Electrochemical oxidation of o-phenylenediamine and 1,3 dihydrospiro[benzo[d]imidazole-2,1′-cyclohexane]. A comprehensive study and introducing a novel case of CE mechanism

Article abstract of DOI:10.1016/j.electacta.2020.136700

Electrochemical behavior of o-phenylenediamine (PDA) and 1,3-dihydrospiro[benzo[d]imidazole-2,1′-cyclohexane] (DBI) was extensively studied in water and water/ethanol mixture using different voltammetric techniques. Our data showed that the oxidation of P

Full text of DOI:10.1016/j.electacta.2020.136700

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Electrochemical oxidation of o-phenylenediamine and 1,3
Dihydrospiro[benzo[d]imidazole-2,10-cyclohexane]. A comprehensive
study and introducing a novel case of CE mechanism
Faezeh Zivari Moshfegh , Davood Nematollahi ,
Mahmood Masoudi Khoram , Abdollah Rahimi
PII:
S0013-4686(20)31093-8
DOI:
Reference:
https://doi.org/10.1016/j.electacta.2020.136700
EA 136700
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26 June 2020
28 June 2020
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Mahmood Masoudi Khoram , Abdollah Rahimi , Electrochemical oxidation of o-phenylenediamine
and 1,3 comprehensive study
Dihydrospiro[benzo[d]imidazole-2,10-cyclohexane].
and introducing novel case of CE mechanism, Electrochimica Acta (2020), doi:
https://doi.org/10.1016/j.electacta.2020.136700
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Highlights


Introducing a rarely studied CE-mechanism.
Comprehensive
electrochemical
studies
of
o-phenylenediamine
and
1,3-
dihydrospiro[benzo[d]imidazole-2,1'-cyclohexane].

Potential-pH studies of o-phenylenediamine and 1,3-dihydrospiro[benzo[d]imidazole-2,1'-
cyclohexane].



Kinetic studies of hydrolysis of 1,3-dihydrospiro[benzo[d]imidazole-2,1'-cyclohexane.
Hydrolysis study of 1,3-dihydrospiro[benzo[d]imidazole-2,1'-cyclohexane].
Mechanistic studies of the oxidation of o-phenylenediamine and 1,3-
dihydrospiro[benzo[d]imidazole-2,1'-cyclohexane.
1

Research Paper
Electrochemical oxidation of o-phenylenediamine and 1,3
Dihydrospiro[benzo[d]imidazole-2,1'-cyclohexane]. A comprehensive study and
introducing a novel case of CE mechanism
Faezeh Zivari Moshfegh, Davood Nematollahi*,a, Mahmood Masoudi Khoram, Abdollah Rahimi
aE-Mail: nemat@basu.ac.ir
Faculty of Chemistry, Bu-Ali Sina University, Hamedan, Iran. Zip Code 65178-38683.
*Corresponding author. Tel.: + 0098 813 8271541; fax: +0098 813 8272404.
2

E-mail addresses: nemat@basu.ac.ir, dnematollahi@yahoo.com (D. Nematollahi).
Fax: +0098 813 8257407, Tel: +0098 813 8282807.
ABSTRACT
Electrochemical
behavior
of
o-phenylenediamine
(PDA)
and
1,3-
dihydrospiro[benzo[d]imidazole-2,1'-cyclohexane] (DBI) was extensively studied in water and
water/ethanol mixture using different voltammetric techniques. Our data showed that the
oxidation of PDA is highly dependent on pH, follows a complex pattern and participate in
following chemical reactions such as polymerization. Unlike acidic and neutral solutions, in
highly alkaline solutions (pH ≥ 11), however, PDA shows a simple reversible redox system.
Contrary to PDA, the presence of the cyclohexyl group in the structure of DBI makes its
oxidation pattern less complex than that of PDA and causes the molecule less susceptible to
participate in the following chemical reactions. Our results showed that DBI in aqueous
solutions is unstable and undergoes acid catalyzed hydrolysis to give PDA. The instability of DBI
in acidic solutions is so high that it turns completely into PDA in the time scale of the
voltammetric experiments. Different from acidic media, in alkaline solutions (pH ≥ 9.0), the
hydrolysis rate is slow, so that DBI shows a reversible redox couple. The kinetic of DBI
hydrolysis using differential pulse voltammetry method was studied and the apparent
hydrolysis rate constants (푘hobs) were found by assuming the pseudo-first order rate kinetics. In
addition, in this work, adsorption activity, diffusion coefficient and pKa values of DBI and PDA
species were determined and the Pourbaix diagrams for these compounds were constructed.
The most important part of this paper is devoted to introducing a rare type of mechanism in
3

electrochemical reactions. In this way, the rarely studied mechanism has been introduced in
relation to the reaction of PDA with cyclohexanone and the formation of DBI.
Keywords: CE mechanism; o-phenylenediamine; 1,3-dihydrospiro[benzo[d]imidazole-2,1'-
cyclohexane]; Cyclic voltammetry; Differential pulse voltammetry; Pourbaix diagram.
4

1. Introduction
Electrochemistry provides powerful techniques for the study and synthesis of organic
compounds [1]. These techniques can be used to obtain some useful data on homogeneous
reactions coupled with electron transfer reactions [2]. In this connection, to illustrate the
mechanisms, E and C are used for the electron transfer and chemical reactions, respectively.
Accordingly, many mechanisms such as EC [3], EE [4], ED (D: dimerization) [5], EC’ (C’: catalytic)
[6], ECE [7], ECEC [8], ECECE [9], ECECEC [10], ECE-ECE [11], ECEC-ECEC [11], have been
introduced and described. Despite the abundance of these mechanisms, however, CE
compared to mechanisms such as EC, EC’ and ECE is an infrequently observed mechanism [12].
In a typical CE mechanism, before the electron transfer step, the primary molecule, which is not
electroactive, is converted to an electroactive compound (EAC). This compound (EAC), is
formed by a reversible chemical reaction. A rare type of CE mechanism (compared to
mechanisms such as EC, EC’ and ECE), is a CE mechanism in which the primary molecule is
electroactive. In this condition, the primary molecule, in addition to participating in the
electrode process through the CE mechanism, can also directly participate in the electrode
process. Of course, in such a situation, the oxidation of the primary molecule is more difficult
than EAC. A rarer type of this mechanism consists of conditions in which the redox reaction of Y
is reversible. In this paper, we are going to introduce one example of such mechanism.
o-Phenylenediamine (benzene-1,2-diamine) (PDA) is a molecule that has been used many
times for electropolymerization [13-15]. However, in those papers, attention has been paid to
the electrochemical properties of the polymer and less attention has been paid to o-
5

phenylenediamine itself. In addition, a literature search indicated that no electrochemical data
were available concerning the voltammetric behavior of o-phenylenediamine derivatives. So, in
this study, we decided that reporting the electrochemical behavior of o-phenylenediamine and
1,3-dihydrospiro[benzo[d]imidazole-2,1'-cyclohexane] (DBI),
a
spiro derivative of o-
phenylenediamine at different pH values, for the first time. In this connection, a novel CE
mechanism has been proposed and characteristics, such as hydrolysis process, adsorption
activity, diffusion coefficient and pKa value and Pourbaix diagram was reported for DBI.
2. Experimental
2.1. Reagents and apparatus
Cyclic voltammetry, differential pulse voltammetry and chronoamprommetry were
performed using an Autolab model PGSTAT 20 potentiostat/ galvanostat. The working
electrode used in the voltammetry experiments was a glassy carbon disc (1.8 mm diameter)
and a platinum wire was used as the counter electrode. The working electrode potentials were
measured vs. SCE The glassy carbon electrode was polished using alumina slurry followed by
washing with water and acetone. o-Phenylenediamine (PDA), cyclohexanone and ethanol were
obtained from commercial sources and used without further purification. Prior to recording
voltammograms, the solutions were purged with high-purity nitrogen gas. For kinetic studies,
reactions were initiated by placing DBI (1.0 mM) in a mixture of water (phosphate buffer, pH,
8.0 or 7.0, c = 0.2 M)/ethanol (90/10 v/v) (10 mL) at room temperature. Then, at specific times,
the corresponding differential pulse voltamgrams were recorded.
2.2. Synthesis of dihydrospiro[benzo[d]imidazole-2,1'-cyclohexane] (DBI).
6

The synthesis of DBI was performed according to a modified procedure [16,17]. In a typical
procedure, 4 mmol PDA and 0.5 ml cyclohexanone were added to 40 ml boiling water and the
solution was stirred about 10 minutes. After completion of the reaction, the solution was
placed at room temperature to obtain the yellow crystal of DBI. The resulting solid was filtered
and washed several times with water (yield, 55%). The melting point of the crystal is 138-139 ͦC
1
(lit. [16] 139-140
ͦ
ͦ
1.57 (m, 8H, CH2), 5.67 (s, 2H, NH), 6.25 (m, 2H, aromatic), 6.34 (m, 2H, aromatic). 13C NMR (75
MHz, DMSO-d6) δ 27.6, 30.1, 44.7, 85.1, 111.7, 122.7, 145.5.
3. Results and discussion
3.1. Mechanistic study of PDA
Herein, the voltammetric behavior of PDA in different pH values is investigated to obtain
detailed information on the pH-dependent features of PDA and to gain insight into the pH-
dependent processes that control chemical processes after electron transfer. Firstly, the
voltammetric behavior of PDA was investigated at pH = 11, at the scan rate of 100 mV s−1 when
using glassy carbon electrode, (Fig. 1). In these conditions, cyclic voltammogram shows a two-
electron reversible couple with E1/2 = 0.029 V vs. SCE. The anodic and cathodic peaks A1 and C1
are assigned to the oxidation of PDA to its oxidized form cyclohexa-o-benzoquinonediimine
(BQD) and vice versa, respectively (Scheme 1) [18,19]. The reversible feature of the
7

voltammogram, confirms the stability of the electrogenerated BQD at the time scale of the
voltammetric experiment.
Scheme 1. Oxidation mechanism of PDA in more basic solutions.
The cyclic voltammograms of PDA at pH, 11 at different scan rates are compared in Fig. 1,
part II. As can be seen, the peak current ratio (IpC1/IpA1) is scan rate dependent and decreases at
lower scan rates. In addition, a new small cathodic peak C2 concomitantly appears, with
decreasing scan rate (Fig. 1, part II).
Figure 1
The decreasing peak current ratio (IpC1/IpA1) with decreasing scan rate, confirms the
participation of BQD in the following chemical reactions such as dimerization and
polymerization [7]. The peak current ratio (IpC1/IpA1) is also strongly related to the pH of the
solution (Fig. 2). By reducing one unit of pH, the peak current ratio (IpC1/IpA1) is drastically
decreased and by decreasing the pH value to 9, the peak C1 is completely eliminated. By further
decreasing the pH, a new anodic peak (A0) appears at more positive potentials than the main
peak and its current showed an increasing trend with decreasing pH up to pH 4. In addition,
with the disappearing of peak C1, two small cathodic peaks C2 and C3 appear at more negative
potentials which their anodic counterparts (A2 and A3) are observed during the second cycle of
the potential sweep. At more acidic conditions, for example pH 2, voltammograms become
simpler and only A1, C3 and A3 peaks are observed.
8

Figure 2
Based on these electrochemical data and also with the help of valuable information
provided by Losito et al. [20,21], we propose the following mechanism for electrochemical
oxidation of PDA (Scheme 2). After generation of BQD at the electrode surface, as a Michael
type acceptor it is attacked by PDA and after aromatization to yield PDA dimer (DPDA). The
oxidation of DPDA due to its structure being easier than the oxidation of PDA and therefore
becomes oxidized to the corresponding p-quinonediimine (PQDI) which is in equilibrium with o-
quinonediimine form (OQDI) during the anodic scanning. The formation of PQDI dimer was
proved by electrospray ionization ion trap sequential mass spectrometry (ESI-ITMSn), during
the oxidation of PDA by Losito et al [20,21]. In the next step, the intramolecular addition of
amine group causes the formation of the corresponding dihydrophenazine (DHP). The
successive inter and intramolecular Michael additions of a bis‐nucleophile would provide a
useful method for the synthesis of the cyclic compounds. On the other hand, the hydrolysis of
PQDI and the formation of the corresponding p-quinoneimine (PQI) is possible [22]. The
formation of PQI dimer was also proved by Losito et al [20].
9

Scheme 2. Oxidation mechanism of PDA.
According to the proposed mechanism, the anodic and cathodic peaks A2/C2 are assigned to
the redox couple phenazine (DHPox)/dihydrophenazine (DHP). In this way, the anodic and
cathodic peaks A3/C3 are assigned to the redox couple p-quinoneimine (PQI)/p-hydroxyaniline
(PHA). Since, the rate of hydrolysis increases with decreasing pH, in more acidic solutions (pH,
2), A3/C3 peaks become more intense while the A2/C2 peaks disappear. On the other hand, it is
10

suggested that peak A0 is associated with over oxidation of DHPox or BDQ. This peak is
dependent on the A2/C2 peaks. It appears with the appearance of A2/C2 peaks and disappears
with the disappearance of them. Oxidized aromatic compounds can usually be further oxidized
at higher potentials. These over oxidations often result in irreversible ring cleavage reactions
[23].
3.2. Pourbaix diagram of PDA
Electrochemical oxidation of organic compounds is often associated with proton
elimination. Fig. 3 part I, shows the linear sweep voltammograms of PDA in various pHs in the
range of 1–11.6. As can be seen the anodic peak (EpA1) shifts to negative potentials with
increasing pH. A Pourbaix diagram was constructed based on the data of part I and is shown in
Fig. 3 part II.
Figure 3
The diagram consists of four zones and identifies four species. Two of these species are in
the oxidation states and the other two are in the reduction forms. At pH values less than 5.2,
the slope of the line is 58 mV/pH, which is close to 59 mV/pH for a redox process with an equal
number of protons and electrons. The equation describing this process is outlined in Scheme 3
(Eq. 1).
11

Scheme 3. Redox and acid/base behaviors of PDA and BQD at different pH values.
At pH values more than 5.2 and less than 8.0, the slope of the line is 24 mV/pH, which is
close to 29 mV/pH for a two-electron/one-proton process. The equation describing this process
is shown in Scheme 3 (Eq. 2). And finally at pH values more than 8.0, the slope of the line is 68
mV/pH, which is in consistence with a two-electron/two-proton process (Eq. 3). The pKa values
for protonated PDA and BQD are determined using the equation of the lines. According to the
calculations, the pKa values for PDA and BQD are 5.2 (Ref. [24], 4.6) and 8.0, respectively (Eqs. 4
and 5).
3.3. Mechanistic studies of DBI
12

The cyclic voltammograms of DBI (1.0 mM) in water (pH=10.0, carbonate buffer, c =0.2
M)/EtOH mixture (90/10, v/v) is shown in Fig. 4. They exhibit one anodic peak (As1) which is
assigned to the two electron oxidation of DBI to its oxidized forms, spiro[benzo[d]imidazole-
2,1'-cyclohexane], (DBIox) and corresponding cathodic peak (Cs1) which is related to the
reduction of DBIox to DBI.
Figure 4
Contrary to PDA, the peak current ratio (IpAs1/IpCs1) is independent of scan rate and equal to
unity which can be considered as a criterion for the stability of DBIox under the experimental
conditions at the time scale of the voltammetric experiments. In other words, any side
reactions, such as hydroxylation, hydrolysis, dimerization, oxidative ring cleavage on
electrochemically generated DBIox are too slow to be observed at these time scales. The
stability of DBIox over BQD at pH 10, can be due to the electron donating effect of the
cyclohexyl group, as well as its steric hindrance.
The cyclic voltamograms of DBI and PDA at various pH values are compared in Figure 5. At
DBI
1/2
pH 10, both DBI and PDA show typical cyclic voltammograms with E1/2 = 퐸1P/D2A - 퐸
= 0.44 v.
The easier oxidation of DBI than PDA can be related to the electron-donating character of the
aliphatic cyclohexane attached to nitrogen atoms. The voltammogram also confirm the stability
of DBI under the conditions of these experiments at pH 10. These conditions are also present at
pH 9. Contrary to the results obtained at pH values 10 and 9, the cyclic voltammogram of DBI at
pH 8 shows a new redox couple (A1 and C1) which their peak potentials are close to that of PDA.
The results obtained at lower pH values (7 to 5) show a decrease in peak currents associated
with the DBI/DBIox couple (As1 and Cs1 peak) parallel with an increase in IpA1. Finally, the cyclic
13

voltammogram of DBI at pH 5, is quite similar to the cyclic voltammogram of PDA (Figs. 2 and
5). These results show that DBI is only stable in alkaline media (pHs > 8), but, its stability due to
hydrolysis to PDA, is strongly affected by decreasing pH. The hydrolysis of DBI to PDA, is shown
in Scheme 4. According to this Scheme, protonation of DBI is an important and rate limiting
step in the hydrolysis of DBI. So, despite its rapid hydrolysis in acidic solution, DBI has relative
stability in the alkaline solutions.
Figure 5
Scheme 4. Hydrolysis of DBI to PDA.
3.4. Pourbaix diagram of PDA
The effect of pH on the differential pulse voltammograms of DBI was studied in the range of
4.0-11.2 (Fig. 6 part I). At the pH values lower than 4.0, DBI completely hydrolyzed to PDA and
do not have any peak in differential pulse voltammetry. The results indicate that at all studied
pHs, the differential pulse voltammograms show one anodic peak (As1) which is assigned to the
oxidation of DBI to its oxidized form, spiro[benzo[d]imidazole-2,1'-cyclohexane], (DBIox).
Increasing in pH values caused the peaks shifted to negative potentials. The Pourbaix diagram
14

of DBI is shown in Fig. 6 part II. It consists of two lines with slopes 56 and 30 mV/pH. As can be
seen, at pH values less than 8.8, the slope of EpAs1-pH line is 56 mV/pH, which is consistent with
the theoretical value for the same numbers of electrons and protons in the redox processes
(59.2 mV/pH). The electrode reaction for this line is given in Scheme 5 Eq. 1. At pH values more
than 8.8, the slope of EpAs1-pH line is 30 mV/pH which is consistent with the theoretical value
for a two-electron/one-proton process (29.6 mV/pH). The electrode reaction for this line is
shown in Scheme 5 Eq. 2. The pKa of DBIH+ is found to be 8.8 by equating the equations of the
two lines. It is close to that of 8.3 predicted for N,N-dimethyl-1,2-phenylenediamine [25]. The
acid/base equilibrium of DBI is shown in Scheme 5 Eq. 3.
Figure 6
Scheme 5. Redox and acid/base behaviors of DBI at different pH values.
3.5. Diffusion/adsorption studies
The diffusion/adsorption properties of DBI and PDA was studied in water (pH = 10.0,
carbonate buffer, c = 0.2 M)/EtOH mixture (90/10 v/v) to comparison of diffusion coefficient
15

and adsorption activity of the compounds in the same conditions at glassy carbon electrode
(see Supplementary data). In this way, log IpAs1 and IpA1 vs. log v for DBI and PDA were plotted,
respectively. The slope of the line is a measure of the diffusion/adsorption activity of the
compound at the electrode surface. The slopes of the pure diffusion and adsorption processes
are 0.5 and 1, respectively [2]. The slopes of the lines for DBI and PDA is 0.53 and 0.51,
respectively, which means that the electrochemical oxidation of both compounds are diffusion-
controlled processes and also shows that the presence of the cyclohexyl group has little effect
on increasing the adsorption capacity of the DBI.
The diffusion coefficient of the DBI was determined by chronoamperometric method (Fig.
7). To achieve this goal the Shoup and Szabo equations were used (Eqs. 1-3) [26].In this way, a
potential of -0.27 V applied for 12 s and the resulting current was measured over time.
I =-4nFrDcf(τ)
Eq. 1
Eq. 2
f(τ) = 0.7584 + 0.8863τ-1/2 + 0.2146 exp (-0.7823τ-1/2)
where D is the diffusion coefficient, n is the number of electrons, r is the radius of the disk
electrode (r = 0.09 cm) which was calculated as described previously [27], c is the initial
concentration, F is the Faraday constant, and τ is the dimensionless time parameter, given by:
τ = 4Dt/r2
Eq. 3
The calculated diffusion coefficients of DBI and PDA in water (pH=10)/ethanol mixture
(90/10, v/v) at room temperature, are 1.55 (±0.24) × 10-5 cm2/s and 2.96 (±0.76) × 10-5 cm2/s,
respectively.
Figure 7
3.6. Kinetic studies
16

As shown in Scheme 5, DBI is hydrolyzed and converted to PDA. So we decided to study the
kinetic of DBI hydrolysis using differential pulse voltammetry method. Fig. 8 part I, shows the
DPVs of DBI (1.0 mM) at pH, 8.0 (phosphate buffer, c = 0.2 M) at different times. The DPVs
show two-anodic peaks at -0.20 and 0.18 V vs. SCE, which correspond to the oxidation of DBI
and PDA, respectively. During the first scan, 퐼PDBI and 퐼PPDA were 10.2 A and 0.9 A (퐼PDBI/퐼PPDA
=
11.3), respectively. Upon further scans at different times, 퐼PDBI decreases and 퐼PPDA increases,
indicating continuous hydrolysis of DBI. At the last scan recorded at 65 min, 퐼PDBI and 퐼PPDA were
4.4 A and 4.8 A (퐼PDBI/퐼PPDA= 0.9), respectively. Based on Scheme 4 and assuming the pseudo
first-order reaction kinetics, the apparent hydrolysis rate constants (푘hobs), was determined
from the slope of ln Ip/Ip0 versus electrolysis time, as shown in the inset of Fig. 8, in accordance
with the kinetic equation (Eqs. 4 and 5):
ln c/c0 = -푘hobs
t
Eq. 4
Eq. 5
ln Ip/Ip0 = -푘hobs
t
where, c0 and Ip0 are concentration and peak current of DBI at the beginning of hydrolysis. Since
the peak current of DBI (Ip) is proportional to its concentration, the hydrolysis rate constant was
obtained by measuring the ln Ip/Ip0 as a function of time. Based on the slope of the line in Fig. 8
inset, 푘hobs is obtained 1.3×10-2 min-1 at pH, 8.0. Fig. 8 inset, shows also the plot of Ip vs. time for
DBI. The influence of pH on the hydrolysis of DBI has also been examined. Fig. 8 part II shows
the effect of initial pH on the peak current ratio (Ip/Ip0) during the hydrolysis time. Our results
show that in more acidic solutions, DBI was hydrolyzed faster than at higher pHs, which
confirms acid catalyzed nature of the reaction (Scheme 4 and Fig. 5). In the same conditions,
the apparent hydrolysis rate constants (푘hobs) for DBI was obtained 7.1×10-2 min-1 at pH 7.0.
17

Figure 8
3.7. CE mechanism
This section deals with the influence of hydrolysis on the electrochemical responses of DBI.
As mentioned in the preceding sections, DBI is hydrolyzed to PDA and cyclohexanone. To
elucidation of the reaction mechanism, cyclic voltammograms of DBI were recorded at different
times and also at different scan rates (after 120 min of preparing the solution) (Fig. 9). Fig. 9
part I shows the cyclic voltammograms of DBI at different times (5, 30 and 120 min) at high
scan rate (40 V/s). At t = 5 min (curve a), the DBI peak currents, As1 and Cs1 are high, while PDA
peak currents, A1 and C1 are much lower than those of DBI. With increasing time to 30 min
(curve b), the currents of DBI peaks decreased, while, the currents of A1 and C1 peaks increased.
With further increasing time, after reaching equilibrium (120 min, curve c), the current of As1
decreased markedly and its shape changes from peak-shape to S-shape which is a sign of a CE
mechanism in kinetic region [2]. Under these conditions, the PDA peaks (A1 and C1) reached to
their maximum amounts while IpCs1 is minimized. The equilibrium between PDA and DBI is
shown below (Eq. 6).
Figure 9
The effect of potential scan rate on the voltammetric responses of DBI after reaching
equilibrium (120 min at pH 8.0), is shown in Fig. 9 part II. This Figure clearly shows the three
18

distinct regions, diffusion pure, kinetic intermediate and kinetic pure that generally characterize
CE mechanism (preceding reaction) in electrochemical oxidation of DBI [2]. At the scan rate of
20 V/s, the system is in the diffusion pure region, and a diffusion-controlled voltammogram (an
unperturbed reversible reaction) including two redox couples (As1/Cs1 and A1/C1) is observed.
As the scan rate increases (for example, 35 V/s), it enters the kinetic intermediate region. In this
region, As1 is partially governed by the rate at which DBI is supplied by the forward reaction
(Eq. 6). At the scan rate of 45 V/s, it enters the pure kinetic region. In this region, the current of
DBI oxidation (As1) is totally controlled by the rate at which DBI is produced by the forward
reaction (Eq. 6). Since the rate of DBI formation is constant, the current attains a steady-state
value and voltammogram takes on an S-shape. The CE mechanism (preceding reaction) for DBI
oxidation at equilibrium condition is shown in Scheme 7.
The formation constant of DBI (Eq. 6) can be roughly calculated by the following equations
(Eqs. 7-11) and using the peak currents of DBI (IpAs1) and PDA (IpA1) in equilibrium condition (Fig.
8, DPV at 65 min), as well as by considering the DBI and PDA diffusion coefficients (D) (1.55 ×
10-5 cm2/s and 2.96 × 10-5 cm2/s, respectively).
[DBI]
푘
푘
푓
K =
=
Eq. 7
[PDA] [Cyclohexanone]
푏
[PDA] = [Cyclohexanone]
[DBI] + [PDA] = 0.001 M
Eq. 8
Eq. 9
IpAs1 = Constant × (DDBI
IpA1 = Constant × (DPDA
)
1/2 × [DBI]
1/2 × [PDA]
Eq. 10
Eq. 11
)
19

Scheme 7. CE mechanism for oxidation of PAD in the presence of cyclohexanone at
equilibrium condition.
Based on calculations, the values of 6.0 × 10-4 M, 4.0 × 10-4 M and 3.75 × 103 M-1, were
calculated for [DBI], [PDA] and K, respectively. In addition, the values of kf and kb can be roughly
calculated by the following equations (Eqs. 12 and 13) [2]:
퐼
퐼
1
푘
=
Eq. 12
Eq. 13
1.02 +0.471/퐾 휆
√
푑
(푘 +푘 )
푅푇
( )
푛퐹
푓
푏

푣
where, Ik is the kinetic peak current, Id is the diffusion-controlled current and v is scan rate. In
fact, Ik is IpAs1 in kinetic region (Fig. 9 part II, v = 45 V/s). On the other hand, Id is IpAs1 in diffusion
region (Fig. 9 part II, v = 20 V/s). But to calculate Ik/Id, the diffusion current (Id) must first be
calculated at v = 45 V/s using Randles-Sevcik equation. According to these equations, the values
of 2.8 × 10-3 M−1 s−1 and 7.5 × 10-6 s−1 were roughly calculated for kf and kb, respectively.
4. Conclusion
20

In this paper, the electrochemical oxidation of PDA and DBI was extensively studied. The
data, contributes to the increasing scientific knowledge about the oxidation mechanism of
these compounds and understanding the role of the cyclohexyl group attached to amine groups
in oxidation pathway of DBI.
Our data showed that the oxidation of PDA is highly dependent on pH and follows a
complex pattern that is shown in Scheme 2. However, the presence of the cyclohexyl group in
DBI makes its oxidation pattern less complex than PDA and also makes the molecule less
susceptible to participate in the following reactions such as polymerization.
One of our objectives in this paper is the evaluation of the stability of DBI in aqueous
solutions. Our results showed that DBI in aqueous solutions is unstable and undergoes acid
catalyzed hydrolysis to give PDA. This converting reaction is highly pH-dependent so that it is
completely performed in acidic solutions, in the time scale of our voltammetric experiments. In
this regards, in this paper we are introducing a rarely studied CE mechanism for oxidation of
PAD in the presence of cyclohexanone at equilibrium condition.
Additionally we provided the specific electrochemical information of these compounds such
as pourbaix diagram, diffusion coefficient and adsorption studies by CV, DPV and
chronoamperomettry methods and also showed that DPV is very useful technique in kinetic
studies and calculating apparent reaction rate constant at different pH values.
Acknowledgements
The authors wish to acknowledge Iran National Science Foundation (INSF) for financial
support of this work. We also acknowledge the Bu-Ali Sina University Research Council and
21

Center of Excellence in Development of Environmentally Friendly Methods for Chemical
Synthesis (CEDEFMCS) for their support of this work.
Credit Author Statement
Faezeh Zivari Moshfegh: Investigation, Writing - Original Draft
Mahmood Masoudi-Khoram: Writing - Original Draft
Davood Nematollahi: Writing - Review & Editing, Supervision, Project administration
Abdollah Rahimi: Writing - Original Draft
Declaration of interests
☒ The authors declare that they have no known competing financial interests or personal relationships
that could have appeared to influence the work reported in this paper.
Appendix A. Supplementary data
Supplementary data including cyclic voltammograms of DBI and PDA at different scan rates,
1H NMR and 13C NMR of DBI associated with this article can be found, in the online version, at
http://dx.doi.org
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Figures
Fig. 1. Cylic voltammograms of PDA (1.0 mM) at glassy carbon electrode in carbonate buffer
solution (pH = 11, c = 0.2 M). Part I: Scan rate 100 mV s−1. Part II: Scan rates are: a) 10 mV s−1, b)
25 mV s−1, c) 50 mV s−1 and d) 100 mV s−1. Temperature: 25 ± 1 ◦C.
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__
Fig. 2. First (---) and second ( ) cyclic voltammograms of 1 mM PDA at glassy carbon electrode
in aqueous solutions at different pH valus. Scan rate: 100 mV s−1 Temperature: 25 ± 1 ◦C.
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Fig. 3. Part I: linear sweep voltammograms of PDA (1.0 mM) in buffer solution with various pH
values. pHs from a to l are: 1.2, 2.2, 3.3, 4.2, 5.2, 6.1, 7.0, 8.0, 9.1, 10.0, 10.7 and 11.6. At the
glassy carbon electrode, temperature = 25 ± 1
ͦ
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