Electrochemical pyridination of hydroquinone in aqueous solution

Article abstract of DOI:10.1007/s00706-011-0539-x

Electrochemical oxidation of hydroquinone was studied in the presence of pyridine and 4-methylpyridine as nucleophiles in aqueous solution. The results indicate the participation of electrochemically generated p-benzoquinone in Michael addition reaction w

Full text of DOI:10.1007/s00706-011-0539-x

Monatsh Chem (2011) 142:1235–1239
DOI 10.1007/s00706-011-0539-x
ORIGINAL PAPER
Electrochemical pyridination of hydroquinone in aqueous solution
•
Davood Nematollahi Bita Dadpou
Received: 8 January 2011 / Accepted: 30 May 2011 / Published online: 28 June 2011
Ó Springer-Verlag 2011
Abstract Electrochemical oxidation of hydroquinone
was studied in the presence of pyridine and 4-methylpyri-
dine as nucleophiles in aqueous solution. The results
indicate the participation of electrochemically generated p-
benzoquinone in Michael addition reaction with pyridine
and 4-methylpyridine, converting it to pyridinated
compounds.
thought that mechanistic studies and synthesis of new N-
pyridinium phenolates would be useful from the point of
view of pharmaceutical or industrial properties. This idea
motivated us to investigate the electrochemical oxidation
of hydroquinone in the presence of pyridine and 4-meth-
ylpyridine as nucleophiles.
Keywords Electrochemical pyridination ꢀ
Results and discussion
Cyclic voltammetry ꢀ Hydroquinone ꢀ Michael addition
The cyclic voltammogram of 1 mM hydroquinone (1) in
aqueous solution containing phosphate buffer (pH = 7.0,
c = 0.2 M) shows one anodic (A1) and the corresponding
cathodic peak (C1), which corresponds to the transforma-
tion of hydroquinone (1) to p-benzoquinone (2) and vice
versa within a reversible two-electron process (Fig. 1,
curve a). The peak current ratio (IpC1/IpA1) is nearly unity,
which can be considered as a criterion for the stability of p-
benzoquinone produced at the surface of the electrode
under the experimental conditions [17, 22]. The oxidation
of 1 in the presence of pyridine (3a) as a nucleophile was
studied in some detail. Figure 1, curve b shows the cyclic
voltammogram obtained for a 1 mM solution of 1 in the
presence of pyridine (20 mM). In this condition, the height
of the cathodic counterpart (C1) decreased, and a new
anodic and cathodic peak couple (A2 and C2) appeared in
more positive potentials. In this figure, curve c is the cyclic
voltammogram of pyridine (3a). This voltammogram
shows that pyridine (3a) is not electroactive in the studied
potential range (Fig. 1, curve c).
Introduction
Pyridinium-substituted hydroquinones have attracted
interest as a partial structure of modified rifamycins with
antibacterial activities [1–3]. Also, N-pyridinium pheno-
lates (betaines of N-pyridinium) are interesting compounds
because of their clear solvatochromism, thermochromism,
and halochromism [4]. The importance of these compounds
has prompted many workers to synthesize [4–11] or study a
number of them [4, 12–16]. To the best of our knowledge,
no reports exist on the electrochemical synthesis or
mechanistic studies of pyridinated hydroquinone. With due
attention to our experiences on mechanistic studies of
organic processes by electrochemical techniques [17, 18]
and electrosynthesis of organic compounds [19–24], we
D. Nematollahi (&)
Faculty of Chemistry, Department of Analytical Chemistry,
University of Bu-Ali Sina, Hamedan, Iran
e-mail: nemat@basu.ac.ir
The existence of a subsequent chemical reaction
between p-benzoquinone (2) and pyridine (3a) is supported
by the following evidences: (a) Decreasing of IpC1 during
the reverse scan, this could be indicative of the fact that
electrochemically generated p-benzoquinone (2) is
B. Dadpou
Faculty of Science, Department of Chemistry,
Arak Branch, Islamic Azad University, Arak, Iran
123

1236
D. Nematollahi, B. Dadpou
a
b
a
b
A1
A1
I/µA
I/µA
I/µA
A1
I/µA
A1
4.0
A2
A2
A2
4
4.0
2.0
3
2.5
2
C2
C2
1.0
C2
0.0
1
c
0
-0.5
-2.0
-4.0
C1
C1
-1
-3
-2.0
-0.25
-2
-0.25
0
0.25
0.5
0
0.25
0.5
E/V vs. SCE
E/V vs. SCE
C1
C1
c
d
-4
A1
A1
I/µA
-0.2
0
0.2
0.4
0.6
-0.2
0
0.2
0.4
I/µA
8.0
E/V vs. SCE
E/V vs. SCE
A2
9.0
4.0
5.0
2.0
Fig. 1 Cyclic voltammograms of (a) 1 mM hydroquinone, (b) 1 mM
hydroquinone in the presence of 20 mM pyridine at a scan rate of
5 mV s-1 in phosphate buffer solution (pH = 7.0, c = 0.2 M) at
glassy carbon electrode, T = 25 1 °C
C2
-1.0
-6.0
-11.0
-1.0
-4.0
-7.0
removed partially by chemical reaction with pyridine (3a).
(b) Dependency of the peak current ratio (IpC1/IpA1) on the
potential sweep rate (Fig. 2). In this case, for the highest
sweep rate employed a well-defined cathodic peak C1 is
observed. For lower sweep rates, the peak current ratio
(IpC1/IpA1) is less than one and increases with increasing
sweep rate (Fig. 2). This is indicative of departure from the
intermediate and arrival to the diffusion reign [25] with an
increasing sweep rate. (c) Appearance of an anodic peak A2
and its cathodic counterpart (C2) in more positive poten-
tials that shows the product of the following chemical
reaction is electroactive [25]. (d) Dependency of the peak
current ratio (IpA2/IpA1) on the potential sweep rate (Fig. 2).
This is related to the decrease of the effect of the following
chemical reaction with increasing sweep rate. (e) Depen-
dency of peak current ratios (IpC1/IpA1) and (IpA2/IpA1) on
the concentration of pyridine (Fig. 3). This is related to the
increase of the homogeneous reaction rate of the following
chemical reaction.
C1
C1
-0.25
0
0.25
0.5
-0.25
0
0.25
0.5
E/V vs. SCE
E/V vs. SCE
Fig. 2 Cyclic voltammograms of 1 mM hydroquinone in the pres-
ence of 20 mM pyridine at various scan rates. Scan rates from a to d:
5, 10, 25, and 50 mV s-1 in phosphate buffer solution (pH = 7.0,
c = 0.2 M) at glassy carbon electrode, T = 25 1 °C
exhaustive oxidation of a solution containing 1 mmol of
hydroquinone (1) in the presence of 3a (1 mmol) or
4-methylpyridine (3b, 1 mmol) by applying 0.15 V versus
SCE allow us to propose the mechanism for the electro-
oxidation of 1 in the presence of 3a and 3b (Scheme 1).
Accordingly, the 1,4-addition (Michael) reaction of 3a,
3b to p-benzoquinone (2) leads to intermediates 4a, 4b. In
the next step, Michael reaction of 4a, 4b to 2 converts 4a,
4b to intermediates 5a, 5b. Oxidation of 5a, 5b and the
next Michael addition leads to the formation of final
products 7a, 7b. The oxidation of 7a, 7b is more difficult
than the oxidation of the starting molecule 1 by virtue of
the presence of the electron-withdrawing pyridine group on
7a, 7b. Therefore, over-oxidation of 7a, 7b was circum-
vented during the reaction because of the presence of the
electron-withdrawing group. As a result, the anodic peak
A1 pertains to the oxidation of hydroquinone 1 to p-
benzoquinone 2. Obviously, the cathodic peak C1 corre-
sponds to the reduction of p-benzoquinone 2. The anodic
peak A2 can be related to electrochemical oxidation of
Controlled-potential coulometry was performed in
aqueous solution (phosphate buffer, c = 0.2 M, pH = 7.0)
containing 0.1 mmol of hydroquinone (1) and 0.1 mmol
of pyridine (3a) at potential of peak A1 (EpA1). Cyclic
voltammetric analysis carried out during the electrolysis
shows the progressive formation of a new anodic peak A2,
parallel to the disappearance of peak A1. The anodic peak
A1 and cathodic peak C1 disappear when the charge
consumption becomes about 3e- per molecule of 1
(Fig. 4).
These coulometry and voltammetry results accompanied
by the spectroscopic data of final products obtained from
123

Electrochemical pyridination of hydroquinone in aqueous solution
1237
a
b
c
I/µA
4
I/µA
I/µA
A1
A1
A1
4
4
A2
A2
A2
3
2
3
2
3
2
1
1
1
C2
C2
C2
0
0
0
-1
-2
-3
-1
-2
-3
-1
-2
-3
C1
C1
C1
-0.2
0
0.2
E/V vs. SCE
0.4
-0.2
0
0.2
E/V vs. SCE
0.4
-0.2
0
0.2
E/V vs. SCE
0.4
Fig. 3 Cyclic voltammograms of 1 mM hydroquinone in the presence of (a) 20, (b) 30, and (c) 40 mM pyridine in phosphate buffer (pH = 7.0,
c = 0.2 M) at glassy carbon electrode, scan rate 5 mV s-1, T = 25 1 °C
O
OH
A1
-4e- -4H+
5.4
3.4
2
2
R
O
2
OH
1
A2
N
+
3a, 3b
OH
OH
OH
OH
OH
O
O
1.4
+H+
N
N
O
2
R
5a, 5b
R
R
-0.6
-2.6
-4.6
4a, 4b
C2
-2e- -2H+
OH
O
OH
OH
N
-H+
R
N
N
R
N
X-
HO
HO
C1
0.1
E/V vs. SCE
OH
O
6a, 6b
R
-0.2
-0.1
0
0.2
0.3
0.4
0.5
7a, X = 2 Cl-
2-
7b, X = HPO4
a, R = H
b, R = CH3
Fig. 4 Cyclic voltammograms of 0.1 mmol hydroquinone in the
presence of 0.1 mmol pyridine in phosphate buffer (pH = 7.0,
c = 0.2 M) at glassy carbon electrode during controlled-potential
coulometry at 0.15 V versus SCE. After consumption of 0, 5, 10, 15,
20, and 25 °C. Scan rate 5 mV s-1, T = 25 1 °C
Scheme 1
and C3) appear in more positive potentials. These peaks
(A3 and C3) are related to the oxidation and reduction of
dipyridinated hydroquinone/dipyridinated p-benzoquinone
redox couple. These effects are enhanced by increasing the
concentration of 3b (Fig. 5).
pyridinated hydroquinone 4a to its related pyridinated p-
benzoquinone. Clearly, the cathodic peak C2 corresponds
to the reduction of the pyridinated p-benzoquinone to
pyridinated hydroquinone 4a.
The same voltammetric behaviors are observed in
electrochemical oxidation of 1 in the presence of 4-meth-
ylpyridine (3b). The only difference in electrochemical
behavior of 1 in the presence of 3b is related to higher
nucleophilicity of 3b in comparison with 3a. This property
of 3b has two effects in the cyclic voltammograms of 1 in
Conclusions
For the first time in this report, the electrochemical pyri-
dination of hydroquinone was studied. The results of this
work show that hydroquinone is oxidized to p-benzoqui-
none, which is then attacked by pyridine. A novel
pyridinated dimer of hydroquinone is obtained after
the presence of 3b (Fig. 5): (a) the peak current ratio (IpC1
I
/
pA1) decreases, and (b) new anodic and cathodic peaks (A3
123

1238
D. Nematollahi, B. Dadpou
b
c
a
I/µA
I/µA
I/µA
A1
A1
A1
4.5
3.5
4.5
3.5
4.5
3.5
A2
A3
A2
A2
A3
A3
2.5
2.5
2.5
C3
C3
C3
1.5
1.5
1.5
C2
C2
C2
0.5
0.5
0.5
-0.5
-1.5
-2.5
-0.5
-1.5
-2.5
-0.5
-1.5
-2.5
C1
C1
C1
-0.2
0
0.2
0.4
-0.2
0
0.2
0.4
-0.2
0
0.2
0.4
E/V vs. SCE
E/V vs. SCE
E/V vs. SCE
Fig. 5 Cyclic voltammograms of 1.0 mM hydroquinone in the presence of (a) 20, (b) 30, and (c) 40 mM 4-methylpyridine in phosphate buffer
(pH = 7.0, c = 0.2 M) at glassy carbon electrode, scan rate 5 mV s-1, T = 25 1 °C
consumption of 3e- per molecule of hydroquinone as final
product. The reaction mechanism for anodic oxidation of
hydroquinone in the presence of pyridine is presented in
Scheme 1. From the point of view of green chemistry, use
of the electrosynthesis method has some important
advantages. Clean synthesis, the use of electricity instead
of chemical reagents, and the achievement of high atom
economy via a one-step process conducted under ambient
conditions are of preeminent green advantages.
stainless steely gauze as cathode at 25 °C. The electrolysis
was terminated when the current decayed to 5% of its
original value. The process was interrupted during the
electrolysis, and the carbon anode was washed in acetone
in order to reactivate it. At the end of electrolysis, the
precipitated solid was collected by filtration and washed
with water. In the case of compound 7a, the resulting
product was diluted with 4 cm3 water, heated, and treated
with 0.26 cm3 of 18% hydrochloric acid. On cooling, sol-
ids precipitated that were filtered off, washed with water,
and dried in vacuum. After drying, products were charac-
1
Experimental
terized by IR, H NMR, 13C NMR, and MS.
1,10-(2,20,5,50-Tetrahydroxy-1,10-biphenyl-4,40-diyl)bispy-
ridinium dichloride (7a, C22H18Cl2N2O4)
Cyclic voltammetry, controlled-potential coulometry, and
preparative electrolysis were performed using an Autolab
model PGSTAT 30 potentiostat/galvanostat. The working
electrode used in the voltammetry experiments was a
glassy carbon disc (1.8 mm2 area), and a platinum wire was
used as a counter electrode. The working electrode used in
controlled-potential coulometry and macroscale electroly-
sis was an assembly of four carbon rods (31 cm2) and a
large steel sheet constituting the counter electrode. The
working electrode potentials were measured vs. SCE and
Ag/AgCl (from AZAR electrode and Metrohm). All
chemicals (hydroquinone, pyridine, and 4-methylpyridine)
were reagent-grade materials, and phosphate salts were of
pro-analysis grade. These chemicals were used without
further purification.
ꢀ
Yield 54.4%; m.p.: [260 °C (dec.); IR (KBr): m = 3,436,
3,073, 1,667, 1,602, 1,495, 1,398, 1,267, 1,187, 862, 837,
779, 682 cm-1; 1H NMR (300 MHz, DMSO-d6): d = 5.91
(t, 2H), 6.76 (d, 4H), 7.02 (t, 4H), 7.36 (s, 4H), 8.24 (s, 2H),
9.22 (s, 2H) ppm; 13C NMR (75 MHz, DMSO-d6):
d = 117.0, 122.1, 123.2, 127.8, 144.7, 146.3, 146.6,
156.2, 159.9 ppm; MS (70 eV): m/z (relative inten-
sity) = 41 (49), 57 (43), 69 (100), 81 (49), 95 (22), 109
(12), 121 (24), 137 (16), 149 (92), 167 (31), 185 (6), 203
(6), 217 (2), 236 (6), 252 (6), 264 (2), 279 (15), 299 (1),
321 (1), 341 (3), 353 (1), 368 (3), 410 (M? ? HCl, 1).
1,10-(2,20,5,50-Tetrahydroxy-1,10-biphenyl-4,40-diyl)bis(4-
methylpyridinium) hydrogenphosphate
(7b, C24H23N2PO8)
ꢀ
Synthesis of compounds 7a, 7b
Yield 60.2%; m.p.:[260 °C (dec.); IR (KBr): m = 3,392,
3,205, 3,120, 1,637, 1,505, 1,446, 1,351, 1,274, 1,202, 827,
614, 502 cm-1; 1H NMR (300 MHz, DMSO-d6): d = 2.70
(s, 6H), 7.03 (m, 4H), 8.07 (d, 4H), 9.00 (d, 4H), 9.28 (br,
about 2H), 10.27 (br, about 2H) ppm; MS (70 eV):
m/z (relative intensity) = 41 (46), 238 (94), 267 (27), 295
(50), 322 (100), 397 (49), 425 (45), 498 (M? ? HPO4, 42).
Aqueous phosphate buffer solution (80 cm3, c = 0.2 M,
pH = 7.0) containing 1 mmol of hydroquinone (1) and
1 mmol of pyridine (3a) or 4-methylpyridine (3b) was
electrolyzed at potential of peak A1 (0.15 V vs. SCE) in an
undivided cell equipped with a carbon anode and a large
123

Electrochemical pyridination of hydroquinone in aqueous solution
1239
Acknowledgments The authors acknowledge the Bu-Ali Sina
University Research Council and Center of Excellence in Develop-
ment of Chemical Methods (CEDCM) for support of this work.
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