Journal of The Electrochemical Society, 159 (4) E82-E86 (2012)
m/z (relative intensity): 318 (M+., 25), 276 (100), 77 (44), 56 (73),
E83
30 (56).
4-(Piperazin-1-yl)-2-tosylphenol (5b).—
13
H3C
12
11
11
10
10
9
O
O
6
S
2
2
4
5
1
1
3
HO
N
NH
8
7
M.p.: 246–247◦C (Dec.). 1H NMR (500 MHz, DMSO-d6) δ (ppm):
2.35 (s, 3H, methyl), 2.83 (s, 4H, aliphatic), 2.92 (s, 4H, aliphatic),
6.74 (d, J = 8.7 Hz, 1H, aromatic), 7.10 (d, J = 6.6 Hz, 1H, aromatic),
7.34 (d, J = 7.7 Hz, 3H, aromatic), 7.76 (d, J = 8.0 Hz, 2H, aromatic).
13C NMR (125 MHz, DMSO-d6) δ (ppm): 20.9 (C-13), 45.4 (C-1),
50.4 (C-2), 114.7 (C-4), 118.2 (C-7), 124.3 (C-5), 126.1 (C-10), 127.8
(C-11), 129.1 (C-6), 138.6 (C-9), 143.4 (C-8), 144.3 (C-3), 148.9
(C-12). IR (KBr) ν (cm−1): 3444 (broad, O-H and N-H), 3043 (weak,
C-H), 2948 (weak, C-H), 1631 (medium C=C), 1453 (strong, C=C),
1302 (strong, S=O), 1284 (strong, C-H, methyl), 1139 (strong, S=O),
1090, 942, 828, 742, 709, 657, 593. MS (EI, 70 eV) m/z (relative
intensity): 332 (M+., 21), 290 (97), 124 (42), 109 (71), 91 (100).
Figure 2. First and second cyclic voltammograms of 1.0 mM 4-(piperazin-1-
yl)phenol (1) in ethanol/water mixture (10/90) with pH = 1.5, at glassy carbon
electrode at various scan rates. T = 25 1◦C.
Fig. 3. The peak current ratio (IpC1/IpA1) decreases with increasing
pH as well as decreasing potential sweep rate. The cathodic peak
C1 disappears in basic solutions and/or in low potential sweep rates.
Also, the cathodic peak current ratio (IpC1/IpC0) is pH dependent and
decreases with increasing pH. This shows that the rate of above men-
tioned reactions (hydroxylation and dimerization) is pH dependent
and enhances by increasing pH (Fig. 3).21–25
In this work, with the aim of decreasing in the rate of introduced
hydroxylation and/or dimerization reactions, a solution containing
perchloric acid (pH = 1.0) has been selected as a suitable solution
for the electrochemical study of 4-(piperazin-1-yl)phenol (1) in the
presence of aryl sulfinic acids, and synthesis of 2-(phenylsulfonyl)-4-
(piperazin-1-yl)phenol derivatives.
2-(4-Chlorophenylsulfonyl)-4-(piperazin-1-yl)phenol (5c).—
Cl
11
12
11
10
10
9
O
O
6
S
2
2
4
5
1
1
3
HO
N
NH
8
7
Mp.: 229–230◦C (Dec.). 1H NMR (500 MHz, DMSO-d6) δ (ppm):
2.88 (s, 4H, aliphatic), 2.97 (s, 4H, aliphatic), 5.38 (broad, 2H,
OH, NH), 6.73 (s, 1H, aromatic), 7.15 (s, 1H, aromatic), 7.36 (s,
1H, aromatic), 7.66 (s, 2H, aromatic), 7.92 (d, 2H, aromatic). 13C
NMR (125 MHz, DMSO-d6) δ (ppm): 45.8 (C-1), 50.8 (C-2), 115.2
(C-4), 119.0 (C-7), 125.5 (C-5), 125.6 (C-10), 129.4 (C-11), 130.3
(C-6), 138.4 (C-9), 140.9 (C-8), 144.3 (C-3), 150.7 (C-12). IR (KBr)
ν (cm−1): 3454 (broad, O-H), 3274 (medium, N-H), 2952 and 2923
(weak, C-H, aliphatic), 2820 (weak, C-H), 1625 and 1582 (medium
C=C), 1474 (strong, C=C), 1303 (strong, S=O), 1278 (medium C-
O), 1147 (strong, S=O), 1087, 1011, 943, 827, 769, 705, 591. MS
(EI, 70 eV) m/z (relative intensity): 352 (M+., 28), 310 (100), 135 (7),
119 (7), 111 (7), 91 (14), 65 (9.3), 56 (21.7).
The E-pH diagram.— It was found that the peak potential for A1
(EpA1) shifted to the negative potentials by increasing in pH. This is
expected because of the participation of proton(s) in the oxidation
reaction of 1 to p-quinone-imine 2. The half-wave potential (E1/2), is
given by:26,27
E1ꢀ /2 = E1/2 − (2.303 mRT/2F)pH
where m is the number of involved protons in the reaction and E1/2 is
the half-wave potential at pH = 0.0, R, T, and F have their usual mean-
ings. The half-wave potentials (E1/2) were calculated as the average
of anodic and cathodic peak potentials of the cyclic voltammograms
{(EpA1 + EpC1)/2}. A potential-pH diagram is constructed for com-
pound 1 by plotting the calculated E1/2 values as a function of pH
(Fig. 4). The E1/2-pH diagram comprises two linear segments with
Results and Discussion
The effect of pH.— Electrochemical generation of p-quinone-
imine (2) and using it as a Michael acceptor for the synthesis of
2-(phenylsulfonyl)-4-(piperazin-1-yl)phenol derivatives on the one
hand, and minimizing its participation in other possible reactions,
on the other hand, are the main goal of this work. Therefore, electro-
chemical oxidation of 1 has been studied in various pHs.
Cyclic voltammograms of 1.0 mM solution of 4-(piperazin-1-
yl)phenol (1) in ethanol/water mixture (10/90) solution at pH = 1.5
are shown in Fig. 2. In this pH value, cyclic voltammograms exhibit
one anodic (A1) and two cathodic peaks (C1 and C0). A1 and C1 Peaks
are related to the transformation of 1 to p-quinone-imine 2 and vice
versa within a reversible two-electron process. In the second cycle, a
new anodic peak (A0) appears at less positive potential (Fig. 2). These
peaks (A0 and C0) show the occurrence of some reactions such as
hydroxylation on the electrochemically generated to p-quinone-imine
2, and/or coupling of 1 (as a nucleophile) with p-quinone-imine 2
(dimerization reaction), under the experimental conditions.
Figure 3. Cyclic voltammograms of 1.0 mM 4-(piperazin-1-yl)phenol (1)
at a glassy carbon electrode, in buffered solutions {ethanol/water mixture
(10/90)} with various pHs and same ionic strength. Scan rate: 1000 mV s−1
T = 25 1◦C.
.
Cyclic voltammograms of 4-(piperazin-1-yl)phenol 1 (1.0 mM)
in ethanol/water (10/90) solution at various pH values are shown in
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