Electroreduction of polychlorobenzenes using either lead or copper electrodes

Article abstract of DOI:10.1002/jccs.200200084

Polychlorobenzenes can be reduced electrolytically to dichlorobenzenes by using either lead or copper as the electrodes in a MeOH/THF solution. Among the resultants of dichlorobenzenes, 1,4-dichlorobenzene is a major product that might be due to a low enthalpy of formation. A chlorine atom situated at the ortho position of another chlorine atom in the benzene ring is removed prior to others. However, the sequence of reactivities of the polychlorobenzenes for electroreducing by lead electrodes in this study is as follows: 1,2,3,4-C ₆H₂Cl₄ > 1,3,5-C₆H ₃Cl₃ > C₆HCl₅ - 1,2,4,5-C ₆H₂Cl₄ > 1,2,3,5-C₆H ₂Cl₄ - 1,2,3-C₆H₃Cl₃ > 1,2,4-C₆H₃Cl₃ > C₆Cl ₆.

Full text of DOI:10.1002/jccs.200200084

Journal of the Chinese Chemical Society, 2002, 49, 539-544
539
Electroreduction of Polychlorobenzenes Using Either Lead or
Copper Electrodes
Shaw-Tao Lina* (
Sun-Che Linb (
), Richard Yung-Ho Chaoa (
) and Shu-Fan Lina (
),
)
aDepartment of Applied Chemistry, Providence University, Sha-Lu, Taichung Hsien 433, Taiwan, R.O.C.
bDepartment of Chemical Engineering, Hsiuping Institute of Technology,
Taichung Hsien 412, Taiwan, R.O.C.
Polychlorobenzenes can be reduced electrolytically to dichlorobenzenes by using either lead or copper as
the electrodes in a MeOH/THF solution. Among the resultants of dichlorobenzenes, 1,4-dichlorobenzene is a
major product that might be due to a low enthalpy of formation. A chlorine atom situated at the ortho position
of another chlorine atom in the benzene ring is removed prior to others. However, the sequence of reactivities
of the polychlorobenzenes for electroreducing by lead electrodes in this study is as follows: 1,2,3,4-C6H2Cl4 >
1,3,5-C6H3Cl3 > C6HCl5 ~ 1,2,4,5-C6H2Cl4 > 1,2,3,5-C6H2Cl4 ~ 1,2,3-C6H3Cl3 > 1,2,4-C6H3Cl3 > C6Cl6.
Diverse organic chlorides, which are wide industrial
useful but toxic and non-biodegradable, have been widely
used as organic solvents, pesticides, and transformer oils, etc.
Numbers of those compounds are stable under ambient con-
ditions and are classified as persistent organic pollutants
(POPs). The process that converts those compounds into less
toxic structures or renders them biodegradable is a crucial is-
sue in environmental remediation. Hydrodehalogenation of
aromatic chlorides has been intensively studied by using
chemical reduction,1 electroreduction,2 photodegradation,3
and degradation using microorganisms.4 Inherent advantages
of the cathodic dehalogenation are: (a) treatment at ambient
temperature and (b) selective removal of chloride while the
organic skeleton remains to be digested by the biological
route.5 This method is often cheap, technically flexible and
readily allows dechlorination of different chlorinated organic
compounds. However, no work has been reported in detail re-
garding dechlorination of polychlorobenzenes. In the present
study, we attempt to search for the dissolvable electrodes in
order to remove the chlorine atom from polychlorobenzenes.
The initial rates for electrodechlorination of the various
polychlorobenzenes in the presence of LiClO4 as an electro-
lyte are also investigated.
(1:1 volume ratio) solution, low solubility of hexachloroben-
zene leads to presence of suspension solid in this study. Car-
bon fiber, graphite, magnesium, aluminum, copper, and lead
were applied as the electrodes to examine their ability to re-
move chlorine atoms. During the electrolysis, an alternative
polarity on electrodes (change every 30 sec) was applied to
avoid electrodeposition on the electrodes. Among them, car-
bon fiber and graphite are unable to remove any of the chlo-
rine atoms from pentachlorobenzene under 15 V applied volt-
age; however, magnesium and aluminum are readily dis-
solved in MeOH/THF during electrolysis to form the collide
resultants. Lead and copper electrodes demonstrate their abil-
ity to remove chlorine atoms from polychlorobenzenes.
However, the dichlorobenzenes are intact during electrolysis
under the same conditions for more than 600 min.
The reaction rates of electrodechlorination of eight
polychlorobenzenes using copper and lead electrodes are
summarized in Table 1. The copper electrodes displayed less
activities to remove chlorine atoms from tri- and tetrachloro-
benzenes than lead electrodes, while the reaction rates were
comparable to lead in the case of penta- and hexa-chloro-
benzene. Therefore, the lead electrodes were used for this
study. The amount of current depended on the concentration
of electrolytes. High concentration of electrolyte provides
more ionic species and forms high current density. Higher
current density produced more electron flow for the reduc-
tion of polychlorobenzenes and led to higher reaction rates
(Table 1) as well as generating more heat. The concentrations
of LiClO4 of 30 mM were used for this study. The relative re-
action rates and the half-wave potentials for chlorinated
benzenes are summarized in Scheme I, where the larger ar-
RESULTS AND DISCUSSION
The solubilities of the polychlorobenzenes in MeOH
decrease upon the increase of the chlorine atoms on the ben-
zene. THF is added to MeOH as a cosolvent to enhance the
solubilities of polychlorobenzenes. Even in the MeOH/THF

540 J. Chin. Chem. Soc., Vol. 49, No. 4, 2002
Lin et al.
a
Table 1. Rate of Hydrodechlorination of Polychlorobenzenes in MeOH/THF Function on the Concentration of LiClO4
Electrode LiClO4
(mM)
reactants
1,2,3-
C6H3Cl3
1,2,4-
C6H3Cl3
1,3,5-
C6H3Cl3
1,2,3,4-
C6H2Cl4
1,2,3,5-
C6H2Cl4
1,2,4,5-
C6H2Cl4
C6HCl5
C6Cl6
Cu
Pb
Pb
Pb
10
10
30
50
0.008±0.002 0.004±0.001 0.007±0.002 0.015±0.004 0.020±0.003 0.010±0.002 0.030±0.003 0.016±0.002
0.032±0.004 0.023±0.004 0.042±0.003 0.047±0.003 0.033±0.003 0.037±0.003 0.037±0.002 0.020±0.003
0.078±0.004 0.057±0.004 0.070±0.004 0.110±0.007 0.072±0.004 0.071±0.003 0.070±0.003 0.062±0.002
0.102±0.006 0.072±0.003 0.103±0.008 0.147±0.006 0.110±0.006 0.090±0.002 0.150±0.004 0.115±0.008
a Reactant (10.0 mmol), internal standard (n-dodecane, 5.0 mmol) in 100 mL (50/50: MeOH/THF) solution, initial electrode area in the
solution is 100 mm2, the average reaction rates (mM/min) were determined at first 10% consumed from three experiments.
Scheme I Relative rates of electrolytic dechlorination of polychlorobenzenes. The heavier arrows indicate higher rates
in that pathways and the dotted arrows indicate no reaction in that pathway. The values in the parenthesis rep-
resented the half-wave potentials9
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
(-1.96)
Cl
(-2.22)
(-1.76)
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
(-2,44)
(-2.20)
(-1.57)
(-1.76)
(-1.81)
(-1.32)
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
(-1.98)
(-2.20)
(-1.79)
rows represent the higher initial rates. According to the litera-
ture, the half-wave potentials were obtained by cyclic vol-
tametry in 0.1 M (C2H5)4N+Br- in dimethylsulfoxide. The re-
duction potentials were determined from the differential i-E
curve against SCE.9 From the scheme, we could find that the
initial rates for removing the chlorine atom from chlorinated
benzenes strongly depended on the half-wave potentials. The
half-wave potentials for dichlorobezenes and chlorobenzene
are smaller than -2.00 volts and were inert during the electro-
reduction in this study.
sumed polychlorinated benzenes were in balance with those
of the products, indicating that stoichiometric dechlorination
occurred in this system. No other by-products other than the
partially dechlorinated benzene derivatives were detected,
indicating that this electrochemical reaction was very selec-
tive. The product distributions from electrolytic hydrode-
chlorination of polychlorobenzenes for 300 min are summa-
rized in Fig. 1 using bar graphics. It is found in the figure that
the predominant products were 1,4-dichlorobenzene deriva-
tives and 1,2,4-trichlorobenzene derivatives, suggesting the
formation of common intermediates in the dechlorination
process for electrochemical systems.
The progress of reactions were monitored by GLC anal-
yses. Under our analytic conditions, all the chlorinated ben-
zenes along with n-dodecane were well separated except for
1,2,3,5- and 1,2,4,5-tetrachlorobenzenes (Fig. 1). In this cir-
cumstance, only an uncertainty could be encountered in the
electrolysis of pentachlorobenzenes. The amounts of con-
During the electrolysis of trichlorobenzenes, 1,3-di-
chlorobenzene was formed as the sole product from 1,3,5-
troichlorobenzene, while 1,4-dichlorobenzene formed as a
major with 1,3-dichlorobenzene as a minor, and 1,3-dichloro-

Electrodechlorination of Polychlorobenzenes
J. Chin. Chem. Soc., Vol. 49, No. 4, 2002 541
benzene formed as the major with 1,2-dichlorobenzene as a
minor for 1,2,4-, and 1,2,3-trichlorobenzenes, respectively.
In the latter two cases, it is suggested that the chlorine atoms
possessing a chlorine atom at the ortho-position will be acti-
vated by each other. Although a chlorine at position-2 of
1,2,3-trichlorobenzene will have more steric hindrance than
others and with 1/3 possibility compared to the 2/3 possibility
for others by accounting for the number of atoms, it seems to
be more likely due to the activation by two chlorine atoms.
This activation phenomenon is the same as the observation
from electrolysis of polyfluorobenzenes using mercury elec-
trodes.6
tion patterns from electrolysis of 1,2,3-trichlorobenzenes,
dechlorination of 1,2,3,5-tetrachlorobenzenes may yield the
production of 1,3,5-trichlorobenzene. (Fig. 4) However, a
chlorine atom is able to activate the meta-position, an electro-
philic substitution of benzene ring, to form 1,2,4-trichloro-
benzene as an initial product.
Electrolysis of pentachloro- and hexachlorobenzenes
led to the same product distribution, while 1,2,4,5-tetrachloro-
and 1,2,4-trichlorobenzenes were formed at the initial time
and 1,4-dichlorobenzene was a final product. (Figs. 5, 6)
Electrolysis of hexachlorobenzene led to 1,2,4-trichloro-
benzene and 1,2,4,5-tetrachlorobenzene as the initial prod-
ucts followed by formation of 1,4-dichlorobenzene. No
pentachlorobenzene observed from GC analysis with forma-
tion of trichlorobenzene as an initial product might indicate
that the polychlorobenzenes are absorbed on the electrode
surface and can remove more than two chlorine atoms before
being desorbed by changing the electrode polarity. Penta- and
hexachlorobenzene were selectively and stepwise de-
chlorinated to form less-chlorinated homologues, through
tetrachlorobenzene, and trichlorobenzene to dichloroben-
zene as the final products.
The chlorine possessing an ortho-chlorine atom is more
easily removed as the situation of trichlorobenzenes is also
observed in the tetrasubstituted compounds. That 1,4-dichlo-
robenzene was formed along with the formation of 1,2,4-
trichlorobezene during electrolysis of 1,2,3,4-tetrachloro-
benzene might suggest that two chlorine atoms were removed
before the chlorinated benzenes desorbed from the elec-
trodes. (Fig. 3) The same reaction results were reported on
dechlorination of 1,2,3,4-tetrachlorobezene photocatalyzed
by ZnS under UV light.7 According to the product distribu-
Fig. 1. Chlorinated benzenes were analyzed by gas chromatography (Hewlett Packard 6890 Series with the autosampler; col-
umn: Alltech ECONOCAPTM #19657 SE-54, 30 m length, 0.53 mm i.d. with 1.2 m film thickness and FID). The flow
rate of the carrier gas (nitrogen) was 26.3 mL/min, the temperature program started at 50 C (constant for 3 min),
ramped at 5 C/min up to 110 C (constant for 3 min), at 3 C/min up to185 C (constant for 1 min), and at 60 C/min
up to 200 C. (1) toluene; (2) monochlorobenzene; (3) 1,3-dichlorobenzene; (4) 1,4-dichlorobenzene; (5) 1,2-dichlo-
robenzene; (6) 1,3,5-trichlorobenzene; (7) 1,2,4-trichlorobenzene; (8) n-dodecane; (9) 1,2,3-trichlorobenzene;
(10,11) 1,2,3,5-tetrachlorobenzene+1,2,4,5-tetrachlorobenzene; (12) 1,2,3,4-tetrachlorobenzene; (13) pentachloro-
benzene; (14) hexachlorobenzene.

542 J. Chin. Chem. Soc., Vol. 49, No. 4, 2002
Lin et al.
Fig. 2. Product distribution from electroreductive dechlorination of various polychlorobenzenes for 6 hrs on lead cathode in
the solution containing 30 mM LiClO4 as an electrolyte (current density = 10 mA/mm2).
CONCLUSION
tials must be larger than -2.00 volt for the reduction to take
place. That 1,4-dichlorobenzene is always formed as a final
product might be due to the lowest enthaphy of formation
among three isomers (7.89, 6.72, and 5.88 Kcal/mol for 1,2-,
1,3-, 1,4-dichlorobenzene, respectively)8 which, in turn, indi-
During the electrolysis of polychlorobenzenes, the ac-
tivities of chlorinated benzenes toward electroreduction de-
pended on their half-wave potentials. The half-wave poten-
12.00
12.00
1,3-dichlorobenzene
1,4-dichlorobenzene
10.00
8.00
6.00
4.00
2.00
0.00
1,3-dichlorobenzene
1,4-dichlorobenzene
10.00
8.00
6.00
4.00
2.00
0.00
1,2,4-trichlorobenzene
1,2.3,4-tetrachlorobenzene
1,2,4-trichlorobenzene
1,2,3,5-tetrachlorobenzene
0
50
100
150
200
250
300
350
0
20
40
60
80
100
120
140
160
180
200
Time of Reaction (min)
Time of Reaction (min)
Fig. 3. Electroreductive dechlorination of 1,2,3,4-tet-
rachlorobenzene on lead plate cathode in the so-
lution containing 30 mM LiClO4 as an electro-
lyte.
Fig. 4. Electroreductive dechlorination of 1,2,3,5-tetra-
chlorobenzene on lead plate cathode in the solu-
tion containing 30 mM LiClO4 as an electrolyte.

Electrodechlorination of Polychlorobenzenes
J. Chin. Chem. Soc., Vol. 49, No. 4, 2002 543
zene, n-dodecane (as internal standard) and LiClO4 were well
mixed by means of ultrasonic irradiation for 5 min before
electrolysis. The electrodes were polished with sandpaper
(400 mesh) and then washed with water, acetone, and fol-
lowed by drying. The terminal potential of 15 volts was ap-
plied on a pair of lead electrodes (10 mm 1 mm 40 mm,
distance between two electrodes 2.0 cm, surface area immer-
sion in the solution about 400 mm2) and the electrode polari-
ties were changed every 30 sec. The aliquots (1.0 mL) were
sampled for tracing the progress of reactions every 30 min.
The samples were directly analyzed with a Hewlett Packard
6,890 series II gas chromatograph (column Alltech Econo-
CapTM SE-54, 30 m 0.53 m 1.2 m film thickness). The
flow rate of the carrier gas (nitrogen) was 26.3 mL/min; the
temperature program started at 50 C, (held for 3 min), heated
at 5 C /min to 110 C, (held for 3 min), heated at 3 C /min to
185 C, (held for 1 min), heated to 200 C, (held for 1 min).
The product ratios were calculated based on the peak areas
and corrected with response factors of products. The injector
and the FID detector were held at 250 C. A Finnigan MD-
800 GC/MS was used to confirm peak identity. The reaction
rates were measured according to the diminishing of reactant
at first 10% consumption.
12.00
1,4-dichlorobenzene
1,2,4-trichlorobenzene
10.00
8.00
6.00
4.00
2.00
0.00
1,2,3,5-tetrachlorobenzene+1,2,4,5-tetrachlorobenzene
pentachlorobenzene
0
50
100
150
200
250
300
350
400
Time of Reaction (min)
Fig. 5. Electroreductive dechlorination of pentachloro-
benzene on lead plate cathode in the solution
containing 30 mM LiClO4 as an electrolyte.
12.00
1,4-dichlorobenzene
1,2,4-trichlorobenzene
1,2,3,5-tetrachlorobenzene+1,2,4,5-tetrachlorobenzene
hexachlorobenzene
10.00
8.00
6.00
4.00
2.00
0.00
0
50
100
150
200
250
300
350
Time of Reaction (min)
Fig. 6. Electroreductive dechlorination of hexachloro-
benzene on lead plate cathode in the solution
containing 30 mM LiClO4 as an electrolyte.
ACKNOWLEDGMENT
Financial support by the National Science Council of
the Republic of China is gratefully acknowledged.
cates a thermodynamic control process is operated for the
product distribution. Strong absorption of polychloroben-
zenes by electrodes lead to more than two chlorine atoms be-
ing removed before the products desorbed during the change
of electrode polarity.
Received December 4, 2001.
Key Words
Electroreduction; Polychlorobenznes; Lead
electrodes; Copper electrodes; Relative initial
rates.
EXPERIMENTAL
Electrolysis of Polychlorobenzenes
Typical procedure (precautions must be taken against
spark formation of locate overheating caused by faulty elec-
trical connections. During electrolysis, potential explosive
mixtures of gases may be formed (H2, O2, Cl2 etc.). Protective
measures must be taken to eliminate the danger of explo-
sions. Dried perchlorates are potential explosive compounds;
overheating a mixture containing perchlorate must be avoided.
All reactions were performed in an undivided glass cell with
a magnetic stirrer and a water-circulation system to control
the temperature at 25 3 C. Solutions of polychloroben-
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