Electrochemical dechlorination of 4,4′-(2,2,2-trichloroethane-1,1-diyl)bis(chlorobenzene) (DDT) at silver cathodes

Article abstract of DOI:10.1016/j.electacta.2014.05.127

Cyclic voltammetry and controlled-potential (bulk) electrolysis have been employed to investigate the reduction of 4,4′-(2,2,2-trichloroethane-1,1-diyl)bis(chlorobenzene) (DDT) at silver cathodes in dimethylformamide (DMF) containing 0.050 M tetramethylammonium tetrafluoroborate (TMABF₄). In addition, this work has been extended to the individual reductions of two degradation products, namely 4,4′-(2,2-dichloroethane-1,1-diyl)bis(chlorobenzene) (DDD) and 4,4′-(ethene-1,1-diyl)bis(chlorobenzene) (DDNU). At a scan rate of 100 mV s^-1, cyclic voltammograms for irreversible reduction of DDT at a silver electrode exhibit four prominent cathodic peaks in DMF and CH₃CN, and three prominent cathodic peaks in DMSO. On the other hand, reduction of DDD and DDNU at silver in DMF-0.050 M TMABF₄displays four and two irreversible peaks, respectively. Carbon-chlorine bonds of the -CCl₃moiety of DDT and of the -CHCl₂moiety of DDD are reduced more easily at silver than at glassy carbon. Bulk electrolyses of DDT at a silver gauze cathode in DMF-0.050 M TMABF₄afford a potential-dependent mixture of products that includes DDD, DDNU, 4,4′-(2,2-dichloroethene-1,1-diyl)bis(chlorobenzene) (DDE), 4,4′-(2-chloroethene-1,1-diyl)bis(chlorobenzene) (DDMU), 4,4′-(2-chloroethane-1,1-diyl)bis(chlorobenzene) (DDMS), 1-chloro-4-(1-phenylvinyl)benzene (PVB), 1,1′-diphenylethylene (DPE), and 1,1′-ethylidenebisbenzene (EBB). However, at more negative potentials, the principal products are completely dechlorinated DPE and EBB. Dechlorination of DDT at silver appears to proceed via a series of steps involving carbanion intermediates arising from direct reduction of alkyl and aryl carbon-chlorine bonds along with hydroxide-promoted E2 elimination of chloride. When DMF-d₇was used as solvent, no evidence for deuterium atom incorporation into any product was seen, which indicates that radical intermediates do not play a significant role in the reduction of DDT.

Full text of DOI:10.1016/j.electacta.2014.05.127

Accepted Manuscript
Title: Electrochemical dechlorination of
4,4ꢀ-(2,2,2-trichloroethane-1,1-diyl)bis(chlorobenzene) (DDT)
at silver cathodes
Author: Caitlyn M. McGuire Dennis G. Peters
PII:
S0013-4686(14)01129-3
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.electacta.2014.05.127
EA 22822
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Electrochimica Acta
Received date:
Revised date:
Accepted date:
20-2-2014
20-5-2014
21-5-2014
Please cite this article as: C.M. McGuire, D.G. Peters, Electrochemical dechlorination
of 4,4ꢀ-(2,2,2-trichloroethane-1,1-diyl)bis(chlorobenzene) (DDT) at silver cathodes,
Electrochimica Acta (2014), http://dx.doi.org/10.1016/j.electacta.2014.05.127
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Electrochemical dechlorination of 4,4′-(2,2,2-trichloroethane-1,1-
diyl)bis(chlorobenzene) (DDT) at silver cathodes
Caitlyn M. McGuire, Dennis G. Peters*
Department of Chemistry, Indiana University, Bloomington, IN 47405, USA
ABSTRACT
Cyclic voltammetry and controlled-potential (bulk) electrolysis have been
employed to investigate the reduction of 4,4′-(2,2,2-trichloroethane-1,1-
diyl)bis(chlorobenzene) (DDT)at silver cathodes in dimethylformamide (DMF)containing
0.050 M tetramethylammonium tetrafluoroborate (TMABF4). In addition, this work has
been extended to the individual reductions of two degradation products, namely 4,4′-(2,2-
dichloroethane-1,1-diyl)bis(chlorobenzene)
(DDD)
and
4,4′-(ethene-1,1-
diyl)bis(chlorobenzene) (DDNU). At a scan rate of 100 mV s–1, cyclic voltammograms
for irreversible reduction of DDT at a silver electrode exhibit fourprominent cathodic
peaks in DMF and CH3CN, and three prominent cathodic peaks in DMSO. On the other
hand, reduction of DDD and DDNU at silver in DMF–0.050 M TMABF4 displays four
and two irreversible peaks, respectively. Carbon–chlorine bonds of the –CCl3 moiety of
DDT and of the –CHCl2 moiety of DDD are reduced more easily at silver than at glassy
carbon. Bulk electrolyses of DDT at a silver gauze cathode in DMF–0.050 M TMABF4
afford a potential-dependent mixture of products that includes DDD, DDNU, 4,4′-(2,2-
dichloroethene-1,1-diyl)bis(chlorobenzene)
(DDE),
4,4′-(2-chloroethene-1,1-
diyl)bis(chlorobenzene) (DDMU), 4,4′-(2-chloroethane-1,1-diyl)bis(chlorobenzene)
(DDMS), 1-chloro-4-(1-phenylvinyl)benzene (PVB), 1,1′-diphenylethylene (DPE), and
1,1′-ethylidenebisbenzene (EBB). However, at more negative potentials, the principal
products are completely dechlorinated DPE and EBB. Dechlorination of DDT at silver
appears to proceed via a series of steps involvingcarbanion intermediates arising from
direct reduction of alkyl and aryl carbon–chlorine bonds along with hydroxide-promoted
E2 elimination of chloride. When DMF-d7 was used as solvent, no evidence for
deuterium atom incorporation into any product was seen, which indicates that radical
intermediates do not play a significant role in the reduction of DDT.
Keywords:DDT; 4,4-(2,2,2-Trichloroethane-1,1-diyl)bis(chlorobenzene); Carbon–
chlorine bond cleavage; Silver cathodes; Electrocatalytic reduction
_______________
* Corresponding author. Tel.: +1 812 855 9671; fax: +1 812 855 8300.
E-mail address:peters@indiana.edu (D.G. Peters).
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1. Introduction
Once
a
celebrated
(DDT),
insecticide,
together with
4,4′-(2,2,2-trichloroethane-1,1-
4,4′-(2,2-dichloroethane-1,1-
diyl)bis(chlorobenzene)
diyl)bis(chlorobenzene) (DDD) and 4,4′-(2,2-dichloroethene-1,1-diyl)bis(chlorobenzene)
(DDE), has become associated with many adverse effects on human health and the
environment. Ingestion of DDT appears to cause major neurophysiological disorders,
and DDT and its metabolites have been linked to breast, pancreatic,and lung cancer and
to multiple myeloma[1–4]. Animal studies have indicated that DDT disrupts reproductive
and developmental systems [3].In response to these dangers, DDT has been banned from
use in many nations, but it remains a common malaria deterrent in some countries. Its
limited natural degradation and its ease of transport have created global contamination
issues that face us today [5]. It has been estimated that 2.4 × 106kg of DDT has
accumulated in Antarctic snow [6].Although DDT is naturally volatile, large bodies of
water continue to accumulate this pesticide. In solutions of pH 9, DDT undergoes
hydrolysis in about 80 days to afford DDE, which does not degrade further [3]. Soil
microorganisms break down DDT; however, DDE, its major metabolite, is resistant to
biodegradation [7].
As a starting point for the present investigation, Figure 1shows the structures,
names, and abbreviations for DDT and the various compounds that can arise from its
electrochemical and base-promoted degradation.
Early efforts to study the
electrochemistry of DDT centered on the use of polarography. Fukami and Nakajima [8]
examined the reduction of DDT at a dropping mercury electrode and reported that DDD
is the major product. In subsequent research, Rosenthal and Lacoste [9] conducted a
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3
systematic polarographic study of the behavior of a number of aromatic chloroethanes,
including DDT.By monitoring the reduction of DDT in alcohol–water media, Reddy and
Reddy [10] determined that DDD is formed via a two-electron process.Tandon and co-
workers [11]examined the reduction of DDT in a dimethylformamide (DMF)–ethanol–
water mixture and concluded that 4,4′-(ethane-1,1-diyl)bis(chlorobenzene) (DDO) is
produced in an overall six-electron process.
Mericaet al. [12] studied the direct reduction of DDT at mercury, lead, and nickel
cathodes in four different media via continuous-flow controlled-current electrolysis;
thetime-dependent mixture of products includedDDD, DDE, DDMU, DDMS, DDNU,
DDO, and EBB. Reduction of DDT at graphite felt cathodes in a bicontinuous
microemulsion consisting of dodecane, didodecyl dimethylammonium bromide, and
water revealed two cathodic peaks; bulk electrolysis at a potential corresponding to the
second peak led to production of EBB in 34% yield [13].
In work performed in our laboratory [14], direct reduction of both DDT and DDD
at glassy carbon electrodes in DMF containing 0.10 M tetramethylammonium
tetrafluoroborate (TMABF4) was probed with the aid of cyclic voltammetry and
controlled-potential electrolysis. A mechanistic picture for the reduction of DDT was
proposed to account for the formation of DDNU as the major product, along with
DDMU, DDE, EBB, and DDO as minor species. It was concluded that DDNU, DDMU,
and DDE arise via elimination reactions that take place when hydroxide ion (derived
from deprotonation of adventitious water by electrogenerated carbanion intermediates)
attacks DDMS, DDD, and DDT, respectively [15].
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4
Catalytic reduction of DDT has been the subject of a number of other studies.
Schweizer and co-workers[13] showed that reduction of DDT in the presence
ofelectrogenerated tris(2,2′-bipyridyl)cobalt(I) affords a product distribution similar to
that observed for the direct reduction of DDT at carbon felt electrodes. Electrogenerated
cobalt(I) salen was used to promote the reductions of DDT, DDD, and DDE in DMF–
0.050 M TMABF4[16].Catalytic reduction of DDT with electrogenerated cobalt(I)
phthalocyanines [17] and cobalt(I) porphyrins [18] in various media has been found to
yield DDD, DDE, and DDMU as major products.Recently, our research group
[19]employed electrogenerated nickel(I) salen as a catalyst for the reduction of DDT;it
was observed that the major product is DDNU, although some completely dechlorinated
DPE was detected.A report by Jabbar and co-workers [20] focused on the catalytic
reduction of DDT in an ionic liquid (1-butyl-3-methylimidazolium tetrafluoroborate)
with an electrogenerated cobalt(I) derivative of vitamin B12; a mixture of products (DDO,
DDNU, and DDMS) was obtained.
To the best of our knowledge, there has been no previous investigation of the
reduction of DDT at a silver electrode, although the literature shows an increasing
number of papersin which organic chlorides can be reduced directly at silver without the
aid of a catalyst. An early study [21], in which the reduction of chloroform at a silver
cathode and at 13 other metal electrodes was examined, revealed that silver has especially
good electrocatalytic activity for the cleavage of carbon–chlorine bonds. Consequently,
the scission of carbon–halogen bonds at silver cathodes occurs at significantly more
positive potentials [22,23]. A recent publication [24] from our laboratory contains an
extensive list of references that deal with the reduction of halogenated organic
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5
compounds at silver. Two of these references, both by Isse and co-workers
[25,26],provide valuable insights concerning the mechanisms of electroreductive carbon–
chlorine bond cleavage at silver cathodes in CH3CN and DMF solvent systems, including
effects attributable to the adsorption of activated complexes and to the presence of
deliberately added proton donors, and with special attention to the relationship between
dissociative electron transfer and the electrocatalytic properties of the cathode material.
In the present work, cyclic voltammetryand controlled-potential (bulk)
electrolysis have been used to investigate the electrochemical reductions of DDT, DDD,
and DDNU at silver cathodes in DMFcontaining 0.050 M TMABF4 as supporting
electrolyte. In addition, the cyclic voltammetric behavior of other metabolites of DDT—
namely DDMU, PVB, DPE, and EBB—wasexamined to understandbetter the processes
corresponding to each stage in the overall reduction of DDT. Gas chromatography (GC)
and gas chromatography–mass spectrometry (GC–MS)have been employed to separate,
identify, and quantitate all products arising from the electrolytic reduction of DDT. A
sequence of mechanistic steps, involving both direct electrolytic cleavage of carbon–
chlorine bonds and base-promoted elimination of chloride ions, is proposed in Figure 1 to
account for the formation of the variousproducts.Most significant is ourdiscovery that
complete removal of all aliphatic and aryl chlorine atoms can be accomplishedwith the
aid of silver cathodes.
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6
2. Experimental
2.1. Reagents
We purchased the following chemicals, which were used without further
purification, from Aldrich Chemical Company: 4,4′-(2,2,2-trichloroethane-1,1-
diyl)bis(chlorobenzene) (DDT, 98%), 1,1-diphenylethylene (DPE, 97%), 4,4′-(2-
chloroethene-1,1-diyl)bis(chlorobenzene) (DDMU, 99.7%), deuterium oxide (D2O, 99.9
atom% D), and n-hexadecane (99%, used as an internal standard for gas
chromatography). In addition, other reagents were acquired commercially, with purities
and sources as indicated: bis(4-chlorophenyl)methanone (DBP, 99.8%) and bis(4-
chlorophenyl)methane (DDM, 99.1%)from Riedel-de Haën; 4,4′-(2,2-dichloroethane-1,1-
diyl)bis(chlorobenzene)
diyl)bis(chlorobenzene) (DDE, 99%)from Supelco Analytical;4,4′-(ethene-1,1-
diyl)bis(chlorobenzene) (DDNU, >95%) and 4,4′-(2-chloroethane-1,1-
(DDD,
97%)
and
4,4′-(2,2-dichloroethene-1,1-
diyl)bis(chlorobenzene) (DDMS, >98%) from American Custom Chemicals Corporation;
1-chloro-4-(1-phenylvinyl)benzene (PVB, 98%) from 3B Scientific Corporation;and 1,1′-
ethylidenebisbenzene (EBB) fromChemSamp Corporation. Solvents for electrochemical
experiments were dimethylformamide (DMF, 99.9%, EMD Chemicals),
dimethylformamide-d7 (DMF-d7, 99.5 atom % D, Cambridge Isotope Laboratories),
acetonitrile (CH3CN, 99.99%, EMD Chemicals), and dimethyl sulfoxide (DMSO, 99.9%,
J. T. Baker). Tetramethylammonium tetrafluoroborate (TMABF4, 98%, GFS Chemicals)
was recrystallized from water–methanol and stored in a vacuum oven at 80 ºCuntil being
used as supporting electrolyte. All deoxygenation was accomplished with zero-grade
argon (Air Products).
Page 6 of 32

7
2.2. Electrodes, cells, and instrumentation
Procedures, cells, and data acquisition methods for cyclic voltammetry have been
described in previous publications [27,28].Silver and glassy carbon working electrodes
(geometric area of 0.071 cm2) were constructed and cleaned as indicated in an earlier
paper [28]. Instrumentation [28] and the two-compartment cell [29] employed for
controlled-potential (bulk) electrolysis are discussed elsewhere.Circular silver gauze
working electrodes (approximate surface areas of 20 cm2) were constructed from
commercially available material (Alfa Aesar, 99.9%, 20 mesh woven from 0.356-mm
diameter wire). For all experiments, the reference electrode was a cadmium-saturated
mercury amalgam [denoted as Cd(Hg) for Figures 2–6] in contact with DMF saturated
with both cadmium chloride and sodium chloride; this electrode has a potential of –0.76
V vs. an aqueous saturated calomel electrode (SCE) at 25 °C [30–32].
2.3.Separation, identification, and quantitation of electrolysis products
Post-electrolysis catholytes were partitioned between brine and diethyl ether, after
which the ether phase was dried over anhydrous sodium sulfate and concentrated for
analysis by means of rotary evaporation.By comparing experimentally acquired mass
spectra with those found in the NIST 2007 mass spectral database, we used gas
chromatography–mass spectrometry (Hewlett-Packard 6890N gas chromatograph, fitted
with a 30 m × 0.25 mm i.d. Agilent HP-5ms capillary column and coupled to a Hewlett-
Packard model 5973 inert mass-selective detector) to identify all electrolysis products.
For the quantitation of products, concentrated ether extracts were injected intoan
Agilent 7890A gas chromatograph equipped with a flame-ionization detector and a 30 m
Page 7 of 32

8
× 0.25 mm capillary column (J & W Scientific) with a DB-5 stationary phase consisting
of 5% phenylpolysiloxane and 95% methylpolysiloxane. Chromatographic peak areas
were measured by integration with respect to an internal standard (n-hexadecane) added
in known amount to each solution at the start of an electrolysis. Product identities were
confirmed via comparison of their retention times with those of commercially available
standards. Procedures used to quantitate the products on the basis of the gas-
chromatographic measurements have been previously described [33]. Product yields
reported in this paper reflect the absolute percentage of starting material incorporated into
a particular species.
3. Results and discussion
3.1.Cyclic voltammetric behavior of DDT and its reduction products
Shown in Figure 2 are cyclic voltammograms recorded at 100 mV s–1 for the
separate reduction of 5.0 mM solutions of DDT(upper panel) and DDD (lower panel) at
glassy carbon and silver electrodes in DMF containing 0.050 M TMABF4. Cyclic
voltammograms for reduction of DDT at glassy carbon in DMF (dotted curve in upper
panel of Figure 2) exhibit five irreversible cathodic peaks at –1.02, –1.17, –1.37, –1.60,
and –1.80 V,the first two of which are not well defined. Depicted by the dotted curve in
the lower panel of Figure 2 is a cyclic voltammogram for reduction of 5.0 mM DDD at
glassy carbon in DMF–0.050 M TMABF4, which shows three irreversible cathodic peaks
at –1.42, –1.57, and–1.80 V. All of these observations are consistent with those reported
in an earlier paper from our laboratory [14].
In experiments never performed before, we examined the cyclic voltammetric
Page 8 of 32

9
behavior of DDT and DDD at silver cathodes in DMF–0.050 M TMABF4. Reduction of
DDT at a silver cathode (solid curve in upper panel of Figure 2) shows four irreversible
cathodic peaks at –0.56, –1.26, –1.58, and –1.80 V. Notice in Figure 2 that the first two
peaks appear at significantly less negative potentials for silver than for glassy carbon;
however, the final two peaks recorded with glassy carbon and silver do not differ
significantly in their respective peak potentials. Illustrated by the solid curve in the lower
panel of Figure 2 is a cyclic voltammogram for reduction of DDD at a silver cathode in
DMF–0.050 M TMABF4 that reveals three prominent irreversible cathodic peaks at
–1.22,–1.56, and –1.79 V, along with a tiny wave at –1.07 V. Comparison of the two
cyclic voltammograms in the lower panel of Figure 2 reveals that (i) reduction of DDD at
silver gives rise to one more cathodic peak than at glassy carbon and (ii) the two cathodic
peaks at the most negative potentials are virtually identical for both glassy carbon and
silver.
However, further scrutiny of cyclic voltammograms for reduction of DDT at
silver reveals that, depending on the scan rate, another small irreversible cathodic peak
can appear on the rising portion of the second peak. Depicted in Figure 3 is a set of
cyclic voltammograms recorded at 25, 75, and 250 mV s–1 for reduction of 5.0 mM DDT
at the same silver electrode. For a scan rate of 75 mV s–1, a small and barely discernible
peak can be seen at approximately –1.17 V; and, for the cyclic voltammogram recorded
at 250 mV s–1, there is a another small peak at –0.88 V. Thus, under some conditions,
there are five discernible stages in the reduction of DDT at silver.
Provided in Figure 4 is a comparison of cyclic voltammograms recorded at 100
mV s–1for reduction of DDT, DDD, DDNU, and DPE at a silver cathode in DMF–0.050
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10
M TMABF4. Behavior of DDT and DDD is identical with that described in the preceding
two paragraphs. For the reduction of DDNU at silver, two irreversible cathodic peaks at
–1.58 and –1.78 V are observed, whereas the reduction of DPE exhibits only a single
irreversible cathodic peak at –1.76 V. From a mechanistic viewpoint, it is significant
thatthe reductions of DDT, DDD, and DDNU give rise to a pair of cathodic peaks at –
1.57 ± 0.01 Vand at –1.79 ± 0.01 V and that reduction of DPE affords a single peak at –
1.76 V. For DDT, DDD, and DDNU, we attribute the peak at –1.57 V to reductive
cleavage of a pair of aryl carbon–chlorine bonds, whereas we assign the peak at a more
negative potential to the conversion of DPE to EBB, the latter being electroinactive.
In addition to an examination of the cyclic voltammetric behavior of DDT at
silver in DMF–0.050 M TMABF4, we have surveyed the use of other solvents for the
reduction of DDT. Depicted in Figure 5 is a comparison ofcyclic voltammograms
recorded at 100 mV s–1for the reduction of 5.0 mM DDT at silver in DMF, DMSO, and
CH3CN, each containing 0.050 M TMABF4.Relative magnitudes of the peak currents are
in accord with how the viscosity (η) of each solvent would be expected to affect the
diffusion coefficient of DDT: at 25 °C, ηDMSO (1.99 cP)> ηDMF (0.80 cP) > ηCH CN (0.35
3
cP).For the experimental conditions employed to obtain Figure 5, three prominent
cathodic peaks are associated with reduction of DDT in the DMSO medium, whereas
four stages of reduction are seen for reduction of DDT in both DMF and CH3CN. For all
three solvents the two cathodic peaks seen at potentials more negative than –1.5 V show
similar characteristics; on the other hand, the cathodic peaks seen at potentials more
positive than –1.5 V are decidedly dependent on the solvent.
Compiled in Table 1 is a summary of cyclic voltammetric peak potentials
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11
obtained at 100 mV s–1for reduction of DDT, DDD, DDNU, and DPE at both glassy
carbon and silver electrodes, together with two other members (DDMU and PVB) of the
family of DDT degradation products. In addition, we present cathodic peak potentials for
reduction of DDT at both glassy carbon and silver cathodes in CH3CN and DMSO, as
well as for reduction of DDT at silver in two different DMF–water mixtures. Notice that,
if the behavior of DDT and its degradation products is compared at glassy carbon and
silver cathodes, peak potentials for cleavage of the aliphatic carbon–chlorine bonds are
always less negative at silver. For the last two entries of Table 1, it is evident that an
increase in the water content of the solvent system causes a positive shift in the cathodic
peak potentials; furthermore, as the water content increases,both the cathodic peak
currents and the width of the potential window diminish.When cyclic voltammograms
were recorded with both glassy carbon and silver electrodes for solutions containing a 5.0
or 10.0 mM concentration of 1,1′-ethylidenebisbenzene (EBB),no cathodic peak was
observed. Finally, as shown in Figure 6, if one compares cyclic voltammograms for
reduction of 5.0 mM concentrations of DDNU and PVB at either a glassy carbon or silver
cathode, two peaks are observed; however, the peak current for the first stage of
reduction of PVB is approximately two thirds of that for the first stage of reduction of
DDNU.
3.2.Controlled-potential (bulk) electrolyses of DDT, DDD, and DDNU
Listed in Table 2 are results obtained from a set of controlled-potential (bulk)
electrolyses of 5.0 mM solutions of DDT, DDD, and DDNU in DMF containing 0.050 M
TMABF4carried out with a silver gauze cathode. Potentials for these electrolyses were
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12
chosen to correspond to values between 50 and 150 mV more negative than the peak
potentials (listed in Table 1) determined from the various cyclic voltammograms for
reduction of these substrates. Included in the table are the measured coulometric n values
and the product distributions; for the majority of these electrolyses, we were able to
account for essentially all of the starting material. Each entry for DDT and DDD is the
average of at least four separate electrolyses, whereas only two trials were done for entry
8.
For bulk electrolyses of DDT (entries 1–6), the coulometric n value and the extent
of dechlorination both increase as the potential is made more negative. When DDT was
electrolyzed at –0.71 V, the measuredn value was 1.6 and the major products were DDE
and DDMU. At –1.15 V, the measured n value was 2.5 and the major products were
DDE, DDNU, and DDMU. When DDT was electrolyzedat –1.40 V, the measured n
value was 5.2 and the starting material was converted mostly to DDMS; however, less-
chlorinated products (including DDMU, DDNU, and DPE) were detected. At –1.62 V
(entry 4), the measured n value was 8.1 and the major products werecompletely
dechlorinated DPE and EBB, although smaller amounts of four other products were
found.For electrolyses of DDT at potentials between –1.62 and –1.85 V, the average n
value did not change significantly, a finding that is consistent with previously published
results obtained with a carbon cathode [14].Electrochemical reduction of DDD at –1.40 V
(entry 7) gave a mixture of products (DDMU, DDNU, DPE, and PVB) as well as a
substantial quantity of unreduced DDD, and the measured n value was only 1.4. For
reduction of DDNU at –1.85 V, the coulometric n value was 5.0 (entry 8). For bulk
electrolyses of DDT and DDNU at silver cathodes held at –1.85 V, we were able to
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13
account only for 81% and 84% of the electrolysis products (entries 6 and 8, respectively);
for both sets of experiments, the major products were EBB and DPE.
For electrolyses conducted at –1.62 V (entry 5), the presence of 1 M D2O in the
solvent–supporting electrolyte caused a slight increaseof the measured n value to 8.3, and
the only electrolysis products were completely dechlorinated EBB (103%) and DPE
(4%). This finding is attributed to a small positive shiftin the reduction potential for DDT
as the availability of protons in the system increased. More importantly, when these
products were characterized by means of GC–MS, we found that the possible sites of
dechlorination were deuterated to an extent greater than 85%, a result that clearly points
to the intermediacy of carbanions.
To explore the possibility that radical intermediates might be involved in the
reduction of DDT, a bulk electrolysis of a 5.0 mM solution of DDT at a silver gauze
cathode held at –1.62 V in DMF-d7 containing 0.050 M TMABF4 was carried out. At the
end of this electrolysis, our GC–MS analysis of the catholyte revealed the presence of a
mixture of DDE, EBB, DDMU, DPE, and DDNU. Most importantly, mass spectral data
indicated that none of these products was deuterated, a result leading us to concludethat,
if radical intermediates arising from one-electron cleavage of carbon–chlorine bonds are
generated during the electrochemical reduction of DDT at silver, those intermediates
must be further reduced efficiently to carbanions. This finding is in contrast with a recent
investigation in our laboratory of the direct reduction of 1R,2r,3S,4R,5r,6S-
hexachlorocyclohexane (lindane) to benzene at a silver cathode [24]. In that research,
bulk electrolyses involving the use of DMF-d7as solvent revealed the presence of two
37
deuterated intermediates (C6H5D35Cl4 and C6H5D35Cl3 Cl), indicating that lindane
Page 13 of 32

14
undergoes a combination of one- and two-electron C–Cl bond cleavages and, therefore,
that both radical and carbanion intermediates are involved.
3.3. Mechanistic aspects of the reduction of DDT and its metabolites at silver cathodes
Shown in Figure 1 is a set of mechanistic steps, compatible with observations
made in this investigation, to account for the formation of the various products listed in
Table 2 that arise from bulk electrolyses of DDT, DDD, and DDNU at silver cathodes.
At –0.71 V, DDT (the only species reducible at this potential)undergoestwo-electron
reductive cleavage of an aliphatic carbon–chlorine bond to yield a carbanion
intermediate,which is protonated by residual water (~40 mM [14]) in the solvent to give
DDD. Alternatively, DDT can eliminate chloride ion in the presence of hydroxide ion
(arising from the deprotonation of adventitious water) to form DDE, and DDD is subject
to an analogous process to afford DDMU.
During an electrolysis of DDT at–1.15 V, the processes described above will
occur and, in addition, DDE is reduced to DDMU. At this potential, DDD undergoes a
two-electron reduction and subsequent protonation to form DDMS, and the latter species
can undergo elimination to form DDNU. Alternatively, DDNU arises via a two-electron
reduction of DDMU. At –1.62 V, reductive cleavage of aryl carbon–chlorine bonds of
DDNU takes place to afford PVB and DPE. Finally, at a potential of –1.85 V, DPE can
be converted to EBB via a two-electron, two-proton process.
A notable feature of the data shown in Table 2 is the lack of correspondence
between the measured n values and the theoretical n values calculated on the basis of the
observed product yields and the number of electrons associated with formation of each
Page 14 of 32

15
product that can be seen in Figure 1. For entries 1, 3, 4, and 6, the measured n values are
high, whereas the measured n values are low for entries 5 and 7, and the measured and
theoretical n values are essentially identical for entries 2 and 8. Such disagreement
between measured and theoretical n values for bulk electrolyses has been encountered in
earlier studies of the electrochemical reduction of chlorinated organic compounds at
silver cathodes, as noted in our previous paper describing the reduction of lindane [24].In
cases where the measurednvalue is larger than the theoretical value, DISP1 processes
might be competitive with the more straightforward two-electron reactions suggested by
Figure 1. Therefore, for the cyclic voltammograms shown in Figure 3, it was of interest
to examine the effect of scan rate on cathodic peak potentials for reduction of DDT by
measurement of Ep/log(v) [34]. For each of the three main cathodic peaks, we found
that this parameter exceeded significantly the theoretical value of –29.6 mV per log unit,
indicating that DISP1 reactions do not play a role in the reduction of DDT at silver.
4. Conclusions
In this investigation the electrochemical reduction of DDT at silver cathodes in
DMF containing 0.050 M TMABF4has been studied, along with a brief overview of the
behavior of two degradation products (DDD and DDNU). Cyclic voltammograms for
irreversible reduction of DDT at a silver electrode exhibit four prominent cathodic peaks
in DMF and CH3CN, whereas only three main peaks are seen in DMSO. Controlled-
potential electrolyses of DDT at a silver gauze cathode in DMF–0.050 M TMABF4 afford
a potential-dependent mixture of products that includes DDD, DDNU, DDE, DDMU,
Page 15 of 32

16
DDMS, PVB, DPE, and EBB. However, at the most negative potentials, the principal
products are completely dechlorinated DPE and EBB.
In comparison with our earlier investigation of the direct reduction of DDT at
glassy carbon cathodes [14], we have found that the aliphatic carbon–chlorine bonds of
DDT and its metabolites are much easier to reduce at silver electrodes in DMF as well as
in CH3CN and DMSO. Further reductive cleavage of the aryl carbon–chlorine bonds is
possible at potentials more negative than –1.62 V. Information about the mechanism and
products for electrochemical reduction of DDT and its metabolites at silver can provide
insights into how these environmentally harmful compounds can be degraded. It is
worthwhile to consider that the total dechlorination of DDT and its less-chlorinated
metabolites leads to the formation of DPE, which is a feedstock for the polymer industry.
Acknowledgments
Dr. Jonathan A. Karty, director of the Indiana University Mass Spectrometry
Facility, assisted with the interpretation of results obtained from the deuterium-labeling
experiments, and Dr. Elizabeth R. Wagoner aided with the quantitation of products
arising from controlled-potential (bulk) electrolyses.
Page 16 of 32

17
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Compounds and Risk of Pancreatic Cancer, Journal of the National Cancer Institute
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[2] B. van Wendel de Joode, C. Wesseling, H. Kromhout, P. Monge, M. García, D.
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[3] Agency for Toxic Substances and Disease Registry. Toxicological Profile for DDT,
DDE, and DDD; U.S. Department of Health and Human Services, Public Health
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[4] J. Beard, DDT and human health, Science of the Total Environment 355 (2006) 78.
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molecular basis of pesticide degradation by microorganisms,Critical Reviews in
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[8]H. Fukami, M. Nakajima, Polarographic analysis of agricultural chemicals. VI.
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[9]I. Rosenthal, R.J. Lacoste, Systematic polarographic study of the aromatic
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[10]A.S. Reddy, S.J. Reddy, Electrochemical reduction of DDT at the hanging mercury
drop electrode, Indian Journal of Chemistry 23A (1984) 619.
[11] R. Tandon, I. Bala, M. Singh, Investigations on the kinetics and mechanism of
polarographic reduction of methoxychlor and DDT, Zeitschrift für Physikalische
Chemie (Leipzig) 267 (1986) 340.
[12]S.G. Merica, W. Jędral, S. Lait, P. Keech, N.J. Bunce, Electrochemical reduction
and oxidation of DDT, Canadian Journal of Chemistry 77 (1999) 1281.
[13]S. Schweizer, J.F. Rusling, Q. Huang, Electrolytic dechlorination of DDT in a
bicontinuous microemulsion, Chemosphere 28 (1994) 961.
[14]M.S. Mubarak, P.C. Gach, D.G. Peters, Electrochemical reduction of 4,4′-(2,2,2-
trichloroethane-1,1-diyl)bis(chlorobenzene) (DDT) and 4,4′-(2,2-dichloroethane-
1,1-diyl)bis(chlorobenzene (DDD) at carbon cathodes in dimethylformamide,
Electroanalysis 18 (2006) 417.
[15]D.M. La Perriere, W.F. Carroll, Jr., B.C. Willett, E.C. Torp, D.G. Peters, Radicals
and carbanions as intermediates in the electrochemical reduction of 1-iododecane at
mercury. Effect of potential, electrolysis time, and water concentration on the
mechanism, Journal of the American Chemical Society 101 (1979) 7561.
[16]P.C. Gach, M.S. Mubarak, J.A. Karty, D.G. Peters, Catalytic reduction of 4,4′-
(2,2,2-trichloroethane-1,1-diyl)bis(chlorobenzene) with cobalt(I) salen
electrogenerated at vitreous carbon cathodes in dimethylformamide, Journal of The
Electrochemical Society 154 (2006) F1.
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19
[17] J.-G. Shao,A. Thomas,B.-C. Han,C.A. Hansen,DDT-reductive dechlorination
catalyzed by cobalt phthalocyanine, 2,3- and 3,4-tetrapyridoporphyrazine
complexes in non-aqueous media, Journal of Porphyrins and Phthalocyanines14
(2010) 133.
[18]W. Zhu, Y. Fang, W. Shen, G. Lu, Y. Zhang, Z. Ou, K.M. Kadish, Reductive
dechlorination of DDT electrocatalyzed by synthetic cobalt porphyrins in N,N′-
dimethylformamide, Journal of Porphyrins and Phthalocyanines 15 (2011) 66.
[19] E.R. Wagoner, J.A. Karty, D.G. Peters, Catalytic reduction of 4,4′-(2,2,2-
trichloroethane-1,1-diyl)bis(chlorobenzene) (DDT) with nickel(I) salen
electrogenerated at vitreous carbon cathodes in dimethylformamide, Journal of
Electroanalytical Chemistry 706 (2013) 55.
[20] M.A. Jabbar, H. Shimakoshi, Y. Hisaeda, Enhanced reactivity of hydrophobic
vitamin B12 towards the dechlorination of DDT in ionic liquid, Chemical
Communications (2007) 1653.
[21]N. Sonoyama, K. Hara, T. Sakata, Reductive electrochemical decomposition of
chloroform on metal electrodes, Chemistry Letters(1997)131.
[22]A.A. Isse, L. Falciola, P.R. Mussini, A. Gennaro, Relevance of electron transfer
mechanism in electrocatalysis: the reduction of organic halides at silver electrodes,
Chemical Communications(2006) 344.
[23]A.A. Isse, A. De Guisti, A.Gennaro, L. Falciola, P.R. Mussini, Electrochemical
reduction of benzyl halides at a silver electrode,Electrochimica Acta 51 (2006)
4956.
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[24] A.A. Peverly, J.A. Karty, D.G. Peters, Electrochemical reduction of
(1R,2r,3S,4R,5r,6S)-hexachlorocyclohexane (Lindane) at silver cathodes in organic
and aqueous–organic media, Journal of Electroanalytical Chemistry 692 (2013) 66.
[25] A.A. Isse, S. Gottardello, C. Durante, A. Gennaro, Dissociative electron transfer to
organic halides: Electrocatalysis at metal cathodes, Physical Chemistry Chemical
Physics 10 (2008) 2409.
[26] A.A. Isse, G. Sandonà, C. Durante, A. Gennaro, Voltammetric investigation of the
dissociative electron transfer to polychloromethanes at catalytic and non-catalytic
electrodes, Electrochimica Acta 54 (2009) 3235.
[27] K.L. Vieira,D.G. Peters,Voltammetric behavior of tertiary butyl bromide at
mercury electrodes in dimethylformamide, Journal of Electroanalytical Chemistry
93 (1985) 93.
[28]M.P. Foley,P. Du, K.J. Griffith, J.A. Karty,M.S. Mubarak,K. Raghavachari,D.G.
Peters, Electrochemistry of substututed salen complexes of nickel(II): Nickel(I)-
catalyzed reduction of alkyl and acetylenic halides, Journal of Electroanalytical
Chemistry 647 (2010) 194.
[29] P. Vanalabhpatana, D.G. Peters, Catalytic reduction of 1,6-dihalohexanes by
nickel(I) salen electrogenerated at glassy carbon cathodes in dimethylformamide,
Journal of The Electrochemical Society 152 (2005) E222.
[30] L.W. Marple, Reference electrode for anhydrous dimethylformamide, Analytical
Chemistry 39 (1967) 844.
[31] C.W. Manning, W.C. Purdy, Reference electrode for electrochemical studies in
dimethylformamide, Analytica Chimica Acta 51 (1970) 124.
Page 20 of 32

21
[32] J.L. Hall, P.W. Jennings, Modification of the preparation of a cadmium amalgam
reference electrode for use in N,N-dimethylformamide, Analytical Chemistry 48
(1976) 2026.
[33] W.A. Pritts, K.L. Vieira, D.G. Peters, Quantitative determination of volatile
products formed in electrolyses of organic compounds, Analytical Chemistry 65
(1993) 2145.
[34]J.-M. Savéant, Elements of Molecular and Biomolecular Electrochemistry. An
Electrochemical Approach to Electron Transfer Chemistry, Wiley, New York,
2006, p. 96.
Page 21 of 32

22
CAPTIONS FOR FIGURES
Fig. 1.DDT and its degradation products: structures, names, abbreviations, and proposed
pathways for electrochemical and base-promoted degradation.
Fig. 2. Cyclic voltammograms recorded at 100 mV s–1for reduction of 5.0 mM DDT at a
glassy carbon cathode (upper panel, dotted curve) and at a silver cathode (upper panel,
solid curve) and for reduction of 5.0 mM DDD at a glassy carbon cathode (lower panel,
dotted curve) and at a silver cathode(lower panel, solid curve) in oxygen-free DMF
containing 0.050 M TMABF4. Areas of electrodes were 0.071 cm2, and scans go from
ca. 0 to –2.15 to 0 V.Potentials are given with respect to a cadmium-saturated mercury
amalgam reference electrode in contact with DMF saturated with both cadmium chloride and
sodium chloride; this electrode has a potential of –0.76 V versus SCE at 25 °C.
Fig. 3. Cyclic voltammograms recorded at scan rates of 25, 75, and 250 mV s–1 for
reduction of 5.0 mM DDT at a silver cathode (area of 0.071 cm2) in oxygen-free DMF
containing 0.050 M TMABF4. Scans go from ca. 0 to –2.0 (or beyond) to 0 V. Notice
the appearance of a shoulder peak at –1.17 V for the cyclic voltammogram recorded at 75
mV s–1, which establishes that DDT can undergo five stages of reduction; see text for
further discussion.Potentials are given with respect to a cadmium-saturated mercury amalgam
reference electrode in contact with DMF saturated with both cadmium chloride and sodium
chloride; this electrode has a potential of –0.76 V versus SCE at 25 °C.
Page 22 of 32

23
CAPTIONS FOR FIGURES (continued)
Fig. 4. Cyclic voltammograms recorded at 100 mV s–1 for reduction of 5.0 mM solutions
of DDT, DDD, DDNU, and DPE at a silver cathode (area of 0.071 cm2) in oxygen-free
DMF containing 0.050 M TMABF4. Scans go from ca. 0 to –2.15 to 0 V.Potentials are
given with respect to a cadmium-saturated mercury amalgam reference electrode in contact with
DMF saturated with both cadmium chloride and sodium chloride; this electrode has a potential of
–0.76 V versus SCE at 25 °C.
Fig. 5. Cyclic voltammograms recorded at 100 mV s–1 for reduction of 5.0 mM solutions
of DDT at a silver cathode (area of 0.071 cm2) in oxygen-free DMF, DMSO, and
CH3CN, each containing 0.050 M TMABF4. Scans go from ca. 0 to –2.15 to 0
V.Potentials are given with respect to a cadmium-saturated mercury amalgam reference electrode
in contact with DMF saturated with both cadmium chloride and sodium chloride; this electrode
has a potential of –0.76 V versus SCE at 25 °C.
Fig. 6. Cyclic voltammograms recorded at 100 mV s–1 for reduction of 5.0 mM solutions
of DDNU and PVB at a silver cathode (area of 0.071 cm2) in oxygen-free DMF
containing 0.050 M TMABF4. Scans go from ca. 0 to –2.15 to 0 V.Potentials are given
with respect to a cadmium-saturated mercury amalgam reference electrode in contact with DMF
saturated with both cadmium chloride and sodium chloride; this electrode has a potential of –0.76
V versus SCE at 25 °C.
Page 23 of 32

24
Figure 1
C. M. McGuire and D. G. Peters
“Electrochemical reduction of
4,4′-(2,2,2-trichloroethane-1,1-
diyl)bis(chlorobenzene) (DDT)
at silver cathodes”
H
Cl
C
Cl
CCl3
_
_
_
_
2 e
,
, H2O
OH
(E2 elimination)
Cl
4,4'-(2,2,2-trichloroethane-1,1-diyl)-
bis(chlorobenzene)(DDT)
H
Cl
C
Cl
Cl
C
Cl
CHCl2
CCl2
4,4'-(2,2-dichloroethane-1,1-diyl)-
4,4'-(2,2-dichloroethene-1,1-diyl)-
bis(chlorobenzene) (DDD)
bis(chlorobenzene)(DDE)
_
_
_
_
_
_
Cl
2 e
,
, H2O
2 e
,
, H2O
Cl
H
Cl
C
Cl
Cl
C
Cl
CH2Cl
CHCl
4,4'-(2-chloroethane-1,1-diyl)-
4,4'-(2-chloroethene-1,1-diyl)-
bis(chlorobenzene) (DDMS)
bis(chlorobenzene) (DDMU)
_
_
_
Cl , H2O
2 e
,
Cl
C
Cl
CH2
4,4'-(ethene-1,1-diyl)-
bis(chlorobenzene) (DDNU)
_
_
_
Cl , H2O
2 e
,
Cl
C
CH2
1-chloro-4-(1-phenylvinyl)benzene (PVB)
_
_
_
2 e
,
, H2O
Cl
C
CH2
1,1-diphenylethylene (DPE)
_
2 e
, 2 H2O
H
C
CH3
1,1'-ethylidenebisbenzene (EBB)
Page 24 of 32

25
Figure 2
C. M. McGuire and D. G. Peters
“Electrochemical reduction of
4,4′-(2,2,2-trichloroethane-1,1-
diyl)bis(chlorobenzene) (DDT)
at silver cathodes”
0
-50
-100
-150
-200
-250
-300
-350
-400
DDT at C
DDT at Ag
-2.0
-1.5
-1.0
-0.5
0.0
0
-50
-100
-150
-200
-250
-300
-350
-400
DDD at C
DDD at Ag
-2.0
-1.5
-1.0
-0.5
0.0
Potential (V vs. Cd(Hg))
Page 25 of 32

26
Figure 3
C. M. McGuire and D. G. Peters
“Electrochemical reduction of
4,4′-(2,2,2-trichloroethane-1,1-
diyl)bis(chlorobenzene) (DDT)
at silver cathodes”
0
-100
-200
-300
-400
-500
-600
-700
25 mV s-1
75 mV s-1
250 mV s-1
-2.0
-1.5
-1.0
-0.5
0.0
Potential (V vs. Cd(Hg))
Page 26 of 32

27
Figure 4
C. M. McGuire and D. G. Peters
“Electrochemical reduction of
4,4′-(2,2,2-trichloroethane-1,1-
diyl)bis(chlorobenzene) (DDT)
at silver cathodes”
0
-50
-100
-150
-200
-250
-300
-350
DDT
DDD
DDNU
DPE
-2.25 -2.00 -1.75 -1.50 -1.25 -1.00 -0.75 -0.50 -0.25
Potential (V vs. Cd(Hg))
Page 27 of 32

28
Figure 5
C. M. McGuire and D. G. Peters
“Electrochemical reduction of
4,4′-(2,2,2-trichloroethane-1,1-
diyl)bis(chlorobenzene) (DDT)
at silver cathodes”
0
-100
-200
-300
-400
-500
-600
-700
DMF
DMSO
CH3CN
-2.0
-1.5
-1.0
-0.5
0.0
Potential (V vs. Cd(Hg))
Page 28 of 32

29
Figure 6
C. M. McGuire and D. G. Peters
“Electrochemical reduction of
4,4′-(2,2,2-trichloroethane-1,1-
diyl)bis(chlorobenzene) (DDT)
at silver cathodes”
0
-50
-100
-150
-200
-250
-300
-350
DDNU
PVB
-2.0
-1.5
-1.0
-0.5
0.0
Potential (V vs. Cd(Hg))
Page 29 of 32