Electrochemical study of 4-chloroaniline in a water/acetonitrile mixture. A new method for the synthesis of 4-chloro-2-(phenylsulfonyl)aniline and: N -(4-chlorophenyl)benzenesulfonamide

Article abstract of DOI:10.1039/d0ra05680d

The electrochemical oxidation of 4-chloroaniline as a model compound in a water/acetonitrile mixture was investigated by cyclic voltammetry and differential pulse voltammetry. It was established that one-electron oxidation of 4-chloroaniline followed by disproportionation reaction affords unstable (4-iminocyclohexa-2,5-dien-1-ylidene)chloronium. In water/acetonitrile mixtures and in the absence of nucleophiles, the most likely reaction on produced chloronium is hydrolysis and p-quinoneimine formation. The electrochemical oxidation of 4-chloroaniline in the presence of arylsulfinic acids was also investigated in a water/acetonitrile mixture at a glassy carbon electrode. It was established that under these conditions, the anodically generated chloronium reacts with benzenesulfinic acid to produce the corresponding diaryl sulfone and N-phenylbenzenesulfonamide derivatives. In addition, Gaussian 09W was applied for prediction of the possible product by the calculation of natural charge, LUMO orbital energies and thermodynamic stability of intermediates and products. This journal is

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Electrochemical study of 4-chloroaniline in
a water/acetonitrile mixture. A new method for the
synthesis of 4-chloro-2-(phenylsulfonyl)aniline and
N-(4-chlorophenyl)benzenesulfonamide†
b
Cite this: RSC Adv., 2020, 10, 31563
Niloofar Mohamadighader,a Mahnaz Saraei,a Davood Nematollahi
*
and Hamed Goljanib
The electrochemical oxidation of 4-chloroaniline as a model compound in a water/acetonitrile mixture was
investigated by cyclic voltammetry and differential pulse voltammetry. It was established that one-electron
oxidation of 4-chloroaniline followed by disproportionation reaction affords unstable (4-iminocyclohexa-
2,5-dien-1-ylidene)chloronium. In water/acetonitrile mixtures and in the absence of nucleophiles, the most
likely reaction on produced chloronium is hydrolysis and p-quinoneimine formation. The electrochemical
oxidation of 4-chloroaniline in the presence of arylsulfinic acids was also investigated in a water/acetonitrile
mixture at a glassy carbon electrode. It was established that under these conditions, the anodically
generated chloronium reacts with benzenesulfinic acid to produce the corresponding diaryl sulfone and
N-phenylbenzenesulfonamide derivatives. In addition, Gaussian 09W was applied for prediction of the
possible product by the calculation of natural charge, LUMO orbital energies and thermodynamic
stability of intermediates and products.
Received 29th June 2020
Accepted 16th August 2020
DOI: 10.1039/d0ra05680d
rsc.li/rsc-advances
electrodimerization mechanism of PCA was studied by Amatore
and coworkers in unbuffered DMF medium. They used
Introduction
conventional and fast voltammetry to show that the primary
radical cation intermediates, formed by the one electron
oxidation of PCA.8 Kadar et al. studied electrochemical oxida-
tion of 4-chloroaniline in acetonitrile and stated that “the
substituent in the para position is not only eliminated at the elec-
trochemically initiated dimerization step, but it was oxidized to
chlorine, which substitutes the free ortho position of the starting
chloroaniline”.9 In addition, other researchers have studied the
oxidation of this compound and published valuable informa-
tion.10–12 Since, a careful choice of electrochemical parameters
are necessary for a successful electrochemical synthesis, in the
rst step, we studied the electrochemical behavior of PCA in the
water/acetonitrile mixture (solvent of synthesis), so that we
could use the data for synthetic purposes.
Since diaryl sulfone and N-phenylbenzenesulfonamide
derivatives have been synthesized in this work, in this section,
we will point out the importance of these compounds as well
as a brief discussion covering other synthetic strategies for the
synthesis of these compounds. Diaryl sulfones are useful
intermediates due to their interesting biological13–17 and
chemical18–20 properties. Diaryl sulfone derivatives are either
an antibiotic like dapsone for the treatment of leprosy13 or
have anti-HIV-1 activity like 2-nitrophenyl phenyl sulfone,14 N-
(5-bromo-3-((4-chlorophenyl)sulfonyl)thiophen-2-yl)
Haloaniline derivatives are important building blocks in the
synthesis of polymers, dyes, agricultural chemicals and phar-
maceuticals.1 Among them, 4-chloroaniline or p-chloroaniline,
PCA, is utilized as an intermediate in the synthesis of a number
of pharmaceuticals, herbicides and insecticides (e.g., monuron,
diubenzuron), pigments, azo dyes and cosmetic products. It is
also used as a precursor in the manufacture of pesticides, such
as chlorphthalim, anilofos, monolinuron and pyraclostrobin.2–5
Despite the large number of studies focused on the synthetic
aspects of PCA, a literature survey shows that the electro-
chemical information on oxidation behavior of PCA in aqueous
solutions is very limited. The rst published paper is related to
the study by Adams et al.6 They have shown that in acidic
solutions, the rst step in the oxidation of PCA is the formation
of PCA radical cation and the second step is dimerization of the
radicals. Reddy and coworker studied oxidation of PCA in
aqueous solutions at a platinum electrode and suggested that
the electrochemical oxidation of PCA in alkaline media, leads to
the
formation
of
4,40-dichloroazobenzene.7
The
aDepartment of Chemistry, Payame Noor University, Tehran, Iran
bFaculty of Chemistry, Bu-Ali Sina University, Hamedan 65174, Iran. E-mail: nemat@
basu.ac.ir
† Electronic supplementary information (ESI) available. See DOI:
10.1039/d0ra05680d
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acetamide,15 2-amino-6-arylsulfonylbenzonitrile,16 pyrrylar-
ylsulfone17 and indolylaryl sulfones14 (Fig. 1).
The classical method for the preparation of diaryl sulfone
involves the reaction between amines,21 boronic acids,22–24 aryl
halides,25 phenyliodonium salts,26 and arenediazonium tetra-
uoroborates and sodium sulnates27 or sulfonyl azides28 as
sulfonyl donors. In addition, the oxidation of sulphides and
sulfoxides are efficient methods for the synthesis of diaryl
sulfone.29,30 In addition, diaryl sulfones were synthesized by Ni-
catalysed photoredox sulnylation of halides,31 cross coupling
of aryl iodide with sulnate salts in the presence of photoredox/
Ni catalyst,32 Cu-catalyzed direct sulfonylation of indolines,33
silver-promoted decarboxylative sulfonylation of aromatic
carboxylic acids with sodium sulnates,34 arenes and 3CdSO4-
$xH2O in the presence of P2O5 via mechanochemistry35 and
electrochemical methods.36–38 All of these methods are efficient
in synthesis of diaryl sulfones. They all have strengths, capa-
bilities and weaknesses. The aim of this study is to provide
a thorough understanding of the oxidation pathways of 4-
chloroaniline as a model compound and establishes a new
strategy for the synthesis of new diaryl sulfone and N-phenyl-
benzenesulfonamide derivatives along with previous methods.
Fig. 2 Part I: cyclic voltammogram of 1.0 mM PCA at scan rate of
100 mV sꢀ1. Part II: cyclic voltammograms of PCA (1.0 mM) at different
scan rates. Scan rates from a to g are: 10, 25, 50, 100, 250, 500 and
1000 mV sꢀ1. Part III: differential pulse voltammogram of PCA (0.1
mM). Solvent: water (phosphate buffer, pH, 2.0, c ¼ 0.2 M)/acetonitrile
(50/50 v/v) mixture at GC electrode. Step potential: 5 mV, modulation
amplitude: 25 mV, modulation time: 0.05 s, at room temperature.
W1/2 ¼ 3.52RT/nF
(1)
The rst and second cycles of the cyclic voltammogram of
PCA at two potential scan rates are shown in Fig. 3 parts I and II.
In the rst cycle, CVs show the mentioned anodic peak A1 and
independent cathodic peak C2. In the second cycle, a new
anodic peak (A2) which is the counterpart of peak C2 appears.
This behavior is a clear example of a typical ECE mechanism.39
Fig. 3 part III shows the second cycle of the cyclic voltammo-
gram of PCA in the similar conditions with that of part I, but
with the difference that p-aminophenol (PAP) (0.25 mM) is
added to the solution. As can be seen, in the presence of PAP,
IpA2 increases, which conrms that peak A2 is related to the
oxidation of PAP.
Results and discussion
The cyclic voltamogram of p-chloroaniline (PCA) (1.0 mM) in
water (phosphate buffer, pH, 2.0, c ¼ 0.2 M)/acetonitrile (50/50
v/v) mixture is shown in Fig. 2, part I. The voltammogram
consists of an irreversible anodic peak (A1) which shows the
instability of oxidized PCA. The irreversible nature of oxidation
of PCA, observed at all studied scan rates (10 to 1000 mV sꢀ1
)
(Fig. 2, part II), clearly conrms the limited stability of oxidized
PCA. In other words, due to the higher reactivity of oxidized PCA
and its conversion to other compounds, there is no sign of the
reduction peak. In order to calculate the number of electrons
exchanged in the oxidation of PCA in a short time scale, the
half-peak width (W1/2) of differential pulse voltammogram of
PCA was used (eqn (1)). The differential pulse voltammogram of
PCA is shown in Fig. 2, part III. The measured half-peak width
(W1/2) is 75 mV. Based on this, the number of exchanged elec-
trons is calculated to be 1.2.
According to these results and previous data,40,41 it can be
concluded that the peaks A2 and C2 are related to the oxidation
of p-aminophenol (PAP) to p-quinoneimine (PQI) and reduction
of PQI to PAP, respectively. Considering all the available infor-
mation, the following mechanism can be considered for elec-
trochemical oxidation of PCA in aqueous solutions (Scheme 1).
According to Scheme 1, the PCA is rst converted to the
corresponding radical (PCAc) aer the loss of an electron and
Fig. 3 Part I, cyclic voltammograms (first and second cycles) of
1.0 mM PCA at scan rate: 100 mV sꢀ1. Part II, similar to part I at scan
Fig. 1 The structures of dapsone (I), 2-nitrophenyl phenyl sulfone (II), rate: 1000 mV sꢀ1. Part III, cyclic voltammogram (second cycle) of
N-(5-bromo-3-((4-chlorophenyl)sulfonyl)thiophen-2-yl)acetamide
1.0 mM PCA in the presence of 0.25 mM PAP at scan rate: 100 mV sꢀ1
.
(III), 2-amino-6-arylsulfonylbenzonitrile (IV), pyrrylarylsulfone (V) and Solvent: water (phosphate buffer, pH, 2.0, c ¼ 0.2 M)/acetonitrile (50/
indolylaryl sulfones (VI).
50 v/v) at GC electrode at room temperature.
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Scheme 1 Electrochemical oxidation pathway of PCA in aqueous
solutions.
Fig. 4 Part I: normalized cyclic voltammograms of 1.0 mM PCA at
different scan rates (10, 25, 50, 100, 250, 500 and 1000 mV sꢀ1). Part II:
plot of log IpA1 vs. log v. Part III: plot of IpA1 vs. v1/2. The data are taken
from cyclic voltammograms shown in Fig. 2 part II.
a proton. The disproportionation reaction in the next step,
converts PCAc into (4-iminocyclohexa-2,5-dien-1-ylidene)
chloronium (ICC). ICC is an unstable compound that in the
time scale of our voltammetric experiments and in acidic
solution, is hydrolyzed to p-quinoneimine (PQI).
In order to obtain more information about the electro-
chemical oxidation of PCA, its diffusion/adsorption character-
istics were examined. For this purpose, the normalized cyclic
voltammograms of PCA is shown in Fig. 4, part I. It should be
noted that the data used in Fig. 4 (parts I and II), are taken from
cyclic voltammograms shown in Fig. 2 part II. It is also neces-
sary to mention that the normalization was carried out by
dividing the current on the square root of the potential scan rate
(I/v1/2).
Two important points can be deduced from Fig. 4 part I.
First, the normalized peak current (IpA1/v1/2) for different scan
rates is almost the same, which is evidence of the diffusion
nature of PCA oxidation under experimental conditions.39 The
second point is that the peak potential (EpA1) shis towards
positive potentials as scan rate increases. This feature is
a characteristic property of an irreversible electron transfer
process.39 On the other hand, the plot of log IPA1 vs. log v is
shown in Fig. 4 part II. As can be seen, log IPA1 is linearly
proportional to log v with the slope of 0.5, revealing a pure
diffusion-controlled electron transfer process according to
Randles–Sevcik equation.39 In addition, the plot of IPA1 vs. v1/2
(Fig. 4 part III) is a straight line, indicating that the process is
totally diffusion controlled.39
The most noticeable change in the cyclic voltammogram of
PCA in the presence of BSA is the decrease in peak C2. As
described in the previous section, peak C2 is due to reduction of
PQI to PAP, therefore, a decrease in peak C2 current is a sign of
a decrease in the concentration of PQI at the electrode surface.
In other words, the presence of BSA in the experimental solu-
tion has caused electrogenerated ICC consumed in a reaction
other than hydrolysis. The general conclusion is that in this
condition, ICC reacts with the BSA. Based on these results,
along with the spectroscopic data of the isolated products, we
propose the following mechanism for the electrochemical
oxidation of PCA in the presence of BSA (Scheme 2).
In subsequent experiments we investigated the effect of the
presence of benzenesulnic acid (BSA) as a nucleophile on the
cyclic voltammograms of PCA. Fig. 5 part I shows the cyclic
voltammogram of PCA (1.0 mM) in the presence of BSA (1.0
mM). This voltammogram is compared with the cyclic voltam-
mogram of PCA in the absence of BSA (Fig. 5 part II).
Fig. 5 Cyclic voltammograms of PCA (1.0 mM): (a) in the absence, (b)
in the presence of BSA (1.0 mM) and (c) cyclic voltammogram of BSA
(1.0 mM). Solvent: water (phosphate buffer, pH, 2.0, c ¼ 0.2 M)/
acetonitrile (50/50 v/v) at GC electrode. Scan rate: 100 mV sꢀ1 at room
temperature.
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According to this mechanism, PCA at the anode surface is
oxidized to its corresponding radical (PCAc). The dispropor-
tionation reaction in the next step, converts PCAc into (4-
iminocyclohexa-2,5-dien-1-ylidene)chloronium (ICC). The reac-
tion of ICC as an acceptor with nucleophile (BSA) affording N-(4-
chlorophenyl)benzenesulfonamide (1b) and 4-chloro-2-
(phenylsulfonyl)aniline (2b). The infrared spectra of these two
compounds clearly show the typical bands of the secondary and
primary amines. The FTIR spectrum of 1b, a secondary amine
shows only a single band at 3248 cmꢀ1, while, the FTIR spec-
trum of 2b, a primary amine shows two bands at 3483 and
3381 cmꢀ1 (see ESI†).
Due to the asymmetry of the ICC, two different paths and in
other words two different vacant sites must be considered for
nucleophilic addition of BSA to ICC. Consequently, ICC can be
attacked by BSA from sites A and B to yield the products 2b and
3b (Scheme 3).
Scheme 3 Possible products from reaction of ICC with BSA.
0.0 kcal molꢀ1, respectively, which agrees with the results of
NBO analysis and conrms the greater stability of 2b.
The LUMO orbital energies of possible products 2b and 3b
are also calculated at B3LYP/6-311G level of theory (Fig. 9). The
calculated LUMO orbital energies for 2b and 3b are ꢀ1.99 and
ꢀ1.91 eV, respectively. This result show that 2b is more stable
than 3b.
In this research, in order to facilitate the synthesis of these
compounds, a constant current method has also been used. In
this method, the current density is one of the most important
factors that should be optimized. This factor plays an important
role in the purity and the yield of product. In this method, the
synthesis of 1b and 2b was performed under the same condi-
tions as described for controlled-potential method. In these
experiments the current density varied from 0.23 to 3.67 mA
cmꢀ2 while the other parameters (temperature ¼ 298 K, PCA
In order to determine the nal product, the 1H NMR spectra
of possible products 2b and 3b were simulated42 and compared
with 1H NMR spectrum of separated product (Fig. 6). This
comparison clearly shows that the spectrum of synthesized
diaryl sulfone is similar to the simulated spectrum for 2b. This
may be due to the formation of intramolecular hydrogen
bonding in 2b which can remarkably stabilize it.
In addition, the Natural Bond Orbital (NBO) analysis was
used for calculation of the partial charge in ICC. The B3LYP/6-
311G level was used for the calculations. As shown in Fig. 7,
the natural charges of carbon-A and carbon-B in ICC are about
ꢀ0.037 and ꢀ0.217 e. This result shows that carbon-A is less
negative than carbon-B. The NBO analysis result conrms that
the carbon-A is more suitable for chemical reaction than
carbon-B.
The thermodynamic stability of two possible products 2b
and 3b was also investigated. Fig. 8 shows the optimized
structure of 2b and 3b. The comparison of the relative Gibbs
free energies of two possible products 2b and 3b, are 25.4 and
Scheme 2 Electrochemical oxidation pathway of PCA in the presence Fig. 6 Simulated and experimental 1H NMR spectra for compounds
of BSA.
2b, 3b and synthesized diaryl sulfone.
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Fig. 7 The natural charge of ICC calculated at B3LYP/6-311G levels of
theory.
Fig. 8 The optimized structures of 2b and 3b at B3LYP/6-311G level of
theory.
amount: 0.25 mmol and BSA amount: 0.5 mmol) are kept
constant. It should also be noted that the amount of electricity
consumed in these optimizations is kept constant and is equal
to the theoretical amount of electricity for 0.25 mmol PCA
(50 C). Our results show that the highest yield (48% for 2b and
40% for) 1b was obtained at a current density of 2.44 mA cmꢀ2
(Fig. 10, part I). The potential–time diagram during constant
Fig. 10 Part I: effect of current density on the yield of 1b and 2b.
Charge consumed, 50 C. Part II: the potential–time diagram during
current electrolysis is shown in Fig. 10, part II. This gure shows constant current electrolysis PCA (0.25 mmol) in the presence of BSA
(0.5 mmol) in water (phosphate buffer, pH, 2.0, c ¼ 0.2 M)/acetonitrile
that the potential created by applying the current intensity of
2.44 mA cmꢀ2 is about 1 V vs. Ag/AgCl. 1 V is a suitable potential
applied in controlled-potential method (see Experimental
section). The lower product yields at low current intensities may
be due to competition between the hydrolysis of ICC (Scheme 1)
and the reaction of ICC with BSA (Scheme 2). On the other hand,
the higher current densities result in an increase in anodic
overpotential and then increase in side reactions such as the
oxidation of products and solvent which will lead to a decrease
in product yield.
(50/50 v/v) solution. Electrode rotation rate: 500 rpm.
Conclusions
In this paper, the electrochemical oxidation of 4-chloroaniline
as a model compound in aqueous solutions was investigated.
Our results show that the 4-chloroaniline is converted to (4-
iminocyclohexa-2,5-dien-1-ylidene)chloronium (ICC) aer the
passage of an electron and a proton and participating in the
disproportionation reaction. ICC is an unstable compound and
is hydrolyzed to p-quinoneimine (PQI) (Scheme 1). Despite this
trend, when BSA is added as a nucleophile, the reaction pathway
changes due to the higher rate of addition reaction between ICC
and BSA (Scheme 2). In this work, we have developed an elec-
trochemical approach for the successful synthesis of potentially
important compounds (1b and 2b). The prominent features of
this work are, provide new data on the oxidation of 4-chlor-
oaniline in aqueous solutions, developed a new method for the
synthesis of potentially important compounds, synthesis at
room temperature and atmospheric pressure, using the
Fig. 9 The LUMO orbital structures of possible products.
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electrode as an electron source instead of toxic reagents, one-
pot synthesis and high energy efficiency.
Acknowledgements
The authors wish to acknowledge Iran National Science Foun-
dation (INSF) for nancial support of this work. The authors
also acknowledge the Bu-Ali Sina University Research Council
and Center of Excellence in Development of Environmentally
Friendly Methods for Chemical Synthesis (CEDEFMCS) for their
support of this work. The authors wish to acknowledge Dr Saber
Alizadeh for his intellectual advice and helpful discussion.
Experimental section
Apparatus and reagents
A SAMA500 potentiostat/galvanostat was used for cyclic vol-
tammetry and controlled-potential coulometry technics. The
working and counter electrodes used in the voltammetry
experiments were a glassy carbon disc (2.6 mm diameter) and
a platinum rod, respectively. The working electrode used in
controlled-potential coulometry and macroscale electrolysis
was an assembly of four carbon rods (31 cm2), while a large
stainless steel plate constituted the counter electrode. The
working electrode potentials were measured vs. Ag/AgCl (all
electrodes from AZAR electrodes). Benzenesulnic acid (sodium
salt), p-chloroaniline, acetic acid, perchloric acid and phos-
phoric acid were obtained from commercial sources.
Notes and references
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General procedure for synthesis of 1b, 2b
In an undivided cell equipped with four carbon rods as anode
and a large stainless steel plate as cathode, a mixture of water
(phosphate buffer, pH, 2.0, c ¼ 0.1 M)/acetonitrile (50/50 v/v)
(ca. 80 mL) containing 4-chloroaniline (1.0 mM) and benzene-
sulnic acid (sodium salt) (2 mM) was electrolyzed at 1 V vs. Ag/
AgCl. The electrolysis was terminated when the current decayed
to 5% of its original value. At the end of the electrolysis, the cell
was placed overnight. The precipitated light brown was
collected by ltration. The purication and separation of two
compounds 1b and 2b was performed by thin layer chroma-
tography (n-hexane/ethyl acetate 8/1).
N-(4-Chorophenyl)benzenesulfonamide (1b). Light brown;
isolated yield 43%, mp ¼ 106–108 ꢁC; 1H NMR: d ppm (500
MHz, CDCl3): 7.07 (d, J ¼ 10 Hz, 2H, aromatic), 7.21 (d, J ¼
10 Hz, 2H, aromatic), 7.40 (broad-s, 1H, N–H), 7.48 (t, 2H
aromatic), 7.58 (t, 1H, aromatic), 7.81 (d, J ¼ 10 Hz, 2H,
aromatic); 13C NMR: d ppm (125 MHz, CDCl3): 123.0, 127.2,
129.3, 129.4, 131.0, 133.3, 134.9, 138.6; IR (KBr) (cmꢀ1): 3248
(medium, N–H), 1490 (medium C]C), 1331 and 1160 (strong,
S]O), 1090, 911, 685, 631, 584; MS (m/z) (EI, 70 eV) (relative
intensity): 77 (100), 126 (95), 99 (70), 51 (68), 267 (M+, 77).
4-Chloro-2-(phenylsulfonyl)aniline (2b). Dark brown, iso-
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7.25 (dd, J ¼ 8.5 and 2.8 Hz, 1H, aromatic), 7.53 (t, 2H,
aromatic), 7.62 (t, 1H, aromatic), 7.84 (d, J ¼ 2.8, 1H, aromatic),
7.96 (d, J ¼ 8.6, 2H, aromatic); 13C NMR: d ppm (125 MHz,
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144.5; IR (KBr) (cmꢀ1): 3483–3381 (medium, NH2), 1446
(medium C]C), 1297 and 1145 (strong, S]O), 1067, 688, 743,
599; MS (m/z) (EI, 70 eV) (relative intensity): 267 (M+, 100), 167
(85), 202 (80), 149 (75), 51 (70).
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Conflicts of interest
The authors declare no conict of interest.
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