Electrochemical Reduction of Trichlorobiphenyls: Mechanism and Regioselectivity

Article abstract of DOI:10.1134/S1070363218100055

The regioselectivity of electrochemical reduction of four trichlorobiphenyls (PCB 28–30 and PCB 37) was studied by cyclic voltammetry and bulk electrolysis. The number of stages and mechanism of electrochemical reduction of each of the examined substrate

Full text of DOI:10.1134/S1070363218100055

ISSN 1070-3632, Russian Journal of General Chemistry, 2018, Vol. 88, No. 10, pp. 2058–2066. © Pleiades Publishing, Ltd., 2018.
Original Russian Text © V.P. Boyarskii, M.V. Sangaranarayanan, I.A. Boyarskaya, E.G. Tolstopyatova, T.G. Chulkova, 2018, published in Zhurnal
Obshchei Khimii, 2018, Vol. 88, No. 10, pp. 1610–1619.
Electrochemical Reduction of Trichlorobiphenyls:
Mechanism and Regioselectivity
a
b
a
V. P. Boyarskii *, M. V. Sangaranarayanan , I. A. Boyarskaya ,
a
a
E. G. Tolstopyatova , and T. G. Chulkova
a
St. Petersburg State University, Universitetskaya nab. 7–9, St. Petersburg, 199034 Russia
e-mail: v.boiarskii@spbu.ru
*
b
Madras Institute of Technology, MIT Road, Radha Nagar, Chromepet, Chennai, Tamil Nadu, 600044 India
Received June 21, 2018
Abstract—The regioselectivity of electrochemical reduction of four trichlorobiphenyls (PCB 28–30 and PCB
7) was studied by cyclic voltammetry and bulk electrolysis. The number of stages and mechanism of
3
electrochemical reduction of each of the examined substrate were inferred on the basis of the experimental
electron transfer coefficients and calculated (DFT) bond lengths and potential energy surface sections. GC/MS
analysis of the controlled potential electrolysis products showed that chlorine atom in the disubstituted ring of
trichlorobiphenyls is reduced more readily than in the monosubstituted ring and that the rate of chlorine
reduction changes in the series o-Cl > p-Cl > m-Cl.
Keywords: polychlorobiphenyls, cyclic voltammetry, preparative electrolysis, DFT calculations, electron
transfer mechanism
DOI: 10.1134/S1070363218100055
Nowadays, the problem of environmental pollution
with persistent organic pollutants (POPs) is considered
by the United Nations to be a priority environmental
problem. The group of persistent organic pollutants
consists of 12 chemical compounds, including poly-
chlorinated biphenyls (PCBs) [1, 2]. The Stockholm
Convention on Persistent Organic Pollutants signed in
order to develop scientifically substantiated recom-
mendations on the use of one or another method for
remediation of PCBs, it is necessary to consider the
problem of the reduction regioselectivity, which has
been insufficiently studied. Taking into account that
many reduction and substitution processes involve inter-
mediate formation of radical anions [7, 20], it seemed
important to study radical anion reactions of PCBs.
2
001 was aimed at not only elimination of the
production and use of POPs but also disposal of the
existing stocks of these compounds.
One of the most obvious ways to study radical
anion reactions of PCBs consists of generation of
radical anions in an electrochemical cell. Furthermore,
in recent years elctroreductive decontamination of
water and soils from halogenated POPs has gained
researchers’ attention [21], which further increases the
importance of studying electrochemical hydrodechlo-
rination of PCBs.
Commercial PCBs are complex mixtures consisting
of 50–70 individual compounds (congeners) with
different numbers and positions of chlorine atoms in
their molecules. Up to now, various methods for
remediation of PCBs have been proposed; they include
biochemical [3, 4], oxidative [5, 6], and reductive
methods [6–12] and those based on nucleophilic
substitution of chlorine by other functional groups
Electrochemical reduction of polychlorinated bi-
phenyls and naphthalenes is often performed by cyclic
voltammetry [22]. It should be noted that the presence
of aromatic radical anion mediators such as biphenyl
and naphthalene increases the rate of reduction of
PCBs [23]. We have recently studied by cyclic
voltammetry regioselective electrochemical reduction
(
[
e.g., methoxycarbonylation [13] or solvolysis
14–19]). Utilization of PCBs is possible only via
reduction and nucleophilic substitution or
a
combination of these methods. Biphenyl or its
functional derivatives thus formed can be used as
starting compounds in industrial organic synthesis. In
2
058

ELECTROCHEMICAL REDUCTION OF TRICHLOROBIPHENYLS
2059
Scheme 1.
_
Clx
ET
_Cl−
а
b, ET
_Cl−
Cl
x−1
Clx
Cl
x−1
of dichlorobiphenyls, and the results of these studies
allowed us to analyze the kinetic and mechanistic
aspects of the process [24, 25]. Electrochemical
reduction of PCBs involves dissociation of the C–Cl
bond, which gives rise to the corresponding aryl
radical according to either stepwise (path a) or
concerted mechanism (path b; Scheme 1). The relative
rates of the two reduction stages, electron transfer and
bond cleavage, determine the reduction mechanism. If
the bond dissociation rate is lower than the rate of the
electron transfer stage (with account taken of solvation
shell reorganization), the reaction follows the stepwise
path. Otherwise (i.e., when the bond dissociation stage
is faster), the concerted mechanism is operative. The
neutral radical formed after departure of chloride ion is
reduced at a lower potential than that required for the
reduction of the initial compound. The resulting aryl
anion abstracts a proton (mostly from a solvent
molecule) [26] to produce hydrodechlorination
product.
The substrates were four PCB congeners 1–4 with
3:0 (2, 3) and 2:1 substitution patterns (1, 4) and
different numbers of chlorine atoms in the ortho
positions.
Cyclic voltammetry. The cyclic voltammograms
for the reduction of PCBs 1–4 were recorded at a
potential scan rate of 0.1–10 V/s. The cyclic
voltammograms of PCBs 28, 29, and 37 (1, 2, 4)
displayed three irreversible electrochemical reduction
peaks (Fig. 1) corresponding to dissociation of three
different C–Cl bonds. The peak potentials are given in
Table 1. The reduction peaks remained irreversible
even at high potential scan rates, which indicated fast
decomposition of radical anions into neutral aryl
radical and chloride ion. At a potential scan rate of
0
.1 V/s, distinct peaks were observed for PCB 37,
whereas those for PCB 29 and PCB 28 were less
distinct. The peak potential separation between the first
and second peaks of PCB 37 was 180 mV, and
between the second and third peaks, 159 mV. Only
two irreversible peaks were observed for PCB 30, and
the peak potential separation was small (105 mV).
Theoretical calculations can also be used to
understand the reaction mechanism. For example, the
C–X bond lengths before and after electron transfer
and spin density on the carbon atom linked to halogen
in the radical anion can be successfully used as
indicators of the reaction mechanism. Even a more
important indicator is the occurrence of local minima
and transition states on the potential energy surface,
which points out the formation of stable radical anions.
Herein we report the kinetic and mechanistic features
of successive hydrodechlorination of four trichlorobi-
phenyls which were studied by complementary methods
including cyclic voltammetry, structure determination
of controlled potential electrolysis products, and DFT
quantum chemical calculations.
The other two parameters important for under-
standing the mechanism of electrochemical reduction
[27] are the peak width (Ep/2 – E
of the peak potential on the potential scan rate
∂E /∂log v. Their values are also given in Table 1. The
peak width was calculated as the difference between
the peak potential E and half-peak potential Ep/2. For
) and the dependence
p
p
p
the accurate determination of the second and third peak
parameters, the procedure for recording baseline for
multicomponent systems was applied [28]. The width
of the first peak for PCB 29 and PCB 30 were 123.5
and 155.7 mV, respectively, whereas the calculated
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 88 No. 10 2018

2
060
BOYARSKII et al.
Scheme 2.
Cl
Cl
Cl
Cl Cl
Cl
Cl
Cl
Cl
PCB 28 (1)
Cl
PCB 29 (2)
Cl
Cl
PCB 30 (3)
PCB 37 (4)
peak widths of the second and third peaks exceeded
00 mV. These findings suggest that the reaction rate
is controlled by electron transfer. The width of the first
peak of PCB 28 was 49.4 mV, and that of PCB 30,
Controlled
potential
electrolysis
(bulk
2
electrolysis). Controlled potential electrolysis was
performed to estimate regioselectivity of the reduction
of C–Cl bonds in PCBs 1–3 possessing a chlorine atom
in the ortho position with respect to the bridging bond.
In all cases, the substrate conversion did not exceed
2% in order to determine which chlorine atom is
cleaved primarily. The electrolysis products were
analyzed by GC/MS, and the resulting di-
chlorobiphenyls were identified by comparing their
retention times with those of reference compounds. No
monochlorobiphenyls were detected in the reaction
mixtures, which indicated stability of the dichloro-
biphenyls under the electrolysis conditions. The
regioselectivity of the reduction in the first stage is
illustrated by Scheme 3.
9
5.0 mV. The calculated peak widths of the second and
third peaks widths ranged from 63.8 to 266.8 mV,
indicating that the reaction rate is controlled by both
processes.
The ∂ E /∂ log v values for trichlorobiphenyls 1, 2,
p
and 4 ranged from 78.2 to 111.3 mV; this means that
the rate-determining stage is the initial electron
transfer. A conclusion on the reaction mechanism can
be drawn on the basis of the electron transfer
coefficients [29]:
1
.856RT
α =
.
F(Ep/2 − Ep)
Obviously, elimination of chlorine from the ortho
position was preferred in all cases. These data are very
consistent with the selectivity observed in other radical
anion reactions of PCBs studied previously, in
The electron transfer coefficients α (Table 1) for
PCB 28 (first peak) and PCB 37 (first and second
peaks) are higher than 0.5, indicating stepwise
mechanism of the electrochemical reduction, whereas
the α values are lower than 0.5 for PCB 29 and PCB 30
particular chemical reduction with
a
sodium–
naphthalene complex [30], electrochemical reduction
of dichlorobiphenyls [24, 25], and methoxycarbo-
nylation of PCB 28 [13].
(
all peaks), which suggests concerted mechanism of
the process.
Table 1. Parameters of the electrochemical reduction of trichlorobiphenyls 1–4 (voltammetric analysis)
a
PCB 28 (1)
PCB 29 (2)
PCB 30 (3)
PCB 37 (4)
Parameter
1
2
3
1
2
3
1
2
1
2
3
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
E
r
, V
–2.252 –2.379
–2.483 –2.248 –2.397
–2.549 –2.391 –2.496 –2.215 –2.395 –2.554
(
v = 0.1 V/s)
, mV
/∂logv, mV
E
p/2–E
p
49.4
82.3
240
88.2
266.8
111.3
0.179
155.7
95.6
258.4 309–410 123.5
201.4
–
95
63.8
92.3
231.1
87.9
∂
E
p
103.3
0.185
99.7
–
78.2
0.501
α
0.972
0.199
0.306
0.154
0.386
0.237
0.747 0.206
a
3
Parameters for the Cl atom cannot be determined.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 88 No. 10 2018

ELECTROCHEMICAL REDUCTION OF TRICHLOROBIPHENYLS
а) (b)
2061
(
+
+
Potential, V (vs. Ag/Ag )
Potential, V (vs. Ag/Ag )
(
c)
(d)
+
+
Potential, V (vs. Ag/Ag )
Potential, V (vs. Ag/Ag )
Fig. 1. Cyclic voltammograms with subtracted background current for the reduction of (a) 2,4,5-trichlorobiphenyl (PCB 29),
b) 2,4,6-trichlorobiphenyl (PCB 30), (c) 2,4,4'-trichlorobiphenyl (PCB 28), and (d) 3,4,4'-trichlorobiphenyl (PCB 37) recorded
against 0.1 M Bu NClO in MeCN; v = 0.1 V/s.
(
4
4
Quantum chemical calculations. During the past
two decades quantum chemical calculations have been
increasingly used to analyze dissociation processes
with electron transfer in organic halogen derivatives
28, PCB 37) give rise to stable π-type radical anions
(Fig. 2) which occupy local energy minima on the
potential energy surface (the corresponding Hessian
matrix contains no imaginary frequencies). This means
that PCB 28 and PCB 37 should react according to the
stepwise mechanism (path a in Scheme 1).
[31–35]. These calculations make it possible to answer
two important questions: (1) does electrochemical
reduction follow stepwise or concerted mechanism and
We also calculated potential energy surface sections
along the reaction coordinate (C–Cl distance) for the
radical anions derived from PCB 28 and PCB 37
(
2) what is the regioselectivity of the reaction. For this
purpose it is necessary to estimate the relative
stabilities of radical anions, bond lengths therein, and
effective charges on the departing halogen atoms [36].
(
Fig. 3). Obviously, dissociation of the C–Cl bond in
radical anions requires overcoming an energy barrier
corresponding to the transition state between π-radical
anion and aryl radical/chloride ion couple. The
Table 2 contains the corresponding parameters
calculated for PCBs 1–4. Geometry optimization of
radical anions showed that PCB congeners containing
no more than two chlorine atoms in a single ring (PCB
≠
activation barriers ΔE are given in Table 2. The
≠
following trends in the variation of ΔE may be noted.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 88 No. 10 2018

2
062
BOYARSKII et al.
Scheme 3.
Cl
Cl
Cl
Cl
Cl
Cl
1
2
Cl
Cl
Cl
Cl
+
Cl
Cl
2.5:1
Cl
Cl
Cl
Cl
Cl
Cl
3
4
Cl
Cl + Cl
3:1
Cl
Cl
Cl
Elimination of the ortho-chlorine atom from the
disubstituted ring of PCB 28 requires an energy lower
by 4–11 kJ/mol than elimination of the other chlorine
atoms. Elimination of chlorine from the monosub-
stituted ring is least probable for both PCB 28 and
PCB 37 (activation barrier 13–18 kJ/mol). In addition,
Table 2 contains some structural parameters of neutral
PCB 28 and PCB 37 molecules and radical anions
derived therefrom. The lowest activation barrier was
found for that C–Cl bond in the π-radical anion which
is elongated most relative to the corresponding bond in
the neutral molecule. Change of the charge on the
chlorine atom (calculated according to both Mulliken
and NBO method) in going from the neutral molecule
to its radical anion cannot be used to determine the
regioselectivity of the radical anion reduction of PCBs.
thermodynamic stability of the product; however, the
differences in the thermodynamic stabilities of
biphenylyl radicals are insignificant (2.4 kJ/mol).
Obviously, this is reflected in the reduced selectivity of
the first stage of preparative electrolysis (Scheme 3).
Our results are analogous to those obtained
previously for dichlorobiphenyls [24, 25]. The
calculated activation barriers are consistent with the
regioselectivity observed in the preparative electrolysis
of PCB 28 and PCB 37. In both cases, the
regioselectivity correlates with the activation barrier.
Furthermore, the theoretical regioselectivity conforms
to the regioselectivity observed experimentally in
another radical anion reaction of PCB 28, namely
methoxycarbonylation [13].
A relation between the potential barrier and
thermodynamic stability of biphenyl radical formed as
a result of dissociation of the C–Cl bond was observed
for PCB 28 (Table 2). In other words, both kinetic and
thermodynamic parameters suggest preferential elimina-
tion of chlorine from the ortho position of the
disubstituted ring. In the case of PCB 37, the activation
barrier changes in the opposite direction to the
Attempted geometry optimization of π-radical
anions derived from PCB 29 and PCB 30 possessing
three chlorine atoms in the same ring led to structures
whose Hessian matrices contained one imaginary
frequency; i.e., these structures are transition states
rather than intermediate products. Therefore, the
reduction of PCB 29 and PCB 30 is a concerted
process. Comparison of the results of calculations with
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ELECTROCHEMICAL REDUCTION OF TRICHLOROBIPHENYLS
2063
Table 2. Selected structure and energy parameters of trichlorobiphenyls 1–4 and π-radical anions derived therefrom
determined by quantum chemical calculations (solvent acetonitrile)
1
2
3
4
Parameter
C–Cl bond length, Å
1.763 (o) 1.790 (o) 1.760 (о) 1.787 (o) 1.759 (o) 1.807 (о) 1.753 (m) 1.774 (m)
.760 (p) 1.773 (p) 1.748 (р) 1.763 (p) 1.756 (p) 1.781 (р) 1.751 (p) 1.764 (p)
.764 (p') 1.777 (p') 1.749 (m) 1.773 (m) 1.758 (o) 1.797 (о) 1.764 (p') 1.777 (p')
1
1
NBO charge on the
chlorine atom, a.u.
0.004 (o) –0.065 (o) 0.012 (o) –0.058 (o) 0.018 (o) –0.098 (o) 0.019 (m) –0.039 (m)
0.000 (p) –0.062 (p) 0.032 (p) –0.040 (p) 0.014 (p) –0.074 (p) 0.020 (p) –0.038 (p)
–
0.013(p') –0.070 (p') 0.028 (m) –0.040 (m) 0.018 (o) –0.070 (o) –0.013(p') –0.070 (p')
Mulliken charge on the
chlorine atom, a.u.
0.192 (o) 0.075 (o) 0.226 (o) 0.095 (o) 0.198 (o) –0.040 (o) 0.209 (m) 0.153 (m)
0.211 (p) 0.104 (p) 0.271 (p) 0.108 (p) 0.239 (p) 0.065 (p) 0.250 (p) 0.159 (p)
0
.193 (p') 0.098 (p') 0.244 (m) 0.134 (m) 0.199 (o) 0.088 (o) 0.196 (p') 0.114 (p')
a
φ, deg
57
28
58
31
90
44
38
0
≠
b
ΔE , kJ/mol
1.9 (o)
2.7 (p)
13.2 (m)
17.9 (p')
6
.0 (p)
13.1 (p')
c
ΔE, kJ/mol
0 (o) min
6.3 (p)
0 (o) min
8.6 (p)
0 (o) min
8.2 (p)
0 (m) min
2.4 (p)
1
16.3 (p')
9.4 (m)
2.4 (p')
ΔG (electron transfer),
–2.05
–2.06
–1.96
–2.23
eV
a
b
c
Dihedral angle formed by the benzene rings. Energy barrier to the fragmentation of radical anion. Relative energy of radical.
the experimental data on the regioselectivity of
preparative electrolysis showed that relative thermo-
dynamic stability of the electrolysis products is the
main factor responsible for the regioselectivity.
30, the regioselectivity depends on the thermodynamic
stability of the aryl radical formed as a result of
elimination of chloride ion.
In summary, by using four different trichloro-
biphenyls as substrates we showed that electro-
chemical reduction provides a convenient method for
selective hydrodechlorination of PCBs. The chlorine
atom in the ortho position with respect to the bridging
bond is reduced most readily, and next follows para-
and then meta-chlorine atom. The data were obtained
in the framework of a complementary approach
including electrochemical measurements, preparative
electrolysis, and quantum chemical calculations. The
validity of this approach is confirmed by similarity of
the conclusions on the regioselectivity and relative
reactivity, which were drawn on the basis of the results
obtained by different methods.
Thus, in keeping with the theoretical data, the
electrochemical reduction of not all of the examined
trichlorobiphenyls follows the stepwise mechanism.
Congeners containing chlorine atoms in both rings
(
PCB 28 and PCB 37) give rise to stable π-radical
anions as intermediate species, and the reduction is a
stepwise process. Electron transfer to PCB 29 and PCB
3
0 molecules leads to barrierless formation of ortho-
biphenylyl radical/chloride ion couple. The
regioselectivity of the reduction of PCB 28 and PCB
3
7 is determined by the activation barrier to dissocia-
tion of the C–Cl bond in intermediate radical anion,
which increases in going from ortho- to para- and then
to meta-chlorine atom. In the case of PCB 29 and PCB
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 88 No. 10 2018

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064
BOYARSKII et al.
(
а)
(b)
Fig. 2. Optimized geometric structures and HOMO isosurfaces of π-radical anions derived from PCBs (a) 1 and (b) 4.
EXPERIMENTAL
as solvent. GC/MS analysis was performed on a
Shimadzu GCMS QP-2010 SE instrument [electron
impact, 70 eV; a.m.u. range 35–500 Da; ion source
temperature 200°C; Rtx-5MS column, 30 m×0.32 mm,
film thickness 0.25 μm; carrier gas argon, flow rate
0.8 mL/min; oven temperature programming from 50°C
(2 min) to 250°C at a rate of 25 deg/min, 20 min at
250°C]. GLC analysis was performed with a Khromatek
Crystall 5000.2 chromatograph equipped with a flame
ionization detector and an Optima 1 capillary column
(25 m×0.32 mm, film thickness 0.35 μm). Electro-
chemical measurements were carried out on a CHI
Pure trichlorobiphenyls PCB 28 (1), PCB 29 (2),
PCB 30 (3), and PCB 37 (4) and dichlorobiphenyls
authentic samples and internal standards for GLC)
were synthesized by the Suzuki reaction from the
corresponding substituted iodobenzenes and aryl-
boronic acids according to the procedure described in
(
[
18] using a carbene palladium complex [37, 38] as
catalyst. The other reagents and solvents were com-
mercial products (Aldrich) which were used without
additional purification.
6
60A workstation (CH Instruments, USA) using a
1
13
The H and C NMR spectra were recorded on a
glassy carbon working electrode with a diameter of
3 mm (CH Instruments, USA), a platinum auxiliary
electrode, and an Ag/Ag (10 mM) reference pseudo-
1
Bruker Avance II+ spectrometer [400.13 ( H),
00.61 MHz ( C)] at room temperature using CDCl
1
3
+
1
3
(
а)
(b)
d(C–Cl), Å
d(C–Cl), Å
Fig. 3. Potential energy surface cross sections along the reaction coordinate for the fragmentation of π-radical anions of PCBs (a) 1
and (b) 4.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 88 No. 10 2018

ELECTROCHEMICAL REDUCTION OF TRICHLOROBIPHENYLS
2065
electrode. The glassy carbon electrode was polished
with alumina and ultrasonically treated prior to use.
The cyclic voltammograms were recorded using a
single-compartment electrochemical cell at 298 K.
Uncompensated resistance of the solution was
measured before each run, and the ohmic drop was
compensated to a residual value of ~35 Ω.
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Bulk electrolysis. A four-necked electrolysis cell
was filled with a solution of trichlorobiphenyl
(
30 mM) and tetrabutylammonium perchlorate (0.2 M)
in anhydrous acetonitrile (20 mL), and the solution
was purged with argon of ultrapure grade. A constant
+
potential (–2.6 V vs. Ag/Ag ) was applied to the
working electrode under continuous stirring and
slowly bubbling argon through the solution. The
electrolysis was terminated when the current decreased
to a background value. The mixture was poured into
water (30 mL) and extracted with n-hexane (30 mL).
The organic phase was separated, washed with brine
8
(
20 mL), dried over Na SO , and analyzed by GC/MS.
2 4
Quantum chemical calculations were performed
9
in terms of the density functional theory using B3LYP
functional and 6-31+G(d) basis set (Gaussian 09 [39]).
Open and closed shell models were applied to calculate
radical anions and neutral molecules, respectively.
Energy minimization was conducted until a state
without imaginary frequencies was obtained. Solvent
effects were included in terms of the polarizable
continuum model. The potential energy surfaces for
radical anions were scanned with respect to the C–Cl
bond length through a step of 0.05 Å with full
geometry optimization at each step.
1
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1
ACKNOWLEDGMENTS
This study was performed under financial support
by the St. Petersburg State University (project no.
14. Zabelina, O.N., Pervova, M.G., Kirichenko, V.E.,
Yatluk, Yu.G., and Saloutin, V.I., Russ. J. Appl. Chem.,
2
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CONFLICT OF INTERESTS
No conflict of interests was declared by the authors.
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