Electrochemical oxidation of 2,5-diethoxy-4-morpholinoaniline in aqueous solutions

Article abstract of DOI:10.1016/j.electacta.2013.09.063

Electrochemical oxidation of 2,5-diethoxy-4-morpholinoaniline (1) has been studied at various pH values using cyclic voltammetry and controlled-potential coulometry. The results indicate that electrochemically generated p-benzoquinonediimine (1ox) is unstable and participates in the two types of reactions based on solution's pH. The results also show that in the acidic media, electrochemically generated 1ox via two successive hydrolysis reactions is converted to 2,5-diethoxy-p-benzoquinone (3ox), while at intermediate pH values, the Michael addition reaction of 1-1ox takes place prior to the hydrolysis reaction. Our data show that in these pH values, 1ox via two successive Michael addition reactions followed by a hydrolysis reaction is converted to 2,5-bis(2,5-diethoxy-4-morpholinophenylamino)-3,6-diethoxy-p- benzoquinone (6ox).

Full text of DOI:10.1016/j.electacta.2013.09.063

Electrochimica Acta 114 (2013) 242–250
Contents lists available at ScienceDirect
Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Electrochemical oxidation of 2,5-diethoxy-4-morpholinoaniline in
aqueous solutions
Hadi Beiginejad, Davood Nematollahi∗
Faculty of Chemistry, Bu-Ali-Sina University, P.O. Box 65174, Hamedan, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 23 June 2013
Received in revised form 3 September 2013
Accepted 7 September 2013
Available online 17 October 2013
Electrochemical oxidation of 2,5-diethoxy-4-morpholinoaniline (1) has been studied at various pH values
using cyclic voltammetry and controlled-potential coulometry. The results indicate that electrochemi-
cally generated p-benzoquinonediimine (1ox) is unstable and participates in the two types of reactions
based on solution’s pH. The results also show that in the acidic media, electrochemically generated 1ox via
two successive hydrolysis reactions is converted to 2,5-diethoxy-p-benzoquinone (3ox), while at inter-
mediate pH values, the Michael addition reaction of 1–1ox takes place prior to the hydrolysis reaction.
Our data show that in these pH values, 1ox via two successive Michael addition reactions followed by
a hydrolysis reaction is converted to 2,5-bis(2,5-diethoxy-4-morpholinophenylamino)-3,6-diethoxy-p-
benzoquinone (6ox).
Keywords:
2,5-Diethoxy-4-morpholinoaniline
Trimerization
Hydrolysis
Para-benzoquinone
Green synthesis
© 2013 Elsevier Ltd. All rights reserved.
1. Introduction
with increasing pH, while it was decreased for some others in
the same condition [13]. In this work electrochemical oxidation of
Electrochemistry presents very versatile means for the electro-
chemical synthesis of organic and inorganic compounds, kinetic
and mechanistic studies of electron transfer reactions [1–7]. Elec-
trooxidation of amines were reported by many workers at different
conditions such as aqueous and non aqueous solvents or at var-
ious pH values [8–11]. It was reported that electrooxidation of
aromatic amines is not simple and leading to the various products
depending on their structure and electrolysis conditions [10,11].
Results of the investigations indicated that depending on the solu-
tion’s pH, the electrochemical oxidation of amines leads to the
dimerization, trimerization, hydrolysis and hydroxylation reac-
tions [12–14]. The hydrolysis and formation of Schiff bases is
important due to its relevance to the transformation of >C O to
>C N [15] and vice versa in biochemical processes [16–19]. It
should be noted that, in the formation of the imine derivatives
(Schiff bases), primary amines, undergo nucleophilic addition with
aldehydes or ketones to give carbinolamines which then dehy-
drate to give substituted imines [20]. Electrochemical evidence for
the formation of carbinolamines as intermediate in the reaction
of hydrazine with various carbonyl compounds has been reported
recently by Zuman and Baymak [21]. Formation and hydrolysis
of imines have also studied by several workers [22,23]. It was
reported that the rate of hydrolysis for some amines increases
2,5-diethoxy-4-morpholinoaniline using cyclic voltammetry (CV)
and controlled-potential coulometry methods with different time
scale was investigated. Since, this compound has two amine groups,
its hydrolysis reaction occurs in two steps. Our results show that
hydrolysis competes with dimerization. In acidic pH values, the
rate of hydrolysis is high, while at intermediate pH values, trimeri-
zation reaction is predominant. Our data also show that although
trimerization at intermediate pH values is predominant, hydroly-
sis reaction also contribute to the formation of the final product.
This work also represents a facile and one-pot electrochemical
method for the synthesis of new p-benzoquinone derivatives via
electrochemical oxidation of 2,5-diethoxy-4-morpholinoaniline at
different pHs under green conditions, without toxic reagents and
solvents at a carbon electrode in a divided cell, using an environ-
mentally friendly method. This mild aqueous reaction expands the
repertoire of aqueous chemistries available for small molecules.
2. Experimental
2.1. Apparatus and reagents
Cyclic voltammetry (CV) and controlled potential coulometry
were performed using a conventional three-electrode potentiostat.
A divided cell was used for coulometry. The working electrode used
in the voltammetry experiments was a glassy carbon disk (1.8 mm
diameter) and a platinum wire was used as the counter electrode.
The working electrode used in controlled-potential coulometry was
∗
Corresponding author. Tel.: +98 811 8282807; fax: +98 811 8257407.
E-mail addresses: nemat@basu.ac.ir, dnematollahi@yahoo.com (D. Nematollahi).
0013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.electacta.2013.09.063

H. Beiginejad, D. Nematollahi / Electrochimica Acta 114 (2013) 242–250
243
an assembly of four carbon rods (31 cm2) and a large platinum
gauze was used as counter electrode. The working electrode poten-
tials were measured vs. Ag/AgCl. More details are described in our
previous paper [24].
6.5
A1
4.5
2.5
2.2. Computational study
0.5
The geometries of all species in the gas phase were fully opti-
mized at Density Functional Theory (DFT), B3LYP [25,26] level of
theory using Gaussian 03 [27]. The standard 6-311G+ (p,d) basis
set was used for all species. Vibrational frequency analysis, calcu-
lated at the same level of theory, indicates that optimized structures
are at the stationary points corresponding to local minima without
any imaginary frequency. Also Natural Bond Orbital (NBO) analyses
[28] were carried out at the mentioned levels of theory. A starting
molecular-mechanics structure for the ab initio calculations was
obtained using the HyperChem 5.02 Program [29].
-1.5
-3.5
C1
-0.1
0
0.1
0.2
0.3
E vs. (AgAgCl)/V
Fig. 1. CVs of 2,5-diethoxy-4-morpholinoaniline (1) (0.5 mM) at glassy carbon elec-
trode, in phosphate buffer solution (c = 0.2 M, pH 6.5); scan rate: 200 mV s−1. Starting
potential = −0.08 V and switching potential = 0.32 V, t = 25 1 ◦C.
2.3. Electroorganic synthesis of 2,5-diethoxy-p-benzoquinone
(C10H4O4) (3ox)
(pH 6.5) at scan rate = 200 mV s−1 is shown in Fig. 1. The voltam-
mogram indicates an anodic peak (A1) in the positive-going scan
and a cathodic counterpart peak (C1) in the negative-going scan
which corresponds to the transformation of 1 to p-quinonediimine
(1ox) and vice versa within a quasi reversible two electron process
Phosphate buffer solution (80 ml, 0.2 M, pH 2.0) containing
0.05 mmol of 2,5-diethoxy 4-morpholinoaniline (1) was subjected
to electrolysis at 0.40 V vs. Ag/AgCl, in a divided cell. The electrol-
ysis was terminated when the current decayed to 5% of its original
value. The product was extracted by ethylacetate and was washed
with water. After drying, the product was characterized by IR,
1H NMR, 13C NMR, and MS (isolated yield 43%). M.p = 183–184 ◦C.
IR(KBr) = 497, 826, 879, 920, 1029, 1107, 1208, 1232, 1288, 1359,
[30–34]. Under these conditions, the peak current ratio (IpC /IpA
)
1
1
is about one can be considered as a criterion for the stability of
p-quinonediimine 1ox produced at the surface of electrode. With
increasing the concentration of 1, or decreasing potential scan rate,
the peak current ratio (IpC /IpA1 ) decreases and a new cathodic
peak appears at less positiv1e potentials.
The CVs of 1 in various pH values are shown in Fig. 2, part I.
It was found that the peak potential for peak A1 (EpA1 ) shifted to
the negative potentials by increasing pH. This is expected because
1412, 1446, 1609, 1673, 2937, 2978, 3060 cm−1 1H NMR (300 MHz,
.
acetone-d6): 1.40 (6H, t, methyl), 4.06 (4H, q, methylene), 5.88 (2H,
s, aromatic). 13C NMR (75 MHz, acetone-d6): 14.1, 65.9, 106.5, 159.4,
182.2. MS (m/e) (relative intensity), 140 (M-C4H6, 100), 112 (21),
95 (8), 94 (12), 84 (30), 71(52), 70 (28), 69 (53). “M” is the molecular
mass of 3ox.
I
2.4. Electroorganic synthesis of 2,5-bis(2.5-diethoxy-4-
morpholinophenylamino)-3,6-diethoxy-p-benzoquinone
(C38H52N4O4) (6ox)
Phosphate buffer solution (80 ml, 0.2 M, pH 6.5) containing
0.25 mmol of 2,5-diethoxy-4-morpholinoaniline (1) was subjected
to electrolysis at 0.23 V vs. Ag/AgCl, in a divided cell. The electrol-
ysis was terminated when the current decayed to 5% of its original
value. Oxidized product extract by ethylacetate and dried (isolated
yield 36%). M.p = 168–169 ◦C. IR(KBr) = 575, 705, 765, 823, 917, 958,
1044, 1119, 1203, 1266, 1295, 1338, 1395, 1450, 1472, 1510, 1588,
1629, 1651, 2817, 2847, 2929, 2977, 3263 cm−1 1H NMR (300 MHz,
.
acetone-d6/D2O): 1.18, 1.28, 1.34, 1.42 (18H, t, t, t, t, methyl), 3.06
(6H, t, methylene), 3.76 (10H, t, methylene), 3.98 (6H, m, methy-
lene), 4.10 (2H, t, methylene), 5.86 (1H, s, aromatic), 5.91 (1H, s,
aromatic), 6.49 (1H, s, aromatic), 6.65 (1H, s, aromatic). 13C NMR
(75 MHz, acetone-d6/D2O): 14.3, 14.4, 15.3, 15.4, 51.8, 51.9, 64.7,
65.2, 65.6, 65.7, 67.6, 100.5, 100.7, 106.6, 109.2, 134.6, 140.7, 142.6,
147.1, 154.4, 154.5, 163.0, 182.1. MS: m/e (relative intensity), 727
(M+3H, 1), 724 (M, 0.6), 699 (100), 670 (22), 625 (21), 596 (7),
450 (26), 421 (9), 376 (7), 352 (44), 271 (21), 242 (20). “M” is the
molecular mass of 6ox.
0.5
EtO
II
0.4
0.3
0.2
0.1
0
O
N
NH
OEt
1ox
E = -0.029pH + 0.393
R2 = 0.9975
EtO
NH
O
NH2
OEt
1H+
EtO
O
N
NH2
OEt
E = -0.054pH + 0.525
R2 = 0.9989
1
3. Results and discussion
0
2
4
6
8
10
12
pH
3.1. Potential-pH study
Fig. 2. (I) CVs of 2,5-diethoxy-4-morpholinoaniline (1) (1.0 mM) in buffer solution
with various pH values at a glassy carbon electrode. pHs from (a) to (e) are 1.4, 3.2,
4.3, 6.2 and 8.3. Scan rate 100 mV s−1. t = 25 1 ◦C. (II) Potential-pH diagram of 1.
The
CV
recorded
for
0.5 mM
of
2,5-diethoxy-4-
morpholinoaniline (1) in aqueous phosphate buffer solution

244
H. Beiginejad, D. Nematollahi / Electrochimica Acta 114 (2013) 242–250
NH2
NH
NH
O
O
-2e-, -2H+
O
O
pH < 5.23
N
NH2
N
NH2
O
O
1ox
O
O
1H+
- H+
O
O
pKa = 5.23
NH2
NH
NH
O
O
O
1
O
1H+
-2e-, -1H+
pH > 5.23
O
O
N
N
O
O
1
1ox
Scheme 1. Mechanism of the electrochemical oxidation of protonated and non protonated of 2,5-diethoxy-4-morpholinoaniline (1).
of the participation of proton(s) in the oxidation reaction of 1 to
p-quinonediimine 1ox.
scale of our experiments is unstable and participates in a chemical
following reaction.
Fig. 3 represents the first and second cycles of CV of 1H+ in
lower scan rate. The voltammogram exhibits one anodic peak (A1)
and three cathodic peaks (C1, C2, and C3). In the second cycle, the
voltammogram shows that, parallel to the decrease in current of A1,
two new anodic peaks (A2 and A3) appear at less positive potentials.
It is also seen that proportional to the increasing of the potential
sweep rate, parallel to the increase in height of the C1 the height of
C2 and C3 decreases. The presence of two redox couples A2/C2 and
A3/C3 peaks confirms two successive following chemical reactions.
To get better and more consistent data on the type of these
following chemical reactions, electrochemical oxidation of 1 was
performed in different concentration of 1. The normalized CV of
1H+ in aqueous phosphate buffer solution (pH 2.0) at various con-
centration is shown in Fig. 4. Normalized cyclic voltammograms
are obtained by dividing the current by the concentration (I/C). The
results demonstrate that the peak current ratio (IpC /IpA1 ) is nearly
constant and independent on the concentration of 11. Dimerization
reactions can be recognized from the first-order one by the depend-
1 ꢀ 1ox + mH+ + 2e−
where m is the number of protons involved in the reaction. The
anodic peak potential (EpA1 ), is given by [12]:
ꢀ
ꢁ
2.303 mRT
2F
Epꢀ A = EpA
−
pH
where EpA is the anodic peak potential at pH 0.0, R, T, and F have their
usual meanings. A potential-pH diagram is plotted for 1 by plotting
the EpA values as a function of pH (Fig. 2, part II). The EpA − pH
1
1
diagrams comprise two linear segments with different equations
and slopes around of pH 5.23.
In pHs < 5.23
54 mV
pH
EpA = 0.52 − 0.054 pH or slope = −
In pHs > 5.23
(1)
ence of the peak current ratio (IpC /IpA1 ) on concentration [8(page
29 mV
pH
1
EpA = 0.34 − 0.029 pH or slope = −
(2)
498)]. Using this data it could be concluded that in this condition,
the chemical following reaction is not the dimerization of 1 [4].
Controlled-potential coulometry was performed in aqueous
solution containing 0.05 mmol of 1H+ in a divided cell at 0.40 V
On the basis of the above mentioned slope, it can be concluded
that the reaction occurring at the pH below 5.23 is a two-electron,
two-proton process involving the oxidation of “protonated” 1 (1H+)
to the p-quinonediimine 1ox in the forward scan and reduction of
1ox to 1H+ in the reverse scan (Scheme 1). Whereas, the electrode
surface reaction at pH > 5.23, corresponds to the two-electron, one-
proton process (Scheme 1). Also, the pKa obtained in this work for
acid/base equilibrium shown in Scheme 1 is 5.23.
A1
9.5
7.5
5.5
3.5
1.5
A2
3.2. Electrochemical study in pHs 1–3
A3
As shown in Fig. 2a, the CV of 2,5-diethoxy-4-morpholinoaniline
(1) exhibits one anodic peak A1 at 0.44 V and two cathodic peaks C1
and C2 at 0.39 and 0.25 V vs. Ag/AgCl, respectively. The anodic and
cathodic peaks A1 and C1 are counterpart and correspond to the
transformation of 1H+ to the p-quinonediimine 1ox and vice versa
within a quasi-reversible two-electron process. The new cathodic
peak C2 can be related to the electrochemical reduction of the
product of side reactions such as hydrolysis, hydroxylation, and
dimerization reaction [12,35–40]. The current of the cathodic peak
C1 strongly depends on the potential scan rate. In lower scan rates,
-0.5
-2.5
-4.5
-6.5
C1
C3
C2
-0.1
0.1
0.3
0.5
0.7
E vs. (AgAgCl)/V
Fig. 3. The first and the second cycle of CVs of 2,5-diethoxy-4-morpholinoaniline (1)
(1.0 mM) at a glassy carbon electrode, in phosphate buffer solution (c = 0.2 M, pH 2.0);
scan rate: 50 mV s−1. Starting potential = −0.07 V and switching potential = 0.70 V,
t = 25 1 ◦C.
the peak current ratio (IpC /IpA1 ) is less than one and increases
1
with increasing scan rate. This result indicates that 1ox in the time

H. Beiginejad, D. Nematollahi / Electrochimica Acta 114 (2013) 242–250
245
vs. Ag/AgCl. Monitoring of the progress of the electrolysis was car-
ried out by CV (Fig. 5). As shown, during coulometry, in parallel
with the decrease in height of the anodic peak A1 and its cathodic
counterpart (C1), new anodic peaks (A2 and A3), and cathodic coun-
terpart of A3 (C3) appear and the height of them increases. At the
end of the coulometry all anodic and cathodic peaks disappear and
only anodic and cathodic peaks A3 and C3 remain. The anodic peak
A1 disappears when the charge consumption becomes about 2e−
per molecule of 1H+. These observations allow us to propose the
pathway in Scheme 1 for the electrochemical oxidation of 1H+ at
pH 2.0. In addition, the results show that the current of starting
potential for curve (a) is zero, but curves (b)–(d) show that pro-
portional to the advancement of coulometry, a cathodic current
appears at starting potential (Fig. 5, curve e). This current increases
with progress of coulometry and shows the progressive forma-
tion of a reducible compound during coulometry. According to our
results, it seems that the hydrolysis reaction of p-quinonediimine
1ox leads to p-quinoneimine 2a or 2b. The hydrolysis of compound
2 in the next step, converts it into the final product p-benzoquinone
3ox.
A1
12
7
2
-3
-8
C1
C2
0
0.1
0.2
0.3
0.4
0.5
E vs. (AgAgCl)/V
Fig. 4. Normalized CVs of 2,5-diethoxy-4-morpholinoaniline (1) at glassy carbon
electrode, in phosphate buffer solution (c = 0.2 M, pH 2.0) at various concentrations
of 1 (0.6, 1.0, 1.25, 1.6 and 2.0 mM). Scan rate = 100 mV s−1. Starting potential = 0.02 V
and switching potential = 0.55 V, t = 25 1 ◦C.
In this section, we tried to identify the more probable path-
way for the synthesis of 3ox. Our previous work explains that
4
4
A1
b
a
3
2
3
A1
2
1
1
A2
A3
0
0
C1
-1
-2
-3
-4
-1
C1
C3
-2
-3
-4
C2
C2
-0.1
0.1
0.3
0.5
0.7
-0.1
0.1
0.3
0.5
0.7
E vs. (AgAgCl)/V
E vs. (AgAgCl)/V
4
3
2
A3
c
3
2
d
1
A3
A1
A2
1
0
0
-1
-2
-3
-4
C1
-1
-2
-3
-4
C2
C3
C3
-0.1
0.1
0.3
0.5
0.7
-0.1
0.1
0.3
0.5
0.7
E vs. (AgAgCl)/V
E vs. (AgAgCl)/V
0.1
-0.1
-0.3
-0.5
-0.7
-0.9
-1.1
-1.3
a
b
e
c
d
-0.08
-0.04
0
0.04
E vs. (AgAgCl)/V
Fig. 5. CVs of 2,5-diethoxy-4-morpholinoaniline (1) (0.05 mmol) during controlled potential coulometry at 0.40 V vs. Ag/AgCl in aqueous solution containing phosphate
buffer (pH 2.0, c = 0.2 M), after the consumption of: (a) 0 C, (b) 3 C, (c) 7 C and (d) 10 C. Scan rate: 100 mV s−1. Starting potential = −0.05 V and switching potential = 0.70 V,
t = 25 1 ◦C.

246
H. Beiginejad, D. Nematollahi / Electrochimica Acta 114 (2013) 242–250
NH
NH2
NH
OEt
OEt
OEt
-2e- -2H+
Path A
EtO
EtO
First hydrolysis
reaction
EtO
N
NH
O
2a
O
O
1H+
1ox
First hydrolysis
Path B
Second hydrolysis
reaction
reaction
O
N
OEt
O
O
OEt
EtO
Second hydrolysis
reaction
EtO
O
2b
3ox
Scheme 2. Oxidation pathway of 2,5-diethoxy-4-morpholinoaniline in acidic pHs.
the N C bond order (WBIs) and charge on the reaction sites (C1,
C4) play significant roles on the rate of hydrolysis reaction [39].
WBIs (Wiberg bond indexes), is an indication of bond order and
it was concluded that increasing in WBIs or bond order leads
to increasing in the strength of bonds, The more positive charge
on the C4 or C1 atom creates a more suitable site for the nucle-
ophilic addition of water to 1ox. Therefore, the rate of hydrolysis
reaction increases by increasing the positive charge. In addition,
the hydrolysis rate is inversely proportional to the strength of
N1 C1 and N2 C4 bonds, so that increase in the WBIs led to the
decrease in the hydrolysis rate [39]. Using NBO analysis charge
on the reaction sites and WBIs of N1 C1 and N2 C4 bonds were
calculated which are shown in Fig. 6. As can be seen not only
positive charge on C4 is higher than C1, but also WBIs of C4 N2
is lesser than N1 C1. So, it can be concluded that hydrolysis rate
on C4 atom is higher than C1. In other word, since the forma-
tion rate of 2a is higher than 2b, we can conclude that, path
A is more probable than B (Scheme 2). Accordingly, the anodic
peak A1 pertains to the oxidation of protonated 2,5-diethoxy-4-
morpholinoaniline (1H+) to the p-quinonediimine 1ox. Obviously,
the cathodic peak C1 corresponds to the reduction of 1ox to 1H+.
The cathodic peaks C2 and C3 can be related to electrochemical
reduction of 2a and 3ox to the hydroquinones 2r and 3r, respec-
tively (Scheme 3). Clearly, the anodic peaks A2 and A3 correspond
to the oxidation of the hydroquinones 2ar and 3r to 2a and 3ox,
respectively.
3.3. Electrochemical study in pHs 4–9
The normalized CVs of 1.0 mM solution of 1 in aqueous phos-
phate buffer solution (c = 0.2 M, pH 6.5) at various potential sweep
rate are shown in Fig. 7. Normalized CVs are obtained by divid-
ing the current by the square root of the scan rate (I/ꢀ1/2). The
voltammograms show one anodic (A1) and two cathodic peaks
(C1 and Ct). According to Scheme 3, the anodic peak A1 pertains
to the oxidation of 1 to the p-quinonediimine 1ox and cathodic
peak C1 is its counterpart. These voltammograms show that, 1ox
at pH 6.5 is also unstable and participates in a chemical following
reaction. The results also show that proportional to the increas-
ing of the potential sweep rate, the cathodic peak C1 appears and
in parallel with the decrease in current of Ct, the height of it
increases.
In a first-order reaction, the characteristic lifetime of a chemical
reaction with k (rate constant) can be taken as t1 = 1/k, while for a
NH
NH2
OEt
OEt
-2e- -2H+
Peak A1
Peak C1
EtO
EtO
N
O
NH
O
1H+
1ox
NH2
NH
OEt
-2e- -2H+
OEt
OEt
Peak A2
Peak C2
NH
EtO
EtO
EtO
EtO
charge on C1 =+0.16801
WBI(C1-N1) =1.8078
WBI(C4-N2) =1.3768
OH
2r
EtO
O
2a
O
1
4
OH
OEt
charge on C4 =+0.31482
OEt
-2e- -2H+
Peak A3
Peak C3
N
O
OH
O
3r
3ox
Fig. 6. Calculated charge on the reaction sites (C4 and C1) and WBIs of C4 N2 and
N1 C1 bonds, using B3LYP level of theory and 6-311G+ (p,d) basis set.
Scheme 3. Redox behavior of couples 1H+/1ox, 2r/2a and 3r/3ox.

H. Beiginejad, D. Nematollahi / Electrochimica Acta 114 (2013) 242–250
247
A1
18
13
8
c
3
a
b
-2
-7
-12
Ct
C1
-0.15 -0.05
0.05
0.15
0.25
0.35
E vs. (AgAgCl)/V
Fig. 8. Normalized CVs of 2,5-diethoxy-4-morpholinoaniline (1) at glassy carbon
electrode, in phosphate buffer solution (c = 0.2 M, pH 6.5) at various concentrations.
(a) 2.0 mM, (b) 1.0 mM and (c) 0.5 mM. Scan rate = 150 mV s−1. Starting poten-
tial = −0.12 V and switching potential = 0.40 V, t = 25 1 ◦C.
Fig. 7. Normalized CVs of 2,5-diethoxy-4-morpholinoaniline (1) (1.0 mM) at glassy
carbon electrode in phosphate buffer solution (c = 0.2 M, pH 6.5), at various scan
rates. Scan rates from a to d are 10, 50, 100 and 500 mV s−1. Starting poten-
tial = −0.20 V and switching potential = 0.50 V, t = 25 1 ◦C.
second-order reaction (such as dimerization), t2 = 1/kCi, where t1 is
the time required for the reactant concentration to drop to 37% of its
initial value in a first-order process, Ci is the initial concentration of
reactant, and t2 is the time required for the concentration to drop to
one-half of Ci in a second-order process. Therefore, dimerization or
trimerization reactions can be recognized from the first-order one
by the dependence of the electrochemical response on Ci [8(page
498)]. Fig. 8 exhibits the effect of concentration of 1 on the normal-
ized CVs of 1 at pH 6.5. Normalized CVs are obtained by dividing the
current by the concentration (I/C). As is shown in Fig. 8, the peak
Electrolysis of 1 was performed in aqueous phosphate buffer
solution (c = 0.2 M, pH 6.5) at 0.25 V vs. Ag/AgCl. Cyclic voltam-
metric analysis was carried out during the electrolysis (Fig. 9). It
was observed that, proportional to the advancement of electrolysis,
anodic peak A1 decreases and peak A2 increases. These observations
allow us to propose the pathway in Scheme 4 for the electrooxida-
tion of 1 at pH 6.5.
According to the obtained electrochemical and spectro-
scopic data, the electrochemical oxidation of 2,5-diethoxy-4-
morpholinoaniline (1) leads to the formation of p-quinonediimine
1ox. In path A, the hydrolysis of p-quinonediimine 1ox in the next
step, converts it into the p-benzoquinone 3ox. The Michael addi-
tion reaction of 1 to p-benzoquinone 3ox and aromatization leads
to hydroquinone 4h. The oxidation of compound 4h is easier than
the oxidation of 1 by virtue of the presence of an electron-donating
current ratio (IpC /IpA1 ) is dependent to the concentration of 1 and
1
proportional to the increasing it, the peak current ratio (IpC /IpA
)
1
decreases. The dependence of peak current ratio (IpC /IpA1 ) 1on the
1
concentration of 1 is indication of dimerization reaction after elec-
tron transfer process [8(page 498),40].
A1
34
34
24
a
b
A1
24
14
4
14
4
C1
C1
-6
-6
-16
-26
-16
Ct
Ct
-26
-0.24
-0.04
0.16
0.36
-0.24
-0.04
0.16
0.36
E vs. (AgAgCl)/V
E vs. (AgAgCl)/V
34
24
14
34
24
14
4
At
c
d
At
A1
4
-6
C1
-6
Ct
Ct
-16
-16
-26
-0.24
-26
-0.24
-0.04
0.16
0.36
-0.04
0.16
0.36
E vs. (AgAgCl)/V
E vs. (AgAgCl)/V
Fig. 9. CVs of 2,5-diethoxy-4-morpholinoaniline (1) during electrolysis at 0.25 V vs. Ag/AgCl in aqueous solution containing phosphate buffer (pH 6.5, c = 0.2 M). (a) At
beginning. (b)–(d) During electrolysis. Scan rate: 50 mV s−1. Starting potential = −0.20 V and switching potential = 0.45 V, t = 25 1 ◦C.

248
H. Beiginejad, D. Nematollahi / Electrochimica Acta 114 (2013) 242–250
O
O
EtO
EtO
OEt
Peak A1
Path A
NH2 -2e-,-H+
OEt
O
N
NH
OEt
N
O
Peak C1
EtO
(+H2O)
Hydrolysis
1ox
3ox
1
EtO
EtO
O
N
NH2
OEt
O
N
NH2
OEt
1
1
EtO
EtO HO
EtO
EtO
EtO H2N
OH
NH OEt
EtO HN
N
O
N
O
-2e-,-H+
O
N
O
N
NH OEt
O
N
NH OEt
OEt
4ox
4
4h
OEt
OEt
P
a
t
h
(
B
+
H
H
-2e-,-2H+
y
d
2
O
EtO
r
EtO
o
)
l
y
s
i
s
O
N
NH2
OEt
Path C
EtO
O
O
NH OEt
1
O
N
OEt
NH
4q
OEt
O
EtO
O
N
OEt
N
NH2
OEt
EtO
H2N
EtO
EtO
N
O
1
NH
OEt
OEt
OEt
OH
NH
NH
N
O
N
HO
EtO
EtO
5
O
OEt
EtO
N
-2e-,-H+
5h
O
-2e-,-2H+
Peak Ct Peak At
OEt
NH
O
N
OEt
N
OEt
O
EtO
HN
EtO
EtO
O
(+H2O)
Hydrolysis
N
NH OEt
NH
EtO
O
O
OEt
EtO
EtO
NH
N
OEt
5ox
O
N
6ox
O
Scheme 4. Electrochemical oxidation pathways of 2,5-diethoxy-4-morpholinoaniline (1) at intermediate pH values.

H. Beiginejad, D. Nematollahi / Electrochimica Acta 114 (2013) 242–250
249
group. In the next step, p-benzoquinone 4q, via a Michael reaction
and aromatization, is converted to hydroquinone 5h. Further oxida-
tion converts hydroquinone 5h into the final product 6ox. In paths
B and C, it seems that 1,4-(Michael) addition reaction of 1 to 1ox is
faster than other secondary reactions such as hydrolysis, hydrox-
ylation [38] and oxidative ring cleavage [41,42] leading to 4. The
oxidation of this compound (4) is easier than the oxidation of 1 by
virtue of the presence of an electron-donating morpholinoaniline
group. Therefore in the applied potential it is converted to 4ox. In
path B, 4ox via hydrolysis reaction is changed to p-benzoquinone
4q. Next Michael addition, aromatization and oxidation processes
converts 4q to the final product 6ox. In path C, the generated
p-quinonediimine 4ox is subjected to another Michael addition
reaction with 1. In the next step, intermediate 5 after two-electron
oxidation is converted to disubstituted p-quinonediimine 5ox. And
in the final step, this compound (5ox) after hydrolysis reaction is
converted to the final product 6ox. Under these conditions (pH 6.5),
it seems that the rate of hydrolysis reaction is less than the rate of
Michael addition reaction of 1 to p-benzoquinone 3ox, thus, it can
be conclude that in Scheme 4, path A is improbable. In addition,
since, the Michael addition reaction takes place prior to the hydrol-
ysis reaction; we think that path C is more probable than path B.
It should be noted that, in the mass spectra of aminoquinonic
compounds such as 6ox, the molecular ions [M+3H] were recorded.
This mass is related to protonation of the quinonic moiety and
amino groups [43–45].
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4. Conclusions
Cyclic voltammetry, controlled potential coulometry and
spectroscopic data indicate that the mechanism of the electro-
chemical oxidation of 2,5-diethoxy-4-morpholinoaniline (1) in
aqueous acidic media (ca pH 2) is hydrolysis. Under these con-
ditions, in the first step 1 is oxidized to p-quinonediimine 1ox
in a two-electron process. In the second step, electrochemically
generated 1ox via two successive hydrolysis reactions is converted
to 2,5-diethoxy-p-benzoquinone (3ox). In addition, in this work,
the electrochemical oxidation of 1 has been investigated at inter-
mediate pH values (ca pH 6.5). Under these conditions, the data
show that 1ox via trimerization and hydrolysis is converted to
2,5-bis(2.5-diethoxy-4-morpholinophenylamino)-3,6-diethoxy-p-
benzoquinone (6ox). Our data show that at intermediate pH values,
the Michael addition reaction takes place prior to the hydrolysis
reaction.
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