Comptes Rendus Chimie

László Kiss

Sándor Kunsági-Máté

lable at ScienceDirect
C. R. Chimie 22 (2019) 557e561Contents lists avaiComptes Rendus Chimie
www.sciencedirect.comPreliminary communication / CommunicationElectrochemical oxidation of benzaldehyde and
hydroxybenzaldehydes in acetonitrile on platinum and glassy
carbon electrodes
Laszlo Kiss a, b, *, Sandor Kunsagi-Mate a, b, c
a University of Pecs, Department of General and Physical Chemistry, Ifjúsag útja 6, H-7624 Pecs, Hungary
b Janos Szentagothai Research Center, Ifjúsag útja 20, H-7624 Pecs, Hungary
c University of Pecs, Department of Pharmaceutical Chemistry, Rokus utca 2, Pecs H-7624, Hungarya r t i c l e i n f o
Article history:
Received 28 February 2019
Accepted 19 June 2019
Available online 30 July 2019
Keywords:
Benzaldehydes
Voltammetry
Fouling
Quinone* Corresponding author. University of Pecs, Depar
Physical Chemistry, Ifjúsag útja 6, H-7624 Pecs, Hun
E-mail address: kissl@gamma.ttk.pte.hu (L. Kiss)
https://doi.org/10.1016/j.crci.2019.06.004
1631-0748/© 2019 Published by Elsevier Masson SAa b s t r a c t
The electrochemical oxidation of benzaldehyde and four hydroxybenzaldehydes was
studied on platinum and glassy carbon electrodes in acetonitrile. A considerable difference
was observed in electrooxidation performed on platinum and glassy carbon electrodes. All
hydroxy derivatives fouled the glassy carbon electrode, but platinum was passivated only
by the electrooxidation of 3-hydroxybenzaldehyde, highlighting the crucial role of the
position of the substituent relative to the hydroxy group. On the glassy carbon electrode,
the formation of the corresponding benzoyl radical could have taken place, which pro-
moted the buildup of polymers on the electrode surface.
© 2019 Published by Elsevier Masson SAS on behalf of Académie des sciences.1. Introduction
The study of electrooxidation of organic substances on
different types of electrodes has gained a lot of scientific
interest because of its importance in the production of new
materials by electropolymerization or electrodeposition.
There is a high potential for their utilization because of the
size exclusion properties of their deposited layers and also
their potential to improve the sorption properties of solid
surfaces.
Hydroxybenzaldehydes are phenol derivatives; there-
fore, their electrochemistry should be similar to that of
phenols. Numerous works can be found in the literature
about the electrooxidation of phenol and phenol de-
rivatives performed in acetonitrile. Phenol and its de-
rivatives get quickly electropolymerized onto platinum andtment of General and
gary.
.
S on behalf of Académie descarbon-based electrodes [1e5], but many derivatives pro-
duce reproducible voltammograms [6].
Because acetonitrile has a wide potential window and
relatively high dielectric constant, it is suitable for the
investigation of a wide range of redox-active materials.
Moreover, many chemicals are soluble in this solvent owing
to its moderate permittivity. Owing to these advantages,
acetonitrile seemed to be an appropriate choice as a me-
dium for our experiments.
In the work of Sharma et al. [7], electrochemical
methoxylation was carried out with benzaldehyde de-
rivatives. However, to the best of our knowledge, benzal-
dehyde and hydroxybenzaldehydes have not yet been
investigated in nonaqueous solvents, and therefore, their
electrochemistry is in the focus of this article.2. Experimental
Analytical-grade chemicals were used to carry out the
experiments. Polyether ether ketone (products of eDAQ)e
sealed platinum and glassy carbon disc electrodessciences.
L. Kiss, S. Kunsagi-Mate / C. R. Chimie 22 (2019) 557e561558(diameter: 1mm)were used as working electrodes to study
the electrooxidation of the selected compounds. The sol-
vent acetonitrile was dry with a water content of around
103 M. Before preparation of solutions, its water content
was further reduced by adding anhydrous Na2SO4 to it.
Before the investigations, both electrodes were polished
with alumina (particle sizes: 1, 0.3, 0.05 mm) on a polishing
cloth and washed thoroughly with deionized water. The
equipment used for carrying out the experiments was a
potentiostat (Dropsens, Spain). The counter electrodewas a
platinum rod, and a silver wire served as the reference
electrode. The solutions were stirred before recording each
curve. Tetrabutylammonium perchlorate (TBuClO4) served
as the supporting electrolyte.3. Results and discussion
3.1. Investigation of the hydroxybenzaldehydes on the
platinum electrode
To get insights into the electrochemical behavior of the
four hydroxy derivatives of benzaldehyde, five cyclic vol-
tammograms were recorded with both electrodes. The so-
lutions were stirred before each scan. The potential
window was set to the range of 0e3 V (v ¼ 0.1 V/s). The
subsequent cyclic voltammograms obtained with the
platinum electrode are shown in Fig. 1. Using the platinum
electrode, only one peak showed up in the voltammograms
that is attributable to the phenolic hydroxy group. The
selected compounds contained a formyl substituent, which
does not react in the chosen potential window. Therefore,Fig. 1. Repetitive cyclic voltammograms of 10 mM 3-hydroxybenzaldehyde
hydroxybenzaldehyde (d) on a platinum electrode (v ¼ 0.1 V/s, supporting electroltheir electrochemistry is very similar to that of phenol
derivatives having electroinactive substituents. The results
clearly show that 3-hydroxybenzaldehyde fouled the sur-
face of platinum, but hardly any evidence of fouling can be
observed in the case of 4-hydroxybenzaldehyde. These re-
sults suggest that the position of the substituent signifi-
cantly influences the behavior of the selected compounds.
This indicates that the radical formed during the charge
transfer process reorganizes mainly to the resonant form
where the unpaired electron is in the para position being
ready to couple with another radical or reactant molecule.
If this position is free from substituents, quick polymeri-
zation can take place.
Contrary to the previous findings, 2-
hydroxybenzaldehyde serves as an example for a deriva-
tive having no substituent in the para position showing
weak passivation. The peak currents decreased slowly by
repeating the scans. An explanation to this findingmight be
the steric hindrance caused by the formyl group located in
the ortho position. When the corresponding radical forms
during the electrooxidation, the other substrate molecules
cannot reach the phenolic oxygen atom properly, but they
can reach the para position of the cyclohexadienone ring
because of the close neighborhood of the substituent. The
radicals can therefore undergo mainly dimerization. As a
result, the peak currents are a little higher than those of the
other monohydroxy derivatives.
3,4-Dihydroxybenzaldehyde has a 1,2-
dihydroxybenzene moiety (the same part as characteristic
for 1,2-dihydroxybenzene), and its voltammograms have
an oxidation peak at 1.5 V attributed to this moiety, yielding(a), 4-hydroxybenzaldehyde (b), 3,4-dihydroxybenzaldehyde (c), and 2-
yte 0.1 M TBuClO4).
L. Kiss, S. Kunsagi-Mate / C. R. Chimie 22 (2019) 557e561 559the corresponding 1,2-quinone. The peak heights were
identical for all five scans as usual in the case of 1,2-
dihydroxybenzene. Interestingly, the anodic peak currents
hardly differed from those of the monohydroxy derivatives,
despite the fact that two electrons are involved in the
charge transfer process. It suggests that its diffusion coef-
ficient is significantly lower than those of themonohydroxy
benzaldehydes.
In summary, it can be concluded that on a platinum
electrode, only the phenolic hydroxy group(s) react; thus,
their electrochemistry is similar to that of other substituted
phenols.3.2. Investigation of the hydroxybenzaldehydes on a glassy
carbon electrode
The cyclic voltammetry of the selected benzaldehydes
was also performed with a glassy carbon electrode in
acetonitrile (Fig. 2). As can be seen, the electrode was
passivated quickly by derivatives containing only one hy-
droxy group. In an earlier work where para-substituted
acetophenones and among them, 40-hydroxyacetophenone
was investigated in acetonitrile at higher anodic potentials,
passivation took place continuously by repeating the scans
[8]. In that work, it was demonstrated that the solvent itself
contributed to the process. This separated experiment
confirmed that the glassy carbon surface at high anodic
potentials changes, which is attributable to the solvent
acetonitrile because after scanning in it, no peak appearedFig. 2. Repetitive cyclic voltammograms of 10 mM 3-hydroxybenzaldehyde
hydroxybenzaldehyde (d) on a glassy carbon electrode (v ¼ 0.1 V/s, supporting elein the voltammogram of the substrate 40-hydrox-
yacetophenone. The observed fouling by the monohydroxy
derivatives of benzaldehyde is more significant than that
observed in the aforementioned earlier work by 40-
hydroxyacetophenone as very low currents were observed
in the fifth cycle. The current enhancement that attributed
to the solvent electrooxidation close to 3 V was not present,
indicating that diffusion of the solvent molecules is also
hindered because of the presence of a very compact film.
An additional smaller voltammetric peak appeared at the
shoulder of the one that is related to the phenolic hydroxy
group (around 2.3 V) in the case of monohydroxy de-
rivatives. The latter peak might be attributed to the
oxidation of formyl group, which takes place on the glassy
carbon electrode. It results in the formation of the corre-
sponding benzoyl radicals coming from the electro-
oxidation of this group. They trigger the formation of chain
polymers, which as radicals can couple with other mono-
mers through the phenolic oxygen-producing ester bonds
or dimerize through the benzoyl moieties. At potentials
close to 3 V, both the phenolic hydroxy and the formyl
group oxidize simultaneously producing biradicals; there-
fore, their coupling might occur in all possible modes.
These products can be adsorbed on the electrode surface.
Mainly, this process is responsible for the rapid deactiva-
tion (Fig. 2).
The anodic peaks of 3,4-dihydroxybenzaldehyde asso-
ciated with the 1,2-dihydroxybenzene moiety around 1.5 V
decreased continuously in subsequent scans, and the peak(a), 4-hydroxybenzaldehyde (b), 3,4-dihydroxybenzaldehyde (c), and 2-
ctrolyte 0.1 M TBuClO4).
Fig. 4. Subsequent cyclic voltammograms of 10 mM benzaldehyde at a
platinum electrode and a glassy carbon electrode (inset graph) (v ¼ 0.1 V/s,
supporting electrolyte 0.1 M TBuClO4).
L. Kiss, S. Kunsagi-Mate / C. R. Chimie 22 (2019) 557e561560potentials shifted continuously to more positive potentials.
It is partly due to the surface deactivation caused by solvent
electrooxidation in the previous scans at higher potentials.
The second peak at around 2.6 V is due to the oxidation of
the formyl group. By the appearance of this peak, the pre-
dominant formation of di(3,4-benzoquinone)-ethanedione
occurs. By comparing its voltammograms with those of
monohydroxy derivatives, it is obvious that the deactiva-
tion is less significant. The two neighboring hydroxy groups
of 3,4-dihydroxybenzaldehyde oxidize rapidly to oxo
groups, not being capable of coupling with radicals. This
indicates that in the case of the monohydroxy derivatives,
only the hydroxy group reacts, producing a phenoxyl
radical that is capable of initializing polymerization. In the
case of monohydroxy derivatives, the formed radical in-
creases the rapid formation of polymer molecules which in
turn are adsorbed onto the glassy carbon surface. The
probable cause for the appearance of second smaller anodic
peaks, in case monohydroxy derivatives are attributed to
the subsequent oxidation of formyl groups at less positive
potentials, attributable to the negative shift caused by the
adsorption of the previously formed polyphenol.
The voltammograms of 3,4-dihydroxybenzaldehyde
were taken with the glassy carbon electrode also between
0 and 1.7 V (Fig. 3). The curves show that lowering the
upper potential limit to the appearance of the first oxida-
tion peak drastically decreases the rate of fouling as
oxidation is limited mainly to the quinone moiety. The
small decrease in anodic peak currents might be because of
a very slow deactivation of the electrode surface. The used
solvent provides an aprotic environment for the formed
quinone molecules as they bound readily to the oxygen-
containing functional groups present on the glassy carbon
surface through hydrogen bonds [9]. The small increase in
the cathodic peak heights seems to confirm it.
3.3. Complementary experiments with benzaldehyde
In the voltammograms of the benzaldehyde derivatives,
a peak showed up at around 2.5 V. Complementary cyclicFig. 3. Subsequent cyclic voltammograms of 10 mM 3,4-
dihydroxybenzaldehyde in acetonitrile between 0 and 1.7 V (v ¼ 0.1 V/s,
supporting electrolyte 0.1 M TBuClO4).voltammetric experiments were carried out also with the
unsubstituted benzaldehyde in acetonitrile between 0 and
3 V. On the platinum electrode, therewas no oxidation peak
at around 2.55 V and no fouling when investigating benz-
aldehyde (close to 3 V, its oxidation begins), but a peak
appeared in the first voltammogram using the glassy car-
bon electrode (Fig. 4.). The maximum currents measured
with this electrode are significantly lower than those
measured with the platinum electrode, indicating surface
passivation. Further unambiguous evidence is the signifi-
cant current decrease in the subsequent scans and the
absence of voltammetric peak in the second scan. In the
first step, benzoyl radicals formwhich dimerize resulting in
the final 1,2-diphenylethanedione product. This compound
might be adsorbed on the glassy carbon surface and further
oxidized on the aromatic ring, resulting in the formation of
polymers. The benzoyl moiety can also react as it was
demonstrated in an earlier work in the case of benzophe-
none: an anodic peak appeared in the first scan with the
glassy carbon electrode [8]. It evidences unambiguously
that in the previous studies with 3,4-
dihydroxybenzaldehyde, oxidation of the formyl group
took place where the second peak appeared and where the
second smaller peak showed up by the monohydroxy de-
rivatives. Moreover, for additional evidence, a voltammo-
gram was also recorded with 2,4-dichlorobenzaldehyde
(not shown) with the glassy carbon electrode. An anodic
peak also appeared, but shifted to 2.75 V, because of the
electron-withdrawing effect of chloro substituents.4. Conclusion
The study of electrooxidation of benzaldehyde and
hydroxybenzaldehydes showed that these derivatives react
on platinum and glassy carbon electrodes in very different
ways. On the glassy carbon surface, the polymer formation
can take place, making possible the preparation of new
types of polymers originating from monomers bearing a
formyl group. Further work is needed to clarify the trans-
port properties of 3,4-dihydroxybenzaldehyde.
L. Kiss, S. Kunsagi-Mate / C. R. Chimie 22 (2019) 557e561 561Acknowledgements
Financial support of the GINOP 2.3.2-15-2016-00022
(Hungary) is highly appreciated.
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Electrochemical oxidation of benzaldehyde and hydroxybenzaldehydes in acetonitrile on platinum and glassy carbon electrodes

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C. R. Chimie 22 (2019) 316e320Contents lists avaiComptes Rendus Chimie
www.sciencedirect.comFull paper/MemoireVoltammetric oxidation of acetophenone derivatives and
benzophenone in acetonitrile on a platinum and glassy
carbon electrode
Laszlo Kiss a, b, *, Sandor Kunsagi-Mate a, b, c
a University of Pecs, Department of General and Physical Chemistry, Ifjúsag Street 6, H-7624 Pecs, Hungary
b Janos Szentagothai Research Center, Ifjúsag útja 20, H-7624 Pecs, Hungary
c University of Pecs, Department of Pharmaceutical Chemistry, Rokus Street 2, H-7624 Pecs, Hungarya r t i c l e i n f o
Article history:
Received 10 December 2018
Accepted 23 January 2019
Available online 28 February 2019
Keywords:
Acetophenone derivatives
Benzophenone
Passivation
Acetonitrile
1,4-Dihydroxybenzene* Corresponding author. University of Pecs, Depar
Physical Chemistry, Ifjúsag street 6, H-7624 Pecs, H
E-mail address: kissl@gamma.ttk.pte.hu (L. Kiss)
https://doi.org/10.1016/j.crci.2019.01.007
1631-0748/© 2019 Académie des sciences. Publishea b s t r a c t
The voltammetric oxidation of five para-substituted acetophenone derivatives (containing
nitro, chloro, bromo, methyl and hydroxyl groups as substituents) and that of benzophe-
none was studied on a platinum and glassy carbon electrode in acetonitrile media. Elec-
trode passivation was observed in the case when all selected compounds are on a glassy
carbon electrode; an oxidation peak appeared between 2.5 and 3 V. The voltammograms of
40-hydroxyacetophenone showed an additional peak around 2 V, which may be associated
with the oxidation of the phenol moiety. In the case of a platinum electrode, no relevant
peaks appeared on the voltammograms, reflecting the absence of passivation. The use of a
redox probe 1,4-dihydroxybenzene showed that the glassy carbon electrode, after cycli-
zation in the solutions of the outlined compounds, has higher sensitivity towards the
oxidation product (1,4-benzoquinone) of this compound.
© 2019 Académie des sciences. Published by Elsevier Masson SAS. All rights reserved.1. Introduction
Acetophenone and its derivatives are widespread ma-
terials in organic synthesis because they can serve as in-
termediate compounds used in cosmetics and perfumes.
Furthermore, they are used as flavouring agents. This group
of compounds belongs to ketones because they have an
acetyl group on the benzene ring.
This group of organic compounds is also studied by
electrochemical methods according to their complexation
ability towards metal ions [1,2]. Only a few works are
presented in the literature regarding the electrooxidation
of benzophenone and acetophenone derivatives in
nonaqueous systems. Acetone is the most investigated
ketone whose electrochemistry is examined extensively intment of General and
ungary.
.
d by Elsevier Masson SAS. Aaqueous solutions but studies are also carried out in
nonaqueous solvents. Iodide ion is a good mediator
because the formation of iodine during its anodic oxida-
tion is able to react with aliphatic ketones on their a
carbon atoms. These properties are widely applied in
synthetic reactions accomplished in alcohols [3e5]. The
final product is the corresponding ester substituting iodide
ion by the nucleophilic attack of alcohol molecules or
alkoxide ions. The BaeyereVilliger electrooxidation of ke-
tones uses a high concentration of oxygen and their
electrocatalytic reactions can result in the formation of
esters and lactones, which were successful in ionic liquids
[6] on activated carbon fibre anode. This procedure
resulted in case of acetophenones the corresponding
methyl esters. There are acetophenone derivatives having
moieties that can be electropolymerized easily, like the
corresponding pyrrole [7] and thiophene [8].
In this study, acetophenone derivatives with different
types of substituents in the para position andll rights reserved.
L. Kiss, S. Kunsagi-Mate / C. R. Chimie 22 (2019) 316e320 317benzophenone were investigated with a platinum and
glassy carbon electrode under ambient conditions.
2. Experimental section
The chemicals and solvent used in the experiments
were of analytical grade and tetrabutylammonium
perchlorate (TBuClO4) was the supporting electrolyte in
solutions prepared with dry acetonitrile (water content
around 103 M). In the aqueous solutions, KCl served as
supporting electrolyte. Platinum and glassy carbon disc
electrodes (1 mm in diameter) were used as the working
electrodes in the electrochemical cell. The counter elec-
trode was a platinum wire and a silver wire was the
reference, whereas in the aqueous solutions saturated
calomel served as a reference electrode. All experiments
were carried out with a potentiostat (Dropsens, Spain).
Polishing of the working electrodes was accomplished with
a polishing cloth by using 1, 0.3 and 0.05 mm alumina
powder. Then, they were thoroughly washed with tap and
deionized water. Before all measurements in acetonitrile
the traces of water were removed by rinsing the electrodes
with dry acetone.
3. Results and discussion
3.1. Voltammetric experiments with a platinum electrode
The selected acetophenones and benzophenone were
examined by cyclic voltammetry between 0 and 4 V with
0.1 V/s scan rate in acetonitrile. The results for the plat-
inum electrode are presented in Fig. 1. The voltammo-
grams of the different compounds are very similar, and
they have a small peak around 3 V except for 40-nitro-
acetophenone and benzophenone. It can be attributed to
the oxidation of methyl group present in the acetyl group.
Formation of the intermediate methyl radicals is followed
by their dimerization resulting in the final product 1,4-
diphenyl-1,4-butanedione containing the corresponding
substituent in the para position in the benzene ring. The
absence of this anodic peak in case of 40-nitro-
acetophenone is because of the strong electron-
withdrawing effect of the nitro group. Consequently, it
reacts at higher potentials. Significant currents flow only
at potentials close to 4 V as a result of the oxidation of
solvent and subsequent oxidation of the substrate on the
carbonyl oxygen. Benzophenone does not have this peak
because it contains an additional aryl moiety instead of
the methyl group. The voltammograms are very different
in case of 40-hydroxyacetophenone as compared with
other derivatives, which have an additional anodic peak at
2 V, and the heights of the subsequent scans are almost
the same indicating the lack of passivation. It has a phenol
moiety and the acetyl group is present in the para posi-
tion. It suggests that this para-substituted phenol deriva-
tive does not foul the surface of the platinum electrode in
acetonitrile. This is because radical formed due to the
electrochemical oxidation contained the unpaired electron
in the para position, which is the most stable form. The
final products can dissolve and diffuse into the bulk.3.2. Investigation of acetophenones and benzophenone with a
glassy carbon electrode
The electrochemical behaviour of the outlined com-
pounds was also investigated with a glassy carbon elec-
trode under the same conditions as in the case of the
platinum electrode (Fig. 2). The remarkable difference as
compared with the platinum electrode is the appearance
of a more enhanced oxidation peak between 2.5 and 3 V
during the first scans of each compounds except for 40-
nitroacetophenone. The curves have similar shape to that
recorded with the platinum electrode, indicating also
that this derivative does not show redox character in this
voltage range. The small peak around 3 V can be attrib-
uted to the solvent, which was verified in a separate
experiment (see below). The voltammograms of 40-
hydroxyacetophenone differ also significantly from the
other ones as it contains also the oxidation peak of
phenolic moiety at 1.8 V in the first scan. This aceto-
phenone derivative can be oxidized through its two
functional groups (hydroxy and methyl) scanning be-
tween 0 and 4 V; therefore, chain polymers can form and
then adsorb on the surface. The subsequent voltammo-
grams contain smaller peaks but after the second scan
they do not decrease as it can be observed generally in
the surface fouling reactions. It suggests that the polymer
formed has a loose structure elevating the diffusion of the
monomers to the electrode surface. Because the hydroxy
group is an activating substituent the oxidation peak
attributable to the methyl group appeared around 2.6 V.
This is also characteristic for 40-methylacetophenone-
containing methyl group being also an activating sub-
stituent. This oxidation peak showed up close to 3 V by
the halogen derivatives because they have electron-
withdrawing character.
Further experiments were performed with 10 mM
acetone in acetonitrile using 0e4 V potential rangewith the
aim to get information whether the methyl group is
oxidized. An oxidation peak appeared around 2.75 V, which
is similar to that of acetophenones reinforcing the earlier
hypothesis.
Interestingly, the oxidation peak above 2.5 V appeared
also approximately in the same potential interval by
benzophenone as in the case of acetophenones. In spite of
that this compound does not have methyl group, only two
phenyl groups. It suggests that benzophenone reacts on its
benzene ring while an aryl radical forms. This radical can
dimerize and then oxidize on the other phenyl group
building up polymer chains. It is also clearly seen from the
figure that the currents measured in the next scans are
significantly lower indicating the deactivation of the glassy
carbon surface. Moreover, this finding suggests that the
examined acetophenones can be oxidized also on their
benzoyl moieties on the glassy carbon electrode, resulting
in polymeric products.
The voltammograms of acetophenones and benzophe-
none suggest that these compounds foul the surface of the
glassy carbon electrode. However, looking at the ranges
between 3 and 4 V where oxidation of solvent molecules
Fig. 1. Repeated cyclic voltammograms of 40-bromo- (a), 40-chloro- (b), 40-nitro- (c), 40-methyl- (d), 40-hydroxyacetophenone (e) and benzophenone (f) in
acetonitrile with a platinum electrode (scan rate 0.1 V/s, c ¼ 10 mM, supporting electrolyte 50 mM TBuClO4).
L. Kiss, S. Kunsagi-Mate / C. R. Chimie 22 (2019) 316e320318occur, the currents do not decrease by repeating the scans
indicating the lack of serious passivation. To get more
insight into the processes on its surface a cycle was taken
between 0 and 4 V with acetonitrile and then, a cyclic
voltammogram in the solutions of the selected compounds.
These scans showed high similarity to the second and
subsequent cycles of acetophenones and benzophenone
obtained previously (see Fig. 2). Two reasons can be
responsible for these observations. Acetonitrile electro-
oxidation at higher potentials deactivates the glassy carbonelectrode making the surface unsuitable for the further
electrochemical reaction of acetophenones and benzo-
phenone. This observation is supported by an earlier study
of Staley et al. [9] with quinones in acetonitrile on the
glassy carbon electrode due to the presence of surface ox-
ides. These surface sites provide appropriate possibilities to
build hydrogen bonds between substrate, products and the
surface. This deactivation process could also be observed by
repeating the scans only to 3 V where the first oxidation
peak appeared in case of each compound.
Fig. 2. Repeated cyclic voltammograms of 40-bromo- (a), 40-chloro- (b), 40-nitro- (c), 40-methyl- (d), 40-hydroxyacetophenone (e) and benzophenone (f) in
acetonitrile with a glassy carbon electrode (scan rate 0.1 V/s, c ¼ 10 mM, supporting electrolyte 50 mM TBuClO4).
L. Kiss, S. Kunsagi-Mate / C. R. Chimie 22 (2019) 316e320 319As could be seen in the voltammograms of 40-hydrox-
yacetophenone, the oxidation peak of phenolic hydroxy
group appeared around 2 V. For obtaining more informa-
tion concerning the electrochemistry of the phenolic moi-
ety, the cyclic voltammetric experiments were repeated
between 0 and 2.2 V with the glassy carbon electrode
(Fig. 3). The peak currents, which are attributed to this
phenol moiety, decreased slowly by repeating the scans.
Previously, when the switching potential was 4 V the
decreasing tendency of this peak was more significant. It
indicates that the electrode becomes deactivated only athigher potentials because of the adsorption of products and
solvent electrooxidation.
3.3. Studies using 1,4-dihydroxybenzene as redox probe
Complementary studies were carried out with the
glassy carbon electrode using a redox probe 1,4-
dihydroxybenzene between 0 and 1 V to get more infor-
mation about the deactivation process. Previously, this
electrode showed passivation and more insights were
needed to understand the surface processes. The 5 mM
Fig. 3. Complementary voltammograms of 40-hydroxyacetophenone (c ¼
10 mM) in acetonitrile with a glassy carbon electrode (supporting electrolyte
50 mM TBuClO4).
Fig. 4. Cyclic voltammograms in aqueous solution of 5 mM 1,4-
dihydroxybenzene with bare glassy carbon electrode (black curve) and
with glassy carbon after five cycles taken in acetonitrile solution of 10 mM
40-chloroacetophenone (red curve).
L. Kiss, S. Kunsagi-Mate / C. R. Chimie 22 (2019) 316e320320concentration aqueous solution of the redox probe was
used and the obtained data are collected in Table 1. Five
cyclic voltammograms were recorded in acetonitrile solu-
tions of the studied compounds between 0 and 4 V and
finally a scan with the redox probe in its aqueous solution,
respectively. For comparison, the data obtained after taking
five cycles in the acetonitrile solvent are also presented. It
can be seen in Table 1 that the anodic peak currents
diminished to ~90% of the value measured with the glassy
carbon electrode without cycling in any acetonitrile solu-
tion. Remarkably, the anodic peak potentials were smaller
and cathodic peak potentials were higher. Two voltam-
mograms are depicted in Fig. 4 for 40-chloroacetophenone
in aqueous solution of 1,4-dihydroxybenzene. The curves
clearly show that the anodic peaks are shifted to less pos-
itive and the cathodic peaks shifted to less negative po-
tentials as compared with bare carbon. These findings
suggest that substrate adsorbs readily on the surface of
pretreated glassy carbon. The more oxidized surface and
the presence of adsorbed products of acetophenone de-
rivatives and benzophenone electrooxidation offer appro-
priate surface sites for adsorption of the substrate and the
product 1,4-benzoquinone because of hydrogen bonds. The
higher cathodic currents indicate that the increased num-
ber of adsorbed 1,4-benzoquinone is capable to reduce in
less negative potential regions. This increase in the current
could also be observed in case of the electrode withTable 1
Measured parameters of cyclic voltammograms of 5 mM 1,4-
dihydroxybenzene in its aqueous solution with a bare electrode and
after cycling in the corresponding acetonitrile solution (acf,
acetophenone).
Epa/V Ipa/mA Epc/V Ipc/mA
Bare electrode 0.51 22.4 0.08 10.08
Acetonitrile 0.49 18.443 0.1 10.821
Benzophenone 0.46 21.5 0.116 12.5
4-NO2-acf 0.484 19.7 0.1 11.367
4-Cl-acf 0.437 20.474 0.137 11.987
4-Br-acf 0.45 20.075 0.118 11.946
4-CH3-acf 0.457 18.9 0.116 11.029
4-OH-acf 0.45 18.717 0.127 11.853modified surface in acetonitrile but pretreatment in solu-
tions of the acetophenones and benzophenone contributed
to an additional small enhancement.
4. Conclusions
The electrochemical oxidation of benzophenone and
acetophenone derivatives showed that their reactions at a
platinum and glassy carbon electrode are remarkably
different, as the studied compounds react at significantly
lower potentials on the latter one. The oxidized forms of
quinones give higher cathodic currents on the pretreated
glassy carbon in the acetonitrile solutions of substrates.
Therefore, the glassy carbon electrode modified in this way
might be useful for a more sensitive detection of quinones.
Acknowledgements
Financial support of the GINOP 2.3.2-15-2016-00022
grant is highly appreciated.
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