-
k2,w
k2,m
∆Hw
∆Hm
∆Sm
J mol
∆Sw
strate
–1
–1
–1
15
25
35
45
15
25
35
45
kJ mol
K
1
9
2.0
1.7
4.0
3.0
6.9
4.5
14.0
9.9
2.5
0.6
4.0
0.9
7.0
1.7
13.5
2.0
46.2
40.8
39.3
30.0
–78.6
–98.9
–100.6
–144.4

1184
Russ.Chem.Bull., Int.Ed., Vol. 50, No. 7, July, 2001
Shagidullina et al.
–1
the activation enthalpy by 7 kJ mol–1 and the activation
kapp/s
0.15
–1
–1
entropy by 22 J mol
Ê
were found (see Table 2).
8
The reaction of more hydrophobic substrate 9 occur
contrary to the Ingold—Hughes concept: only the con-
centrating factor contributes positively (Fc ≈ 52—57)
and the micellar microenvironment affects negatively
the reactivity (Fm ≈ 0.2—0.4). A transfer of the reaction
of substrate 9 into the micellar pseudo-phase is accom-
panied (see Table 2) by a decrease in the activation
enthalpy by 10.8 kJ mol–1 and the activation entropy by
7
0.10
0.05
0
4
3
6
–1
–1
45.5 J mol Ê .
2
5
Thus, one can conclude that the transfer of ion-
molecular reactions from water to the less polar pseudo-
phase results in a decrease in the activation enthalpy
according to the Ingold—Hughes rule. Meanwhile, a
passage from diluted aqueous solutions to concentrated
solutions with the micellar pseudo-phase, is accompa-
nied by a decrease in entropy apparently at the expence
of a decrease in the entropy of mixing, a decrease in the
mobility of reactants, etc. In the reaction of substrate 1,
changes in the activation enthalpy and entropy are
mutually compensated and hence, the reactivity remains
practically the same in both pseudo-phases. An increase
in the concentrating effect in the reaction of compound
9 results in the more significant loss in the activation
entropy, which quantitatively exceeds the gain in the
activation enthalpy and manifests itself in an apparent
violating the Ingold—Hughes rule.
An increase in the hydrophobicity of a substituent in
the alkoxyl group is accompanied by an increase in kapp
by nearly an order of value (for substrates 3 and 8, see
Fig. 1). The rate growth mentioned in the case of
substrate 8 is due to a simultaneous increase in the
factors of concentrating and micellar microenvironment
(see Table 1). As we have shown previously,8 an in-
crease in the length of the alkoxyl group in p-nitrophenyl
esters affects more weakly the rate of alkaline hydrolysis
in the CPB micellar solution: kapp for substrate 9 this is
only twice as high as the kapp value for 1. The difference
in kapp for phosphonates 1 and 9 in the DPB micelles is
still less (Fig. 3). Examination of the kinetic data (see
Table 1) leads to a conclusion that in the last two cases,
a substantial increase in the binding constants in the
reaction of substrate 9 compared to 1 is compensated by
a decrease in the k2,m value for substrate 9. Hence,
distinguishing the hydrophobic substrates by micelles
depends on both the surfactant nature and substrate
structure, particularly on the arrangement of the hydro-
phobic moiety.
1
–1
0.05
[DPB]/mol L
Fig. 3. The apparent rate constants for alkaline hydrolysis of
substrates 1 (14) and 9 (5—8) in the DPB micellar solutions
at different temperatures in the presence of 0.001 mol L
NaOH at 15 (1, 5), 25 (2, 6), 35 (3, 7) and 45 °C (4, 8),
respectively.
–1
reactivity is due to the more favorable effect of the
micellar environment of the CPB aggregates on the
reaction transition state compared to the CTAB mi-
celles. Table 1 demonstrates that k2,m for substrate 1 in
the CPB micelles is nearly an order of magnitude higher
than k2,m for this substrate in the CTAB micellar pseudo-
phase, whereas the concentrating factors change scarcely.
An increase in the chain length of the surfactant, as a
rule, increases the micellar effect.16 The same trend is
found when the kinetic data in the micellar solutions of
DPB and CPB are compared. An increase in the micel-
lar effect by 4 times (for substare 1) and by 5 times (for
substrate 9) when the chain length of the surfactant by
kapp•102/s
–1
6
3
4
2
2
1
Note a considerable effect of the nature of the head
group in a surfactant on the reactivity of the compounds
under study. Figure 4 presents the kinetic data for
substrate 1 in the CTAB, CPB, and DPB micelles.
Comparison of the maximal values of kapp for CPB and
CTAB at the same concentration of alkali (see Fig. 4)
shows that the kapp value in the CPB micellar solutions
is nearly 7 times higher. Analysis of the kinetic data
using Eq. (1) indicates that such a difference in the
0
–1
0.005
0.010
0.015 [Surf]/mol L
Fig. 4. The apparent rate constants for alkaline hydrolysis of
substrates 1 in the micellar solutions of surfactants (Surf):
1, CTAB, 0.0005 mol L
–1
–1
NaOH, 2, CTAB, 0.001 mol L
–1
NaOH, 3, CPB, 0.0005 mol L NaOH.

Russ.Chem.Bull., Int.Ed., Vol. 50, No. 7, July, 2001
1185
Reactivity of phosphorus acid esters in micelles
6. L. Ya. Zakharova, F. G. Valeeva, R. A. Shagidullina, and
L. A. Kudryavtseva, Izv. Akad. Nauk, Ser. Khim., 2000,
1366 [Russ. Chem. Bull., Int. Ed., 2000, 49, 1360].
7. L. Ya. Zakharova, R. A. Shagidullina, F. G. Valeeva, E. P.
Zhil'tsova, and L. A. Kudryavtseva, Izv. Akad. Nauk, Ser.
Khim., 2000, 1712 [Russ. Chem. Bull., Int. Ed., 2000, 49,
1696 (Engl. Transl.)].
8. R. A. Shagidullina, L. Ya. Zakharova, and L. A.
Kudryavtseva, Izv. Akad. Nauk, Ser. Khim., 1999, 279 [Russ.
Chem. Bull., 1999, 48, 278 (Engl. Transl.)].
9. D. F. Toy and K. H. Rattenbury, US Pat. 2922810, 1960;
Chem. Abstrs., 1960, 54, 9848.
10. K. Martinek, A. K. Yatsimirsky, A. V. Levashov, and I. V.
Beresin, Micellization, Solubilization, and Microemulsions,
Ed. K. L. Mittal, Plenum Press, New York—London,
1977, 2, 489.
four methylene groups is due to an increase in
the concentrating factor in the CPB micelles by
12—17 times and the compensating decrease in the
factor of a micellar microenvironment by 3—5 times
(see Table 1).
Thus, the mechanism of the catalytic effect of the
CTAB micelles in the alkaline hydrolysis of a wide
series of phosphonic esters was studied. A significant
effect of the substrate structure, the nature of the head
group, and the chain length on the reactivity was found.
A widely reported trend of a decrease in the reactivity of
compounds in the micellar pseudo-phase was critically
examined for the first time, and this phenomenon was
interpreted on the basis of calculations of the activation
parameters for the reaction in both preudo-phases.
This work was financially supported by the Russian
Foundation for Basic Research (Project No. 99-03-
32037a).
11. L. Ya. Zakharova, F. G. Valeeva, L. A. Kudryavtseva, and
Yu. F. Zuev, Zh. Fiz. Khim., 2000, 74, 2007 [Russ. J. Phys.
Chem., 2000, 47 (Engl. Transl.)].
12. C. A. Bunton and G. Savelli, Adv. Phys. Chem., 1986,
22, 213.
13. L. S. Romsted, Surfactants in Solution, Ed. K. L. Mittal,
Plenum Press, New York—London, 1984, 4, 1015.
14. C. K. Ingold, Structure and Mechanism in Organic Chemis-
try, Clarendon Press, London, 1968.
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Received June 8, 2000;
in revised form March 6, 2001