Kinetics of the autoxidation of adrenaline and [copper(II)(adrenaline)] 2+ in alkaline aqueous and micellar media

Article abstract of DOI:10.1007/s11243-012-9675-3

The kinetics of autoxidation of adrenaline and [Cu(adrenaline)] ²⁺ complex by dissolved oxygen in alkaline aqueous and micellar media has been studied. The reaction is initiated by the removal of amino-H ⁺ protons of adrenaline by hydroxide ion, followed by cyclization. The values of (1/k _obs) for the autoxidation of both species were found to be linearly dependent upon 1/[OH^-]. The reaction follows a consecutive pathway in which the intermediate adrenochrome remains stable for few minutes, and then undergoes further reactions to yield adrenolutin and other products. The [Cu(adrenochrome)]⁺ complex is stable for a few hours. Studies on the effects of cetyltrimethylammonium bromide (CTAB) and sodium dodecyl sulfate (SDS) on the reactivity of both species revealed different behaviors. The micelles of CTAB catalyzed the rates of autoxidation for both species, whereas SDS micelles inhibited the autoxidation of adrenaline but catalyzed the rate of autoxidation of [Cu(adrenaline)]²⁺. Addition of the reactive counterion surfactant, cetyltrimethylammonium hydroxide (CTAOH) initially increased the rate constant with the increasing [CTAOH], until it reached a plateau for k _ψ -[CTAOH]. Salts such as NaCl, NaBr, tetramethyl ammonium bromide, and tetraethyl ammonium bromide increased the rate when added at lower concentrations, but had negligible effect at higher concentrations. The results obtained in micellar media were treated according to Berezin's Pseudophase Model. The various kinetic parameters for the reactions occurring in aqueous and in micellar media are reported.

Full text of DOI:10.1007/s11243-012-9675-3

Transition Met Chem (2013) 38:173–181
DOI 10.1007/s11243-012-9675-3
Kinetics of the autoxidation of adrenaline
2
+
and [copper(II)(adrenaline)] in alkaline aqueous
and micellar media
Abdullah S. Al-Ayed
•
•
Hamad A. Al-Lohedan
•
•
M. Z. A. Rafiquee
Zuheir A. Issa
Mohd Sajid Ali
Received: 1 October 2012 / Accepted: 12 November 2012 / Published online: 1 December 2012
Ó Springer Science+Business Media Dordrecht 2012
Abstract The kinetics of autoxidation of adrenaline and
[
Introduction
2
?
Cu(adrenaline)] complex by dissolved oxygen in alka-
line aqueous and micellar media has been studied. The
?
reaction is initiated by the removal of amino-H protons of
Adrenaline (Epinephrine) and norepinephrine are cate-
cholamines containing oxidizable catechol and basic amine
groups and are of clinical interest in the investigation of
tumors of neurological origin. These catecholamines reg-
ulate myocardial contractility and metabolism; they play a
prominent role in cardiac physiology [1] and have been
used effectively for the treatment of heart failure. The
autoxidation of the amine group, with the formation of
oxygen free radical byproducts, is viewed as a major event
in catecholamine cardiomyopathy during postischemic
reperfusion [2, 3]. The autoxidation of catecholamines at
neutral pH is extremely slow or nonexistent, but becomes
faster at pH values above 8 [4]. The oxidation of cate-
cholamines is a rather complex process which involves for-
mation of quinone, hydroquinone, and semiquinone species
[5, 6]. Catecholamines, after conversion to semiquinone,
disproportionate to form the corresponding ortho-quinone;
the latter undergoes an irreversible 1,4-intramolecular cycli-
zation leading to the formation of an unstable leucoamino-
chrome which is rapidly oxidized to an aminochrome [7, 8].
The major products formed during the autoxidation of
adrenaline are adrenochrome and adrenolutin. However, other
products are also formed, but are relatively unstable and
undergo further transformation to dimers and oligomers [9].
Sokoloski and Higuchi [10] reported the kinetics of degra-
dation of adrenaline but were unable to present a complete
mechanistic sequence. They suggested that the reaction is
extremely complex in nature and involves free radical path-
ways. The antioxidant activity and radical scavenging activity
of l-adrenaline have been investigated by Ilhami [11], who
studied the antioxidant activity and mechanism using various
in vitro antioxidant assays such as 1,1-diphenyl-2-picryl-
hydrazyl (DPPHꢀ), 2,2-azino-bis(3-ethylbenzthiazoline-6-
adrenaline by hydroxide ion, followed by cyclization. The
values of (1/kobs) for the autoxidation of both species were
-
found to be linearly dependent upon 1/[OH ]. The reaction
follows a consecutive pathway in which the intermediate
adrenochrome remains stable for few minutes, and then
undergoes further reactions to yield adrenolutin and other
?
products. The [Cu(adrenochrome)] complex is stable for a
few hours. Studies on the effects of cetyltrimethylammo-
nium bromide (CTAB) and sodium dodecyl sulfate (SDS)
on the reactivity of both species revealed different behav-
iors. The micelles of CTAB catalyzed the rates of autoxi-
dation for both species, whereas SDS micelles inhibited the
autoxidation of adrenaline but catalyzed the rate of
2
?
autoxidation of [Cu(adrenaline)] . Addition of the reac-
tive counterion surfactant, cetyltrimethylammonium
hydroxide (CTAOH) initially increased the rate constant
with the increasing [CTAOH], until it reached a plateau for
kw-[CTAOH]. Salts such as NaCl, NaBr, tetramethyl
ammonium bromide, and tetraethyl ammonium bromide
increased the rate when added at lower concentrations, but
had negligible effect at higher concentrations. The results
obtained in micellar media were treated according to
Berezin’s Pseudophase Model. The various kinetic
parameters for the reactions occurring in aqueous and in
micellar media are reported.
A. S. Al-Ayed ꢀ H. A. Al-Lohedan (&) ꢀ
M. Z. A. Rafiquee ꢀ M. S. Ali ꢀ Z. A. Issa
Department of Chemistry, College of Science, King Saud
University, Riyadh 11451, Saudi Arabia
e-mail: hlohedan@ksu.edu.sa
123

1
74
Transition Met Chem (2013) 38:173–181
sulfonic acid) (ABTS), N,N-dimethyl-p-phenylenediamine
?
multitemperature water bath. The observed first-order rate
constant for the formation of yellow adrenochrome was
(
DMPDꢀ ), superoxide anion radicals, hydrogen peroxide,
total antioxidant activity, iron (II) and iron (III), copper (II)
chelating activity etc.
obtained from slopes of plots ln (A?-A ) versus time. The
t
kinetic experiments were performed until the reaction was
completed to 3–4 half-lives. All the values of rate constants
Micelles and other associated colloids have been widely
used as reaction media [12–14] as they are capable of both
catalyzing and inhibiting [15, 16] reaction rates, depending
upon the nature of interaction between the surfactant and
reactants. They play a critical role in the dispersion and/or
localization of charges on the activated complex and/or
intermediates. Thus, the micelles can influence the reaction
rates, equilibria, and concentrations of reactants in the
interfacial region [19–25]. Surfactant aggregates can also
serve as mimic for biological systems [15].
2
(in the absence and presence of Cu ) and kw (in the
?
presence of surfactants) were obtained from linear plots
2
with average linear regression coefficient, r C 0.99. The
autoxidation of adrenaline was carried out under pseudo
first-order conditions using sodium hydroxide in large
excess over adrenaline. Each kinetic run was repeated at
least 3 times, and the observed results were consistent to
within ± 5 %. Critical micelle concentrations for the sur-
factants under the reaction conditions were determined
using a Kruss Type 10 tensiometer.
Copper(II) plays an important role in the reactions of
catecholamines and catechol–amino acids in neurological
systems [16]. Copper(II) can form complexes with adren-
aline [17, 18] and so modify its reactivity. Therefore,
keeping in view of the significance of copper(II) and
adrenaline in the biological systems, we have studied the
kinetics of autoxidation of adrenaline in the absence and
the presence of copper(II). The influence of copper(II) on
the autoxidation and binding of adrenaline with micelles
have been explored and are presented in this paper.
Results and discussion
Autooxidation of adrenaline in aqueous alkali
Repetitive scans for the autoxidation of adrenaline
-
(1.0 9 10 mol dm ) in 5.0 9 10 mol dm NaOH
4
-3
-3
-3
(Fig. 1) indicated that an intense yellow color with kmax at
300 nm reaches a maximum in *30 min, remains stable
for few minutes, and then starts to decay. The reaction
followed a consecutive pathway in which an intermediate
adrenochrome is formed (species 5 in Scheme 1). The
adrenochrome undergoes further reactions to yield adre-
nolutin, dimers, and other products [9]. The pathway is
summarized in Scheme 1.
Experimental
Materials
1
-(3,4-Dihydroxyphenyl)-2-methylamino ethanol (adrena-
line, 95 %, Aldrich, USA), sodium dodecyl sulfate (SDS,
9 %, BDH, England), cetyltrimethylammonium bromide
CTAB, 99 %, BDH, England), sodium chloride (99.9 %,
The repetition of the same experiment in the presence of
-3
-
3
2?
9
4.0 9 10
mol dm
Cu
(Fig. 2) showed that the
(
Merck, Germany), and sodium bromide (99 %, BDH, Eng-
land) were used without further purification. Cetyltrimeth-
ylammonium hydroxide (CTAOH) was synthesized and
recrystallized in the laboratory. Tetramethyl ammonium
bromide (99 %, Sigma, USA), tetraethyl ammonium bro-
mide (99 %, Sigma, USA), copper(II) sulfate (CuSO .5H O,
4
2
9
9 %, BDH, England), and sodium hydroxide of AnalaR
grade were used for the experiments. Deionized
-
6
double-distilled water (specific conductance: 1–2 9 10
-
1
-1
cm ) was used as a solvent.
X
Kinetic measurements
The kinetics was followed by monitoring the absorbance at
(290 and 300 nm in the presence and the absence of
k
max
2
?
Cu , respectively) for the appearance of an intense yellow
color as a function of time, using a PerkinElmer Lambda
Fig. 1 Repetitive scan for the autoxidation of adrenaline for the
formation of adrenochrome. Reaction conditions: [adrenaline] =1.0 9
6
50 UV–visible spectrophotometer. The temperature was
-4
-3
mol dm , [NaOH] = 5.0 9 10
-3
-3
mol dm , temperature =
1
0
maintained at 25.0 ± 0.1 °C using an L.K.B. 2209
25.0 ± 0.1 °C
1
23

Transition Met Chem (2013) 38:173–181
175
Scheme 1 The proposed
mechanism of the autoxidation
of adrenaline catalyzed by
NaOH
OH
OH
HO
HO
HO
HO
N
NHMe
K1
Me
+
OH-
+ H2O
2
1
k1, slow
OH
OH
O
HO
HO
k2
HO2-
+
O2
+
fast
N
N
HO
Me
Me
3
4
HO2-
k3
fast
OH
O
O
-
+
OH + H O
2
N
Me
5
intermediate adrenochrome is somewhat more stable and the
yellow color (with kmax = 290 nm) was observed to be
stable over the next 2 h (the time up to which spectra were
recorded). Plots of log (absorbance) versus time for different
Scheme 2, which shows our proposed mechanism in the light
of the result obtained.
-
The reaction is initiated by the attack of OH on
2
?
[Cu(adrenaline)]
to give intermediate 7. The anionic
complex slowly undergoes cyclization to give
-
4
initial concentrations (in the range of 1.0–12.0 9 10
7
-
3
mol dm ) of adrenaline and [Cu(adrenaline)] at fixed
2?
-
concentration of sodium hydroxide (5.0 9 10 mol dm
3
-3
)
at 25.0 ± 0.1 °C gave parallel straight lines. Thus, the
constant values of kobs indicate that the reaction follows
first-order kinetics with respect to adrenaline or its copper
complex. The values of the rate constant increased linearly
-
3
with increasing [NaOH] from 1.0 9 10
2
to 1.0 9
at fixed concentration of adrenaline
-
-3
1
0
mol dm
-
4
-3
2?
(
1.0 9 10 mol dm ) or [Cu(adrenaline)]
-3
(1.0 9
mol dm ). Plots of log k versus log [adrenaline] (or
-
4
1
0
obs
2
log [Cu(adrenaline)] ) gave straight lines with slopes 0.99
?
and 0.78, indicating that the reaction is first order in
2
adrenaline] and fractional order in [Cu(adrenaline)] .
?
[
West [26] proposed that the presence of oxygen is essential
for the autoxidation of adrenaline to form adrenochrome.
The dissolved O present in solution oxidizes the two phe-
2
2
?
Fig. 2 Repetitive scan for the autoxidation of [Cu(adrenaline)]
2? -4
complex. Reaction conditions: [Cu(adrenaline)] = 1.0 9 10
nolic hydroxyl groups of adrenaline. The complexation of
2
Cu with the amino group of adrenaline is rapid [18, 27, 28],
?
-
3
-3
-3
mol dm , [NaOH] = 5.0 9 10 mol dm , temperature =25.0 ±
0.1 °C
and the species 6 existing in alkaline medium is shown in
123

1
76
Transition Met Chem (2013) 38:173–181
Scheme 2 The proposed
mechanism of the autoxidation
of [Cu(adrenaline)] catalyzed
by NaOH
OH
OH
HO
HO
HO
2?
K2
+
OH-
+
H2O
+
NHMe
NMe
HO Cu OH2
OH
HO
HO Cu OH2
OH
6
7
k4
slow
OH
OH
O
HO
HO
k5
HO2-
+
O2
+
fast
N
Me
N Me
HO
HO Cu OH2
OH
HO Cu OH2
OH
8
9
HO -
2
k6
fast
OH
O
O
OH-
+
H O
2
+
N
Me
HO Cu OH2
OH
1
0
3
.5
intermediate 8. Molecular oxygen then attacks the quinol
group of adrenaline to yield the adrenoquinone complex of
copper 9, followed by the adrenochrome complex 10. The
reaction follows free radical pathways [1, 29] for the for-
mation of 10. The following rate Eqs. (1 and 2) were
obtained for the autoxidation of adrenaline and copper
adrenaline complex, respectively, corresponding to the
proposed mechanisms (Schemes 1, 2):
3.0
2.5
2
1
1
.0
.5
.0
1
1
1
¼
¼
þ
ð1Þ
ð2Þ
kobs
K1½OHꢁꢂ k1
0.5
.0
1
1
1
0
þ
kobs
K2½OHꢁꢂ k4
0.0
0.2
0.4
0.6
0.8
1.0
-
3
-1
3
According to the both rate equations, a plot of 1/k
obs
-
versus 1/[OH ] should give a straight line (Fig. 3) with
10 /[NaOH] (mol dm )
Fig. 3 Plot of 1/kobs versus 1/[NaOH] for the autoxidation of
slope (1/K ) and intercept (1/k ). The values of k and k
1
1
1
4
2?
adrenaline (open circle) and [Cu(adrenaline)] (filled circle) in the
for adrenaline and the Cu(II)-adrenaline complex were
obtained from the intercept and are given in Table 1. The
-4
aqueous medium. Reaction conditions: [adrenaline] = 1.0 9 10
2?
mol dm , [C(adrenaline)] = 1.0 9 10 mol dm , temperature =
25.0 ± 0.1 °C
-3
-4
-3
values of the equilibrium constants, K (for adrenaline) and
1
K (for copper complex), were obtained from the slope and
2
are also presented in Table 1.
obtained values of k
are actually lower for [Cu(adrenaline)] than for adrena-
(Table 1) for the cyclization reaction
4
2
?
Coordination of adrenaline to the copper(II) center
results in a decrease in electron density on the nitrogen
atom, facilitating release of the NH proton. Hence, the
reaction rate in the presence of copper increases. The
2
?
line. This is because coordination to Cu may lower the
electron density on the nitrogen anion, making it a poorer
nucleophile for attack on the ring. The value of the
1
23

Transition Met Chem (2013) 38:173–181
177
2
Table 1 Values of rate constants and equilibrium constants for adrenaline and [Cu(adrenaline)] in the aqueous NaOH and micellar media
?
2
10 k
-1
(s
-3
2
K (mol dm )
Medium
1
)
2
?
2?
Adrenaline
[Cu(adrenaline)]
Adrenaline
[Cu(adrenaline)]
NaOH
CTAB
SDS
34.56 ± 1.34
a
12.36 ± 0.87
2.03 ± 0.32
3.00 ± 0.22
2.19 ± 0.14
0.33 ± 0.02
3.02 ± 0.06
0.21 ± 0.04
1.85 ± 0.62
12.01 ± 0.91
6.14 ± 0.74
-3
0.81 ± 0.02
-
4
-3
-2
-3
-2
Reaction conditions: [adrenaline] = 1.0 9 10 mol dm , [SDS] = 1.0 9 10 mol dm , [CTAB] = 2.0 9 10 mol dm
a
-2
-3
2?
-4
-3
[
CTAB] = 1.0 9 10 mol dm , [Cu(adrenaline)] = 1.0 9 10 mol dm , temperature = 25.0 ± 0.1 °C
2
2
2
.9
.8
.7
Table 2 Values of the observed rate constants and calculated rate
2
?
constants for the autoxidation of adrenaline and [Cu(adrenaline)] in
the alkaline medium using Eqs. 1 and 2 and the parameters given in
Table 1 at different concentrations of NaOH
2.6
2
2
2
2
2
2
1
.5
.4
.3
.2
.1
.0
.9
3
3
3
3
3
1
[
(
0
10 (kadr
)
obs
10 (kadr
)
calc
10 (kCu-
10 (kCu-
-1
-1
-1
obs (s
NaOH]
mol dm
(s
)
(s
)
adr
)
)
adr
-1
)
calc
-3
)
(s )
1
5
1
1
2
3
5
.0
0.33 ± 0.06
1.62 ± 0.11
3.28 ± 0.09
4.87 ± 0.14
6.55 ± 0.12
9.76 ± 0.16
0.33
1.64
3.27
4.88
6.48
9.63
1.74 ± 0.08
4.26 ± 0.14
7.96 ± 0.16
1.70
5.35
8.67
.0
0.0
5.0
0.0
0.0
0.0
1.8
1
1
1
.7
.6
.5
14.22 ± 0.12 13.72
17.34 ± 0.13 18.10
22.54 ± 0.12 24.86
32.12 ± 0.14 33.66
0
50
100
150
200
250
300
3
-3
15.98 ± 0.15 15.75
10 [salt] (mol dm )
2
?
Fig. 4 Plot of kobs versus [salt] for the autoxidation of adrenaline in
the aqueous medium. Reaction conditions: [adrenaline] =
equilibrium constants (K ) is higher for [Cu(adrenaline)]
2
2
?
than for adrenaline (K ) because Cu facilitates the removal
?
of H ions from the amino nitrogen, as described earlier.
-
4
-3
-3
-3
1
1.0 9 10
(filled circle), NaCl (open circle), Tetraethyl ammonium bromide
filled triangle), Tetramethyl ammonium bromide (open triangle),
mol dm , [NaOH] = 5.0 9 10
mol dm , NaBr
(
Thus, the overall value of k is higher for [Cu(adrena-
obs
2
?
temperature = 25.0 ± 0.1 °C
line)] than for adrenaline. The observed and calculated
values of rate constants (kobs and kcalc) are presented in
Table 2. The values of the rate constants were calculated
from the parameters given in Table 1. Increasing concen-
trations of sodium chloride, sodium bromide, tetramethyl
ammonium bromide, and tetraethyl ammonium bromide
autoxidation of adrenaline was studied at varying concen-
trations of surfactant. The addition of CTAB increased the
values of pseudo first-order rate constants in the lower
concentration range for both the adrenaline and
2
?
[Cu(adrenaline)] . Further increases in the concentration
of CTAB increased the rate constant to a maximum value,
and then, it decreased with still further increase in [CTAB].
The decrease in the rate constant was sharp for adrenaline,
(
Fig. 4) slightly increased the values of rate constant in the
-
3
lower concentration range (i.e. 5–30 mmol dm ); similar
behavior of these salts was observed in the autoxidation of
benzoin in alkaline solution [24, 30]. The formation of an ion
pair helps to stabilize the anion as shown in Scheme 3 and so
increases the rate of reaction. Further increases in [salt] did
not influence the rate of reaction, indicating that the autox-
idation of adrenaline follows either a molecular or free rad-
ical mechanism.
2
?
while for [Cu(adrenaline)]
the rate constant did not
decrease much. This behavior is typical of a bimolecular
reaction catalyzed by non-reactive counter ion micelles
(Fig. 5). SDS inhibited the rate of autoxidation of adrena-
line (Fig. 6), while CTAOH gave a plateau-like curve for
the plot of k versus [CTAOH] (Fig. 7). The variation in
w
rate constant for CTAB and SDS for the autoxidation of
2
[Cu(adrenaline)] displayed a maximum type behavior
?
Autoxidations in micellar media
-
with maximum values at 2.0 9 10 mol dm for [SDS]
2
-3
-
3
-3
and 7.0 9 10 mol dm for [CTAB].
The influence of the cationic surfactants cetyltrimethyl-
ammonium bromide (CTAB), cetyltrimethylammonium
hydroxide (CTAOH), and anionic surfactant sodium
dodecyl sulfate (SDS) on the reaction rate for the
The results obtained for the autoxidation of adrenaline
2
?
and [Cu(adrenaline)] in the presence cationic micelles of
CTAB can be explained on the basis of the pseudophase
123

1
78
Transition Met Chem (2013) 38:173–181
Scheme 3 Formation of an ion
?
pair between cations (Na and
H
C
H
OH
OH
O
?
O
(
anion 4
CH
3
)
4
N
the intermediate
C
CH2
CH2
+
Na+
:-O
(CH )
:-
3 4
N
N
O
N
H
H
CH3
CH3
1
1
1
1
1
8
6
4
2
0
70
6
5
0
0
40
8
6
4
2
0
3
2
1
0
0
0
0
0
20
40
60
80
100
0
10
20
30
40
50
3
-3
1
0 [CTAB] (mol dm )
3
-3
1
0 [CTAOH] (mol dm )
Fig. 5 Plot of k
w
versus [CTAB] for the autoxidation of adrenaline
Fig. 7 Plot of k
in the absence (filled circle) and the presence of NaOH (open circle;
w
versus [CTAOH] for the autoxidation of adrenaline
2
open circle) and [Cu(adrenaline)] (filled circle). Reaction condi-
?
(
tions: [adrenaline] = 1.0 9 10
-
4
-3
mol dm , [Cu(adrenaline)]
2?
=
mol dm , temper-
-3
.0 9 10 mol dm NaOH, filled triangle: 2.0 9 10 mol dm
-3
-3
-3
1
NaOH, open triangle; 3.0 9 10 mol dm NaOH), [adrenaline] =
-
4
-3
mol dm , [NaOH] = 5.0 9 10
-3
-3
1
.0 9 10
-3
-3
ature = 25.0 ± 0.1 °C
-4
-3
1
.0 9 10 mol dm , temperature = 25.0 ± 0.1 °C
which the surfactant aggregates act as an ion exchanger.
Scheme 4 is used to describe the reactions occurring in the
aqueous and micellar phases:
1
1
2
0
In this Scheme 4, D is the micellized surfactant (i.e.
n
[
total surfactant]—cmc), k ’ and k ’ are the first-order rate
w m
8
6
4
2
0
constant for the autoxidation (of adrenaline or [Cu(adren-
?
aline] ) in the aqueous and micellar pseudophases,
2
respectively, and K is the binding constant for the sub-
s
2?
strate (adrenaline or [Cu(adrenaline] ), S to the micelle,
which is given by
½
S ꢂ
m
Ks ¼
ð3Þ
½S ꢂ½D ꢂ
w
n
0
20
40
60
80
100
2
?
3
-3
Sm and Sw represent adrenaline or [Cu(adrenaline]
bound to the micellar and aqueous pseudophases,
1
0 [SDS] (mol dm )
Fig. 6 Plot of k
w
versus [SDS] for the autoxidation of adrenaline (open
respectively.
2?
circle) and [Cu(adrenaline)]
[
(filled circle). Reaction conditions:
-3
-4
2?
[Cu(adrenaline)] =1.0 9
The observed rate constant in the presence of surfactant
is given by
adrenaline] = 1.0 9 10
-3
mol dm
mol dm , [NaOH] = 5.0 9 10
,
-3
-4
0
-3
mol dm , temperature =
1
2
5.0 ± 0.1 °C
0
0
k þ k Ks½Dnꢂ
w
m
kw ¼
ð4Þ
1
þ Ks½Dnꢂ
ion-exchange model, in which the substrate is considered to
be distributed between the aqueous and micellar phases.
0
0
and k in terms of reactant OH ion concentration can
m
-
k
w
This distribution is governed by the values of the binding
be given by
Â
-
constant, K . The distribution of reactive OH ions in both
pseudophases is based on the principle of ion exchange in
Ã
s
0
ꢁ
w
k ¼ k OH
ð5Þ
w
w
1
23

Transition Met Chem (2013) 38:173–181
179
Â
Ã
Â
Ã
Â
Ã
ꢁ
ꢁ
ꢁ
m
Ks
Br ¼ Br þ Br
ð14Þ
T
w
Sw
+
Dn
Sm
The following quadratic expression was used to evaluate
k'w
k'm
the value of mOH for the determination of micelle-bound
hydroxide ions by considering the ion-exchange model for
ion-exchange resins, keeping b constant
Products
Scheme 4 Mechanism of the micelles catalyzed autodixation of
ꢀ
Â
Ã
Â
Ã
ꢁ
Â
Ã
ꢁ
OH
Br
ꢁ
ꢁ
OH þ K
Br
b OH
OH
T
T
T
2
?
2
OH
adrenaline and [Cu(adrenaline)]
m
þ mOH
ꢁ b ꢁ
OH
Br
ðK ꢁ 1Þ½Dnꢂ
ðKBr ꢁ 1Þ½Dnꢂ
Â
Ã
¼ 0
ð15Þ
ꢁ
m
km OH
0
k ¼
ð6Þ
m
The experimental observations demonstrate that the
½Dnꢂ
presence of micelles influences the reaction rate by
-
where k is defined in terms of the molar ratio of OH ions
bound to the micellar head-groups.
-
In the present case, Br (the counterion of CTAB) and
-
reactive OH compete for the charged micellar surface.
m
inhibiting (adrenaline in SDS) or by accelerating
2?
(adrenaline in CTAB and [Cu(adrenaline)] in CTAB and
SDS) the rate of autoxidation. The micelles are dynamic in
nature, and the association of surfactant molecules into
micelles and dissociation into monomers are faster than the
rate of autoxidation. Therefore, it can be assumed that a large
proportion of reactions occur in the micellar media, and the
reactions occurring in aqueous media can be neglected by
comparison. Hence, Eq. (12) can be rewritten as
Their distribution in the Stern region of the micelles can be
predicted using a model proposed by Romsted [12, 13], in
which the ions bind to the micelle through a process of ion-
exchange similar to that occurring in ion-exchange resins.
-
-
For OH as reactive ion and Br as micelle counterion, the
ion-exchange equilibrium can be expressed as
kmKsmOH½Dnꢂ
OH
kw ¼
ð16Þ
K
ꢁ
ꢁ
Br
ꢁ
ꢁ
W
1 þ K ½D ꢂ
s
n
OH þ Br ꢁꢁꢁ*OH þ Br
ð7Þ
ð8Þ
W
m )ꢁꢁꢁ
m
Rearranging Eq. (16) gives
Hence
Â
ÃÂ
Ã
mOH
1
1
ꢁ
ꢁ
OH Br
¼ k þ
ð17Þ
OH
Br
m
w
K
¼ Â
ÃÂ
Ã
kw
k K ½D ꢂ
ꢁ
OH Br
w
ꢁ
m m
s
n
m
Thus, plots of mOH/k versus 1/[D ] for the autoxidation
w
n
ꢁ
ꢁ
Here, Br and Br are the surfactant counterions in the
2?
m
w
of adrenaline and [Cu(adrenaline)]
occurring in the
micellar and aqueous phases, respectively. Similarly, the b
parameter may be defined as the neutralized fraction of the
micellar head-groups
micellar media of SDS and CTAB gave straight lines. The
values of k and K were calculated from the intercept and
m
s
slope, respectively, and are given in Table 3. The observed
and calculated values of rate constants (using Eqs. 10–15)
are compared in Table 4 and show good agreement.
The results for the autoxidation of adrenaline in reactive
micelles of CTAOH were treated by using the following
relationship of the mass action model [30–32]
0
b ¼ mOH þ mBr
where mOH and mBr are the neutralized fractions of the
ð9Þ
-
micellar head-group with reactive (OH ) and non-reactive
-
Br ) ions. Then,
(
Â
Ã
ꢁ
OH
m
k
0
mOH ¼
ð10Þ
ð11Þ
ꢁ
ꢁ
OH
ꢁ
OH þ D ꢁꢁꢁ*OH
ð18Þ
½
Br
D ꢂ
W
n )ꢁꢁꢁ
M
n
Â
Ã
ꢁ
Or,
m
mBr
¼
Â
Ã
½
Dnꢂ
ꢁ
OH
M
0
OH
K
¼ Â
ÃÀ
Â
ÃÁ
ð19Þ
ꢁ
ꢁ
M
OH
½D ꢂ ꢁ OH
According to Eqs. 5–8 and Scheme 3, the first-order rate
W
n
constant equation can be written as
Â
Rearrangement of Eq. (19) gives Eq. (20)
Ã
ꢁ
ꢂ
ꢃ
kw OH þ ðk K ꢁ k ÞmOH½Dnꢂ
Â
Ã
Â
à Â
Ã
T
m
s
w
0
OH
2
0
0
ꢁ
ꢁ
ꢁ
OH
M
kw ¼
ð12Þ
ð13Þ
K
OH ꢁ 1 þ K ½D ꢂ þ K OH
M
OH
n
OH
T
1
þ Ks½Dnꢂ
0
Â
Ã
ꢁ
T
þ K ½D ꢂ OH
ð20Þ
OH
n
where
Â
Ã
Â
Ã
Â
Ã
The value of [OH ] was obtained on solving the
m
ꢁ
ꢁ
ꢁ
m
OH ¼ OHw þ OH
T
quadratic Eq. (20). Equation (12) on rearrangement and in
terms of [OH ] can be written as
Similarly,
m
123

1
80
Transition Met Chem (2013) 38:173–181
2
Table 3 Fitted parameters of the kinetic results for the base catalyzed autoxidation of adrenaline and [Cu(adrenaline)] in CTAB and SDS
?
3
10 cmc (mol dm
-3
)
2?
Surfactant
b
Adrenaline
[Cu(adrenaline)]
OH
X
2
10 k
-1
OH
X
2
10 k
-1
K
(X = Br/Cl)
m
(s
)
K
s
K
(X = Br/Cl)
m
(s
)
K
s
CTAB
SDS
0.8
0.8
0.80
0.81
16
6
85 ± 2
1.38 ± 0.04
49 ± 4
16
6
348 ± 2
51 ± 2
9 ± 2
28 ± 4
-3
157 ± 6
-
3
-3
-4
-3
2?
-4
Reaction conditions: [NaOH] = 5.0 9 10 mol dm , [adrenaline] = 1.0 9 10 mol dm , [Cu(adrenaline)] = 1.0 9 10 mol dm
temperature = 25 ± 0.1 °C
,
Table 4 Values of the observed
rate constants and calculated
rate constants for the
3
0 [Surfactant]
2?
1
(
Adrenaline
[Cu(adrenaline)]
-3
mol dm )
3
10 (k ) (s
-1
)
3
10 (k )
-1
3
10 (k ) (s
-1
3
-1
)
w obs
w calc
(s
)
w obs
)
w calc
10 (k ) (s
autoxidation of adrenaline
2
?
and [Cu(adrenaline)] in
the micellar medium by
using Eq. 12 and the parameters
given in Table 3
(i) CTAB
1
3
5
7
07.56 ± 0.25
12.84 ± 0.28
16.79 ± 0.26
15.48 ± 0.32
13.21 ± 0.34
10.17 ± 0.21
08.4 ± 0.22
08.25 ± 0.28
08.13 ± 0.31
09.18
14.95
15.83
12.85
11.60
09.10
06.88
04.90
04.19
05.24 ± 0.23
08.66 ± 0.29
10.71 ± 0.24
–
05.51
09.69
11.19
–
10
30
50
70
90
13.36 ± 0.32
16.32 ± 0.26
16.10 ± 0.32
15.61 ± 0.31
14.22 ± 0.29
14.20
16.64
16.22
15.86
15.04
(
ii) SDS
1
3
5
7
1
3
5
8
9
1.25 ± 0.06
1.19 ± 0.04
1.13 ± 0.04
1.06 ± 0.05
0.93 ± 0.03
0.64 ± 0.04
0.42 ± 0.03
0.30 ± 0.02
–
1.36
1.17
1.08
0.88
0.84
0.52
0.36
0.25
–
10.46 ± 0.23
11.6 ± 0.32
12.2 ± 0.31
11.78 ± 0.37
11.70 ± 0.32
10.07 ± 0.28
09.93 ± 0.26
09.89 ± 0.28
09.87 ± 0.27
10.21
11.58
12.14
11.57
11.68
10.45
09.82
09.75
09.70
0
0
0
0
0
½
OHmꢂ
kw
1
½Dnꢂ
km
near the Stern or Gouy Chapman region of the micelles. As
a result, the value of binding constant is decreased, and the
reaction rate is increased. The increase in concentration of
NaOH in the CTAOH system increases the rate of reaction,
but the overall value of rate constant in the micellar region
remains in the same order. The binding constant is slightly
¼
þ
ð21Þ
kmKs
Thus, a plot of [OH ]/k versus [D ] gave a straight line
n
m
w
with slope = 1/km and intercept = 1/k K . The values of
0
k , K and K were obtained from the plot and are given
OH
m
s
m
s
-
in Table 5. The values of the binding constant of
adrenaline in CTAB are found to be higher than for its
copper complex, indicating that the cationic micelles of
CTAB have less affinity for the positively charged copper
complex. The autoxidation in SDS micelles is extremely
slow and almost ceases at high [SDS], while the reaction
affected due to high concentration of OH in the Stern
region.
Conclusions
2
?
rate increases for [Cu(adrenaline)]
in the same SDS
Adrenaline and its copper complex undergo autoxidation
reaction in alkaline media with different rates in the pres-
ence of dissolved oxygen. The reaction rates of both spe-
micelles and reaches a plateau at higher [SDS]. It may be
that the location or orientation of the molecules in SDS
micelles is such that it does not favor the reaction. The
complexation with copper makes the adrenaline more
-
cies were found to be dependent upon [OH ]. The addition
2
?
of Cu to the adrenaline solution stabilized the interme-
diate adrenochrome in alkaline medium. Added salts
2
?
hydrophilic, and the [Cu(adrenaline)] complex may lie
1
23

Transition Met Chem (2013) 38:173–181
181
Table 5 Fitted parameters of
3
10 [NaOH]
3
10 cmc
2
10 k
-1
Surfactant
B
K
OH
m
(s
)
K
s
the kinetic results for the base
catalyzed autoxidation of
adrenaline in CTAOH
-3
-3
(mol dm )
(
mol dm
)
CTAOH
CTAOH
0.0
1.0
2.0
0.8
0.8
0.8
0.8
0.80
0.80
0.80
0.80
34
34
34
34
3.51 ± 0.04
6.09 ± 0.06
5.71 ± 0.07
5.91 ± 0.06
30 ± 3
15 ± 2
24 ± 4
25 ± 3
Reaction conditions:
-4
[
mol dm
adrenaline] = 1.0 9 10
-
3
,
3
.0
temperature = 25 ± 0.1 °C
increased the rate of reaction slightly in the lower con-
centration range by the formation of ion pairs with the
added cations in the aqueous medium. Higher concentra-
tions of salts elicited no further change in the rate. The
autoxidation reactions follow molecular or free radical
pathways. CTAB and CTAOH micelles increased the rate
of autoxidation of adrenaline, while SDS micelles inhibited
the rate. SDS and CTAB micelles catalyzed the rate of
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autoxidation of [Cu(adrenaline)] in the lower concen-
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trations. Further increase in [surfactant] resulted in a slight
lowering of the rate constant. The binding constant for
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?
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Acknowledgments The authors extend their appreciation to the
Deanship of Scientific Research at King Saud University for funding
the work through the research group project No. RGP-VPP-148.
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