Colloidal state of the system 4-octylphenol-NaOH-water and its catalytic activity in nucleophilic substitution in phosphorus acid esters

Article abstract of DOI:10.1023/B:RUGC.0000007611.57619.fb

The kinetics of decomposition of 4-nitrophenyl ethyl ethylphosphonate in the system 4-octylphenol-NaOH-water was studied spectrophotometrically. Electrophoretic, tensimetric, and kinetic studies proved formation of cationic associates in which phenolysis

Full text of DOI:10.1023/B:RUGC.0000007611.57619.fb

Russian Journal of General Chemistry, Vol. 73, No. 7, 2003, pp. 1057 1061. Translated from Zhurnal Obshchei Khimii, Vol. 73, No. 7, 2003,
pp. 1117 1122.
Original Russian Text Copyright
2003 by Bakeeva, Kosacheva, Bilalov, Kudryavtseva, Barabanov, Sopin.
Colloidal State of the System 4-Octylphenol NaOH Water
and Its Catalytic Activity in Nucleophilic Substitution
in Phosphorus Acid Esters
R. F. Bakeeva, E. M. Kosacheva, A. V. Bilalov,
L. A. Kudryavtseva, V. P. Barabanov, and V. F. Sopin
Kazan State University of Technology, Kazan, Tatarstan, Russia
Arbuzov Institute of Organic and Physical Chemistry, Kazan Scientific Center,
Russian Academy of Sciences, Kazan, Tatarstan, Russia
Received July 5, 2001
Abstract The kinetics of decomposition of 4-nitrophenyl ethyl ethylphosphonate in the system 4-octyl-
phenol NaOH water was studied spectrophotometrically. Electrophoretic, tensimetric, and kinetic studies
proved formation of cationic associates in which phenolysis or base hydrolysis occurs, depending on the
concentration ratio of 4-octylphenol and NaOH.
Interest in phenol and its alkyl derivatives is due
to their significance as raw materials for preparing
surfactants, stabilizers, additives, and extraction sys-
tems with ion-exchange and micellar action [1]. In
addition, phenolic compounds widely occur in ani-
mals and plants; they are secondary metabolites [2].
Therefore, simulation of the action of biostructures
based on associated phenols is an urgent problem.
using decomposition of 4-nitrophenyl ethyl ethyl-
phosphonate I as example. In this system, formation
of the 4-octylphenolate ion (4-C8H17PhONa) by the
reaction of 4-C8H17PhOH with NaOH should be ex-
pected [scheme (1), first stage].
Introduction of alkyl groups into the phenol mole-
cule increases the electron density on the hydroxyl
oxygen atom; associative processes in this case can
give rise to micellar catalytic effects. However, in
aqueous solution the oxygen center of phenolate ions
is involved in hydrogen bonding with the solvent. The
oxygen center can also be blocked in ion pairs of
alkali metal phenolates.
Phenol and its derivatives have several reactive
sites. Chemical bonds can form both by reaction with
the hydroxy group and by electrophilic substitution
at the o- and p-positions of the benzene ring. Also,
phenol can form hydrogen bonds via hydroxy group
and complexes by interaction of the benzene sys-
tem with cations; long-chain alkyl substituents en-
hance hydrophobic interactions with phenols. All these
features results in diverse types of association [3, 4].
In the system 4-C8H17PhOH NaOH water, sub-
strate I can undergo phenolysis and base hydrolysis
[scheme (2)].
It is known that, in molecular solutions, reactions
of four-coordinate phosphorus acid 4-nitrophenolates
with phenolates and hydroxide ion follow the pattern
of bimolecular nucleophilic substitution at the P atom
(SN2P), with the release of 4-nitrophenolate ion. The
observed rate constants are linear functions of the
reactant concentrations [6]. In our case, the second
product of phenolysis is 4-octylphenyl ethyl ethyl-
phosphonate II, and the second product of hydrolysis,
ethyl hydrogen ethylphosphonate III [scheme (2)].
It seemed interesting to find how association phe-
nomena in systems containing long-chain alkylphen-
ols affect their reactivity in processes used for detox-
ification of phosphorus acid esters [5].
Unsubstituted phenol, being a weak acid (pKa 10),
readily dissolves in alkalis to form water-soluble
phenolates, which are strong nucleophiles. Data on the
colloidal properties and catalytic behavior of long-
chain alkylphenols in aqueous alkalis are virtually
lacking [6].
The presence of a long-chain alkyl radical makes
4-C8H17PhOH virtually insoluble in water, and the
system 4-C8H17PhOH water undergoes phase separa-
tion (4-C8H17PhOH is not dispersed in water). On
We studied the capability of the system 4-octyl-
phenol (4-C8H17PhOH, pOP) NaOH water to control
the rate and direction of nucleophilic substitution
1070-3632/03/7307-1057$25.00 2003 MAIK Nauka/Interperiodica

1058
BAKEEVA et al.
NaOH
NaOH
C8H17C6H4ONa+2 ,
O
C8H17C6H4OH
C8H17C6H4ONa
(1)
(2)
C2H5O
C2H5
O
OH
+ H+
P
+
C2H5
C2H5O
P
O
O
O
NO2
O
C2H5
C2H5O
P
O
O
NO2
O
+
<
>
<
>
NO2
<
>
<
>
<
>
`
<
>
`
adding an alkali, the solubility of 4-C8H17PhOH
increases, and a colloidal solution stable for 2 months
is formed. Using tensimetric, potentiometric, and con-
ductometric methods, we proved formation of associ-
ates in the system 4-C8H17PhOH NaOH water and
determined the critical association concentrations.
Figure 1 illustrates determination of the critical asso-
ciation concentrations at n = CNaOH/CpOP = 3 by
these methods. The results obtained by different meth-
ods are in good agreement. Figure 2 shows that, in
going from n = 1 to n = 2, the critical association
concentration sharply decreases, after which it weakly
grows and then flattens out.
NaOH water at various n are listed in Table 1. These
data show that, at a constant 4-C8H17PhOH concentra-
tion of 0.005 M, at n = 1 or 2, the reaction occurs as
phenolysis, yielding ester interchange product II. The
occurrence of phenolysis is confirmed by existence of
4-C8H17PhONa at low n. Starting from n = 3, the
reaction occurs as base hydrolysis, and acid III be-
comes the only reaction product. With increasing
alkali concentration, i.e., with accumulation of reac-
tive counterions, decomposition of I follows the hy-
drolysis pathway.
The dependence of the observed rate constant of
1
decomposition of I (kobs, s ) in the system 4-C8H17
The direction and rate of the reaction in the system
PhOH NaOH water on the 4-C8H17PhOH concentra-
tion at various n is nonlinear, which is typical of
micellar catalysis (Fig. 3). The rate constants in the
colloidal system first vary only slightly and then
increase, reaching a limiting value at CpOP 0.005 M.
4-C8H17PhOH NaOH water depends on the ratio n.
The 31P NMR chemical shifts in the spectra of the
reaction products obtained in aqueous and aqueous-
alkaline solutions and in the system 4-C8H17PhOH
CAC 104
pH
pH
14
16
3
14
12
10
12
10
8
70
30
8
6
4
1
6
2
0
4
1
2
3
4
5
6
0
2
4
6
8
10 12 14 16 n
log CpOP
1
1
Fig. 1. pH, specific electrical conductivity ( ,
cm ),
1
and surface tension ( , dyne cm ) as functions of the
Fig. 2. Critical association concentration (CAC, M) as a
function of the ratio n (C 0.005 M, 40 C).
4-C H PhOH concentration at n = 3 (40 C).
8
17
pOP
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 73 No. 7 2003

COLLOIDAL STATE OF THE SYSTEM 4-OCTYLPHENOL NaOH WATER
1059
Table 1. 31P NMR chemical shifts ( P, ppm) of 4-nitrophenyl ethyl ethylphosphonate and products of its decomposition
in various media (40 C)
Medium
n
P, ppm
Reaction product
H2O
NaOH H2O
34.65
29.81
35.77
35.53
30.09
30.98
30.90
EtOEtP(O)OC6H4NO2-p (I)
EtOEtP(O)OH (III)
EtOEtP(O)OC8H17 (II)
EtOEtP(O)OC8H17 (II)
EtOEtP(O)OH (III)
4-C8H17PhOH NaOH H2O
4-C8H17PhOH NaOH H2O
4-C8H17PhOH NaOH H2O
4-C8H17PhOH NaOH H2O
4-C8H17PhOH NaOH H2O
1
2
3
8
30
EtOEtP(O)OH (III)
EtOEtP(O)OH (III)
Assuming that the reaction occurs in an aggregate and
in the bulk of solution, we can use the equation for
micelle-catalyzed reactions [7]:
Table 2. Kinetic parameters of micelle-catalyzed decom-
position of 4-nitrophenyl ethyl ethylphosphonate in the
system 4-C8H17PhOH NaOH water at various n, 40 C
kobs = (k0 + kasKSC)/(1 + KSC),
CpOP : CNaOH
,
CAC 104,a
kas 103,
KS,
l mol
1
1
n
M
s
where k0 is the rate constant of the reaction in the
bulk; kas, rate constant of the reaction in the aggre-
gate; KS, constant of substrate binding with the asso-
ciate; and C, concentration of 4-C8H17PhOH minus
critical association concentration.
1: 1
1: 2
1: 3
1: 4
1: 8
1: 16
17.20
1.39
4.68
7.58
2.62
1.00
4.06
4.59
7.81
11.90
22.81
27.20
104.23
127.70
287.90
219.14
268.72
259.45
The kinetic parameters of associate-catalyzed de-
composition of I in the system 4-C8H17PhOH NaOH
water at various n, calculated with this equation, are
listed in Table 2. It should be noted that, in the asso-
ciate, the rate of hydrolysis of I (n > 3) is higher than
the rate of its phenolysis (n = 1 or 2).
a
(CAC) Critical association concentration.
colloidal particles in this system considerably exceeds
the length of the hydrocarbon radical 4-C8H17PhOH.
The dependence R = f(n) passes through a minimum
at a 30-fold excess of alkali (Table 3). The experimen-
tal data suggest the presence of several ranges of
alkali concentration in which the dependences of the
Acceleration of hydrolysis of I in the associate
with increasing n (CpOP 0.005 M, Fig. 2, Table 2) is
difficult to explain assuming formation of aggregates
of the anionic surfactant from 4-C8H17PhONa mono-
mers. It is known that anionic micelles drastically
decrease the rate of nucleophilic substitution in four-
coordinate phosphorus acid esters owing to electro-
static repulsion of hydroxide ions, decreasing the ef-
fect of their concentrating [8].
kobs 103
6
5
12
10
8
Electrophoretic studies showed that the dispersed
phase is positively charged at any n (Table 3), which
suggests incorporation into the surface layer of mixed
aggregates consisting, along with 4-C8H17PhONa and
4-C8H17PhOH, of monomers containing excess sodi-
um ions, 4-C8H17PhONa+n.
4
6
3
4
2
0
2
1
0
1
2
3
4
5
Figure 4 shows the optical density D as a function
of wavelength (nm) for the systems with different n
at a constant concentration CpOP 0.005 M. From these
data, using the Rayleigh Debye approximation for the
condition of optically soft particles, we calculated the
characteristic size R of dispersed phase particles [9,
10] (Table 3). It is seen that the characteristic size of
CpOP 103
3
1
Fig. 3. Observed rate constant (k
10 , s ) as a
obs
function of the 4-C H PhOH concentration (C
8
17
pOP
3
10 , M) at various n (40 C). n: (1) 1, (2) 2, (3) 3, (4) 4,
(5) 8, and (6) 16.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 73 No. 7 2003

1060
BAKEEVA et al.
Table 3. Colloidal properties of aggregates in the system 4-C8H17PhOH NaOH water as influenced by the ratio n =
CNaOH/CpOP (40 C)
Parameter
Phenolysis
Hydrolysis
n
1
2
3
4
+1.5
5
+1.2
6
7
+1.3
8
16
+2.5
30
+3.59
40
50
60
B 104,
cm2 V 1 s
R, nm
+3.7 +3.2 +1.8
+1.4
560
1
640 680 768
680
704
696
560
200
80
102 133 248
electrophoretic mobility B, characteristic radius R
(Table 3), and electrical conductivity on n (Fig. 4)
noticeably differ. This fact suggests that the formed
colloidal structures are different. At n < 3, mixed ion
molecule aggregates consisting of the monomers
Q(4-C8H17PhO)Na+n, 4-C8H17PhONa, and 4-C8H17
PhOH are formed. According to the kinetic studies at
n = 1 and 2, the head groups of 4-C8H17PhONa ex-
hibit nucleophilic properties and participate in phenol-
ysis (Table 1). Starting from n = 3, the head groups of
the 4-C8H17PhONa monomers become inacccessible.
At n = 3 and 4, the radius R and electrophoretic mo-
bility decrease. At 4 < n < 8, the electrical conductiv-
ity grows (Fig. 5), with the electrophoretic mobility
remaining constant (Table 3).
persed phase and dispersion medium. The conditions
become unfavorable for phenolysis, and substrate I
undergoes hydrolysis.
At n 20, formation of a uniform cationic associate
consisting of Q(4-C8H17PhO)Na+n is apparently com-
plete, and at n > 20 the degree of ionization of the
associates decreases, which results in their certain
coarsening (Table 3). This is favored by the fact that
the fraction of counterions (OH ions) bound to the
micelle surface increases, and the micelle charge and
electrostatic repulsion forces between the head groups
of the amphiphilic substance on the micelle surface
decrease [11 13].
Thus, the role of alkali in our system consists in
that initially it acts as a component forming an ionic
surfactant, [Q(4-C8H17PhO)Na+nOH ], and then, as an
electrolyte.
At 8 < n < 20, addition of alkali does not cause
a noticeable increase in the electrical conductivity
(Fig. 5), suggesting intense binding of charge carriers.
The relative content of cationic monomers in the asso-
ciate grows, causing a decrease in the aggregation
number and aggregate size and an increase in the elec-
trophoretic mobility. In the process, the counterion
layer of the aggregate becomes much denser, since
there is a considerable gradient in the distribution of
the hydroxy groups and Na+ ions between the dis-
The possibility of such a mechanism of structural
rearrangements is confirmed by the following reason-
ings. If aggregates were built exclusively of 4-C8H17
PhONa molecules, then the charge of the dispersed
phase would be either negative or neutral owing to
blocking of the negative charge of the phenoxyl oxy-
gen by the sodium ion in the ion-pair associate. If ag-
D
5
3
6
4
2
1
30
25
20
15
10
5
1
11
7
10
9
8
0
400
450
500
550
600
0
0
5
10 15 20 25 30 n
Fig. 4. Optical density D of the system 4-C H PhOH
8
17
NaOH H O as a function of the wavelength (nm). n:
(1) 1, (2) 2, (3) 3, (4) 4, (5) 5, (6) 6, (7) 16, (8) 30,
(9) 40, (10) 50, and (11) 60.
2
1
1
Fig. 5. Specific electrical conductivity
(
cm ) as a
function of n at C 0.005 M (40 C).
pOP
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 73 No. 7 2003

COLLOIDAL STATE OF THE SYSTEM 4-OCTYLPHENOL NaOH WATER
1061
gregates were built exclusively of Q(4-C8H17PhO)Na2+
cations, with a counterion layer of hydroxide ions,
then it would be difficult to explain the decrease in
the size R and increase in the electrophoretic mobility
of the agregates with increasing concentration of alka-
li in the range 8 < n < 20.
The reaction rate constants were determined under
pseudomonomolecular conditions at a large (no less
than tenfold) excess of the nucleophile from the first-
order relationship
log (Dt
D1) = 0.434kobs + const,
Thus, our study revealed formation in the system
4-C8H17PhOH NaOH water of mixed associates and
of cationic aggregates enriched in sodium ions in the
associated state. We also found that the substrate
decomposes by the pathway of ester interchange or
base hydrolysis, depending on the ratio n in the sys-
tem 4-C8H17PhOH NaOH water. Associates exhibit
different catalytic activities in decomposition of 4-ni-
trophenyl ethyl ethylphosphonate I.
where Dt an D1 are the optical densities at time t and
after reaction completion, respectively.
The pseudophase model equation was solved by the
least-squares method. Only the constants for which
the correlation coefficient was 0.99 were included.
ACKNOWLEDGMENTS
The study was financially supported by the Russian
Foundation for Basic Research (project no. 01-03-
33224).
EXPERIMENTAL
The UV spectra were recorded on a Specord UV-
Vis spectrophotometer.
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RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 73 No. 7 2003