Electroreduction of Organic Compounds, 34 [1]. Cathodic Dehalogenation of Chloroarenes with Electron-Donating Substituents

Article abstract of DOI:10.1515/znb-2003-1206

The electrochemical reduction of chlorinated arenes with electron-donating substituents, i.e. chlorotoluenes, -anisoles and -phenols, is studied. Preparative electrolyses are run in various solvent-supporting electrolytes under potentiostatic and galvanostatic conditions at lead or carbon cathodes. A partial and mostly regioselective hydrodechlorination of compounds with two or more chloro substituents is possible under suitable conditions. The replacement of one single chloro substituent, in particular in a para-position, is difficult. Highly toxic and persistent oligochloro derivatives are thus transformed into less problematic compounds with a low degree of chlorination. The chlorine content of real-life materials such as extracts of soil contaminated with chlorinated phenols and Nitrofen can also be significantly decreased by electroreduction.

Full text of DOI:10.1515/znb-2003-1206

Electroreduction of Organic Compounds, 34 [1].
Cathodic Dehalogenation of Chloroarenes with Electron-Donating
Substituents
Olaf Kranz and Jürgen Voss
Institut für Organische Chemie der Universität Hamburg,
Martin-Luther-King-Platz 6, D-20146 Hamburg, Germany
Reprint requests to Prof. Dr. Jürgen Voss. Fax: +49 (0) 40 42838 5592.
E-mail: voss@chemie.uni-hamburg.de
Z. Naturforsch. 58b, 1187 – 1200 (2003); received September 16, 2003
The electrochemical reduction of chlorinated arenes with electron-donating substituents, i. e.
chlorotoluenes, -anisoles and -phenols, is studied. Preparative electrolyses are run in various solvent-
supporting electrolytes under potentiostatic and galvanostatic conditions at lead or carbon cathodes.
A partial and mostly regioselective hydrodechlorination of compounds with two or more chloro sub-
stituents is possible under suitable conditions. The replacement of one single chloro substituent, in
particular in a para-position, is difficult. Highly toxic and persistent oligochloro derivatives are thus
transformed into less problematic compounds with a low degree of chlorination. The chlorine content
of real-life materials such as extracts of soil contaminated with chlorinated phenols and Nitrofen® can
also be significantly decreased by electroreduction.
Key words: Electrolysis, Dehalogenation, Chloroarenes
Introduction
are also environmentally relevant compounds. They
are expected to be more difficult to reduce because
their electron density is decreased. We present here
our results on the cathodic reduction of these classes
of compounds.
Now as before, chlorinated organic compounds play
a role in technical syntheses as they represent useful
materials with favourable properties which are suitable
intermediates for synthesizing follow-up compounds
with wide-spread applications. However, these prod-
ucts themselves and some of the by-products such as
polychlorinated dibenzofurans and dibenzo-p-dioxins
which are formed during the technical syntheses and
during uncontrolled incineration processes are not only
toxic but also in many cases very persistent. Some of
them have become nearly ubiquitous and the develop-
ment of methods for their degradation is therefore im-
portant and necessary [2 – 4].
Over the last decade we have investigated the
cathodic reduction of chlorinated arenes such as
oligochlorobenzenes [5, 6], polychlorinated biphenyls
(PCB) [5, 6], naphthalenes (PCN) [6, 7], dibenzofurans
(PCDF) [6, 8, 9], and dibenzo-p-dioxins(PCDD) [6, 8].
Besides the direct electroreduction we also studied a
Ni(0)-mediated variant [6, 10] and the electrocarboxy-
lation process [1, 6]. All these substrates are more or
Results and Discussion
Electroreduction of chlorinated toluenes
Chlorinated toluenes, e. g. 2-chloro- (2) and 4-
chlorotoluene (4), are especially used for the indus-
trial production of pesticides and dyes [11]. Polychlo-
rinated toluenes have also been proposed as substitutes
for PCB in hydraulic systems [12]. Suitable procedures
for the degradation of these compounds are therefore
required. For example, chlorotoluenes can be dechlori-
nated by catalytic hydrogenation [13 – 17].
Having performed some preliminary experiments
14 years ago [18], we have now studied the cathodic
dechlorination of the monochloro- (2 – 4), dichloro-
(5 – 7), and trichlorotoluenes (8 – 10) in a comprehen-
sive way.
Table 1 shows the polarographic half-wave reduc-
less electron-deficient compounds which exhibit rel- tion potentials of chloroarenes 2–10. Expectedly, the
atively low reduction potentials. On the other hand, number of steps corresponds to the number of chloro
chlorinated arenes with electron-donating substituents substituents. According to the cyclic voltamograms all
c
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1188
O. Kranz – J. Voss · Electroreduction of Organic Compounds, 34
Table 1. Polarographic half-wave potentials E1/2 vs. Ag/AgBr
of chlorotoluenes and chloroanisoles in DMF.
Compound
E1 [V]
/2
2-Chlorotoluene (2)
3-Chlorotoluene (3)
4-Chlorotoluene (4)
−2.10
−2.09
−2.10
2,3-Dichlorotoluene (5) −2.10 −1.89
2,4-Dichlorotoluene (6) −2.09 −1.84
3,4-Dichlorotoluene (7) −2.10 −1.88
2,3,4-Trichlorotoluene (8) −2.08 −1.82 −1.53
2,4,5-Trichlorotoluene (9) −2.10 −1.85 −1.62
2,4,6-Trichlorotoluene (10)−2.10 −1.83 −1.60
2-Chloroanisole (12)
3-Chloroanisole (13)
4-Chloroanisole (14)
−2.04
−2.03
−2.05
2,3-Dichloroanisole (15) −2.05 −1.81
2,4-Dichloroanisole (16) −2.03 −1.77
3,4-Dichloroanisole (17) −2.06 −1.78
3,5-Dichloroanisole (18) −2.04 −1.80
2,3,4-Trichloroanisole (19)−2.02 −1.73 −1.49
2,4,6-Trichloroanisole (20)−2.04 −1.74 −1.51
2,3,4,5-Tetrachloroanisole −2.06 −1.78 −1.58 −1.33
2,3,4,6-Tetrachloroanisole −2.06 −1.78 −1.61 −1.35
2,3,5,6-Tetrachloroanisole −2.08 −1.84 −1.61 −1.37
Pentachloroanisole (21) −2.08 −1.82 −1.61 −1.35 −1.10
Fig. 1. Results of the electroreduction of the monochloro-
toluenes 2 – 4.
Fig. 2. Course of the potentiostatic electroreduction of 2,3-
dichlorotoluene (5) at −2.10 V in 0.1 M methanolic TEAB
solution.
steps are irreversible, i. e., the electron uptake is ac-
companied by the rapid elimination of a chloride an-
ion. An inspection of the reduction potentials does not
allow a prediction of any regioselectivity.
Equimolar mixtures of the three isomers 2 – 4 were
electrolysed in methanol on a preparative scale under
potentiostatic and galvanostatic conditions. Tetraethyl-
ammonium bromide (TEAB) or potassium hydroxide
were used as electrolyte. The results are shown in
Fig. 1.
tho-methyl substituent. The total product yields were
64% of toluene (1) at −2.20 V vs. Ag/AgBr and 79% of
1 at −2.30 V vs. Ag/AgBr, and the current efficiencies
were 7 – 8% for the electrolyses under potentiostatic
conditions. Due to the volatility of toluene (1), the re-
covery was only 60 – 70% depending on the duration of
the electrolyses. The yield of 1 was only 18% when the
Obviously the dechlorination of 3-chlorotoluene electroreduction was performed under the more prac-
(3) and 4-chlorotoluene (4) occurs to a similar ex- tical galvanostatic conditions in methanolic potassium
tent whereas the turnover of 2-chlorotoluene (2) is hydroxide solution. The selectivity in favour of the de-
lower. The same order, meta = para > ortho, was ob- halogenation of 3 and 4 was more pronounced in this
served for the hydrogenation under rhodium- [14] or case. Due to the predominant hydrogen evolution, the
palladium/C-catalysis [15]. This selectivity can be ex- current efficiency for the desired dechlorination pro-
plained by an effect of steric hindrance through the or- cess dropped to only 1%.
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O. Kranz – J. Voss · Electroreduction of Organic Compounds, 34
1189
Table 2. Electroreduction of the
dichlorotoluenes 5 – 7 and the
trichlorotoluenes 8 – 10 with
Pb-cathodes.
Substrate
Charge Recovered
consumption substrate
Product distribution [%]
Current
efficiency
a
[F mol−1
7.1b
34c
]
[%]
6
2
3
31
8
4
–
–
1
23
18
n.d.d
2
[%]
33
4
46
3
30
5
12
5
5
5
6
6
7
7
8
8
9
–
–
–
–
–
40
25
6
32
–
49
46
47
36
36
n.d.d
8
2.3b
33c
–
–
12
21
5
2
21
–
48
19
46
26
60
12
37
28
4.7b
35c
6
–
–
17
23
3
36b
n.d.d
59e
11f
25g
12
16
26
45
36c
18b
n.d.d
2
4
20
19
10
18b
n.d.d
a
Dichlorotoluenes formed from the trichlorotoluenes; b potentiostatic conditions (−2.10 to −2.20 V
c
vs. Ag/AgBr, cf. Experimental), 0.1 M TEAB in MeOH; galvanostatic conditions (1.0 A,
509 A m−2), 1 M KOH in MeOH; not detectable; 3% 5 + 55% 6 + 1% 7; 4% 6 + 1% 7 +
d
e
f
6% 2,5-dichlorotoluene; g 6% 6 + 19% 2,6-dichlorotoluene.
In order to learn more about the regioselectivity
we studied the dechlorination of the dichlorotoluenes
5 – 7. The reduction potential was adjusted at −2.10 V.
In addition, galvanostatic electrolyses were performed.
The results are compiled in Table 2.
No significant product selectivity was observed in
the case of 2,3-dichlorotoluene(5). Since, however, the
ratio of the three products 1, 2, and 3 remained nearly
constant during the electrolysis (cf. Fig. 2 which corre-
sponds to entry 1 in Table 2), again a preferred reduc-
tion of the meta-substituent is probable.
A turnover of 51% of 5 and a current efficiency
of 4% was achieved in methanolic potassium hydrox-
ide solution under galvanostatic conditions. The elec-
troreduction of 2,4-dichlorotoluene (6) led to 4 as the
main product. Obviously, in this case the steric ef-
fect of the methyl group is over-compensated by the
electronic substituent effect of the second chloro sub-
stituent. As one would expect, the electroreduction of
3,4-dichlorotoluene (7) also yielded 46% of 4 as the
main product besides 12% of 3 and 6% of 1. Galvano-
static electroreduction of 6 and 7 led to mixtures of
products. Possibly due to the absence of the easily re-
ducible intermediate 3, only a very small amount of 1
was formed from 6.
Finally, the trichlorotoluenes 8 – 10 were elec-
trolysed. The potentiostatic reduction of 2,3,4-
trichlorotoluene (8) at −2.20 V (entry 7 in Table 2)
was remarkably regioselective. Mainly 4 was formed
with 60% yield besides considerable 23% of toluene
(1) but only 12% of 3 and 5% of 2. The successive
Fig. 3. Course of the potentiostatic electroreduction of 2,3,4-
trichlorotoluene (8) at −2.20 V in 0.1 M methanolic TEAB
solution.
to entry 7 in Table 2). The galvanostatic reduction of
2,3,4-trichlorotoluene (8) (entry 8 in Table 2) led to 6
as the main product (55%).
When 2,4,5-trichlorotoluene (9) was electrolysed at
−1.65 V a partial dechlorination occurred which was,
however, not regioselective. Continuation of the elec-
trolysis at −2.10 V (entry 9 in Table 2) led to the
expected products 4 (37%), 2 (26%), 3 (21%) and 1
(4%). The electroreduction of 2,4,6-trichlorotoluene
(10) at −1.65 V resulted in the formation of 2,6-
dichlorotoluene (33%) and 6 (8%); this mixture was
further dechlorinated at −2.10 V (entry 10 in Table 2)
to form, finally, 2 (45%) and 4 (28%).
Electroreduction of chlorinated anisoles
The single electron transfer, which is the initial
elimination of chloro substituents thus occurs via 6 and step of the electroreduction of an arene, is impeded
4 whereas 5 and 7 are minor products even during the by the strong +M-effect of a methoxy substituent.
course of the electrolysis (cf. Fig. 3 which corresponds The elimination of chloride anions from chloroanisoles
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1190
O. Kranz – J. Voss · Electroreduction of Organic Compounds, 34
Table 3. Potentiostatic electroreduction (−2.20 V vs. Ag/
AgBr) of the monochloro- 12 – 14, dichloro- 15 – 18 and
trichloroanisoles 19 and 20 in methanolic TEAB solution at
a Pb-cathode.
Substrate
Charge
Product distribution [%]
Current
consumption
efficiency
[F mol−1
31
]
12
62
–
–
–
13
–
21
–
14
–
–
89
75
–
> 99
92
–
–
–
90
79
> 99
11
38
79
[%]
2.5
5.5
< 1
8
6.5
5
6
8
5
5
9
12
13
31
31
48
38
38
38
38
58
58
45
16
45
14a
14b
15
16
17
11
–
20c
20 – 27
n.d.d
5
20 – 26 48 – 60
< 1
–
–
–
–
1
4
< 1
Fig. 4. Cyclic voltamogram of pentachloroanisole (21) in
–
3
52
45
64
2
DMF at a scan rate of 400 mV s−1
.
18e
18e
18f
19g
19h
20
48
55
36
5
should therefore occur at substantially more negative
potentials as compared with arenes carrying electron-
withdrawing substituents such as chlorobenzoic esters
[19]. Unexpectedly however, this effect turned out to
be not more pronounced than that of a methyl sub-
stituent in the chlorotoluenes as demonstrated by the
polarographic reduction potentials shown in Table 1.
One reduction step is found for each chloro sub-
stituent in the molecule. This is exemplified in Fig. 4
showing the five irreversible reduction steps of pen-
tachloroanisole (21). The first reduction potential is
shifted to less negative values with an increasing num-
ber of chloro substituents (200– 300 mV each), but
no significant effect of the position of the substituents
on the half-wave potentials is observed (cf. Table 1).
Regioselective dechlorination was found when the se-
lected congeners 12 – 21 were electrolysed on a prepar-
ative scale under protic conditions [20].
1
–
13
26
9
n.d.d
a
b
c
200 mg 14; 1000 mg 14; + 4% 1-chloro-4-methoxy-1,4-cy-
clohexadiene; d not detectable; e membrane: Permion 4035; f mem-
brane: Reichelt MA 3475; g 200 mg 19; h 600 mg 19.
count of the +M-effect of the methoxy group [21]. The
yields of anisole (11) were 79%, 38%, and 11%, re-
spectively, when 3-chloro- (13), 2-chloro- (12), and 4-
chloroanisole (14) where electroreduced with the same
consumption of electricity (31 F mol−1). The current
efficiencies of only 1 – 6% could be improved to 8 –
15% by increasing the normally applied substrate con-
centrations of 14 mM to 70 mM. In case of 14 also the
chemical yield increased from 11% to 20% of 11.
As it was expected from the results above, 14 was
also formed as the main final product by the electro-
lysis of 2,4-dichloro- (16), 3,4-dichloro- (17), 2,3,4-
trichloro- (19) and 2,4,6-trichloroanisole(20), whereas
2,3-dichloroanisole (15) gave a mixture of 11, 12, and
13. About equal amounts of 11 and 13 were formed
from 3,5-dichloroanisole (18). For comparison, we
varied the charge consumption and used two different
anion exchange membranes in this case. However, the
result was not improved substantially. All yields are
compiled in Table 3.
According to its electroanalytical data (cf. Table 1
and Fig. 4), pentachloroanisole (21) should rather eas-
ily release three or four chloro substituents on elec-
troreduction. In fact, the potentiostatic electrolysis at
−2.20 V predominantly led to 4-chloroanisole (14)
(51%), and anisole (11) (26%), with smaller amounts
In general, the observed order of dechlorination was of 2-chloro- (12, 5%) and 3-chloroanisole (13, 14%).
meta > ortho > para as one would expect on ac- As we have found in other cases [6], a trace (< 2%) of
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O. Kranz – J. Voss · Electroreduction of Organic Compounds, 34
1191
Table 4. Potentiostatic elec-
troreduction (−2.20 V vs. Ag/
AgBr) of an equimolar mixture
of the three chloroanisoles 12 –
14 at a lead or carbon cathode
in varying SSE.b−d
Cathode
material
Substrate
amount
[mmol]
2.1
10.9
2.1
2.2
10.0
10.8
Charge
Total
turnover
[%]
32
9
Relative turnover [%]
Current
efficiency
[%]
2
2
0.2
1
10
3
a
consumption
[F mol−1
35
]
12
31
8
13
37
14
29
7
Leadb
Leadc
Leadd
Carbonb
Carbonb
Carbonc
a
8.5
40
30
9.3
12
4
n.d.e
19
n.d.e
23
n.d.e
11
38
9
18
48
14
c
52
15
56
20
8.6
Total yield of 11; b 0.1 M TEAB in MeOH; 0.1 M TEAB in MeOH/H2O 80:20 (v/v); 0.1 M
d
TEAB in MeOH/H2O 50:50 (v/v); e not detectable.
heavy metal, is not a good choice in view of a possible
contamination of the environment. We explored there-
fore the suitability of carbon felt as cathode material,
which has also been used for the electrodechlorination
of dichloro- and trichlorophenoxyacetic acids [22]. In
addition, we wanted to substitute our standard solvent-
supporting electrolyte (SSE), TEAB in dry methanol,
and tried to use the cheaper sodium bromide or even
potassium hydroxide in aqueous methanol instead. Ta-
ble 4 shows the results of our experiments with an
equimolar mixture of 12, 13, and 14 as the test sub-
strate.
The product distribution was determined by GLC.
The turnover degree was calculated for each compo-
nent separately. The amount of 11 as well as the cur-
rent efficiencies correspond to the total turnover of the
particular electrolysis. The data of Table 4 demonstrate
that the total turnover and the current efficiencies, as
referred to the rate of dehalogenation, significantly de-
crease with increasing content of water in the elec-
trolyte, irrespective of the cathode material and the
concentration of starting material used. Due to the en-
hanced decomposition of water, the turnover dropped
to 4% in 50% aqueous methanol and no dechlorina-
tion at all took place in pure water. The use of a carbon
felt cathode instead of a lead foil also led to a decrease
of the turnover from 32% to 18%. It could, however,
be improved to 48% at 10% current efficiency by ap-
plying a higher substrate concentration (entry 5 in Ta-
ble 4). We have observed this effect in other cases too
(see above). It may be due to a decrease of the protic
nature of the double layer. The observed order of the
dechlorination rates was meta > ortho > para in all
cases.
Fig. 5. Course of the potentiostatic electroreduction of pen-
tachloroanisole (21) at −2.20 V in 0.1 M methanolic TEAB
solution.
a hydrogenated species, possibly the Birch-type prod-
uct 1-chloro-4-methoxy-1,4-cyclohexadiene, was de-
tected. The current efficiency was 10%, i. e. higher than
in the case of anisoles with fewer chloro substituents.
Fig. 5 shows a typical course of an electrolysis of 21.
The formation of 14 as the main final product is
obvious, whereas its two isomers 12 and 13 appear
as intermediates, which are further reduced to anisole.
Only small amounts of dichloro-, trichloro- and tetra-
chloroanisoles (< 5% in total) can be detected during
the starting phase of the electrolysis. Even at a reduc-
tion potential of only −1.10 V it was not possible to
eliminate one chloro substituent selectively. Instead,
a very complex product mixture was obtained. Once
again, it contained 31% of 14 and 21% of 16 + 20, i. e.,
products that result from the predominant elimination
of meta-chloro substituents. Only 3% of different tetra-
chloroanisole isomers were detected (cf. Experimental
Section).
In the case of chlorinated anisoles, we have per-
formed electrolyses under varying experimental condi-
tions (cf. Table 4 and 5) in order to find a method which
would be more suitable for practical applications. The
commonly used lead cathodes tend to be corroded dur-
Further experiments aimed at an optimisation of
the electrolyses were performed with 2,3,4-trichloro-
anisole (19). The results are shown in Table 5.
As expected, a higher degree of dehalogenation (i. e.
ing the electrolysis. Furthermore lead, being a toxic the sum of the weighted percentages of dichloro-,
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1192
O. Kranz – J. Voss · Electroreduction of Organic Compounds, 34
Table 5. Electroreduction of 2,3,4-trichloroanisole 19 under varying conditions.
Cathode
Charge
Degree of
Product distribution [%]
Current
material consumption
dechlorination
efficiency
[F mol−1
16
]
[%]
68
54
66
35
53
42
21
42
35
58
16
< 1
25
1
61
4
19
32
34
29
11
17
< 1
14
< 1
31
3
15
24
34
32
13
14
79b
54
91
7
54
38
3
11
13
4
[%]
26
20
25
7
20
11
2
Leada
Carbona
Leadc
16
16
29
16
2
Carbonc
Carbonc
Leade
n.d.d
14
2
24
Carbone
Leadf
52
n.d.d
1
106
189
711
25
20
62h
2
1
0.5
Carbonf
Leadg
< 1
2
a
Potentiostatic conditions (−2.20 V vs. Ag/AgBr), 0.1 M TEAB in MeOH;b + 4% 12 and 1% 13; c potentiostatic conditions (−2.20 V vs.
Ag/AgBr), 0.1 M TEAB in MeOH/H2O 80:20 (v/v); d not detectable; e potentiostatic conditions (−2.20 V vs. Ag/AgBr), 1 M NaOH + 0.1
M NaBr in MeOH/H2O 80:20 (v/v); f galvanostatic conditions (2.0 A or 3.0 A), 1 M KOH in MeOH; g galvanostatic conditions (3.0 A), 1
M KOH in MeOH/H2O 75:25 (v/v); h + 1% 12 + 9% 13.
monochloro- and unsubstituted anisoles) was achieved [(36), cf. Electroreduction of real-life material], as pro-
with a lead cathode as compared with the carbon felt. tecting material for wood, and for other purposes [23].
The amount of remaining dichloroanisoles was signi- Although the production of chlorinated phenols is de-
ficantly higher when carbon felt was used. Similar re- clining and has been totally discontinued in several
sults were observed after the addition of 20% of wa- countries, they still remain severe environmental pol-
ter to the solvent. The electrolyses at carbon cathodes lutants because of their pronounced persistency and
were also less reproducible due to the ill-defined sur- toxicity [24], their ubiquitous occurrence and because
face of fibre tissue and hence the varying current densi- they represent the precursors of the polychlorinated
ties. In addition, the lower overall conductivity of car- dibenzofurans and dibenzo-p-dioxins. The biological
bon compared to a metal led to a considerable warming [25,26] and the chemical degradation of chlorinated
in case of high currents applied.
phenols by reductive [27 – 30] or oxidative [31, 32]
processes have therefore been the target of several dif-
ferent investigations. Electrochemical procedures have
also been studied [33]. Besides the anodic oxidation
[34 – 38], which seemed promising for the degrada-
tion of mono- and dichlorophenols, the mechanism
of the cathodic reduction of chlorophenols [39] and
the preparative electroreduction of pentachlorophenol
32 [40] have been explored. Most of these methods
exhibit significant drawbacks with a view to practi-
cal applications. The direct electrooxidation of 25, for
instance, yields 2,7-dichlorodibenzo-p-dioxin as the
product [34]. We were therefore interested in the be-
haviour of chlorinated phenols during the cathodic re-
duction and have studied the mono-, di-, tri-, tetra-
and pentachloro derivatives 23 – 34, of which 27 is the
starting material for the production of 2,4-D and Nitro-
fen® (36).
As a further experimental parameter, the supporting
electrolyte was changed since the bromide anions of
TEAB are oxidised to bromine at the anode, which
should be avoided in view of a technical application.
We tested sodium and potassium hydroxide. The de-
gree of dechlorination was, however, much lower even
at a lead cathode (see Table 5). This could be due to
the smaller size of the cations, which influences the
polarity of the double layer. Furthermore, the lack of
a well-defined anodic reaction may influence the elec-
troreduction. Last but not least, corrosion of the lead
cathode is considerably higher in the strongly alkaline
media. Interestingly, also the regioselectivity is lost,
i. e., 2,4-dichloro-(16) and 3,4-dichloroanisole(17) are
formed in equal amounts.
Electroreduction of chlorinated phenols
The strong +M-effect of the hydroxy substituent
Large quantities of chlorinated phenols have been causes a very pronounced negative shift of the re-
produced industrially for decades. They were used duction potential of chlorophenols. In addition, the
as intermediates for the synthesis of pesticides, e. g. phenols are deprotonated under the basic conditions
dichlorophenoxyacetic acid (2,4-D) and Nitrofen® in the electrolyte and the resulting phenolate anions
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O. Kranz – J. Voss · Electroreduction of Organic Compounds, 34
1193
Table 6. Galvanostatic elec-
troreduction of the dichloro-
phenols 26 – 29, the trichloro-
phenols 30 and 31 and pen-
tachlorophenol (32) in varying
SSE.
Substrate Cathode
material
Charge
Recovered
Product distribution [%]
Current
efficiency [%]
consumption substrate 27
[F mol−1
683
23
24 25
22
]
[%]
66
11
n.d.c
8
43
57
30
1
6
3
43
3
26
27
28
28
28
29
29
30
30
31
31
32
Carbona,b
Carbona,b
Leadb
–
–
–
–
–
–
–
17
4
10
–
–
76
7
9
0.1
0.3
0.4
0.3
0.2
0.1
0.1
1.3
0.5
2
730
730
730
547
608
1094
291
620
146
–
–
–
–
–
6
3
10
16 39
45
8
7
2
8
14
4
3
5
4
Carbonb,d
Carbona,b
Leadb
9
9
41
62
2
73
41
–
a
Carbon felt; b aqu. 0.1 M NaOH
+ 0.1 M Na2SO4; c not detectable;
Carbonb,d
Leade
–
d
e
carbon fibres; 1 M KOH in
9
68
20
51
13
MeOH; + 2% 26 + 11% 28; g
+
f
Carbonb,d
Leade
53f
33
1
14% 2,5-dichlorophenol.
–
–
Carbona,b
Leade
660
103
39 n.d.c
0.4
6
29g 15 n.d.c 28
are even less easily to reduce although their solubil- electroreduction was, however, regioselective. The
ity is increased in hydroxylic solvents. The reduc- conversion of 23, 24 and 25 was 21%, 15% and
tion potentials of 23 – 31 could therefore not be de- 2%, respectively, i. e., the ease of dechlorination
termined in our standard SSE, which is decomposed follows the order ortho > meta ꢀ para. Due to the
at −2.2 V. In an inert SSE the half-wave potentials of decomposition of the SSE at the necessary, extremely
the monochlorophenols are found at ca. −2.8 V (vs. negative potentials, a satisfactory controlled potential
Ag/AgCl) [41].
electrolysis in TEAB/methanol was not possible.
2,3-Dichloro- (26), 2,4-dichloro- (27), 3,4-dichloro-
(28) and 3,5-dichlorophenol (29) were electrolysed as
single compounds. Galvanostatic conditions with lead
or carbon cathodes were applied and aqueous sodium
hydroxide containing sodium sulfate to increase the
conductivity was used as SSE. The results are shown
in Table 6.
Again, the current efficiency was poor (0.1– 0.4%)
due to pronounced hydrogen evolution. The rate of de-
halogenation was, however, higher. At least, one chloro
substituent was removed easily from most substrates.
As expected from the electrolysis results of 23 – 25, an
ortho-chloro substituent was first split off selectively
followed by a meta-chloro substituent if present. But
at a lead cathode even the para-chloro substituent of
28 could be replaced by hydrogen to a considerable
extent, with 45% of phenol being formed (entry 3 in
Table 6).
The preparative electroreduction of the three
monochlorophenols 23 – 25 was performed in
methanolic potassium hydroxide solution at a lead
2,3,4-Trichlorophenol (30) and 2,4,6-trichlorophe-
cathode under galvanostatic conditions. The anodic nol (31) were electrolysed galvanostatically, each of
and cathodic compartments of the cell were divided by the two both in methanolic potassium hydroxide at a
a cation exchange membrane. The three isomers were lead cathode and in aqueous sodium hydroxide/sulfate
electrolysed as an equimolar mixture. Phenol (22) was at a carbon felt cathode (Table 6). Expectedly, 30 led
found as the only product. Hydrogenated compounds to 25 (68%) and 27 (9%) as the main products. Smaller
could not be detected. The low total yield of 12% of 22 amounts of 23 and 24 as well as 14% of 21 were found
and the extremely low current efficiency of only 0.3% as by-products (lead cathode), with only 1% of start-
could not be increased by a change of the conditions, ing compound 30 being recovered (entry 8 in Table 6).
e. g. the use of a carbon felt instead of a lead cathode In the case of 31, the product mixture (lead cathode)
or aqueous sodium hydroxide as electrolyte. The also consisted of 25 (51%) and 27 (33%) as the main
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1194
O. Kranz – J. Voss · Electroreduction of Organic Compounds, 34
products besides 23 (10%) and phenol (21, 3%) as by- duction of pentachlorophenol (32) led to a good to-
products (entry 10 in Table 6). Only 3% of the start- tal turnover of 97%. Expectedly, 4-chloro- (25, 28%)
ing compound 31 was recovered. The results were less and 2,4-dichlorophenol (27, 29%) were the main prod-
satisfactory when carbon fibres and water were used ucts (entry 12 in Table 6). Only traces of trichloro- and
(entries 9 and 11 in Table 6). In summary, one can tetrachlorophenols were found in the product mixture.
state that a partial dechlorination of the dichloro- and
trichlorophenols to form mainly 4-chlorophenol (25)
is well possible by cathodic reduction in methanolic
potassium hydroxide whereas the use of an aqueous
electrolyte and carbon cathodes is less favourable.
The electroreduction of pentachlorophenol (32)
should lead to a complex multi-component mixture
Putting together these results, a reasonable pathway
of the consecutive steps can be deduced [42]. From a
practical point of view on the other hand, the electrore-
duction under galvanostatic conditions is more impor-
tant. Fortunately, the galvanostatic electroreduction of
32 in methanolic potassium hydroxide at a high cur-
rent density of 255 A m−2 (see above) provided a
more satisfying result. In summary, one can state that
the electroreductive transformation of pentachlorophe-
nol 32 into mono- and dichlorophenols, which are
much less dangerous pollutants, can be achieved un-
der suitable and practical conditions. Removal of the
last chloro substituent to form phenol (22) is however
not possible.
of products, and this is indeed found. Considerable
analytical problems arise from this fact and we have
reported on these in detail [42]. The results can
be summarized as follows. Potentiostatic electroly-
ses in methanolic solution at a low reduction po-
tential of −1.50 V and at low current densities led
to a total turnover of only 48%. Mainly one meta-
or the para-chloro substituent was removed and, in
accordance with the statistical expectation, 2,3,4,6-
tetrachlorophenol (33) and 2,3,5,6-tetrachlorophenol
(34) were formed with 26% and 13% yield besides
5% of 31 and 1% of its isomer 2,3,6-trichlorophenol
(35). If the controlled potential electroreduction of 32
was run for a prolonged time (charge consumption of
65 F mol−1) at −2.20 V, 96% of the starting com-
pound was consumed. However, a disappointing com-
plex mixture of 12% tetrachlorophenols (33, 34), 34%
trichlorophenols (20% 31, 11% 35), 42% dichlorophe-
nols and 8% 2-chlorophenol (23) but without phe-
nol (22) was formed [42]. Substitution of chlorine in
the meta-position also occurred during the indepen-
dent potentiostatic electroreduction of the two tetra-
chlorophenols 33 and 34 at −1.70 V, which we have
studied for comparison. After a charge consumption
of 11 F mol−1 34 was transformed into 35 (13%) and
26 (3%) under meta-substitution whereas 33 gave 31
(41%) and 27 (6%) under meta-substitution besides
only 6% of 35 (para-substitution) and 5% of 2,4,5-
trichlorophenol (ortho-substitution). Complex prod-
uct mixtures resulted from electrolyses of the tetra-
chlorophenols at −2.20 V with charge consumptions
of 35 F mol−1 (cf. Experimental Section). In the
case of 33 the yields of dichloro- and trichlorophe-
nols were 45% and 47%, respectively, besides 3% of
Electroreduction of real-life material
Within the context of a governmental project on soil
remediation in which we were involved, we have stud-
ied the electrochemical reduction of the extract of a
highly contaminated soil. The soil specimen from a
former industrial site at Bitterfeld, Germany, was ex-
tracted with toluene in a Soxhlet apparatus. A care-
ful analysis of this extract revealed a high content
of chlorinated nitrodiphenyl ethers including the no-
torious [43] Nitrofen® [2,4-dichloro-4’-nitrodiphenyl
ether (36)], chloronitrobenzenes and chlorophenols
(Table 7). It has been shown that Nitrofen® is one of
the most abundant contaminants in soils at the agro-
chemical production centres of the former GDR [44].
As recently as in 2002 public concern arose in Ger-
many when considerable amounts of this pesticide
were found in a granary near Berlin which was still
in use.
Polarographic half-wave potentials of 36 and related
2-chlorophenol (23) and 4% of recovered 33. Simi- compounds are also given in Table 7. These data and
larly, 34 yielded 8% monochloro- (mainly 23), 41% the respective cyclic voltamograms of chlorinated ni-
dichloro- and 35% trichlorophenols with 16% recov- troarenes show that as expected, nitro groups are re-
ered starting material 34. The galvanostatic electrore- duced first (in two steps) before dechlorination occurs.
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O. Kranz – J. Voss · Electroreduction of Organic Compounds, 34
1195
Table 7. Concentrations of the main contaminants in the soil
complete removal of the chloro substituents is more
difficult. The best results are obtained by use of lead
cathodes with a methanolic tetraethylammonium bro-
mide solution as solvent-supporting electrolyte. Car-
bon felt cathodes and potassium or sodium hydroxide
solutions with substantial water contents can also be
applied, although the efficiency is considerably lower.
Real-life material, such as extracts of soils that are con-
taminated with chloroarenes can also be treated, with a
significant decrease of the degree of chlorination being
achieved.
specimen studied and polarographic half-wave potentials E1
/2
vs. Ag/AgBr in DMF.
Compound
Concentration
E1/2 [V]
[mg/kg dry soil]
®
Nitrofen (36)
8600
750
2000
1800
−2.01 −1.72 −1.27 −0.59
−1.29 −0.51
4-Chloronitrobenzene
2,4,6-Trichlorophenol (31)
2,4-Dichlorophenol (27)
–
–
–
–
–
–
–
–
After removal of the solvent the soil extract
was electrolysed under galvanostatic conditions in
methanolic potassium hydroxide at a lead cathode. Af-
ter a suitable work-up procedure the product distri-
bution was determined by GLC and GLC-MS cou-
pling. Three products had been formed from Nitrofen®
(36): 4-amino-2’,4’-dichlorodiphenyl ether (37, 6%),
4-amino-4’-chlorodiphenyl ether (38, 76%) and 4-
aminodiphenyl ether (39, 18%). Thus, the nitro group
was completely reduced and mostly one ortho- or
both the chloro substituents were removed. No starting
material could be detected. The 4-chloronitrobenzene
was totally transformed into 4-chloroaniline with-
out any dechlorination. As expected from the above
mentioned results, the chlorinated phenols 27 and
31 were partially dechlorinated, with 24% of 23
being formed mainly at the expense of 31, the
amount of which decreased from 37% to 6% by the
electrolysis.
Experimental Section
Melting points (corrected): Electrothermal. – NMR spec-
tra: AMX 400 (Bruker). GC: GC 4200 (Carlo-Erba) and GC
8000 (Fisons) equipped with a DB 1701 column (J & W Sci-
entific). – GC-MS: GC 8000 (Fisons) equipped with a 30 m
DB 5 MS or a 50 m FFAP MS column in combination with
an MD 800 quadrupole-MS (EI, 70 eV, Fisons).
Differential pulse polarography: VA 663/Polarecord
626 (Metrohm). Cylic voltammetry: VA-Scanner E 612
(Metrohm) with plotter Servotec 720 (Metrawatt). Potentials
were measured in DMF vs. an internal Ag wire as reference
electrode, which corresponds to the Ag/AgBr couple in the
TEAB solution. They were not converted into potentials vs.
the usually applied SCE because we have used different and
varying solvents for the preparative scale electrolyses. This
would require solvent corrections in each case to get compa-
rable potentials.
Materials: 2,3,4-Trichlorotoluene (8) [45, 46], 2,4,5-
trichlorotoluene (9) [47, 48] and 2,4,6-trichlorotoluene (10)
[47, 49] were prepared according to literature procedures.
2,4-Dichloroanisole (16) [50, 51], 3,4-dichloroanisole (17),
3,5-dichloroanisole (18) [52, 53], 2,4,6-trichloroanisole (20)
[54] and pentachloroanisole (21) [55, 56] were prepared by
methylation of the corresponding phenols with dimethyl sul-
fate in aqu. NaOH [45]. 2,3,4,5-Tetrachlorophenol [57] was
obtained by demethylation of 2,3,4,5-tetrachloroanisole with
aqu. 48% HBr [45]. The other substrate compounds were
commercially available.
Electrolyses (general procedure): The electrolyses on a
preparative scale were performed either potentiostatically
(Wenking HP 88, Bank Electronic) or galvanostatically (DC
Power Supply 6247B, Hewlett Packard) in the divided cell
(volume of the cathodic and anodic compartment: 100 mL
each) shown in Fig. 6 [9]. The potentials and currents were
measured by use of a digital voltmeter (Conrad-Electronic).
The consumption of electric charge was monitored with a
DC current counter (Conrad-Electronic). The cathodic and
the anodic compartments were separated by anion- or cation-
exchange membranes. Lead cathodes (Pb foil Merck, analyt-
Conclusions
The cathodic reduction of chloroarenes is, as ex-
pected, impeded if substituents with inductive (methyl)
or mesomeric (methoxy, hydroxy) electron-donating
effects are present. This is evident from their half-
wave reduction potentials. Nevertheless, a preparative
scale electroreduction is possible under potentiostatic
as well as under galvanostatic conditions. In particular,
compounds with two or more chloro substituents are
dechlorinated to a large extent. In nearly all cases the
monochloro derivatives can be obtained, whereas the ical grade, 99.97%) were purified before use by subsequent
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1196
O. Kranz – J. Voss · Electroreduction of Organic Compounds, 34
Electroreduction of the chlorinated toluenes
All electrolyses were performed at Pb-cathodes with Rei-
chelt Chemietechnik MA 3475 anion exchange membranes.
A 0.1 M TEAB solution in MeOH was used as SSE in the
case of controlled potential electrolyses (CPE). A 1 M KOH
solution in MeOH was used in the case of controlled current
electrolyses (CCE). The charge consumptions, current effi-
ciencies for 5 – 10 and amounts of recovered substrate are
given in Table 2.
The monochlorotoluenes were electrolysed as mixtures of
each 203 mg (1.60 mmol) of 2, 3 and 4. CPE (−2.30 V,
20 F mol−1); recovery: 66%; current efficiency: 8%; yield:
79% 1; turnover: 71% 2, 84% 3, 83% 4. CCE (1.0 A,
29 F mol−1); recovery: 96%; current efficiency: 1.3%; yield:
18% 1; turnover: 9% 2, 25% 3, 21% 4.
2,3-Dichlorotoluene (5). CPE (−2.10 V); 507 mg
(3.15 mmol) 5; recovery: 84%; yields: 40% 2, 31% 3, 23% 1.
CCE (1.0 A); 531 mg (3.30 mmol) 5; recovery: 76%; yields:
25% 2, 8% 3, 18% 1.
2,4-Dichlorotoluene (6). CPE (−2.10 V); 517 mg
(3.21 mmol) 6; recovery: 83%; yields: 6% 2, 48% 4. CCE
(0.5 A); 550 mg (3.42 mmol) 6; recovery: 72%; yields: 32%
2, 19% 4, 2% 1.
3,4-Dichlorotoluene (7). CPE (−2.10 V); 511 mg
(3.17 mmol) 7; recovery: 77%; yields: 12% 3, 46% 4, 4% 1.
CCE (1.0 A); 509 mg (3.16 mmol) 7; recovery: 77%; yields:
21% 3, 26% 4, 17% 1.
2,3,4-Trichlorotoluene (8). CPE (−2.20 V); 201 mg
(1.03 mmol) 8; recovery: 81%; yields: 12% 2, 5% 3, 60%
4, 23% 1. CCE (1.0 A); 201 mg (1.03 mmol) 8; recovery:
77%; yields: 3% 5, 55% 6, 1% 7, 16% 2, 2% 3, 12% 4, 3% 1.
2,4,5-Trichlorotoluene (9). CPE (−2.20 V); 202 mg
(1.04 mmol) 9; recovery: 71%; yields: 4% 6, 1% 7, 6% 2,5-
dichlorotoluene, 26% 2, 21% 3, 37% 4, 4% 1.
2,4,6-Trichlorotoluene (10). CPE (−2.20 V); 204 mg
(1.04 mmol) 10; recovery: 69%; yields: 19% 2,6-
dichlorotoluene, 6% 6, 45% 2, 28% 4.
Fig. 6. Schematic setup of the divided batch cell.
rinsing with aqua regia, conc. HCl and hot water. Carbon
felt cathodes (R&G GmbH Faserverbundwerkstoffe), in cer-
tain cases disentangled to fibre bunches, were extracted with
acetone prior to use in order to remove textures and coating
materials. They were fitted into the batch cell with a glass
support.
Analytical protocol: During the electrolyses, samples of
ca. 0.2 – 1.0 ml were taken from the catholyte for monitoring.
They were diluted with the threefold amount of de-ionised
water, acidified with half-conc. HCl and extracted with hex-
ane (toluenes and anisoles) or diethyl ether (phenols and
electrolysis products of the soil extract). The samples were
dried over MgSO4 and analysed by GLC.
After the end of the electrolysis the whole catholyte was
treated in the same way. The product mixtures were iso-
lated and analysed qualitatively and quantitatively by cap-
illary GLC. The yields are corrected with respect to the re-
covery rates. The assignment of the peaks was performed by
co-injection of reference compounds and comparison of the
retention times. Standard solutions of the respective refer-
ence compounds with 1,2,4,5-tetramethylbenzene as internal
standard were measured repeatedly for the quantitative deter-
minations. The evaluation was performed automatically by
use of the ChromCard software (Fisons). Relative response
factors of the components were calculated and the peak areas
corrected accordingly for the determination of the product
distribution.
Electroreduction of the chlorinated anisoles
In addition, selected samples and the respective reference
compound were studied by the GLC-MS coupling method.
Controlled potential electrolyses of the monochloro-,
In several cases, the phenols were transformed into the dichloro- (Table 3) and trichloroanisoles (Table 3) and
corresponding anisoles for the GLC analysis [58]. The elec- of pentachloroanisole (21) were performed at Pb-cathodes
(working potential −2.20 V) with Permion 4035 anion ex-
trolyte was extracted with diethyl ether and the extract was
concentrated. The residue was dissolved in 20 ml acetone, an change membranes if not stated otherwise. A 0.1 M TEAB
excess of iodomethane and K2CO3 were added and the mix- solution in methanol was used as SSE. Charge consumptions
and current efficiencies are given in Table 3.
ture was refluxed for 12 h. After filtration the solution was
diluted with diethyl ether and analysed by GLC as described
2-Chloroanisole (12). 200 mg (1.40 mmol) 12. Recovery:
above. The recovery was tested on examples and was found 57%; yield: 38% 11.
to be 75 – 88%. No selectivity for different chlorinated phe-
nols was observed.
3-Chloroanisole (13). 200 mg (1.40 mmol) 13. Recovery:
57%; yield: 79% 11.
The capillary zone electrophoretic (CZP) analysis of chlo-
rinated phenols has been described elsewhere [42].
4-Chloroanisole (14). – a) 200 mg (1.40 mmol) 14. Re-
covery: 38%; yield: 11% 11. – b) 1000 mg (7.00 mmol) 14.
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O. Kranz – J. Voss · Electroreduction of Organic Compounds, 34
1197
Recovery: 46%; yield: 30% 11 + 4% 1-chloro-4-methoxy- recovery: 52%; yield 14% 11; turnover: 15% 12, 20% 13,
1,4-cyclohexadiene {MS (EI, 70 eV) m/z (%) = 146 (11) 9% 14.
[M+], 145 (5), 144 (35) [M+], 143 (6), 142 (2), 129 (3) [M+–
Controlled potential (working potential: −2.20 V, Rei-
CH3], 128 (3), 114 (11), [M+–CH2O], 109 (84) [M+–Cl],
chelt Chemietechnik MA 3475 membrane) and currency
108 (19), 101 (13), 99 (8), 94 (35), 79 (26), 78 (21), 77 (70),
65 (100)}.
controlled electrolyses of 2,3,4-trichloroanisole (19) under
varying conditions. Charge consumptions and current effi-
2,3-Dichloroanisole (15). 200 mg (1.13 mmol) 15. Recov- ciencies are given in Table 5. – a) CPE 1 (entry 2 in Ta-
ery: 58%; yields: 20 – 27% 11, 20 – 26% 12, 48 – 60% 13.
2,4-Dichloroanisole (16). 200 mg (1.13 mmol) 16. Recov-
ery: 49%; yields: < 1% 11, > 99% 14.
3,4-Dichloroanisole (17). 200 mg (1.13 mmol) 17. Recov-
ery: 75%; yields: 5% 11, 3% 13, 92% 14.
3,5-Dichloroanisole (18). – a) 200 mg (1.13 mmol) 18.
Entry 8 in Table 3; recovery: not determined; yields: 48% 11,
52% 13. – b) Entry 9 in Table 3; recovery: 47%; yields: 55%
11, 45% 13. – c) Reichelt Chemietechnik MA 3475 mem-
brane; recovery: 62%; yields: 36% 11, 64% 13.
2,3,4-Trichloroanisole (19). – a) 200 mg (0.95 mmol) 19.
Entry 11 in Table 3; recovery: 58%; yields: 5% 11, 1% 12,
2% 13, 90% 14, 2% (16 + 17). – b) 600 mg (2.85 mmol) 19.
Entry 12 in Table 3; recovery: 49%; yields: 13% 11, 4% 12,
1% 13, 79% 14, < 1% (16 + 17).
2,4,6-Trichloroanisole (20). 200 mg (0.95 mmol) 20. Re-
covery: not determined; yields: < 1% 11, > 99% 14.
Pentachloroanisole (21). – a) 123 mg (0.44 mmol) 21.
Reichelt Chemietechnik MA 3475 membrane; charge con-
sumption: 85 F mol−1; recovery: 62%; current efficiency:
10%; yields: 26% 11, 5% 12, 14% 13, 51% 14, 2% 1-chloro-
4-methoxy-1,4-cyclohexadiene. – b) 51 mg (0.18 mmol) 21.
Reichelt Chemietechnik MA 3475 membrane; working po-
tential: −1.10 V; charge consumption: 57 F mol−1; recovery:
not determined; current efficiency: 10%; yields: 2% 12, 5%
13, 31% 14, 21% 16 + 20, 13% 2,5-dichloroanisole, 12% 17,
4% 2,4,5-trichloroanisole, 5% 21.
ble 5); 600 mg (2.84 mmol) 19; 0.1 M TEAB/MeOH; car-
bon felt cathode; recovery: 40%; 20%; yields: 4% 11, 2%
13, 54% 14, 25% 16, 14% 17. – b) CPE 2 (entry 3 in Ta-
ble 5); 613 mg (2.90 mmol) 19; 0.1 M TEAB in MeOH/H2O
80:20; Pb-cathode; recovery: 77%; yields: 2% 11, 2% 12, 3%
13, 91% 14, 1% 16. – c) CPE 3 (entry 4 in Table 5); 604 mg
(2.86 mmol) 19; 0.1 M TEAB in MeOH/H2O 80:20; carbon
felt cathode; recovery: 46%; yields: 7% 14, 61% 16, 31%
17. – d) CPE 4 (entry 5 in Table 5); 620 mg (2.93 mmol)
19; 0.1 M TEAB in MeOH/H2O 80:20; carbon felt cath-
ode; recovery: 65%; yields: 14% 11, 1% 13, 54% 14, 4%
16, 3% 17, 23% 19. – e) CPE 5 (entry 6 in Table 5); 593 mg
(2.81 mmol) 19; 1 M NaOH + 0.1 M NaBr in MeOH/H2O
80:20; Pb-cathode; recovery: 75%; yields: 2% 11, 3% 12, 4%
13, 38% 14, 19% 16, 15% 17, 18% 19. – f) CPE 6 (entry 7
in Table 5); 625 mg (2.96 mmol) 19; 1 M NaOH + 0.1 M
NaBr in MeOH/H2O 80:20; carbon felt cathode; recovery:
48%; yields: 3% 14, 32% 16, 24% 17, 41% 19. – g) CCE
1 (2.0 A, entry 8 in Table 5); 598 mg (2.83 mmol) 19; 1 M
KOH/MeOH; Pb-cathode; recovery: 73%; yields: 1% 11, 2%
13, 25% 14, 34% 16, 34% 17, 3% 19. – h) CCE 2 (3.0 A);
250 mg (1.18 mmol) 19; carbon felt cathode; recovery: 86%;
yields: 2% 13, 20% 14, 29% 16, 32% 17, 16% 19. – i) CCE
3 (3.0 A); 100 mg (0.47 mmol) 19; 1 M KOH in MeOH/H2O
75:25; Pb-cathode; recovery: 75%; yields: 2% 11, 1% 12, 9%
13, 62% 14, 11% 16, 13% 17, 2% 19.
Controlled potential electrolyses (working potential:
−2.20 V, Reichelt Chemietechnik MA 3475 membranes) of
equimolar mixtures of the monochloroanisoles 12, 13 and
14 under varying conditions. Charge consumptions and cur-
rent efficiencies are given in Table 4. – a) CPE 1 (entry 1 in
Table 4); each 102 mg (0.71 mmol) 12, 13 and 14; 0.1 M
TEAB/MeOH; Pb-cathode; recovery: 57%; yield: 32% 11;
turnover: 31% 12, 37% 13, 29% 14. – b) CPE 2 (entry 2 in
Table 4); each 531 mg (3.71 mmol) 12, 13 and 14; 0.1 M
TEAB in MeOH/H2O 80:20; Pb-cathode; recovery: 57%;
yield: 9% 11; turnover 8% 12, 12% 13, 7% 14. – c) CPE
3 (entry 4 in Table 4); each 106 mg (0.74 mmol) 12, 13 and
14; 0.1 M TEAB/MeOH; carbon felt cathode; recovery: 57%;
yield: 18% 11; turnover: 19% 12, 23% 13, 11% 14. – d) CPE
4 (entry 5 in Table 4); each 476 mg (3.33 mmol) 12, 13 and
14; 0.1 M TEAB/MeOH; carbon felt cathode; recovery: 50%;
Electroreduction of the chlorinated phenols
The monochlorophenols 23, 24 and 25 were electrolysed
galvanostatically (2.0 A/1020 A m−2, Pb-cathode, Reichelt
Chemietechnik MC 3470 cation exchange membrane) as a
mixture of each 105 mg (0.82 mmol) in 1 M KOH/MeOH.
Charge consumption: 93 F mol−1; recovery: 72%; current
efficiency: 0.3%; turnover: 21% 23, 15% 24, 2.5% 25; yield:
12% 22.
The dichlorophenols 26–29 and the trichlorophenols 30
and 31 were electrolysed galvanostatically (3.0 A, Reichelt
Chemietechnik MC 3470 cation exchange membrane) in 0.1
M NaOH/0.1 M Na2SO4/H2O if not stated otherwise. The
charge consumptions and current efficiencies are given in
Table 6.
2,3-Dichlorophenol (26). 107 mg (0.66 mmol) 26. 1 M
yield: 48% 11; turnover: 52% 12, 56% 13, 38% 14. – e) CPE NaOH/1 M Na2SO4/H2O; carbon felt cathode; recovery:
5 (entry 6 in Table 4); each 515 mg (3.60 mmol) 12, 13 and 82%; yields: 7% 22, 17% 23, 10% 24, 66% 26 (entry 1 in
14; 0.1 M TEAB in MeOH/H2O 80:20; carbon felt cathode; Table 6).
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O. Kranz – J. Voss · Electroreduction of Organic Compounds, 34
2,4-Dichlorophenol (27). 100 mg (0.61 mmol) 27. 1 M 78%; current efficiency: 6%; yields: 4% 22, 15% 23, 28% 25,
NaOH/1 M Na2SO4/H2O; carbon felt cathode; recovery: 29% 27, 14% 2,5-dichlorophenol, 2% 2,4,5-trichlorophenol,
81%; yields: 9% 22, 4% 23, 76% 25, 11% 27 (entry 2 in 1% 31, 3% 32.
Table 6).
3,4-Dichlorophenol (28). 100 mg (0.61 mmol) 28. –
a) CCE 1 (entry 3 in Table 6); Pb-cathode; recovery: 91%;
yields: 45% 22, 16% 24, 39% 25. – b) CCE 2 (entry 4 in
Table 6); carbon fibre bunch cathode; recovery: 76%; yields:
8% 22, 9% 24, 73% 25, 8% 28. – c) CCE 3 (entry 5 in Ta-
ble 6); carbon felt cathode; recovery: 69%; yields: 7% 22,
9% 24, 41% 25, 43% 28.
3,5-Dichlorophenol (29). – a) CCE 1 (entry 6 in Table 6);
200 mg (1.23 mmol) 29; Pb-cathode, 5.0 A; recovery: 57%;
yields: 2% 22, 41% 24, 57% 29. – b) CCE 2 (entry 7 in Ta-
ble 6); 100 mg (0.61 mmol) 29; carbon fibre bunch cathode;
recovery: not determined; yields: 8% 22, 62% 24, 30% 29.
2,3,4-Trichlorophenol (30). – a) CCE 1 (entry 8 in Ta-
ble 6); 101 mg (0.51 mmol) 30, 1 M KOH/MeOH; Pb-
cathode, 2.0 A; recovery: 66%; yields: 14% 22, 6% 23, 2%
24, 68% 25, 9% 27. – b) CCE 2 (entry 9 in Table 6); 107 mg
(0.54 mmol) 30; carbon fibre bunch cathode; recovery 54%;
yields: 4% 22, 3% 23, 1% 24, 20% 25, 2% 26, 57% 27, 11%
28, 6% 30.
2,4,6-Trichlorophenol (31). – a) CCE 1 (entry 10 in Ta-
ble 6); 201 mg (1.02 mmol) 31; 1 M KOH/MeOH; Pb-
cathode, 2.0 A; recovery: 62%; yields: 3% 22, 10% 23, 51%
25, 33% 27, 3% 31. – b) CCE 2 (entry 11 in Table 6); 134 mg
(0.68 mmol); carbon felt cathode; recovery 78%; yields: 5%
22, 13% 25, 39% 27, 43% 31.
The tetrachlorophenols 33 and 34 were electrolysed po-
tentiostatically at a Pb-cathode in 0.1 M TEAB/MeOH. The
current efficiencies are given in Table 6.
Electroreduction of a soil extract
Dried soil (16.5 g) from the Bitterfeld site was extracted
for 48 h with toluene in a Soxhlet extractor. The solvent
was evaporated to yield a residue (411 mg) which contained
143 mg of Nitrofen® (36), 12 mg of 4-chloronitrobenzene,
30 mg of 2,4-dichlorophenol (27), and 33 mg of 2,4,6-
trichlorophenol (31) according to a GLC analysis. This ex-
tract was electrolysed galvanostatically (1.0 A, 509 A m−2
)
in 1 M KOH/MeOH by use of a Pb-cathode and a cation
exchange membrane (Reichelt Chemietechnik MC 3470).
After a total charge consumption of 18000 A s the elec-
trolyte was diluted with H2O and first extracted with diethyl
ether, then acidified with HCl and again extracted. Both ex-
tracts (295 mg in total) were analysed qualitatively (GC-MS
coupling) and quantitatively (GLC). The first extract con-
tained 7 mg (6%) of 4-amino-2’,4’-dichlorodiphenylether
(37), 75 mg (76%) of 4-amino-4’-chlorodiphenylether (38),
15 mg (18%) of 4-aminodiphenylether (39) and 9 mg of 4-
chloroaniline. The acidic extract contained 0.3 mg of phenol
(22), 5 mg of 2-chlorophenol (23), 11 mg of 4-chlorophenol
(25), 34 mg of 2,4-dichlorophenol and 4 mg of 2,4,6-
trichlorophenol (31).
37: MS (EI, 70 eV): m/z (%) = 257 (3) [M+], 255 (14)
[M+], 253 (17) [M+], 219 (1), 183 (3), 154 (3), 147 (8), 145
(11), 135 (4), 133 (8), 127 (4), 111 (11), 109 (31), 108 (100)
[C6H6NO+], 91 (6), 85 (6), 84 (6), 80 (75), 72 (25), 74 (28),
73 (15), 65 (39), 63 (28), 53 (28), 52 (22).
38: MS (EI, 70 eV): m/z (%) = 221 (11) [M+], 219 (44)
[M+], 184 (1), 183 (1), 156 (6), 128 (4), 127 (3), 111 (13),
108 (100) [C6H6NO+], 80 (47), 75 (19), 65 (17), 63 (10), 53
(11), 52 (7).
2,3,4,6-Tetrachlorophenol (33). 153 mg (0.66 mmol) 33. –
a) CPE 1 (−1.70 V); recovery not determined; yields; 6% 27,
6% 35, 5% 2,4,5-trichlorophenol, 41% 31, 40% 33. – b) CPE
2 (−2.20 V); recovery not determined; yields; 3% 23, 7% 26
+ 2,5-dichlorophenol, 27% 27, 11% 2,6-dichlorophenol, 5%
35, 3% 2,4,5-trichlorophenol, 39% 31, 4% 33.
2,3,5,6-Tetrachlorophenol (34). 156 mg (0.67 mmol) 34. –
a) CPE 1 (−1.70 V); recovery: not determined; yields: 3%
26, 13% 35, 82% 34. – b) CPE 2 (−2.20 V); recovery:
not determined; yields: 7% 23, 1% 24, 29% 26, 12% 2,6-
dichlorophenol, 2% 2,3,5-trichlorophenol, 33% 35, 16% 34.
39: MS (EI, 70 eV): m/z (%) = 186 (7) [M+], 185 (42)
[M+], 169 (1), 168 (1), 157 (6), 156 (8), 130 (3), 129 (4),
128 (4), 109 (14), 108 (100) [C6H6NO+], 92 (3), 91 (3), 80
(94), 77 (64), 65 (28), 63 (17), 53 (26), 52 (17), 51 (42),
50 (17).
4-Chloroaniline: MS (EI, 70 eV): m/z (%) = 130 (3),
129 (22) [M+], 128 (8), 127 (100) [M+], 102 (6), 100 (19)
[M+–NH2], 99 (8), 92 (28) [M+–Cl], 91 (8), 78 (3), 75 (8),
74 (6), 73 (11), 65 (56), 63 (36).
Pentachlorophenol (32) was electrolysed at
a Pb-
cathode. – a) CPE 1 (−1.50 V); 156 mg (0.58 mmol) 32;
0.1 M TEAB/MeOH; recovery: not determined; current effi-
ciency: 5%; yields: 5% 31, 1% 35, 26% 33, 13% 34, 52%
32. – b) CPE 2 (−2.20 V); 156 mg (0.58 mmol) 32; 0.1
M TEAB/MeOH; recovery: 77%; current efficiency: 8%;
yields: 8% 23, 8% 26 + 2,5-dichlorophenol, 17% 27, 12%
Acknowledgements
Support of this work by the “Universität Hamburg”,
2,6-dichlorophenol, 11% 35, 1% 2,3,5-trichlorophenol, 20% the “Deutsche Forschungsgemeinschaft” (SFB 188) and the
31, 5% 33, 7% 34, 4% 32. – c) CCE (entry 12 in Table 6); “Bundesministerium für Bildung und Forschung” (BMBF) is
144 mg (0.51 mmol) 32; 1 M KOH/MeOH, 0.5 A; recovery: gratefully acknowledged.
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