Condensation of p-dimethylaminocinnamaldehyde with aniline and substituted anilines in micellar media

Article abstract of DOI:10.1007/s11176-005-0210-5

Optimal conditions were found for the reactions of aniline and its hydroxy-, carboxy-, methyl-, and nitro-substituted derivatives with p-dimethylaminocinnamaldehyde in the presence of sodium dodecyl sulfate micelles in the pH range 1-6. A correlation was revealed between the optimal pH value pK _a of aromatic amines. The reaction in the model system aniline-p-dimethylaminocinnamaldehyde-sodium dodecyl sulfate in micelles formed by anionic surfactants is accelerated more than 1000-fold due to increased concentration of the reactive species in sodium dodecyl sulfate micelles. 2005 Pleiades Publishing, Inc.

Full text of DOI:10.1007/s11176-005-0210-5

Russian Journal of General Chemistry, Vol. 75, No. 2, 2005, pp. 261 267. Translated from Zhurnal Obshchei Khimii, Vol. 75, No. 2, 2005,
pp. 288 294.
Original Russian Text Copyright
2005 by Doronin, Chernova, Gusakova.
Condensation of p-Dimethylaminocinnamaldehyde with Aniline
and Substituted Anilines in Micellar Media
S. Yu. Doronin, R. K. Chernova, and N. N. Gusakova
Chernyshevskii Saratov State University, Saratov, Russia
Received May 13, 2003
Abstract Optimal conditions were found for the reactions of aniline and its hydroxy-, carboxy-, methyl-,
and nitro-substituted derivatives with p-dimethylaminocinnamaldehyde in the presence of sodium dodecyl
sulfate micelles in the pH range 1 6. A correlation was revealed between the optimal pH value pKa of
aromatic amines. The reaction in the model system aniline p-dimethylaminocinnamaldehyde sodium dodecyl
sulfate in micelles formed by anionic surfactants is accelerated more than 1000-fold due to increased con-
centration of the reactive species in sodium dodecyl sulfate micelles.
Condensation of primary aromatic amines with
aldehydes is widely used in organic synthesis (prepara-
tion of secondary amines and heterocyclic compounds,
protection of aldehyde group, etc) [1] and analytical
chemistry (formation of colored Schiff bases) [2 6]. It
is known that micellar media formed by surface-active
substances are capable of essentially changing the
state of reactants in solution, the rate and mechanism
of reactions, and their direction [7]. The catalytic
effect of surfactant micelles in the condensation of
aldehydes with aliphatic and aromatic amines was the
subject of a number of publications [8 12]. For ex-
ample, Martinek et al. [8] and Krivova et al. [9] re-
ported on the effect of sodium dodecyl sulfate (I)
micelles on the condensation of aromatic amines (ani-
line, benzidine, p-anisidine) with p-dimethylamino-
benzaldehyde (II). The authors showed that the reac-
tion rate increases due to increase in the reactant
concentrations in the micellar pseudophase. The in-
fluence of salt I on the mechanism and equilibrium in
the condensation of hydrazine and phenylhydrazine
with vanillin [10] and p-dimethylaminobenzaldehyde
(II) [11, 12] was also studied. It was found that the
apparent rate constants increased by more than 2 or-
ders of magnitude for the reaction with hydrazine and
by a factor of 20 30 for phenylhydrazine, which was
attributed to increase of the reactant concentrations in
the surfactant micelles.
In the present work we examined the effect of sur-
factant micelles on the condensation of p-dimethyl-
aminocinnamaldehyde (III) with aniline, nitroanilines,
aminophenols, toluidines, and aminobenzoic acids.
The mechanism of condensation of primary aromatic
amines with aldehydes was studied in sufficient detail
[13 16]. The condensation includes two steps: the
first step is nucleophilic addition of amine at the
carbonyl carbon atom to give intermediate -amino
alcohol, and the second step is elimination of water
with formation of C=N bond (Scheme 1). The final
products are the corresponding Schiff bases.
Scheme 1.
H
H
+
O
H
H
H
HO
O
H+
R C=N R
R C N R
H
R C N R
H
R C X + H2N R
H2O,
H+
Figure 1a shows the electronic absorption spectra
of the system aniline aldehyde III at various pH
values. It is seen the medium-intensity absorption
band with its maximum at 520 nm, which is ob-
served in the pH range from 2 to 6, corresponds to the
condensation product. The optimal conditions for the
reaction of aniline with aldehyde III (citrate buffer,
pH 2 6; ionic strength 0.1 0.4; temperature 20 C;
1070-3632/05/7502-0261 2005 Pleiades Publishing, Inc.

262
DORONIN et al.
(a)
(b)
A
A
5
1.6
0.8
0
1
2
1.2
4
4
1.0
0.8
0.6
0.4
3
3
4
3
2
1
0.2
1
400
450
500
550 , nm
2
0
250 300 350 400 450 500 550 , nm
Fig. 1. Electronic absorption spectra of the system aldehyde III aniline (a) in the absence of salt I at pH (1) 2, (2) 3, (3) 4,
4
3
3
3
(4) 5, and (5) 6 (c = caniline = 10 M) and (b) in the presence of (1) 0, (2) 10 , (3) 2 10 , and (4) 5 10 mol/l of salt
III
6
I (c
= c
= 5 10 M, pH = 4.0).
aniline
III
absorption of the product at
520 nm develops
amino group radically changes the behavior of the
condensation product with aniline in acid medium,
where quinoid form IV of the protonated Schiff base
is stable [17] (Scheme 2). Hydrolysis is a competing
process which reduces the concentration of N,N-di-
methyl-4-(3-phenylamino-2-propenylidene)-2,5-cyclo-
hexadienylideneammonium ion (IV).
max
during 20 min) differ from those ensuring formation
and stability of most known Schiff bases [1]. It is
known that a number of Schiff bases are stable in
alkaline medium due to their low basicity and that
they undergo hydrolysis in acid medium. The pre-
sence in molecule III of an electron-donor dimethyl-
Scheme 2.
H3C
H3C
H3C
H
H+
+
N
CH CH=CH NH
+
H2N
N
CH=CH C
H2O
H3C
O
III
IV
Depending on the nucleophile nature, the rate and
mechanism of the reaction are strongly influenced by
pH. Here, two main factors are to be taken into ac-
count: first, the nucleophile should exist in its active
form, i.e., it should possess an unshared electron pair
capable of attacking the carbonyl group, and second,
the process is catalyzed by solvated proton. Substi-
tuents in the nucleophile differently affect the reaction
rate, for example, the presence of an electron-donor
substituent in a primary aromatic amine increases its
basicity, which should increase the reaction rate due
to acceleration of the nucleophile addition at the
carbonyl group. On the other hand, increased basicity
tions, we examined the effect of various substituents
in the benzene ring of the aromatic amine (Table 1).
Each of the above amines (except for o-nitroaniline)
reacted with aldehyde III in a citrate buffer to give
colored Schiff bases with
505 560 nm. From the
max
obtained data we have drawn a correlation between
the optimal pH value (pHopt) and pKa of amines:
pHopt = (pKamine + 3.18)/2, where 3.18 is the basicity
constant of aldehyde III (Table 1). Using this correla-
tion we can predict optimal conditions for the reaction
of aldehyde III with any primary aromatic amine. The
lesser the pKa value, the lesser pHopt and the shorter
the time necessary for the reaction equilibrium to
of an arylamine requires greater pH for its protonation, establish; correspondingly, the absorption maximum
which reduces the rate of dehydration of intermediate
amino alcohol.
of the product shifts to longer wavelengths.
As follows from Fig. 1b, a strong absorption band
with its maximum at 540 nm appears in the elec-
tronic absorption spectra starting from a sodium do-
In order to reveal some general relations holding
in the reactions under study and find optimal condi-
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 75 No. 2 2005

CONDENSATION OF p-DIMETHYLAMINOCINNAMALDEHYDE WITH ANILINE
(a) (b)
263
A
A
1
5
0.4
4
3
2
3
4
1.0
1
0.3
2
0.8
0.6
0.4
0.2
0
1
4
5
2
0.2
0.1
0
3
1
2
5
3
4
5
220 230 240 250 260 270 280 290 300
, nm
220 250 280 310 340 370 400 430 460 , nm
Fig. 2. Electronic absorption spectra in citrate buffer of (a) aldehyde III (pH = 3.0) and (b) aniline (pH = 4.0) in the presence
3
3
3
3
of (1) 0, (2) 10 , (3) 2 10 , (4) 4 10 , and (5) 4 10 mol/l of salt I.
3
decyl sulfate concentration of 2 10 M. It reaches
conditions sodium dodecyl sulfate micelles stabilize
the reactive protonated form of aldehyde III via
electrostatic interaction, which leads to acceleration of
the process.
3
the maximal intensity at cI = 5 10 M. The optimal
concentration of salt I for the reaction of aniline with
aldehyde III is considerably lower than critical mi-
celle concentration (CMC) of sodium dodecyl sulfate
The absorption pattern of aniline also changes with
rise in micelle concentration (Fig. 2b). However, the
3
in water, (8 9) 10 M [19]. Many factors are
capable of affecting CMC of surfactants [7]; therefore,
it was reasonable to determine the CMC of I under the
optimal conditions, i.e., in a citrate buffer at pH 4.0.
intensity of absorption of both neutral (
280 nm)
max
Table 1. Reactions of aniline and its derivatives with
p-dimethylaminocinnamaldehyde (III)
The measurements were performed using visco-
metric and spectrophotometric techniques [20]. In the
first case, the CMC of salt I was estimated at 1.8
0.3 mM (number of parallel runs n = 3, confidence
probability p = 0.95); in the second case, a value of
1.60 0.05 mM (n = 3, p = 0.95) was obtained. Thus
the optimal concentration of sodium dodecyl sulfate
b
pHopt
a
,
,
eq
pKa
[18]
max
nm
Aromatic amine
min
experi- calcu-
mental
lated
3
(5 10 M) exceeds its critical micelle concentration
Aniline
520 15 20
520 15 20
525 15 20
515 50 60
505 15 20
525 15 20
520 25 30
4.0
4.0
3.7
4.3
3.9
4.0
4.0
2.75
3.88
3.95
3.68
4.34
3.79
3.94
4.15
2.65
4.58
4.72
4.17
5.50
4.39
4.69
5.12
2.11
in citrate buffer; this means that micelles of salt I are
formed in the given concentration range. Reduction
in the CMC of I is likely to result from the high ionic
strength created by citrate buffer.
o-Aminophenol
m-Aminophenol
p-Aminophenol
o-Toluidine
m-Toluidine
p-Toluidine
One of the main factors determining the rate of the
reaction under study is the state of the reactants in
aqueous and micellar media at different pH values.
Therefore, we examined the electronic absorption spec-
tra of aldehyde III and aniline in citrate buffers at pH
1 6 in the presence of micelles I.
o-Aminobenzoic 545
2 5
acid
m-Aminobenzoic 530
2 5
3.3
2.9
3.15
2.80
3.12
2.41
acid
p-Aminobenzoic 545 30 35
Figure 2a shows that the equilibrium between the
acid
o-Nitroaniline
m-Nitroaniline
p-Nitroaniline
neutral (
400 nm) and protonated (
280 nm)
1.44
2.84
2.1
0.29
2.50
1.02
max
forms of aldehyde III shifts toward the mlaatxter as the
concentration of micelles rises. This follows from the
corresponding redistribution of the absorption inten-
sities and from the presence of an isosbestic point at
310 nm. We can conclude that under the optimal
540 25 30
560 5 10
3.0
2.0
a
is the time necessary for equilibrium to establish.
eq
b
Calculated by the correlation equation given in the text.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 75 No. 2 2005

264
DORONIN et al.
235 nm) decreases, and
and protonated species (
The hydrolysis of IV is a process concurrent with
its formation, which reduces the concentration of the
product. Therefore, we examined the kinetics of the
hydrolysis of the Schiff base prepared from aniline
and aldehyde III as described in Experimental. The
kinetic equation for the hydrolysis of IV looks as
follows:
max
no definite conclusions on the state of aniline in
micellar solution can be drawn. Therefore, we tried to
determine pKa of aniline in micelles by potentiometric
titration. The titration was performed using the half-
neutralization technique which is analogous to the
procedure described in [21]. As titrant we used solut-
ions of NaOH and of NaOH and salt I, which were
freed from CO2. The results showed an apparent shift
of the pK value for protonation of aniline in the
presence of micelles, i.e., pKa increased relative to the
corresponding value in aqueous phase (5.30 0.05
against 4.60 0.02; n = 3, p = 0.95). This may be due
to binding of the reactant by micelles according to the
following scheme:
+
.
dc/d = k2c3cH
(2)
Here, c3 is the concentration of protonated Schiff
base IV (quinoid form). The calculated rate constant
for the hydrolysis of IV in aqueous medium k2 is
equal to (1.6 0.2) 103 min 1 mol 1 l (n = 5, p =
0.95). The equilibrium constant for the reaction of
aniline with aldehyde III in aqueous medium (Kaq)
was calculated as the ratio of rate constants k1 and k2:
Kaq = k1/k2 = (4.1 0.5) 102.
(PhNH+3)aq + (Na+DDS )micelle
(Na+)aq + (PhNH3+ DDS )micelle
.
The kinetics of the reaction of aniline with al-
dehyde III in surfactant micelles were studied in
terms of the micellar catalysis concept [8, 23] accord-
ing to which increase in the order of a reaction by
unity leads to a P-fold increase of its efficiency; here,
P is the coefficient of reactant distribution between
the aqueous and micellar phases. Preliminarily, we
found that the general appearance of the kinetic equa-
tions for the formation and hydrolysis of Schiff base
IV does not change in going from aqueous to micellar
phase.
Deprotonation of aniline is favored by specific
features of its solubilization in the bulk micelle due to
hydrophobic properties of the aromatic ring and rela-
tively small size of the molecule. Our results are
consistent with the data of [22].
Thus, the concentrations of the protonated form of
aldehyde III and neutral form of aniline increase in
going to sodium dodecyl sulfate micelles. Insofar as
both these are reactive species and protolytic equilib-
rium is displaced toward their formation in the pre-
sence of micelles I, the rate of the condensation of
aniline with aldehyde III increases.
We then calculated the rate constants of the for-
ward and reverse reactions in micelles I. For this
purpose, we determined by spectrophotometry the
rates of formation and hydrolysis of IV upon variation
The reactant distribution over aqueous micellar
system can be estimated directly from the kinetic data
obtained by studying the formation and hydrolysis of
protonated Schiff base IV.
5
of the concentrations of aniline (from 0.8 10 to
5
5
6.0 10 M), Schiff base (from 0.8 10 to 4.0
5
2
2
10 M), and salt I (from 0.2 10 to 4.0 10 M).
For each series of solutions we measured the slope of
the linear part of the kinetic curve and calculated the
apparent molar absorption coefficient from the limit-
Before proceeding to kinetic studies in the presence
of surfactant I, we deduced a general kinetic equation
for the condensation of aniline with aldehyde III in
aqueous medium under the optimal conditions. For
this purpose, we plotted the kinetic curves in the
coordinates A t (using spectrophotometric technique)
upon variation of the reactant concentrations and
calculated partial orders of the reaction with respect
to each reactant. As a result, the following equation
was obtained (pH 2.5 6.0):
ing optical density at
540 nm. The results are
summarized in Table 2. mAasx follows from the plots of
km1 and k2m vs. cI (Fig. 3a), increase in the con-
centration of salt I initially leads to increase of the
formation rate constant km1 (curve 1) and decrease of
the hydrolysis rate constant km2 (curve 2); the optimal
3
ratio is reached at cI = 5 10 M. Further rise in the
concentration of salt I (cI > 5 10 M) is accom-
3
+
.
dc/d = k1c1c2cH
(1)
panied by increase of the hydrolysis rate constant and
decrease of the formation rate constant. Figure 3b
shows the dependence of the apparent equilibrium
constant in the micellar medium (Kmap) on the salt I
concentration, which clearly indicates that the optimal
Here, c1 and c2 are the concentrations of aldehyde
III and aniline, respectively. Using this equation, we
calculated the rate constant for the forward reaction in
105
aqueous medium: k1 = (6.5 0.9)
(n = 5, p = 0.95).
min 1 mol 2 l2
3
concentration of I equals 5 10 M.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 75 No. 2 2005

CONDENSATION OF p-DIMETHYLAMINOCINNAMALDEHYDE WITH ANILINE
265
(a)
(b)
km2 103
km1 108
Kmap 105
4.5
7
7
4.0
3.5
6
6
3.0
2.5
2.0
1.5
1.0
0.5
5
4
5
4
1
2
3
2
1
3
2
1
0 2 4 6 8 10
20
40
cI, M
cI, M
m
m
m
Fig. 3. Plots of (a) (1) k and (2) k and (b) K
versus concentration of salt I.
1
2
ap
According to the pseudo-two-phase model of
micelle effects [14], the apparent constant Kmap is
related to the equilibrium constant of the same reac-
tion in aqueous medium (Kaq) and binding constants
of aldehyde III (K1), aniline (K2), and protonated
Schiff base IV (K3) in aqueous micellar medium by
equation (3):
constants are as follows: K1 = (2.9 0.7) 102, K2 =
(2.1 0.5) 102, K3 = (9.1 1.1) 105 (n = 3, p = 0.95).
Using the formula K = (P
1)/V, where K is the
binding constant of a reagent, and V is the molar
volume of micelles I (V = 0.29 l/mol [8]), we calcu-
lated the corresponding distribution constants P
between the aqueous and micellar phases: aldehyde
III, P1 = (1.00 0.02) 103; aniline, R2 = (0.81
0.05) 103; protonated Schiff base IV, R3 = (3.10
0.06) 106 (n = 3, p = 0.95). On the basis of these
values we determined the true equilibrium constant
Km for the condensation of aniline with aldehyde III
in the micellar medium, which is related to the equilib-
rium constant in aqueous medium Kaq by the follow-
ing equation:
1 + K3[I]
Kmap = Kaq
.
(3)
(1 + K1[I])(1 + K2[I])
Here, [I] is the equilibrium concentration of salt I,
calculated from the difference between the total salt
concentration and its critical micelle concentration in
citrate buffer: [I] = cI CMCI. Equation (3) allows
us to calculate the binding constants of the reactants
under the constraint K3 [I] >> 1 since the apparent rate
constant km1 in the presence of micelles I is greater by
three orders of magnitude than in aqueous medium
(Table 2). Then, Eq. (3) may be represented as
follows:
P3
Km = Kaq
.
(6)
P1P2
Thus the calculated true equilibrium constant in
Kaq[I]
K2K
K3
K2 + K1
Table 2. Rate constants for the formation (k m1 ) and hydro-
lysis (k m2 ) of compound IV in the micellar phase (n = 5,
p = 0.95)a
1
K3
F1 =
=
+
[I] +
1 [I]2.
(4)
Kmap
K3
If the equilibrium concentration of salt I is not high,
the dependence F1 = f([I]) is linear, and the intercept
on the y axis is equal to 1/K3. As shown in Fig. 4a,
the F1 = f([I]) plot is linear until the overall concentra-
2
cI 10 ,
(k m1 k m1 ) 108,
(k m2 k m2 ) 103,
M
min 1 mol 2 l2
min 1 mol 1 l
3
tion of I does not exceed (6 8) 10 M. Therefore,
0.2
0.3
0.4
0.5
0.6
0.8
1
2.5 0.3
5.8 0.4
6.4 0.5
6.9 0.4
6.6 0.4
6.5 0.3
6.2 0.3
3.8 0.4
2.2 0.9
2.2 0.9
2.1 0.8
1.5 0.8
1.5 0.7
2.1 0.9
2.3 0.7
2.5 0.9
in the subsequent calculations we used the first five
points of this dependence, and K3 was estimated at
9.09 105. Equation (4) can be transformed into
Eq. (5):
Kaq[I]
1
K2K1[I]
K3
K2 + K1
F2 =
=
+
.
(5)
Kmap
K3
K3[I]
2
The plot of F2 versus [I] is shown in Fig. 4b. Here,
the intercept on the y axis is equal to (K2 + K1)/K3,
and the slope, to K2 K1/K3. The calculated binding
a
(n) is the number of parallel runs, and (p), confidence
probability.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 75 No. 2 2005

266
DORONIN et al.
(a)
(b)
F1
12
F2
10
8
12
10
8
6
4
6
2
4
0
1
2
3
4
5
6
0
[I], M
2
3
4
5
6
7
8
1
[I], M
Fig. 4. Plots of (a) F [Eq. (4)] and (b) F [Eq. (5)] vs. [I].
1
2
sodium dodecyl sulfate micelles, Km = 1.57 103,
exceeds the corresponding value for aqueous phase
by a factor of 4, while the rate constant for formation
of IV in the micellar phase is greater by 3 orders of
magnitude than that in aqueous phase.
tion from a viscometric capillary was measured. The
ratios of the specific viscosities of the solutions to the
concentration of salt I ( sp/cI) vs. cI were plotted,
and CMC was determined by the graphical technique.
Spectrophotometric determination of CMC of salt I
was performed by measuring the optical density at
Our results led us to conclude that the observed
acceleration of the model reaction originates mainly
from increased concentration of the reactive aniline
and p-dimethylaminocinnamaldehyde species in mi-
celles formed by anionic surfactants, as follows from
the calculated distribution coefficients in aqueous
micellar system. The general relations found in the
present study can be applied to condensations of other
primary aromatic amines with p-dimethylaminocin-
namaldehyde in micellar media containing anionic
surfactants.
540 nm for a series of solutions containing 5
10 mol/l of the reactants and 1.0 to 2.0 mol/l of salt
I through a step of 0.1 mol/l, followed by plotting
differential dependences in the coordinates dA/dcI cI.
max
6
Synthesis of Schiff base from aniline and p-di-
methylaminocinnamaldehyde. A solution of equi-
molar amounts of aniline and aldehyde III in 20 ml of
toluene containing 3 drops of glacial acetic acid as
catalyst was heated for 0.5 h under reflux with simul-
taneous removal of the liberated water as azeotrope.
The product was purified by quadruple recrystalliza-
tion from 1-butanol until constant melting point (mp
133 1 C).
EXPERIMENTAL
The electronic absorption spectra were recorded on
SF-46 and Specord M-40 spectrophotometers using
quartz and glass cells with an absorbing layer thick-
ness of 1 5 cm. The optical densities of solutions
were measured with KFK-2 and KFK-3 photoelectro-
colorimeters. The pH values were monitored using
pH-121 and pH-673 pH meters.
ACKNOWLEDGMENTS
This study was performed under financial support
by the Russian Foundation for Basic Research (project
no. 04-03-33077).
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CONDENSATION OF p-DIMETHYLAMINOCINNAMALDEHYDE WITH ANILINE
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