Aminolysis of carboxylic acid esters in direct, bicontinual, and inverse microemulsions based on cetyltrimethylammonium bromide

Article abstract of DOI:10.1007/s11176-005-0376-x

The reactivity of primary alkylamines in cleavage of p-nitrophenyl esters of carboxylic acids in microemulsions of different structures based on cetyltrimethylammonium bromide was studied. The aminolysis rate considerably increases in going from inverse to direct microemulsions, mainly owing to the concentration of the reactants in the boundary layer. 2005 Pleiades Publishing, Inc.

Full text of DOI:10.1007/s11176-005-0376-x

Russian Journal of General Chemistry, Vol. 75, No. 7, 2005, pp. 1108 1112. Translated from Zhurnal Obshchei Khimii, Vol. 75, No. 7, 2005,
pp. 1171 1176.
Original Russian Text Copyright
2005 by Mirgorodskaya, Kudryavtseva, Shtykova, Bogomolova, Shtykov.
Aminolysis of Carboxylic Acid Esters in Direct, Bicontinual,
and Inverse Microemulsions Based
on Cetyltrimethylammonium Bromide
A. B. Mirgorodskaya*, L. A. Kudryavtseva*, L. S. Shtykova**,
I. V. Bogomolova**, and S. N. Shtykov**
* Arbuzov Institute of Organic and Physical Chemistry, Kazan Research Center,
Russian Academy of Sciences, Kazan, Tatarstan, Russia
** Chernyshevskii Saratov State University, Saratov, Russia
Received February 27, 2004
Abstract The reactivity of primary alkylamines in cleavage of p-nitrophenyl esters of carboxylic acids in
microemulsions of different structures based on cetyltrimethylammonium bromide was studied. The amino-
lysis rate considerably increases in going from inverse to direct microemulsions, mainly owing to the con-
centration of the reactants in the boundary layer.
Detergent microemulsions are macroscopically uni-
form, thermodynamically stable, self-organizing dis-
persions with aqueous and hydrocarbon (oil) phases,
in which the phase boundary is occupied by molecules
of micelle-forming surfactants and cosurfactants.
Variation of the water-to-oil volume ratio significantly
alters the structure and properties of the microemul-
sions, with the phase inversion and rearrangement of
the phase boundary [1]. Both inverse, bicontinual, and
direct microemulsions can be formed, with the system
as a whole remaining macrohomogeneous and optical-
ly transparent and preserving a high solubilizing ca-
pacity toward both polar and nonpolar substances.
Unique solubilizing properties of microemulsions,
along with exceedingly developed phase boundary
surface, provide efficient contact between hydrophilic
and hydrophobic reagents and allow successful use of
these systems as nanoreactors for various chemical
processes. With such systems, it is possible to control
not only the reaction rates but even the reaction mech-
anisms [2 5].
separation when the content of the two other compo-
nents, water and hexane, was varied in a wide range.
For kinetic experiments, we used the microemulsions
with the water volume fraction ( ) of 0.77, 0.30, and
0.14, which corresponds to the direct, bicontinual, and
inverse structures, respectively [8, 9].
In molecular aqueous solutions, the cleavage of
carboxylic acid esters in the presence of primary
amines mainly occurs along two pathways: aminolysis
(major reaction) and base hydrolysis (side reaction):
R NH2
RC(O)NHR + HOC6H4NO2-p.
RC(O)OC6H4NO2-p
2OH
RC(O)O + OC6H4NO2-p,
R and R = CnH2n+1
.
The kinetic data characterizing the reaction of
p-nitrophenyl acetate I and p-nitrophenyl laurate II
with primary amines in microemulsions of various
types are given in Table 1. It is seen from these results
that, at equal concentrations of amines (Cam), the
observed rate constants (kobs) only slightly depend on
the hydrophobicity of the amine. At the same time,
the rate constants depend on the structure of the mi-
croemulsions, increasing in going from the inverse
to direct microemulsion. In all the microemulsions
studied, acetate I is cleaved by a factor of 15 20 more
readily than laurate II.
This study continues our previous investigations
concerning the catalytic properties of microemulsions
in nucleophilic substitutions [5 8]. To reveal how the
structure of the reaction medium affects the rate of
ester cleavage, we studied the reactions of p-nitro-
phenyl esters of carboxylic acids with primary aliphat-
ic amines in microemulsions consisting of water,
cetyltrimethylammonium bromide, n-butanol, and
n-hexane. The content of the surfactant and cosurfac-
tant in the microemulsions was 9.42 wt % each (molar
ratio 1 : 5), which made it possible to avoid phase
The concentration dependences of kobs in the range
of amine concentration from 0.005 to 0.04 M in all
1070-3632/05/7507-1108 2005 Pleiades Publishing, Inc.

AMINOLYSIS OF CARBOXYLIC ACID ESTERS IN DIRECT, BICONTINUAL
1109
the microemulsions studied are linear and pass virtual-
ly through the origin (Fig. 1). This fact indicates that
the contribution of base hydrolysis to the ester cleav-
age is insignificant. This contribution can be quantita-
tively estimated from the kinetics of hydrolysis of
esters I and II at the same pH as those of the micro-
emulsions containing amines. Depending on the basic-
ity of the amine and its concentration in the system,
the apparent pH values of the microemulsions, deter-
mined with a pH meter, are in the range 9.8 10.5.
The highest pH was observed with hexylamine at its
0.04 M content in the direct microemulsion. The pH
dependences of the observed rate constant of the base
hydrolysis of esters (k0) in various microemulsions
are shown in Fig. 2. In the systems under considera-
tion, the contribution of hydrolysis to the overall rate
of cleavage of the carboxylic acid esters does not
exceed 10%; nevertheless, it should be taken into
account at low amine concentrations or at high pH.
0.20
0.15
0.10
0.05
0
1
2
3
4
5
6,7
0.01 0.02 0.03 0.04 0.05 0.06
am, M
C
We have shown previously that the hydrolysis of
carboxylic acid esters is usually characterized by a
high activation energy, whereas in aminolysis of these
compounds the rate constants only weakly depend on
temperature [5]. A kinetic study of the cleavage of
ester I at different temperatures in various microemul-
sions based on cetyltrimethylammonium bromide in
alkaline media and in the presence of amines allowed
evaluation of the activation energy (Ea) of hydrolysis
and aminolysis (Table 2).
Fig. 1. Influence of the amine concentration on the
observed rate constant k of cleavage of carboxylic
acid esters in microemulsions based on cetyltrimethyl-
ammonium bromide: (1) hexylamine, p-nitrophenyl
acetate, 0.77; (2) decylamine, p-nitrophenyl acetate,
obs
0.77; (3) decylamine, p-nitrophenyl acetate,
(4) decylamine, p-nitrophenyl acetate, 0.14; (5) decyl-
amine, p-nitrophenyl laurate, 0.77; (6) decylamine,
0.30; and (7) decylamine, p-
0.14.
0.30;
p-nitrophenyl laurate,
nitrophenyl laurate,
The activation energies are apparent quantities and
depend on the thermodynamic parameters of the inter-
mediate equilibrium processes. Hydrolysis of ester I
in all the microemulsions is characterized by high
activation energies, exceeding Ea of this reaction in
0.020
0.015
1
1
water (45.8 kJ mol [5]). The activation energy of the
aminolysis in microemulsions is considerably lower
than that of hydrolysis, although it is essentially
nonzero, in contrast to that of the process in molecular
solutions (Table 2). Thus, at the same pH, the contri-
bution of base hydrolysis to the cleavage of esters
2
0.010
0.005
0
1
Table 1. Observed rate constants (kobs, s ) of the reac-
3
tions of carboxylic acid esters I and II with amines in
microemulsions of various types (25 C, Cam 0.04 M)
4
0.14
II
0.30
II
0.77
Amine
9.2
9.6 10.0
10.4
10.8 11.2
pH
I
I
I
II
Fig. 2. Rate constant of hydrolysis of carboxylic acid
esters in microemulsions based on cetyltrimethylam-
monium bromide as a function of pH: (1, 3) p-nitro-
Hexylamine 0.059 0.0027 0.068 0.0035 0.20 0.014
Octylamine 0.050 0.0023 0.065 0.0036 0.19 0.014
Decylamine 0.045 0.0025 0.055 0.0033 0.18 0.013
Cetylamine 0.030 0.0028 0.038 0.0039 0.18 0.011
phenyl acetate and (2, 4) p-nitrophenyl laurate;
(1, 2) 0.77 and (3, 4) 0.30.
:
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 75 No. 7 2005

1110
MIRGORODSKAYA et al.
1
Table 2. Influence of temperature and composition of the
microemulsion on the observed rate constants of hydrolysis
and aminolysis of p-nitrophenyl acetate
Table 3. Second-order rate constants (k2, , l mol 1 s ) of
aminolysis of carboxylic acid esters I and II in the non-
aqueous phase of microemulsions of various compositions
(25 C)a
t,
Base hydrolysis
Aminolysis
Param-
eter
0.14
II
0.30
II
0.77
C
0.14 0.30 0.77 0.14 0.30 0.77
Amine
I
I
I
II
k01, kobs 25 0.00010 0.0007 0.0010 0.023 0.028 0.09
s
32 0.00043 0.0015 0.0021 0.025 0.032 0.13
44 0.00078 0.0029 0.0046 0.033 0.044 0.14
Hexylamine 1.27 0.058 1.15 0.060 1.27 0.090
Octylamine 1.07 0.050 1.10 0.061 1.20 0.088
Decylamine 0.97 0.053 0.94 0.056 1.13 0.081
Ea,
80.6
57.4 62.7 16.2 18.9
17.3
1
kJ mol
Cetylamine
0.65 0.060 0.65 0.097 0.73 0.066
a
a
1
1
The rate constant of base hydrolysis was determined in micro-
emulsions with a buffer solution of sodium tetraborate (pH
9.2) as aqueous phase.
k
of aminolysis of acetate I, l mol
s : in hexane, 0.018
2
(hexylamine) and 0.02 (cetylamine); in butanol, 0.28 (hexyl-
amine) and 0.33 (cetylamine).
increases with temperature. Hence, in oil/water micro-
emulsions, the temperature, along with the concentra-
tion of the hydroxide ion and amine, can be an im-
portant parameter governing the amide : acid ratio in
the final reaction product.
emulsions based on cetyltrimethylammonium bromide
was characterized with the ET(30) solvatochromic
probe,
2,6-diphenyl-4-(2,4,6-triphenylpyridinio-1)-
phenoxide existing in alkaline solutions in the form of
a zwitter ion:
C6H5
O
C6H5
N+
C6H5
second-order rate constants of aminolysis (k2) aver-
aged over the emulsion volume. Low-polarity long-
chain amines are virtually insoluble in water and con-
centrate in the oil phase or at the phase boundary. The
second-order rate constants determined for the non-
C6H5
C6H5
This compound is very sensitive to the polarity of
the medium, in contrast to other spectroscopic probes
[10]. The ET(30) probe is used for characterizing the
polarity of not only homogeneous solutions but also
microheterogeneous media, in particular, micellar
solutions and microemulsions [11, 12]. A combination
of the hydrophilic and lipophilic properties ensures
the localization of ET(30) in the phase boundary layer
[13, 14]. The hydroxy group of the probe, dissociated
in an alkaline solution, will interact electrostatically
with the head groups of a cationic surfactant, and the
hydrophobic moiety will be oriented toward the oil
phase. Such localization and orientation in the micro-
emulsions under consideration can also be presumed
for long-chain amines exhibiting pronounced amphi-
philic properties.
aqueous part of the microemulsions [k2, = (1
)k2]
allow the effect of this factor on the reaction rate to
be taken into account; these quantities should reflect
changes in the microenvironment in the reaction zone
(factor of the medium).
The k2, values obtained (Table 3) appeared to be
similar for all the types of microemulsions. This fact
suggests that, irrespective of the microemulsion struc-
ture, the reaction occurs in areas of similar micro-
polarity. To estimate preliminarily the site of localiza-
tion of the reaction components, we determined k2
of aminolysis of ester I in nonpolar hexane and low-
polarity butanol (see note to Table 3), which appeared
to be considerably lower than k2, in microemulsions.
Hence, in microemulsions the reaction occurs in a
relatively polar boundary layer. Within this layer, the
reaction zone can probably shift somewhat toward the
oil or aqueous phase depending on the hydrophobicity
of the reactants. This is suggested, e.g., by an increase
in k2, of aminolysis of acetate I in going from cetyl-
amine to hexylamine (Table 3).
The influence of the water content in the system on
the parameter ET is shown in Fig. 3. Three portions
of the plots can be distinguished. In the initial portion
< 0.22), ET is independent of the water content in
the system. Then, in the narrow interval 0.22 <
0.25 (which corresponds to a transition from the in-
verse to bicontinual emulsion, as we determined pre-
viously by a number of physical methods [8]), ET
(
<
The micropolarity of the phase boundary in micro-
sharply increases. At
> 0.25, ET becomes again
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 75 No. 7 2005

AMINOLYSIS OF CARBOXYLIC ACID ESTERS IN DIRECT, BICONTINUAL
independent of the volume fraction of water, since the
1111
probe used is apparently insensitive to further struc-
tural rearrangements in the system and does not allow
detection of the transition from the bicontinual to
direct microemulsion. We have determined the inter-
val of this transition previously ( 0.4 0.5) by vis-
1
53.2
52.8
1
2
cometry and H NMR spectroscopy with pulse mag-
netic field gradient and confirmed it by the method of
chemical kinetics [8]. This fact emphasizes once again
that it is appropriate to use an integrated approach
when studying the structural properties of microemul-
sions.
52.4
52.0
51.6
3
A comparison of the ET values in hexane (31.0),
n-butanol (50.2), water (63.1) [10], and the systems
under consideration (52.2 53.2) shows that the probe
is localized in the boundary layer containing butanol
and certain amount of water.
0.2
0.4
0.6
0.8
In the presence of n-decylamine, the dependence of
ET on the water content in the microemulsions is
different (Fig. 3). This amine makes the probe more
sensitive to structural changes. In this case, we can
distinguish in the plot the portions corresponding to
the existence of inverse ( < 0.2), bicontinual (0.5 >
> 0.25), and direct ( > 0.6) microemulsions, which
is consistent with the data from [8]. We have shown
previously that introduction of long-chain amines into
microemulsions based on cationic surfactants results
in the redistribution of the components between the
phases [15]. Changes in ET depending on the decyl-
amine concentration (Fig. 3) reflect, apparently, the
rearrangements occurring in the system and resulting
in changes in the microenvironment of the probe. In
the presence of decylamine, the intervals of the phase
transitions slightly shift toward higher water content.
We have detected such a shift previously in conducto-
metric studies and attributed it to a change in the po-
rosity and flexibility of the boundary layer with varia-
tion of the cosurfactant (butanol) concentration [16].
Fig. 3. E as a function of water content in micro-
T
emulsions based on cetyltrimethylammonium bromide:
(1) no amine, (2) 0.02 M decylamine, and (3) 0.04 M
decylamine.
sions of different types, kobs is the higher, the smaller
the nonaqueous phase volume (Table 1). Hence, to
attain the highest rates of aminolysis of carboxylic
acid esters, preference should be given to direct mi-
croemulsions.
At the same time, when using microemulsions as
reaction medium, there may be a problem with their
limited solubilizing capacity for long-chain amines,
especially in the case of direct microemulsions. We
determined by potentiometric titration the amount of
amines that can be added to a homogeneous micro-
emulsion without causing visible changes (Table 4).
We found that the solubility of amines in the micro-
emulsions drastically decreased with an increase in the
length of the hydrocarbon substituent in the amine.
It is also seen that the solubilizing capacity of the
bicontinual microemulsion is approximately three
times higher than that of the direct microemulsion.
The solubility of amines in inverse microemulsions is
so high that it cannot be estimated by visual observa-
tion of the state of the microemulsion.
A comparison of the results of the solvatochromic
and kinetic experiments shows that microemulsions
with the volume fraction of water of 0.77, 0.30, and
0.14 in the presence of decylamine are characterized
by virtually equal ET values (Fig. 3) and similar bi-
molecular rate constants of aminolysis in the nonaque-
ous phase (Table 3). For the reaction of decylamine
with p-nitrophenyl acetate in a microemulsion with
0.25, characterized by the highest ET (53.15), we
It should be noted that acidification of the medium,
which is commonly used to increase the solubility of
amines in aqueous solutions and is accompanied by
amine protonation, leads to an opposite result in the
case of microemulsions. Such an effect is due to the
unfavorable influence of the electrostatic repulsion
between the positivelly charged head groups of the
surfactant and protonated nitrogen atom of the amine,
preventing incorporation of the amine in the boundary
layer.
1
obtained k2, as high as 1.56 l mol 1 s .
Thus, the influence of the microenvironment (factor
of the medium) is reflected in k2, of the aminolysis
of the esters. At the same time, the apparent rate con-
stants of the process are governed by the concentrating
effect: With the same amount of an amine, in emul-
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 75 No. 7 2005

1112
MIRGORODSKAYA et al.
Table 4. Solubilizing capacity of microemulsions for long-
chain primary amines
ACKNOWLEDGMENTS
The study was financially supported by the Russian
Foundation for Basic Research (project nos. 03-03-
32952 and 04-03-32946).
Amine content, M (%)
Amine
0.77
0.30
REFERENCES
Octylamine
Decylamine
Dodecylamine
Cetylamine
0.194 (2.50)
0.135 (2.12)
0.097 (1.70)
0.033 (0.71)
0.88 (11.3)
0.40 (6.87)
0.243 (4.50)
0.10 (2.10)
1. Microemulsions: Structure and Dynamics, Fri-
berg, S.E. and Bothorel, P., Eds., Boca Raton: CRC,
1987.
2. Menger, F.M. and Rourk, M.J., Langmuir, 1999,
vol. 15, no. 2, p. 309.
3. Zhang, P. and Gao, L., Langmuir, 2003, vol. 19,
no. 1, p. 208.
EXPERIMENTAL
4. Schwuger, M.J., Stichdom, K., and Schomacker, R.,
Chem. Rev., 1995, vol. 95, no. 4, p. 849.
5. Mirgorodskaya, A.B. and Kudryavtseva, L.A., Zh.
Obshch. Khim., 2002, vol. 72, no. 8, p. 1343.
6. Mirgorodskaya, A.B., Kudryavtseva, L.A., Zu-
ev, Yu.F., Idiyatullin, B.Z., and Fedotov, V.D.,
Zh. Obshch. Khim., 2002, vol. 72, no. 7, p. 1077.
7. Mirgorodskaya, A.B., Kudryavtseva, L.A., Zu-
ev, Yu.F., Idiyatullin, B.Z., and Fedotov, V.D., Izv.
Ross. Akad. Nauk, Ser. Khim., 2001, no. 2, p. 238.
8. Zuev, Yu.F., Mirgorodskaya, A.B., Kudryavtse-
va, L.A., Idiyatullin, B.Z., and Khamidullin, R.N.,
Zh. Obshch. Khim., 2004, vol. 74, no. 7, p. 1140.
9. Bhattacharya, S. and Senhalatha, K., J. Chem. Soc.,
Perkin Trans. 2, 1996, no. 9, p. 2021.
10. Reichardt, Ch., Solvents and Solvent Effects in Or-
ganic Chemistry, Weinheim: VCH, 1988.
11. Lay, M.B., Drummond, C.J., Thistlethwaite, P.J., and
Grieser, F., J. Colloid Interface Sci., 1989, vol. 128,
no. 2, p. 602.
Microemulsions were prepared from commercial
cetyltrimethylammonium bromide (Sigma) containing
99.9% main substance; the surfactant was used with-
out additional purification. Butanol and hexane were
purified by standard procedures. Solutions were pre-
pared in double-distilled water.
For the kinetic studies, we used p-nitrophenyl
esters of carboxylic acids (Fluka), recrystallized by
common procedures. The reaction kinetics was studied
spectrophotometrically (Specord UV-Vis) in tempera-
ture-controlled cells, using freshly prepared micro-
emulsions. The reaction progress was monitored by
variation of the optical density of the solutions at
400 nm (formation of p-nitrophenolate anion). The
6
initial concentration of the substrate was 5 10 M,
and the conversion was more than 90%.
The observed pseudo-first-order rate constants kobs
were determined by the least-squares method from the
relationship log(D
where D and D are the optical densities of the solu-
tions at time and after the reaction completion.
D ) = 0.434kobs + const,
12. Mchedlov-Petrosyan, N.O., Isaenko, Yu.F., and Ty-
china, O.N., Zh. Obshch. Khim., 2000, vol. 70, no. 12,
p. 1963.
The optical density of the solutions and the elec-
tronic absorption spectra of ET(30) in the visible and
UV ranges were measured with a Hewlett Packard-
8452A spectrophotometer in 1-cm quartz cells (Per-
kin Elmer). The empirical parameter was calculated
13. Mchedlov-Petrossyan, N.O., Vodolazkaya, N.A., and
Reichardt, Ch., Colloids Surf. A, 2002, vol. 205,
p. 215.
14. Novaki, L.P. and El’Seoud, O., Langmuir, 2000,
vol. 16, no. 1, p. 35.
15. Zuev, Yu.F., Vylegzhanina, N.N., Idiyatullin, B.Z.,
and Mirgorodskaya, A.B., Appl. Magn. Reson., 2003,
vol. 25, no. 1, p. 65.
16. Mirgorodskaya, A.B., Kudryavtseva, L.A., and
Zuev, Yu.F., Zhidk. Krist. Ikh Prakt. Ispol’z., 2003,
no. 1, p. 68.
1
3
by the formula ET(kcal mol ) = 2.859 10 , where
1
is the frequency (cm ), from the experimental
wavelengths of the absorption maxima of the probe,
which were determined with an error of 1 nm. The
ET values plotted in Fig. 3 have the generally ac-
1
cepted dimension of kcal mol [10].
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 75 No. 7 2005