Kinetics of oxidation of atropine by K2Cr2O7 in acidic aqueous solutions

Article abstract of DOI:10.14233/ajchem.2018.21463

Oxidation of atropine (ATN) by K2Cr2O7 has been studied kinetically in aqueous strongly acidic solutions. The kinetics of this reaction was investigated at different temperatures. A constant pH and constant ionic strength were maintained constant throughout all measurements. The concentration of atropine was around an order of magnitude greater than that of K2Cr2O7, that is under pseudo first order kinetics. Rate constants were obtained by monitoring change in absorbance of K2Cr2O7 with time at its predetermined maximum wavelength. Overall rate constant and the orders of the reaction in terms of concentrations of both atropine and K2Cr2O7 were determined. Reaction runs at different temperature allows activation energy of the process to be convoluted from temperature effect on the rate of the reaction. A resonable mechanism for the reaction was proposed in accordance with kinetics results.

Full text of DOI:10.14233/ajchem.2018.21463

Asian Journal of Chemistry; Vol. 30, No. 9 (2018), 2103-2106
A
SIAN
J
OURNAL OF HEMISTRY
C
https://doi.org/10.14233/ajchem.2018.21463
Kinetics of Oxidation of Atropine by K2Cr2O7 in Acidic Aqueous Solutions
1
1
HAMZEH M. ABDEL-HALIM , ABDULLAH I. SALEH1,2,* and SANAD K. AL-GHREIZAT
1Department of Chemistry, Faculty of Science, The Hashemite University, Az-Zarqa, Jordan
2College of Pharmacy, Al Ain University of Science & Technology, Al Ain, UAE
*Corresponding author: E-mail: a-saleh@hu.edu.jo
Received: 21 May 2018;
Accepted: 16 July 2018;
Published online: 31 July 2018;
AJC-19028
Oxidation of atropine (ATN) by K2Cr2O7 has been studied kinetically in aqueous strongly acidic solutions. The kinetics of this reaction
was investigated at different temperatures.A constant pH and constant ionic strength were maintained constant throughout all measurements.
The concentration of atropine was around an order of magnitude greater than that of K2Cr2O7, that is under pseudo first order kinetics.
Rate constants were obtained by monitoring change in absorbance of K2Cr2O7 with time at its predetermined maximum wavelength.
Overall rate constant and the orders of the reaction in terms of concentrations of both atropine and K2Cr2O7 were determined. Reaction
runs at different temperature allows activation energy of the process to be convoluted from temperature effect on the rate of the reaction.
A resonable mechanism for the reaction was proposed in accordance with kinetics results.
Keywords: Kinetics, Atropine, K2Cr2O7, H2CrO4.
Chromium(VI) is known to be mutagenic and carcino-
genic, hence, posing a serious environmental hazard [15]. It is
used for many applications [16-25]. Consequently, it is of crucial
importance gaining as much information as possible regarding
its various forms and their interaction with organic substrates.
INTRODUCTION
Reaction kinetics of transition metal complexes by various
reducing agents has been widely studied. Different methods
were employed for kinetics measurements. Methods including:
stopped-flow spectrophotometry, chemical analysis of products
and the use of radioactive and stable isotope tracers. Details
of these methods along with much of data produced are given
elsewhere [1]. Of a special interest and relation to the present
work, several papers have been published on kinetics study
of oxidation by potassium dichromate in its different forms.
In particular, kinetics of oxidation of various substrates by
chromic acid has been reported [2-12]. Reaction kinetics of
oxidation of atropine by alkaline potassium permanganate has
been investigated recently [13]. However, detailed mechanistic
study of oxidation of atropine by powerful oxidizing reagents
including potassium dichromate in appropriate pH’s are still
limited.
H3C
N
OH
· H2SO4.H2O
O
Ph
O
2
Fig. 1. Atropine sulfate monohydrate
Potassium dichromate in strongly acidic media can be
present in the form of chromate ion (CrO42–), bichromate ion
(HCrO4–) and chromic acid (H2CrO4). Which species presumed
present in aqueous media is a function of pH and ionic strength
and largely still an open discussion subject [21]. However at
the Cr(VI) concentration used herein, the chromate ion is
There are several medicinal uses of atropine. In the warfare,
atropine in the salt form as atropine sulfate monohydrate (ASM),
Fig. 1, mixed with pralidoxime chloride, 2-PAM chloride, is
commonly used as an antidote for poisoning by organophosphate
insecticides and nerve gases [14].
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2104 Abdel-Halim et al.
Asian J. Chem.
absent [22]. It is also demonstrated that the HCrO4– species is
spectrophotometrically undetectable in the UV-visible region.
That will leave chromic acid as the predominant and might be
the only detectable species under the highly acidic conditions
of this study.
In the present work and as an extension of our previous
kinetics studies on biologically important molecules [13], a
mechanistic aspect of oxidation reaction of atropine by chromic
acid has been investigated. The kinetics of the reaction has
been determined spectrophotometrically. Reaction rates have
been determined by monitoring the decrease of absorption of
Cr(VI) at its absorption maximum, λmax = 350 nm. It was verified
that at this wavelength there are no interferences from either
the reactants, products or any other reagent involved. Scheme-I
represents the overall reactions involved in this study.
In a strongly acidic solution, a condition of this study, atropine
sulfate monohydrate is converted to atropine and K2Cr2O7 is
converted to H2CrO4. Diode Array Spectrophotometer model
8453E from HP Agilent Technologies was used. Reactions
were monitored by measuring the absorbance of chromic acid
with time at its wavelength of maximum absorbance difference,
λ
max. This wavelength was predetermined from the absorption
spectra for chromic acid alone and for its mixture with atropine
sulfate monohydrate after completion of the reaction. Fig. 2
shows a scan at different time intervals for reactants and products,
from which λmax was determined to be at 350 nm. Absorbance
at this wavelength is mainly due to chromic acid. However
absorbance at this wavelength of organic compounds, reactant
or products is negligible.
1.000
K2Cr2O7
2 min
EXPERIMENTAL
0.900
4 min
6 min
8 min
0.800
0.700
0.600
0.500
0.400
0.300
0.200
0.100
0
The following set of preliminary experimental runs was
designed to sort out the different reactions taking place under
the strongly acidic conditions used herein.
Acidic hydrolysis: Atropine sulfate monohydrate (0.10
M) was treated with concentrated H2SO4 (1.0 M) solution for
25 min. Products were then isolated, purified and characterized
by various NMR and IR spectroscopic techniques. There is
apparently no hydrolysis to any measurable extent under reaction
conditions used in this study.
10 min
12 min
14 min
16 min
18 min
20 min
22 min
24 min
300
400
500
600
700
800
Cr(VI) in strongly acidic medium:A blank run of mixing
all ingredients [K2Cr2O7 (0.001M), H2SO4 0.10M)] except
atropine sulfate monohydrate, has shown that the change in
absorption by H2CrO4 is negligible throughout the measuring
period. No spectrophotometrical evidence was obtained for the
presence of any other species apart from chromic acid.
Acidic K2Cr2O7 oxidation: Approximately 10 folds in
excess of atropine sulfate monohydrate solution (0.01 M) was
treated with the acidic K2Cr2O7 (0.001 M) solution for 25 min.
Spectroscopic data of the products have shown the presence of
excess atropine and the oxidized product (Scheme-I).A radical
involvement in this reaction was ruled out based on the tests
of adding mercuric chloride or acrylonitrile monomer to the
reaction. Neither induced a white precipitate formation or
polymerization, respectively [17-19].
Wavelength (nm)
Fig. 2. Absorbance vs. λ for K2Cr2O7 and for the reaction mixture; λmax
350 nm [K2Cr2O7] = 1.00 × 10-3 M, [ATN] = 1.00 × 10-2
=
M
Reaction rates were determined under pseudo-first order
condition, The concentrations of atropine [10–2 to 10–1 mol
dm–3] were an order of magnitude greater than those of K2Cr2O7
[10–4 to 10–2 mol dm–3]. All runs were done at constant pH and
ionic strength (ionic strength was maintained using NaClO4).
Rates were also determined at various temperatures. The temp-
erature for each run was thermstatically controlled within
0.1 ºC.
RESULTS AND DISCUSSION
Chromium(VI) in strongly acidic medium exists in the
form of chromic acid H2CrO4 [17].Atropine on the other hand
will exist in the hydrated form. The UV/visible absorption
spectrum taken directly after mixing atropine with Cr(VI) in a
H2SO4/H2O medium shows an absorbance band at 350 nm and
a shoulder at 420–500 nm, characteristic of Cr(VI) in acidic
medium (Fig. 2). The absorbencies at 350 nm and 420–470
As a result, it is presumed that the reaction under investi-
gation is not accompanied by hydrolysis of atropine. No species
other than chromic acid has an absorbance at the selected
wavelength throughout the course of the reaction.
Kinetics measurements: Different concentrations of atro-
pine sulfate monohydrate and K2Cr2O7 were used in this study.
Fast
H3O+
+
Cr2O7
-
H2O + 2H2CrO4
OH
OH
CHO
H3C
H3C
H3C
O-
O
O
O
H2CrO4
N
N
N
CrIII
H2O
+
HO
+
+
O
O
O
1 equivalant
OH
Atropine
~ 10 equivalents
excess atropine
Scheme-I: Overall reactions involved

Vol. 30, No. 9 (2018)
Kinetics of Oxidation of Atropine by K2Cr2O7 in Acidic Aqueous Solutions 2105
nm decay with time, while the absorbance at 570 nm increases.
The observation of isosbestic point (centered at λ = 520 nm)
indicates that the stoichiometry of the reaction remains un-
changed during the chemical reaction and that no competing
reactions occur during the considered time range. At the end
of the redox reaction, the bands from 520-700 nm indicates
the formation of Cr(III) product [20]. It is well-established that
both Cr(V) and Cr(IV) species absorb at 350 nm, consequently,
Cr(V)/Cr(IV) contributions have to be taken into account when
interpreting the absorbance decay [21]. Nevertheless Cr(IV)
species are unstable oxidation state throughout the reduction
of chromium ion from hexavalent to trivalent states. It implies
that the decay rate of Cr(IV), if present, will take place at a
much higher rate compared to that of Cr(VI) which will ultimately
lead to a little contribution to the absorbance measured at 350
nm.
were carried out for each concentration and the average value
of the observed rate is reported. Experimental errors in value
of rate constant are estimated to be ~ 10 %.
The observed rates constant of the reaction (kobs) at various
concentrations of Cr2O72– and constant [ATN], with [ATN] 〉〉
[Cr2O72–] are shown in Table-1.
TABLE-1
KINETICS RESULT FOR THE OXIDATION OF
ATROPINE BY K2Cr2O7 AT CONSTANT [ATN];
[ATN] = 1.00 × 10–2 M. Average kobs = (7.54 0.8) × 10–3 s–1
Run No.
[Cr2O72–] (mol L–1)
2.00 × 10–3
kobs × 103 (s–1)
1
2
3
4
5
7.55
8.27
6.82
7.50
1.75 × 10–3
1.50 × 10–3
1.00 × 10–3
6.70 × 10–4
7.54
The involvement of free radicals in this reaction has been
ruled out by addition of acrylonitrile or HgCl2 to the reaction
mixture with the observations that are consistent with that
conclusion.
Kinetics: The rate of oxidation of atropine (ATN) by
H2CrO4 is given by:
Table-1 shows that the values of kobs are constant within
experimental errors, indicating that the oxidation rate of
atropine by K2Cr2O7 is first order with respect to [K2Cr2O7]
according to eqn. 4.
The effect of concentration of atropine on reaction rate
was studied. The observed rate of reaction was measured at
various concentrations of atropine while keeping the concen-
tration K2Cr2O7 constant. Results are shown in Table-2.
Rate = k [ATN]a[H2CrO4]b
(1)
where k is the reaction rate constant, a & b are orders of reaction
with respect to concentrations of atropine and H2CrO4, respect-
ively. The concentration of H+ was kept constant at 17 M through-
out all kinetics measurements. Under pseudo-first order conditions
in which [ATN] 〉〉 [H2CrO4], the concentration of atropine is
essentially constant throughout the reaction. The reaction rate
is thus given by:
TABLE-2
KINETICS RESULT FOR THE OXIDATION OF
ATROPINE BY K2Cr2O7 AT CONSTANT [Cr2O72–];
[Cr2O72–] = 1.00 × 10–3
M
Run No.
[ATN] (mol L–1)
2.00 × 10–2
1.75 × 10–2
1.50 × 10–2
1.00 × 10–2
0.75 × 10–2
0.50 × 10–2
kobs × 103 (s–1)
14.9
1
2
3
4
5
6
d[H2CrO4 ]
13.5
10.7
7.50
5.74
Rate ≈ −
= kobs[H2CrO4 ]b
(2)
(3)
dt
where kobs is the observed rate of reaction, given by:
kobs = k [ATN]a
3.19
where k is the rate constant of reaction (1).
The overall rate constant for oxidation of atropine by
K2Cr2O7, k, and the order of the reaction with respect to [ATN],
a, were found from the intercept and the slope, respectively,
of a plot of ln (kobs) versus ln [ATN] according to eqn. 3. The
slope represents the order (a ≈ 1) and eintercept represent k, equals
(1.04 0.1) × 10–1dm3 mol–1 s–1.
For a first-order dependence of reaction rate on [H2CrO4],
i.e. b = 1, experimental absorbance-time data pairs were fitted
to the exponential function:
(A∞ - At) = (A∞–A0) exp(–kobs t)
or
Effect of temperature: Effect of temperature on reaction
rate was studied at various temperatures keeping concentrations
of atropine and K2Cr2O7 constant. As expected, the observed
rate and the rate constant of the reaction increase as temperature
increases.






(A∞ − At )
(A∞ − A0 )
ln
= −kobst
(4)
where At is the absorbance of the reaction mixture which is
mainly due to H2CrO4 at a given time (t) through the reaction,
A0 is its initial absorbance (t = 0) and A∞ is the absorbance of
the mixture at the end of the reaction, that is when the absor-
bance no longer changes with time (t = ∞).
Using Arrhenius equation:
k = A e-Ea/RT
Experimental results showed that a plot of ln [(A∞ – At)/
(A∞ – A0)] vs. time gives a straight line. Its slope represents
the rate of disappearance of H2CrO4, The value of kobs (s–1)
was obtained from the slope, according to eqn. 4. Using eqn.
3, a plot of ln kobs vs. ln [ATN]a is a straight line that gives the
reaction rate constant, k, in units of dm3 mol–1 s–1 (intercept) and
the order of the reaction (a) with respect to [ATN] (slope). At
specific concentrations of substrate and oxidant, several trials
or
Ea
ln k = ln A −
(5)
RT
The activation energy (Ea) and the Arrhenius factor (A)
were obtained by plotting ln k versus (1/T). Results are shown
in the Table-3. It was found that Ea = 33. 70 kJ, A = 8.14 × 105
dm3 mol–1 s–1.

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KINETICS RESULTS FOR THE OXIDATION OF
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