606
NEMATOLLAHI, ARDAKANI, AND SHEKARLAB
efficient synthesis of some new organic compounds, ki-
netic and mechanistic study of the electrochemical ox-
idation of catechols in the presence of 1,3-indandione,
and the estimation of the observed homogeneous rate
constants (kobs) of reaction of electrochemically gener-
ated o-benzoquinones with this nucleophile by digital
simulation of cyclic voltammograms.
EXPERIMENTAL
Apparatus
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 ex-
periments was a glassy carbon disk (1.8 mm2 area),
and a platinum wire was used as a counter electrode.
The working electrode used in controlled-potential
coulometry and macroscale electrolysis was an assem-
bly of four carbon rods (31 cm2), and a large platinum
gauze constitutes the counter electrode. The work-
ing electrode potentials were measured versus SCE
(all electrodes were obtained from AZAR Electrode,
Orumiyeh, Iran). The homogeneous rate constants
were estimated by analyzing the cyclic voltammetric
responses using the simulation CVSIM software [8].
Figure 1 Cyclic voltammograms of 1.0 mM catechol:
(a) in the absence, (b) in the presence of 1.0 mM 1,3-
indandione, and (c) cyclic voltammogram of 1.0 mM 1,3-
indandione at glassy carbon electrode, in water–acetonitrile
mixture (16% v:v) containing 0.2 M acetate buffer (pH
4.8). Scan rate: 50 mV s−1; t = 25
1◦C. [Color figure
can be viewed in the online issue, which is available at
www.interscience.wiley.com.]
as well as by a decrease in the potential sweep rate and
an increase in catechol’s concentration [17–19]. These
can be related to the coupling of anionic or dianionic
forms of catechols with o-quinones (the dimerization
reaction) [17–19].
Reagents
Catechols were of reagent grade and were purchased
from Aldrich (Milwaukee, WI, USA). 1,3-Indandione,
sodium acetate, acetic acid, and acetonitrile were of
proanalysis grade and were purchased from E. Merck
(Darmstadt, Germany). These chemicals were used
without further purification.
In same conditions, the oxidation of catechol (1a) in
the presence of 1,3-indandione (3) as nucleophile was
studied in some detail. Figure 1 (curve b) shows the
cyclic voltammogram obtained for a 1-mM solution of
1a in the presence of 1 mM 1,3-indandione (3). The
voltammogram exhibits one broad anodic peak arising
from overlap of two adjacent peaks (A1 and A2), and
the cathodic counterpart of the anodic peak A1 disap-
pears. The positive shift of the A1 peak in the presence
of 1,3-indandione (3) is due to the formation of a thin
film of product at the surface of the electrode, inhibit-
ing to a certain extent the performance of electrode
process that was enhanced during the repetitive recy-
cling of potential [12–18]. In this figure, curve c is the
voltammogram of 1,3-indandione (3) in the absence of
catechol (1a).
RESULTS AND DISCUSSION
Cyclic Voltammetry Considerations
Cyclic voltammetry of 1 mM of catechol (1a) in water–
acetonitrile mixture (16% v:v), containing 0.2 M ac-
etate buffer (pH 4.8), shows one anodic (A1) and corre-
sponding cathodic peak (C1) (E1/2 = 0.29 V vs. SCE),
which corresponds to the transformation of catechol
(1a) to o-benzoquinone (2a) and vice versa within a
quasi-reversible two electrons process [9–16].
Furthermore, proportional to the augmentation of
potential sweep rate (Fig. 2) or decreasing 1,3-
indandione’s concentration (Fig. 3), the height of the
C1 peak increases. A plot of peak current ratio (IpCl/IpAl)
versus concentration (Fig. 3, inset) or scan rate (Fig. 2,
inset) for a mixture of catechol (1a) and 1,3-indandione
In this condition, the peak current ratio (IpCl/IpAl) is
nearly unity, but is related to solution pH, potential
scan rate, and catechol’s concentration. On the other
hand, in basic solutions, the peak current ratio (IpCl/IpAl)
is less than unity and decreases with an increase in pH
International Journal of Chemical Kinetics DOI 10.1002/kin