Direct Reduction of 1-Bromo-6-chlorohexane and 1-Chloro-6-iodohexane at Silver Cathodes in Dimethylformamide

Article abstract of DOI:10.1016/j.electacta.2016.09.066

Cyclic voltammetry and controlled-potential (bulk) electrolyses have been employed to probe the electrochemical reductions of 1-bromo-6-chlorohexane and 1‐chloro-6-iodohexane at silver cathodes in dimethylformamide (DMF) containing 0.050?M tetra-n-butylammonium tetrafluoroborate (TBABF₄). A cyclic voltammogram for reduction of 1-bromo-6-chlorohexane shows a single major irreversible cathodic peak, whereas reduction of 1-chloro-6-iodohexane gives rise to a pair of irreversible cathodic peaks. Controlled-potential (bulk) electrolyses of 1-bromo-6-chlorohexane at a silver gauze cathode reveal that the process involves a two-electron cleavage of the carbon–bromine bond to afford 1-chlorohexane as the major product, along with 6-chloro-1-hexene, n‐hexane, 1‐hexene, and 1,5-hexadiene as minor species. In contrast, bulk electrolyses of 1-chloro-6-iodohexane indicate that the first voltammetric peak corresponds to a one-electron process, leading to production of a dimer (1,12-dichlorododecane) together with 1-chlorohexane and 6-chloro-1-hexene as well as 1‐hexene and 1,5-hexadiene in trace amounts. At potentials corresponding to the second cathodic peak, reduction of 1-chloro-6-iodohexane is a mixture of one- and two-electron steps that yields the same set of products, but in different proportions. Mechanistic schemes are proposed to explain the electrochemical behavior of both 1‐bromo-6-chlorohexane and 1-chloro-6-iodohexane.

Full text of DOI:10.1016/j.electacta.2016.09.066

Electrochimica Acta 218 (2016) 311–317
Contents lists available at ScienceDirect
Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Direct Reduction of 1-Bromo-6-chlorohexane and 1-Chloro-6-
iodohexane at Silver Cathodes in Dimethylformamide
John A. Rosea, Caitlyn M. McGuirea, Angela M. Hansena, Jonathan A. Kartya,
Mohammad S. Mubarakb, Dennis G. Petersa,
*
a
Department of Chemistry, Indiana University, Bloomington, IN 47405, USA
Department of Chemistry, The University of Jordan, Amman, 11942, Jordan
b
A R T I C L E I N F O
A B S T R A C T
Article history:
Cyclic voltammetry and controlled-potential (bulk) electrolyses have been employed to probe the
electrochemical reductions of 1-bromo-6-chlorohexane and 1-chloro-6-iodohexane at silver cathodes in
dimethylformamide (DMF) containing 0.050 M tetra-n-butylammonium tetrafluoroborate (TBABF4). A
cyclic voltammogram for reduction of 1-bromo-6-chlorohexane shows a single major irreversible
cathodic peak, whereas reduction of 1-chloro-6-iodohexane gives rise to a pair of irreversible cathodic
peaks. Controlled-potential (bulk) electrolyses of 1-bromo-6-chlorohexane at a silver gauze cathode
reveal that the process involves a two-electron cleavage of the carbon–bromine bond to afford
1-chlorohexane as the major product, along with 6-chloro-1-hexene, n-hexane, 1-hexene, and
1,5-hexadiene as minor species. In contrast, bulk electrolyses of 1-chloro-6-iodohexane indicate that
the first voltammetric peak corresponds to a one-electron process, leading to production of a dimer
(1,12-dichlorododecane) together with 1-chlorohexane and 6-chloro-1-hexene as well as 1-hexene and
1,5-hexadiene in trace amounts. At potentials corresponding to the second cathodic peak, reduction of
1-chloro-6-iodohexane is a mixture of one- and two-electron steps that yields the same set of products,
but in different proportions. Mechanistic schemes are proposed to explain the electrochemical behavior
of both 1-bromo-6-chlorohexane and 1-chloro-6-iodohexane.
Received 14 June 2016
Received in revised form 11 August 2016
Accepted 12 September 2016
Available online 13 September 2016
Keywords:
Carbon-halogen bond cleavage
Silver cathodes
Electrochemical reduction
1-Bromo-6-chlorohexane
1-Chloro-6-iodohexane
ã 2016 Elsevier Ltd. All rights reserved.
1. Introduction
1,6-dibromohexane. In hindsight, it is surprising that the
electrochemical behavior of no other dihalohexane at a silver
In a recent publication [1] from our laboratory, the electro-
chemical reduction of a pair of dibromoalkanes, namely 1,2- and
1,6-dibromohexane, at silver cathodes in dimethylformamide was
examined. Cyclic voltammograms for each compound exhibit a
single irreversible peak, but the cathodic peak potentials are
significantly different. Moreover, for identical concentrations of
each compound, the cathodic peak current for 1,2-dibromohexane
is one half of that for 1,6-dibromohexane, an observation directly
related to the number of electrons associated with reduction of
each species and thus with the mechanism for reduction of each
substrate. Whereas only one product (1-hexene) is formed via bulk
electrolysis of 1,2-dibromohexane, a mixture of five different
species (1-hexene, n-hexane, 1,5-hexadiene, 5-hexen-1-ol, and
n-dodecane) can arise from the controlled-potential reduction of
cathode has been investigated. However, in relatively recent
work, Simonet [2] has probed the electrochemical reduction of
1,3-dibromopropane at silver cathodes in dimethylformamide
containing tetra-n-butylammonium salts; this compound was
found to undergo two-electron reduction to afford a diradical
that undergoes polymerization, as well as cyclization to yield
cyclopropane. Aside from these two publications, we are not aware
of any other work that deals with reduction of
at a silver cathode.
a,v-dihaloalkanes
In much earlier work, our laboratory carried out a series of
studies [3–7] that involved the direct reduction of a family of
a,v-dihaloalkanes (ranging from compounds with n-propyl to
n-decyl moieties and with identical or dissimilar halogens)
at carbon cathodes in dimethylformamide. Whereas bulk
electrolyses of various
to the formation of cyclopropane [3], the
-dihalopentanes afforded the corresponding cycloalkanes
in modest yield [4,5]. On the other hand, preparative-scale
a
,
v
-dihalopropanes led almost exclusively
a,v
-dihalobutanes and
a,v
* Corresponding author.
E-mail address: peters@indiana.edu (D.G. Peters).
http://dx.doi.org/10.1016/j.electacta.2016.09.066
0013-4686/ã 2016 Elsevier Ltd. All rights reserved.

312
J.A. Rose et al. / Electrochimica Acta 218 (2016) 311–317
electrolyses of
a
,
v
-dihalogenated hexanes, octanes, and decanes
sources being given in parentheses: 1-bromo-6-chlorohexane (1)
(95%, Sigma Aldrich), 1-chloro-6-iodohexane (2) (96%, Sigma
Aldrich), 1,12-dichlorododecane (96 +%, Acros Organics), n-hexa-
decane (99%, Sigma Aldrich), n-heptane (99%, Mallinckrodt), n-
hexane (96%, EMD), 1-hexene (98%, Alfa Aesar), 1,5-hexadiene
(98%, Sigma Aldrich), deuterium oxide (D2O, 99.9 atom % D, Sigma
Aldrich), and diethyl ether (absolute, anhydrous, EMD). Dime-
thylformamide (DMF, 99.99%, Omnisolv) was used as the solvent
for all electrochemical experiments. Tetra-n-butylammonium
tetrafluoroborate (TBABF4, 98 +%, TCI), used as the supporting
electrolyte, was recrystallized from an ethyl acetate–hexane
mixture, and then stored in a vacuum oven at 80 ꢀC to exclude
traces of water. Before use, 1-bromo-6-chlorohexane (1) was
vacuum distilled. Deaeration of all solutions was accomplished
with zero-grade argon (Air Products).
produced no carbocyclic products, but only mixtures of straight-
chain alkanes and alkenes, along with some dimeric species arising
from radical-coupling reactions [6,7].
Inthelasttwodecades,therehasbeenconsiderableinterestinthe
use of silver cathodes to investigate electron-transfer events
centered on the electrochemical reduction of carbon–halogen
bonds. Investigations have focused on (a) mechanistic features of
these processes, (b) applications in the field of electrosynthesis, and
(c) uses of electrochemical reductions at silver electrodes for the
dehalogenation of environmental pollutants. These studies have
revealed the catalytic ability of silver as a cathode for the reductive
cleavage of carbon–halogen bonds at much less negative potentials
than for other electrodes such as glassy carbon or mercury [8].
Without being encyclopedic, but hoping to provide some guidance
to interested readers, we summarize briefly in the next paragraph
some recent literature that identifies both important contributors
to and applications in this expanding field of research.
2.2. Electrodes, cells, and instrumentation
Utilizing surface-enhanced Raman spectroscopy [9], density
functional theory [10], and digital simulation [11], Amatore and
co-workers investigated the mechanism for reduction of benzyl
chloride at a silver cathode in acetonitrile, particularly with respect
to the existence and detection of adsorbed benzyl chloride as well
as benzyl radicals and benzyl anions. Rondinini et al. [12] studied
the direct reduction of haloadamantanes at silver to assess how the
position of the halogen moiety affects the yield of the dimer
formed via controlled-potential (bulk) electrolysis. More recently,
the same laboratory has prepared silver nanoparticles and
employed them as composite-supported catalysts for the reduc-
tion of chloroform to methane in an aqueous medium [13]. In other
work by the group of Simonet [14,15], the behavior of alkyl iodides
at silver electrodes was probed, and it was observed that homo-
dimerization is the dominant process. A number of publications
related to the reduction of halogenated organic compounds at
silver cathodes by Isse, Gennaro, and their co-workers include
investigations of the reduction of benzyl halides [16], the
mechanism of dissociative electron transfer to organic chlorides
[17–19], and the carboxylation of activated carbon–halogen bonds
[20–22]. In our laboratory, a recent study of the reduction of an
assortment of primary, secondary, and tertiary alkyl monohalides
at silver cathodes was conducted [23]. In addition, electrochemical
reduction (remediation) of some well-known environmentally
hazardous and halogenated pollutants at silver cathodes has been
examined; these compounds include freons such as CFC-113 [24–
27], pesticides such as lindane [28] and DDT [29], and flame
retardants such as decabromodiphenyl ether [30] and hexabro-
mocyclododecane [31].
Cyclic voltammetry was performed in a cell previously
described [32]. A silver (3.0-mm diameter, Alfa-Aesar, 99.9%)
working electrode with a circular planar area of 0.071 cm2 was
used along with a coil of platinum wire which acted as the auxiliary
(counter) electrode. Before each cyclic voltammogram was
acquired, the working electrode was cleaned on a polishing pad
(Buehler) with 0.05-mm aqueous alumina suspension and washed
in an ultrasonic bath with DMF. Cyclic voltammetry experiments
were performed with a Princeton Applied Research Corporation
(PARC) model 2273 or 273A potentiostat that utilized a Power-
Suite1 software package. All data were processed with the aid of
OriginPro 9 or OriginPro 2016 software.
Controlled-potential (bulk) electrolyses were performed in a
two-compartment cell [33]. Silver working electrodes, each with
an estimated surface area of 45 cm2, were constructed from silver
mesh (99.9%, Alfa Aesar) woven from 0.356-mm-diameter wire,
with an additional single wire for the electrical lead. Prior to each
electrolysis, the silver working cathodes were cleaned by ultra-
sonication for at least 30 min in an aqueous sodium bicarbonate
paste, then washed with distilled water and dried for 20 min in an
oven at 180 ꢀC. Owing to the volatility of products arising from
reductions of 1 and 2, sparging of argon through the cathode
compartment to remove oxygen was stopped when the back-
ground current of a pre-electrolysis reached a baseline level. Then
the cell was sealed with parafilm, and the desired starting material
was injected and electrolyzed. A carbon rod auxiliary (counter)
electrode was immersed in DMF–TBABF4 in the anode compart-
ment that was separated from the cathode compartment by a
sintered-glass disk backed by a methyl cellulose plug containing
solvent–electrolyte. Bulk electrolyses were conducted with the aid
of a PARC model 173 potentiostat that utilized a locally written data
collection LabView program; the acquired data were processed
with OriginPro 9 or OriginPro 2016 software.
In the present study, we have sought to extend our knowledge
of the electrochemical reduction of a,v-dihaloalkanes by utilizing
cyclic voltammetry and controlled-potential (bulk) electrolysis to
investigate the direct electrochemical reductions of 1-bromo-6-
chlorohexane (1) and 1-chloro-6-iodohexane (2) at silver cathodes
in dimethylformamide (DMF) containing tetra-n-butylammonium
tetrafluoroborate (TBABF4) as the supporting electrolyte. Products
resulting from bulk electrolyses of 1 and 2 have been separated,
identified, and quantitated with the aid of gas chromatography
(GC) and gas chromatography–mass spectrometry (GC–MS).
Utilizing the information acquired, we propose mechanistic
schemes for the reduction of 1 and 2.
All potentials in this paper are given with respect to a reference
electrode consisting of a cadmium-saturated mercury amalgam in
contact with DMF saturated with both sodium chloride and
cadmium chloride; this electrode has a potential of ꢁ0.76 V versus
the aqueous saturated calomel electrode (SCE) at 25 ꢀC [34–36].
2.3. Separation, identification, and quantitation of products
All product identification was accomplished with the aid of gas
chromatography–mass spectrometry (GC–MS); an Agilent 6890N
gas chromatograph, fitted with a 30 m ꢂ 0.25 mm capillary column
2. Experimental
2.1. Reagents
with a 0.25 mm DB-5 stationary phase consisting of 5% phenyl-
polysiloxane and 95% methylpolysiloxane (J & W Scientific), was
used in tandem with an Agilent 5973 inert mass-selective detector
operating in electron ionization mode (70 eV). Deuterium
Each of the following chemicals was purchased and used as
received unless otherwise indicated, their purities and commercial

J.A. Rose et al. / Electrochimica Acta 218 (2016) 311–317
313
incorporation was measured with a Thermo MAT-95 XP magnetic
sector mass spectrometer with a ThermoTrace GC as an inlet under
conditions similar to those described above for the Agilent system;
the degree of isotopic incorporation was computed as described
elsewhere [37].
Before each bulk electrolysis, known amounts of internal
standards (n-heptane and n-hexadecane) were injected into the
catholyte. At the end of each experiment, the catholyte was
partitioned between cold water and diethyl ether; the resulting
organic layer was washed twice with cold water, dried over
anhydrous sodium sulfate, and concentrated via rotary evapora-
tion. Products were separated and quantitated by use of an Agilent
7890A gas chromatograph, equipped with a flame-ionization
detector; 1,12-dichlorododecane, 1-chlorohexane, and 6-chloro-1-
hexene were separated with the DB-5 column described above,
whereas more volatile products (1,5-hexadiene, 1-hexene, and
n-hexane) required separation by a 12.5-m (including a 2.5-m
particle trap) ꢂ 0.53-mm ꢂ 20-
mm PLOT fused-silica capillary
column (Varian). Using a procedure described in earlier research
[38], we determined the product distributions in this report from
gas chromatographic response factors and peak areas measured
with the aid of Agilent ChemStation software.
Fig. 2. Cyclic voltammograms recorded with
a
silver electrode (geometric
area = 0.071 cm2) at 100 mV sꢁ1 in oxygen-free DMF containing 0.050 M TBABF4
(curve A) and 5.0 mM 1-chloro-6-iodohexane, 2 (curve B). Scans go from ca. 0 to
ꢁ2.2 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.
3. Results and discussion
3.1. Cyclic voltammetric behavior of 1-bromo-6-chlorohexane (1) and
1-chloro-6-iodohexane (2)
voltammograms for this compound with the same electrode
and with different electrodes; the cyclic voltammogram shown in
Fig. 2 is the most “representative” of what we typically observed.
Unfortunately, we do not yet have an explanation for this
phenomenon. For example, the appearance or absence of a
“double” peak does not seem related in any obvious way to the
polishing procedure done between scans. Moreover, for bulk
electrolyses of 2 discussed in the next section of this paper, the
potential was always set at a value (–1.60 V) to be significantly
more negative than the second (single or double) major peak.
In addition, we recorded cyclic voltammograms for reduction of
both 1 and 2 at scan rates ranging from 25 to 250 mV sꢁ1, as shown
in Figs. 3 and 4, respectively. For each of the predominant second
cathodic peaks for reduction of 1 and 2, we found that Ipc increases
Fig.1 depicts a cyclic voltammogram recorded at 100 mV sꢁ1 for
the reduction of a 5.0 mM solution of 1-bromo-6-chlorohexane (1)
at a silver electrode in DMF containing 0.050 M TBABF4. One major
irreversible cathodic peak is observed at ꢁ1.45 V, and there is a
small (possibly reversible) peak on the rising portion of the large
peak. Shown in Fig. 2 is a cyclic voltammogram recorded at
100 mV sꢁ1 for the reduction of a 5.0 mM solution of 1-chloro-6-
iodohexane (2) at a silver electrode in DMF containing 0.050 M
TBABF4. Two irreversible peaks are seen at ꢁ0.63 and ꢁ1.36 V, the
second of which is clearly dominant and which, for some recorded
cyclic voltammograms, appears as two closely overlapped peaks.
In fact, we spent significant time recording many cyclic
Fig. 1. Cyclic voltammograms recorded with
a
silver electrode (geometric
Fig. 3. Cyclic voltammograms recorded at scan rates of 25, 50, 75,100, 150, 200, and
250 mV sꢁ1 for reduction of 5.0 mM 1-bromo-6-chlorohexane (1) at a silver cathode
(area = 0.071 cm2) in oxygen-free DMF containing 0.050 M TBABF4. Scans go from ca.
0 to ꢁ2.0 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.
area = 0.071 cm2) at 100 mV sꢁ1 in oxygen-free DMF containing 0.050 M TBABF4
(curve A) and 5.0 mM 1-bromo-6-chlorohexane, 1 (curve B). Scans go from ca. 0 to
ꢁ2.2 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.

314
J.A. Rose et al. / Electrochimica Acta 218 (2016) 311–317
orbital makes the formation of a radical–anion unfavorable; the
incoming electron must move into an elongated carbon–halogen
bond, which is cleaved upon injection of the electron, resulting in a
concerted process.
3.2. Controlled-potential (bulk) electrolyses of 1-bromo-6-
chlorohexane (1) and 1-chloro-6-iodohexane (2)
Summarized in Table 1 are coulometric data and product
distributions arising from bulk electrolyses of 1 and 2 at silver
gauze electrodes in DMF containing 0.050 M TBABF4 with applied
potentials chosen to correspond to the major cathodic peaks
revealed by cyclic voltammetry (Figs. 1 and 2). Each entry
corresponds to the average of at least three reproducible experi-
ments. Percentages represent the amount of starting material
converted into each product.
Reduction of 1 results in a coulometric n value of 1.4, indicating
the likely generation of carbanion intermediates; 1-chlorohexane
(3) was the major product, along with 6-chloro-1-hexene (4), 1,5-
hexadiene (5), 1-hexene (6), and n-hexane (7). Increasing the
concentration of 1 had little effect on the product distribution
(Table 1, entry 2), except that the yields of 3 and 4 were increased
and decreased, respectively. Because the potential for complete
reduction of 1-chlorohexane is more negative than the potential
used to reduce 1, we are not surprised to find 1-chlorohexane as
the major product. When 1 was electrolyzed at ꢁ1.60 V in the
presence of a 100-fold excess of D2O, we found that the resulting
1-chlorohexane was monodeuterated to an extent of 52%, an
observation that supports the intermediacy of carbanions.
A reviewer of the first version of our manuscript raised an
interesting question concerning whether bromide ion, arising from
the reduction of 1, might be adsorbed onto the silver cathode such
that the reduction of 1 is affected. One piece of evidence against
this proposal is that no unreduced 1 remained at the end of bulk
electrolyses conducted at ꢁ1.60 V (Table 1, entries 1 and 2). Even
more compelling information is provided in a publication by
Falciola and coworkers [41] which indicates that, at potentials
more negative than ꢁ1.4 V vs. SCE (or ꢁ0.64 V vs. our reference
electrode), bromide ion is desorbed completely from silver.
Therefore, for the reduction of 1, we believe that adsorption–
desorption of bromide ion does not play a significant role. An
analogous situation was encountered in an earlier study [31] from
our laboratory of the reduction of 1,2,5,6,9,10-hexabromocyclodo-
decane at silver in dimethylformamide containing 0.10 M tetra-
methylammonium tetrafluoroborate.
Fig. 4. Cyclic voltammograms recorded at scan rates of 25, 50, 75,100, 150, 200, and
250 mV sꢁ1 for reduction of 5.0 mM 1-chloro-6-iodohexane (2) at a silver cathode
(area = 0.071 cm2) in oxygen-free DMF containing 0.050 M TBABF4. Scans go from ca.
ꢁ0.07 to ꢁ1.85 to ꢁ0.07 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.
in proportion to v
1/2. On the other hand, the first peak for reduction
of 2 is proportional to v between 25 and 250 mV sꢁ1; it is
noteworthy that this first peak is symmetrical in shape, which may
be indicative of species adsorption onto the surface of the silver
cathode. For the specified range of scan rates, Epc shifted (by
various increments) to more negative values: (a) for 1, Epc shifted
from ꢁ1.41 to ꢁ1.82 V (a change of 410 mV); (b) for 2, Epc for the
first peak shifted from ꢁ0.65 to ꢁ0.77 V (a change of 120 mV) and
Epc for the second peak shifted from ꢁ1.36 to ꢁ1.59 V (a change of
230 mV). Thus, in all cases, the value of
DEp/Dlog(v) greatly
exceeded the diagnostic (–59.2 mV per log unit) which would be
expected for a stepwise mechanism where the initial electron
transfer is the rate-determining step (with a transfer coefficient,
close to 0.5) [39]. However, using the equation = –1.15(RT/F)
log Epc), we calculated transfer coefficients lower than 0.5
a,
a
(
D
n/D
that ranged from 0.07 to 0.24. These small transfer coefficients
are indicative of the large driving force required to cleave
carbon–halogen bonds and are consistent with a concerted
mechanism [39]. This finding is in accord with previous reports
which indicate that reduction of aliphatic halides at silver cathodes
occurs via a concerted electron-transfer mechanism. Isse and
coworkers [40] comment that the absence of an energetically
Electrolyses of 2 at a potential of ꢁ0.95 V (corresponding to the
first voltammetric peak) resulted in an
n value of 1.1, an
observation indicating the dominance of radical intermediates.
Major products were 1-chlorohexane (3), 6-chloro-1-hexene (4),
accessible
p* orbital and the high energy of the s* carbon–halogen
Table 1
Coulometric n values and product distributions for electrochemical reduction of 1-bromo-6-chlorohexane (1) and 1-chloro-6-iodohexane (2) at silver mesh cathodes in
oxygen-free DMF containing 0.050 M TBABF4.
Entry
Substrate
Concentration (mM)
E (V)
na
Product distribution (%)b
3
4
5
6
7
8
Total
1
2
3
4
5
6
1
1
2
2
2
2
5.0
10.0
5.0
10.0
5.0
10.0
–1.60
–1.60
–0.95
–0.95
–1.60
–1.60
1.4
1.4
1.1
1.1
1.3
1.1
48
63
27
34
52
43
21
10
22
19
21
23
4
4
7
5
19
18
NDc
NDc
5
NDc
NDc
33
40
8
99
100
82
93
94
96
TRd
TRd
3
TRd
TRd
5
2
1
5
22
3 = 1-chlorohexane; 4 = 6-chloro-1-hexene; 5 = 1,5-hexadiene; 6 = 1-hexene; 7 = n-hexane; 8 = 1,12-dichlorododecane.
a
Average number of electrons per molecule of 1 or 2.
Yield expressed as the percentage of 1 or 2.
ND = not detected.
TR = trace.
b
c
d

J.A. Rose et al. / Electrochimica Acta 218 (2016) 311–317
315
and 1,12-dichlorododecane (8). As anticipated, doubling the
concentration of 2 resulted in an increase in the production of
the dimer (8). In addition, we observed small amounts of
completely dehalogenated products [1,5-hexadiene (5) and 1-
hexene (6)]. Reduction of 2 at a potential of ꢁ1.60 V (corresponding
to the second voltammetric peak) resulted in an n value of 1.3,
suggesting a mixture of one- and two-electron processes. Addition
of D2O for the reduction of
2
at ꢁ0.95 V resulted in no
incorporation of deuterium into the resulting 1-chlorohexane
(3); however, we did find that 1-chlorohexane (3) was mono-
deuterated to an extent of 56% when 2 was reduced at a potential of
ꢁ1.60 V in the presence of a 100-fold excess of D2O. These results
support the conclusion that the reduction of 2 at ꢁ0.95 V occurs via
a radical mechanism, whereas the electrolysis of 2 at ꢁ1.6 V
proceeds through a mixture of one- and two-electron transfers.
Reduction at the more negative potential generated the same set of
products, but with a different distribution. When the concentra-
tion of 2 was 5 mM, a dramatic decrease in dimer formation
occurred, allowing the production of 1-chlorohexane to increase.
At a higher concentration (10 mM) of 2, formation of the dimer
increases and the n value decreases correspondingly, which
indicates the predominance of radical processes and enhanced
radical coupling.
Fig. 5. Cyclic voltammograms recorded with
a
silver electrode (geometric
area = 0.071 cm2) at 100 mV sꢁ1 in oxygen-free DMF containing 0.050 M TBABF4.
Curve A: no added 1-chlorohexane; curve B: with 5.0 mM 1-chlorohexane. Scans go
from ca. 0 to ꢁ2.0 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.
3.3. Mechanistic aspects of the reduction of 1-bromo-6-chlorohexane
(1)
be deprotonated by 9 to eliminate a chloride or bromide ion,
respectively, and to yield 3. Finally, as shown in reaction 6, 3 can be
reductively dechlorinated to give n-hexane (7). To confirm the
occurrence of this last process, we carried out additional cyclic
voltammetry and bulk electrolysis experiments. Fig. 5 shows a
comparison between cyclic voltammograms for the solvent–
supporting electrolyte alone (curve A) and solvent–supporting
electrolyte containing 5.0 mM 1-chlorohexane (curve B). Although
no obvious voltammetric peak is seen in curve B, it is nevertheless
evident that some reduction of 1-chlorohexane does occur at
ꢁ1.60 V. Indeed when we electrolyzed a solution containing
Depicted in Scheme 1 is a suggested set of mechanistic steps for
the reduction of 1. In the first reaction, 1 accepts a pair of electrons,
resulting in loss of a bromide ion, to give 9, which can be
subsequently protonated by residual water in the solvent–
electrolyte to give 1-chlorohexane (3). It is definitely conceivable
that the overall two-electron process occurs in a stepwise fashion
to produce a transient radical intermediate that is immediately
reduced to a carbanion via uptake of a second electron to afford 9.
In reaction 2, carbanion 9 (acting as a base) deprotonates a
molecule of the starting material (1), leading to expulsion of a
bromide ion and formation of 6-chloro-1-hexene (4). Alternatively,
as shown in reaction 3, 1 and 9 can interact to afford 6-bromo-1-
hexene (10) as well as 3 and a chloride ion. As depicted in
reaction 4, 10 can undergo a classic two-electron cleavage of its
carbon–bromine bond (with protonation by residual water) to
produce 1-hexene (6). In reaction 5, we show that either 4 or 10 can
5.0 mM 1-chlorohexane in DMF–TBABF4 at
a potential of
ꢁ1.60 V, subsequent gas chromatographic analysis revealed the
presence of n-hexane. Therefore, we conclude that, on the time
scale of a bulk electrolysis of 1, production of n-hexane can occur,
as evidenced by the product distributions compiled in Table 1.
A final mechanistic point pertains to the roles played by OHꢁ
and 9 as bases in Scheme 1. As seen in reactions 1, 4, and 6, OHꢁ
arises via protonation of electrogenerated alkyl carbanions by
residual water that is present in the solvent–supporting electro-
lyte; typically, the concentration of this adventitious water is
approximately 40 mM [42]. On the basis of the roster of products
observed for the reduction of 1, we conclude that OHꢁ is not a
viable base in terms of its SN2 attack on carbon–halogen bonds,
because no alcohols were detected as products. In two earlier
papers [42,43] from our laboratory, hydroxide ions arising from the
protonation by residual water of n-decyl carbanions arising from
the reductive cleavage of 1-iododecane did afford 1-decanol, the
production of which was ascribed to attack of OHꢁ on yet
unreduced alkyl iodide. However, in the present work, we regard 9
as a much more potent base than OHꢁ for the deprotonation of 1, 4,
and 10 (as shown for reactions 2, 3, and 5 in Scheme 1) that leads to
the formation of olefins.
3.4. Mechanistic aspects of the reduction 1-chloro-6-iodohexane (2)
Scheme 2 provides a suggested sequence of mechanistic steps
for the reduction of 2; reactions 1 through 4 involve radical
intermediates that arise from one-electron reduction of 2 at
Scheme 1. Proposed mechanistic steps involved in the reduction of 1-bromo-6-
chlorohexane (1) at silver cathodes in dimethylformamide (DMF) containing
0.050 M TBABF4.

316
J.A. Rose et al. / Electrochimica Acta 218 (2016) 311–317
ꢁ0.95 or ꢁ1.60 V affords mainly 1-chlorohexane, 6-chloro-1-
hexene, and 1,12-dichlorododecane, along with traces of 1,5-
hexadiene and 1-hexene, whereas reduction of 2 at ꢁ1.60 V forms
1,12-dichlorododecane in substantial yield. Electrolyses of 2 at
ꢁ0.95 V (first cathodic peak) in the presence of excess D2O led to no
incorporation of deuterium, which indicates the absence of
carbanion intermediates. However, when both 1 and 2 were
electrolyzed at ꢁ1.60 V in the presence of excess D2O, there was
approximately 50% incorporation of deuterium into 1-chlorohex-
ane, which is evidence for a carbanion intermediate.
Another matter for brief mention here is the possibility or
likelihood that one or both of the parent compounds (as well as
electrogenerated intermediates) undergo adsorption onto a silver
cathode. In the course of preliminary research [44] in our
laboratory, early evidence [(a) electrochemical surface-enhanced
Raman spectroscopy (EC–SERS), (b) calculations based on the use
of density functional theory (DFT), and (c) surface interrogation
mode of scanning electrochemical microscopy (SI–SECM)] has
offered enticing results pertaining to possible adsorption of
1-bromodecane and 1-iododecane (and perhaps intermediates
arising from their reduction) onto silver cathodes. At a future time,
we hope to report results from this work and to extend these
efforts to the reduction of other halogenated compounds at silver
electrodes.
Scheme 2. Proposed mechanistic steps involved in the reduction of 1-chloro-6-
iodohexane (2) at silver cathodes in dimethylformamide (DMF) containing 0.050 M
TBABF4.
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