Spectroscopic investigation on kinetics and mechanistic aspects to electron-transfer process into quinolinium dichromate oxidation of a high blood pressure drug captopril in acidic medium

Article abstract of DOI:10.1016/j.molliq.2014.12.017

This study investigated on kinetics of oxidation of captopril by QDC was studied spectrophotometrically in acidic medium along with its mechanistic pathway. Such studies are greatly helpful in gaining an insight into the interaction of metal ions through the study of the mechanistic pathway of CPL in redox reactions. The oxidative product of captopril was found to be captopril disulfide was separated, and identified by FT-IR. A suitable free radical mechanism was proposed. The reaction exhibited first-order kinetics with respect to [oxidant] and fractional order in CPL. Consequently, the interaction between the complex species and CPL is supported kinetic orders of reaction by spectrophotometric verification, positive entropy of activation and the first-order rate constant increased with the increase in the dielectric constant and increase ionic strength of the medium. The reaction constants involved in the mechanism were computed and the overall activation parameters were evaluated which lend support to the proposed mechanism.

Full text of DOI:10.1016/j.molliq.2014.12.017

Journal of Molecular Liquids 203 (2015) 1–6
Contents lists available at ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
Spectroscopic investigation on kinetics and mechanistic aspects to
electron-transfer process into quinolinium dichromate oxidation of a
high blood pressure drug captopril in acidic medium
a,b
Abdullah M. Asiri a,b, Aftab Aslam Parwaz Khan a,b, , Anish Khan
⁎
a
Chemistry Department, King Abdulaziz University, Jeddah 21589, Saudi Arabia
Center of Excellence for Advanced Materials Research (CEAMR), King Abdulaziz University, Jeddah 21589, Saudi Arabia
b
a r t i c l e i n f o
a b s t r a c t
Article history:
This study investigated on kinetics of oxidation of captopril by QDC was studied spectrophotometrically in acidic
medium along with its mechanistic pathway. Such studies are greatly helpful in gaining an insight into the inter-
action of metal ions through the study of the mechanistic pathway of CPL in redox reactions. The oxidative prod-
uct of captopril was found to be captopril disulfide was separated, and identified by FT-IR. A suitable free radical
mechanism was proposed. The reaction exhibited first-order kinetics with respect to [oxidant] and fractional
order in CPL. Consequently, the interaction between the complex species and CPL is supported kinetic orders
of reaction by spectrophotometric verification, positive entropy of activation and the first-order rate constant in-
creased with the increase in the dielectric constant and increase ionic strength of the medium. The reaction con-
stants involved in the mechanism were computed and the overall activation parameters were evaluated which
lend support to the proposed mechanism.
Received 23 November 2014
Received in revised form 7 December 2014
Accepted 11 December 2014
Available online 23 December 2014
Keywords:
CPL
Kinetics
Mechanism
Polymerization
Ionic strength
Rate constant
Activation parameters
© 2014 Elsevier B.V. All rights reserved.
1. Introduction
CPL has three different potential donor groups (Sthiol, Oacid, and
Oamide) which may bind with metal ions to form 1:1 stoichiometric
Captopril-(2S)-3-mercapto-2-methyl propionyl-L-proline (CPL) is a
well-known drug applied therapeutically as an antihypertensive agent
[1], it is also prescribed for the treatment of congestive heart failure [2].
A chemical synthesis of captopril (Fig. 1) has been reported elsewhere
[3].
It inhibits the active sites of a zinc glycoprotein, the angiotensin
converting enzyme (ACE), blocking the conversion of Angiotensin(I) to
Angiotensin(II), the level of which is elevated in patients with hyperten-
sions. Captopril exists in solution as an equilibrium mixture of trans and
cis isomers with respect to the conformation across the amide bond [4].
ratio in acidic medium and 1:2 stoichiometric ratio in nearly basic me-
dium [5].
The sulfhydryl group is also the key functional group in the metabo-
lism of CPL. CPL is oxidized after dissolution in water to form its disulfide
and mixed disulfides with thiol-containing amino acids, peptides, and
proteins [6,7].
The chemistry of the reduction of Cr(VI) to Cr(III) with a selec-
tion of organic and inorganic reductants be able to occur by a multi-
plicity of mechanisms which depend on the nature of the reducing
agent [8]. The continuation of different species of Cr (VI) in acid so-
lutions, unstable oxidation states [Cr (IV) and Cr (V)] and the incli-
nation of Cr (III) to form a range of complexes, all merge to provide
systems of significant complexity [9]. Attempts have been done to con-
firm the intermediacy of Cr (IV) and Cr (V) by utilization of competitive
experiments [10,11].
The oxidation capacity completely depends on their redox potential.
The reported reduction potential of the Cr(VI)/Cr(III) couple is 1.33 V
[12]. It is widely known that the redox potential depends upon the pH
dependent. Thus, this potent oxidant is used mainly as an electron
abstracting/proton abstracting agent, or in free radical generation dur-
ing the oxidation of numerous organic compounds. The reagent,
quinolinium dichromate (QDC) is an adaptable oxidant so as to deserve
further study, which has been used extensively as a suitable oxidant for
⁎
Corresponding author at: Chemistry Department, King Abdulaziz University, Jeddah
21589, Saudi Arabia.
E-mail address: draapk@gmail.com (A.A.P. Khan).
http://dx.doi.org/10.1016/j.molliq.2014.12.017
0167-7322/© 2014 Elsevier B.V. All rights reserved.

2
A.M. Asiri et al. / Journal of Molecular Liquids 203 (2015) 1–6
(Fig. 2). The pseudo-first-order rate constants, kobs, calculated from the
slopes of plots were reproducible within 4% error.
3. Results and discussion
3.1. Effect of acids on oxidation of CPL by QDC
Acidity measurements were performed by monitoring absorbance
changes on reaction mixtures in different acids like HClO4·H2SO4, HCl
and H3PO4 medium, respectively. The reaction mixtures were allowed
to stand in the presence of 0.1 mL acid at room temperature for comple-
tion of the acidity. After completion of the reaction, the product was
monitored spectrometrically from its absorbance maxima. It was
found that the oxidation of CPL by QDC is slow in hydrochloric acid,
phosphoric acid, sulfuric acid comparison to perchloric acid (Fig. 3).
Therefore, HClO4 was chosen for further work. Nitric acid was not tested
because it is an oxidizing agent and gets disturbed with the oxidation
state.
Fig. 1. Chemical synthesis of captopril.
studying the oxidation of many organic and inorganic substrates under
both acidic and alkaline conditions [13–20].
Several methods have been reported in the literature that are based
on the reaction of the CPL with many reagents in acidic and alkaline
medium [21–28], etc. No kinetic spectrophotometric method has been
reported in the literature for the oxidation of captopril by QDC. There-
fore, in the present paper, we attempt to investigate in detail study on
the rate of the oxidation of CPL by QDC in acid medium.
3.2. Preparation of reaction product for infrared (IR) measurements
Aqueous solutions of CPL, HClO4 and QDC were transferred into a 10-
2. Materials and methods
mL calibrated flask and diluted to the mark, to obtain the following con-
centration: [CPL] = 4 × 10−3 mol L−1; QDC = 5.0 × 10−3 mol L−1
;
2.1. Materials
HClO4 = 3.0 mol L−1. The mixture was shaken occasionally for
30 min. The obtained oxidation product between CPL and QDC was
then extracted in CH2Cl2. After elimination of the solvent (under vacu-
um conditions), the dried material was analyzed by Fourier Transform
Infrared Spectroscopy (FTIR). Captopril, as for all thiols, was expected
to undergo some degree of oxidative degradation such as the formation
of disulfide [30] as can be seen from the FTIR spectra, where the charac-
teristic band of S\H, from 2565 cm−1 disappears. A new band, charac-
The main reagents used for the analysis of the kinetics were obtained
from Sigma and CDH (used as received). All other reagents and
chemicals were of analytical grade. Double distilled water was used
throughout the study.
2.2. Preparation of quinolinium dichromate
teristic for S\S frequency, appears at 620 cm−1
.
The oxidant, QDC, was prepared by the reaction of quinoline with
chromium trioxide, following the procedure reported earlier [29]. Purity
of the product was verified by IR, 1H-NMR and elemental analysis. Even
though QDC in aqueous solutions appeared to be stable for more than 3
months, it was prepared as fresh and used.
3.3. Dependence of rate on the reactants, substrate and oxidant
The effect of [H+] on the rate of the reaction was studied by vary-
ing the concentration of HClO4. The examination of the rate data in-
dicates that the reaction rate is increased with an increase in the
concentration of HClO4 (Table 1). Plots of log 1/kobs vs. 1/[H+]
(Fig. 4) were linear with a unit slope showing the first-order behav-
ior of H+ for the CPL. At constant [QDC], [H+] and temperature, the
rate constant increased with an increase in [substrate] (Table 1). Fur-
ther, the plot of first-order rate constant against the substrate con-
centrations is good with unity. At lower concentration of substrate,
the reaction was first order, and at higher concentration of substrate
the reaction was independent. The oxidation kinetics was studied
2.3. Preliminary studies
An absorption spectra of QDC solution, recorded using a UVD-2960
double beam PC connected UV–vis spectrophotometer with quartz
cells of 1 cm path length, showed a λmax at 440 nm. Aqueous solutions
of QDC were standardized iodometrically. Similarly, aqueous solutions
of CPL of desired strength were freshly prepared prior to the experi-
ment. The required acidity was maintained using a standardized
HClO4 solution.
0.6
0.5
0.4
0.3
0.2
0.1
0
2.4. Kinetic measurements
Kinetic measurements were performed by monitoring absorbance
changes on a UVD-2960 double beam PC connected UV–vis spectropho-
tometer. The reactions were followed under pseudo-first-order condi-
tions of [substrate] N N [QDC]. Mixtures of solutions containing the
requisite amounts of substrate, perchloric acid (to maintain a known
H+ concentration), and water (to keep the total volume constant)
were placed in stoppered boiling tubes. Each mixture was thermally
equilibrated in a water bath at 25 °C. To initiate the redox reaction in
each solution mixture in the tube, was added a known amount of pre-
equilibrated, standard QDC stock solution and stirred to give a known
overall concentration. The progress of the reaction was monitored for
two half-lives by withdrawing aliquots at varying time intervals and
measuring the absorbance of the unreacted QDC in each sample at
440 nm. The reaction was quenched appropriately prior to the absor-
bance measurement. Plots of log (absorbance) vs. time were linear
0
5
10
15
20
Time (min)
A
B
C
D
Fig. 2. Log absorbance versus time plots for the first-order plots of oxidation of CPL by QDC
in acidic medium at 25 °C.

A.M. Asiri et al. / Journal of Molecular Liquids 203 (2015) 1–6
3
-2
1/[H+]2x 10-2 dm6 mol
12
10
8
6
4
2
6
5
4
3
2
1
0
5.5
4.5
3.5
2.5
1.5
0.5
2.3
1.9
1.5
1.1
0.7
0.3
Fig. 3. Acidic effects observed upon the oxidation of CPL by QDC at room temperature.
-1
1/[CPL] x 10-2dm3mol
under pseudo-first-order conditions of [substrate] N N [QDC] by vary-
ing the concentration of QDC at constant substrate and [H+] at 25 °C
temperature (Table 1). Plots of rate constant against the concentra-
tion of QDC were calculated that the rate of reaction is the first
order in QDC (Table 1) was calculated from the plot of rate constant
against the concentration of QDC with straight line confirming the
first order with respect to oxidizing concentrations (Table 1).
Fig. 4. Effects of [H+] and [CPL] on the reaction rate at 25 °C.
equilibrium with the oxidant. Furthermore, the rate of reaction
remained unaffected with increasing ionic strength of reaction medium
effected by adding salts such as NaCl and NaClO4 indicating that the rate
determining step does not involve ion–ion interactions.
3.4. Influence of temperature and ionic strength
3.6. Influence of solvent composition or dielectric constant
The effect of temperature was examined in the range of 15–40 °C.
The reaction was also studied at higher temperatures but it was found
that the complex undergoes degradation. The reaction rate is increased
with increasing temperature that means the reaction followed the Ar-
rhenius equation. The reaction rate proceeds at 25 °C. Therefore, room
temperature was recommended as the optimum temperature, for the
reaction system. The effect of ionic strength (I) on the reaction was
also studied, in 0.01–0.3 mol L − 1 range, using KNO3. Then ionic
strength of the reaction medium 0.05 mol L − 1 was selected for further
study as it provided suitable change in absorbance value.
The effect of dielectric constant (D) on the rate was studied by vary-
ing the composition of the CH3CN–water mixture in the solvent. The
rate increased with increasing proportion of CH3CN–water. Plots of
kobs vs. 1/D were linear (Fig. 5) with a positive slope suggesting that
the relatively less polar transition state as compared to the ground
state is stabilized with decreasing solvent polarity [31].
4. Influence of acrylonitrile study for the free radicals
Free radical measurements were performed by monitoring precipi-
tation on reaction mixtures taking CPL, QDC and HClO4 in a round bot-
tom with screw cap and rubber liner test tube and the acrylonitrile
solution in another test tube. After equilibrated the system, the solu-
tions were mixed and the reaction mixture was kept for 2 h in dark in
an inert N2 atmosphere at room temperature. A white precipitate of
the CPL product formed clearly indicating the in situ formation of free
radicals in the reaction was routed. It is also ascertained by the decrease
in rate with initial addition of monomer.
3.5. Influence of the rate on Cr(III) and added salts
The effect of varying concentrations of Cr(III) (which is the reduction
product of the oxidant) on the rate was investigated at constant [oxi-
dant], [substrate], and ionic strength. The reaction rate was not affected
by the presence of Cr(III) indicating that it was not involved in a pre-
Table 1
Effects of varying concentrations of QDC, CPL, and perchloric acid on the rate in acid solu-
4.1. Stoichiometric studies
tions at 25 °C and at I = 2.50 mol dm−3 (Scheme. 2).
[QDC] × 104
(mol dm−3
[CPL] × 102
(mol dm−3
[HClO4]
Kexp × 103
kcal × 103
The stoichiometry of the reaction was studied by taking number of
reaction mixtures containing large excess of QDC over CPL. The reaction
mixtures were allowed to stand in the presence of HClO4 at 25 °C for
completion. After completion of the reaction, the concentration of the
)
)
(mol dm−3
)
(s−1
)
(s−1
)
Variation of [QDC]
2.0
4.0
5.0
8.0
10.0
12.0
0.8
0.8
0.8
0.8
0.8
0.8
3.00
3.00
3.00
3.00
3.00
3.00
4.28
6.25
8.12
8.43
8.56
8.74
8.14
8.14
8.15
8.14
8.14
8.15
1/D x 103
22
20
18
16
14
12
10
1.8
1.6
1.4
1.2
1
0.8
0.6
0.4
0.2
2
Variation of [CPL]
1.8
1.6
1.4
1.2
1
5.0
5.0
5.0
5.0
5.0
5.0
0.4
0.6
0.8
1.0
1.5
3.0
3.00
3.00
3.00
3.00
3.00
3.00
4.40
7.23
8.12
8.45
8.84
9.20
4.42
7.25
8.10
8.46
8.85
9.19
0.8
0.6
0.4
0.2
0
Variation of [HClO4]
5.0
5.0
5.0
5.0
5.0
5.0
0.8
1.50
2.50
3.00
3.50
3.70
4.00
4.52
6.77
8.12
8.67
9.11
9.23
4.55
6.78
8.11
8.66
9.12
9.25
0.8
0.8
0.8
0.8
0.8
2
1.8
1.6
1.4
1.2
1
I1/2
Fig. 5. Effects of dielectric constant (D) and ionic strength (μ) on the reaction rate at 25 °C.

4
A.M. Asiri et al. / Journal of Molecular Liquids 203 (2015) 1–6
product formed was calculated spectrometrically from its absorbance
maxima and the known value of molar extinction coefficient. The results
indicate two moles of CPL is consumed per one mole of oxidant. The
overall reaction may be represented by Scheme 1 and also optimized
structure of disulfide of CPL represented in Fig. 6.
The main oxidative product of CPL was found to be captopril disul-
fide which was separated as solid by ether extraction. The product, cap-
topril disulfide is further confirmed by its characteristic peaks in FT-IR
spectrum. The disappearance of SH vibration peak at 2565 cm−1 and
appearance of new peak at 620 cm−1 for S\S stretching, clearly indicate
the formation of captopril disulfide.
The results from kinetic and product analysis study presented
above clearly indicate that the QDC is capable of transforming CPL
to their disulfide via electron transfer. The kinetic data point to
Michaelis–Menten behavior i.e., a complex formation between the
substrate and the oxidant before the rate determining step and the
reaction is the first order in the oxidant and fractional order with re-
spect to CPL. The spectral studies of the chromium species has been
confirmed by spectrophotometrically. The color in the solution indi-
cates the formation of Cr(VI)-species and the slowly disappearance
with appearance of Cr(III)-species at λ = 530 nm recorded the
isosbestic point. Form one isosbestic point indicates low concentra-
tion of the intermediates chromium species like (V), (IV) [32] and
their slowly decompose to (III). The electronic spectrum of Cr(III)-
species lies in the range of 320–600 nm. Here QDC in the presence
of two moles of acid to give species H2CrO4. There have been reports
of the involvement of species in aqueous acid solution chromium
(VI) exists in the form of H2CrO4 [33,34].
The stoichiometry of the oxidation of QDC with CPL is found to be
2:1. As the acid concentration is increases, the rate of reaction also
increased (Table 1). No effects of initially added Cr(III) and quinoline
to the reaction mixture of the QDC and the substrate on the rate of the
reaction was observed and this rules out the presence of the pre-equi-
librium before the rate determining step in the mechanism, and the
oxidant reacts only after protonation of QDC. In Scheme 2, the results
suggest that H2CrO4 combines with a molecule of CPL to form a com-
plex. Further, which decomposes in the rate determining step to give
an intermediate chromium(IV) species and a free radical generated
from the substrate as a result of oxidation by H2CrO4 at a slow step.
The free radical formation is detected by the polymerization test
where copious precipitation was observed in the reaction mixture, to
which a known quantity of acrylonitrile has been added initially, was
kept in an inert atmosphere at room temperature for 4 h. Upon diluting
the reaction mixture with methanol, a white precipitate resulted
suggesting that was participation of free radicals in the reaction system.
In the next step another molecule of acid chromate combine with
one molecule of CPL producing another molecule of an intermediate,
chromium(III) and product of CPL. In a further fast step, two mole-
cules of free radical generated in Scheme 2 are combining to give
CPL disulfide. Further, it is supported by FTIR spectra that, the disap-
pearance of SH vibration peak at 2565 cm−1 and appearance of new
peak at 620 cm−1 for S\S stretching, clearly indicates the formation
of disulfide.
Fig. 6. A 3D view of the optimized structure of the product of disulfide CPL by QDC.
According to Scheme 1, the rate laws given for the formation of the
complex can be written as:
Rate ¼ k½Complexꢀ
h
i
2
ð1Þ
−d½QDCꢀ
Rate ¼
¼ kK1K2½CPLꢀ ½QDCꢀ Hþ
f
f
dt
f
½QDCꢀt ¼ ½QDCꢀf þ ½H2CrO4ꢀ þ ½Complexꢀ
h
i
h
i
2
½QDCꢀ ¼ ½QDCꢀ þ K2½QDCꢀ Hþ 2 þ K1K2½CPLꢀ ½QDCꢀ Hþ
t
f
f
f
f
f
f
ꢀ
ꢁ
h
i
h
i
½QDCꢀt ¼ ½QDCꢀf þ 1 þ K2 Hþ 2 þ K1K2½CPLꢀ Hþ
2
ð2Þ
f
f
f
½QDCꢀt
½QDCꢀf
¼
2
2
1 þ K2½Hþꢀf þ K1K2½CPLꢀf ½Hþꢀf
½CPLꢀt ¼ ½CPLꢀf þ ½Complexꢀ
h
i
2
½CPLꢀ ¼ ½CPLꢀ þ K1K2½CPLꢀ ½QDCꢀ Hþ
t
f
f
f
f
ꢀ
ꢁ
h
i
2
½CPLꢀt ¼ ½CPLꢀf þ 1 þ K1K2½CPLꢀ Hþ
ð3Þ
ð4Þ
f
f
½CPLꢀf
½CPLꢀt ¼
2
1 þ K1K2½CPLꢀf ½Hþꢀf
½CPLꢀt ¼ ½CPLꢀf
h
h
h
i
i
i
h
h
h
i
2
2
2
Hþ
Hþ
Hþ
¼
¼
¼
Hþ 2 þ ½H2CrO4ꢀ þ ½Complexꢀ
t
t
t
f
i
h
i
h
i
2
Hþ 2 þ K2½QDCꢀ Hþ 2 þ K1K2½CPLꢀ ½QDCꢀ Hþ
f
f
f
f
f
f
i
ꢂ
ꢃ
Hþ 2 þ 1K2½QDCꢀ þ K1K2½CPLꢀ ½QDCꢀ
ð5Þ
f
f
f
f
ꢄ
ꢅ
Hþ
h
i
2
t2
Hþ
¼
f
1 þ K2½QDCꢀf þ K1K2½CPLꢀf ½QDCꢀf
h
i
h
i
2
2
Hþ
¼
Hþ
ð6Þ
ð7Þ
t
f
ꢄ
ꢅ
2
kK1K2½CPLꢀ½QDCꢀ Hþ
−d½QDCꢀ
Rate ¼
¼
2
2
1 þ K2½Hþꢀ þ K1K2½CPLꢀ½Hþꢀ
dt
The rate law is in agreement with the observed experimental results
showed a first order with respect to each concentration of acidity, oxi-
dant and substrate.
ꢄ
ꢅ
2
kK1K2½CPLꢀ Hþ
Rate
½QDCꢀt
¼ Kobs
¼
ð8Þ
2
2
1 þ K2½Hþꢀ þ K1K2½CPLꢀf ½Hþꢀ
Scheme 1. Stoichiometric reaction for the CPL oxidation by QDC.

A.M. Asiri et al. / Journal of Molecular Liquids 203 (2015) 1–6
5
Scheme 2. A detailed plausible mechanism of disulfide of CPL by QDC.
1
Kobs
1
1
1
k
conditions and compared with experimental kobs values. They are found
to be in reasonably agreeing with each other which fortifies the mecha-
nism as in Scheme 2. The thermodynamic quantities for the first and
second equilibrium steps of Scheme 2 can be evaluated.
¼
2 þ
þ
:
ð9Þ
kK1K2½CPLꢀ½Hþꢀ
K1K2½CPLꢀ
(i) From the plot of 1/kobs versus 1/[CPL]
(Intercept)1 = 1/k
The reactions were studied at different temperatures to obtain vari-
ous values of activation parameters (Table 2). From the Arrhenius plots
of log k versus 1/T, activation energy, and other thermodynamic
Therefore, k = 1/(Intercept)1 = 2.123 × 10−2 s−1
(Slope)1 = 1/kK1K2[H+]2 + 1/k.
(ii) From the plot of 1/kobs versus 1/[H+]2
(Intercept)2 = 98.21 = 1/kK1K2[CPL] + 1/k.
Table 2
Effect of temperature on the reaction rate for the oxidation of CPL by QDC in acid solutions
[CPL]0 = 4.00 × 10−3 M, [QDC] = 5.00 × 10−4 M, and [H+] = 3.00 M.
From (Intercept)1, the value of k = 2.123 × 10−2 s−1
.
Temperature (K)
k × 102 (s−1
)
log k
1/T × 103
Hence,
(a) Effect of temperature with respect to slow step of Scheme 1
98.21 = 1/2.123 × 10−2 × K2 × 4.0 10−3 + 1/1.328 × 10−2
293
298
303
308
7.42
8.34
10.12
12.22
0.870
0.921
1.005
1.087
3.413
3.356
3.300
3.246
1 + K2(4.0 × 10−3) = 98.21 × 2.123 × 10−2 K2 × 4.0 × 10−3
1 + .004 K2 = .008399932 K2
1 = .008399932 K2 − 004 K2
1 = K2(.008399932 − .004)
Parameters
Values
1 = K2(.004399932)
K2 = 230.41 dm3 mol−1
(b) Activation parameters
Ea (kJ mol−1
ΔH (kJ mol−1
)
68.5
45.8
−76.4
52.5
(Slope)2 = 608.5 = 1/kK1K2[CPL]
608.5 = 1/2.123 × 10−2 × 230.41 dm3 mol−1 × 4.0 · 10−3 × K1
K1 = 8.34 × 10−2 dm6 mol−2
At 25 °C:
)
ΔS (J K−1 mol−1
)
ΔG (kJ mol−1
)
(c) Temp. (K)
K
1 × 102 dm6 mol−2
K2 × 10−2 dm3 mol−1
K1 = 8.34 × 10− dm6 mol−2
293
298
303
308
5.52
6.34
7.22
8.34
1.52
2.10
1.18
2.30
K2 = 230.41 dm3 mol−1
k = 2.123 × 10−2 s−1
.
(d) Thermodynamic parameters
Using K1 values
Using K2 values
The other conditions being constant, plots of 1/kobs versus 1/[CPL]
and 1/kobs versus 1/[H+] are linear (Figs. 4 and 5). The slopes and inter-
cepts of such plots yield K1, K2 and k (Table 2). These constants were
used to calculate the rate constants in Eq. (9) at different experimental
ΔH (kJ mol−1
)
−74.5
−232
−1.50
11.5
105
−13.2
ΔS (J K−1 mol−1
ΔG (kJ mol)
)

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A.M. Asiri et al. / Journal of Molecular Liquids 203 (2015) 1–6
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A comparison of these values with those obtained for the slow step
of the reaction shows that they mainly refer to the rate-limiting step,
supporting the fact that the reaction before the rate determining step
is fairly fast and the value of energy of activation shows that the reaction
is slow and enthalpy is controlled [36]. An increase in the volume of
acetic acid leads to an increase in the reaction rate. A plot of logkobs ver-
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active species to a greater extent in a low dielectric constant medium
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the oxidation presumably occurs via an inner-sphere mechanism. This
conclusion is supported by observations made earlier.
5. Conclusion
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In summary, the oxidation of captopril by QDC was found to proceed
by the formation of free radical generated from captopril followed by
dimerization of the free radicals. The product of captopril was found
to be captopril disulfide which was characterized by TLC and FT-IR.
The rate constant of slow step and other equilibrium constants involved
in the mechanism are evaluated and activation parameters with respect
to slow step of reaction were computed. The overall mechanistic
sequence described here, is consistent with the products formed.
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Acknowledgments
[35] A.A.P. Khan, A.M. Asiri, N. Azum, M.A. Rub, A. Khan, A.O. Al-Youbi, Ind. Eng. Chem.
Res. 51 (2012) 4819.
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Siddiqi, K.A. Alamry, J. Ind. Eng. Chem. 19 (2013) 595.
This paper was funded by the Deanship of Scientific Research (DSR),
King Abdulaziz University, under grant No. (D-004/431). The authors,
therefore, acknowledge technical and financial support of KAU.
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