Direct reduction of 1,2- and 1,6-dibromohexane at silver cathodes in dimethylformamide

Article abstract of DOI:10.1016/j.electacta.2015.10.164

Electrochemical reduction of 1,2-dibromohexane (1) and 1,6-dibromohexane (2) at silver cathodes in dimethylformamide (DMF) containing tetramethylammonium perchlorate (TMAP) has been investigated with the aid of cyclic voltammetry and controlled-potential electrolysis. Cyclic voltammograms for reduction of 1 and 2 both exhibit a single irreversible cathodic peak associated with reduction of carbon-bromine bonds; however, the cathodic peak potential (-0.33 V) for 1 is significantly less negative than that (-1.00 V) for 2, and the peak current for 1 is approximately half of that for 2. Cyclic voltammograms for 0.5-20.0 mM solutions of 1 and 2, separately, show that the parameter I_pc/C?increases as the concentration (C?) decreases; this trend is likely due to a combination of adsorption phenomena and a potential-dependent transfer coefficient (α). Coulometric n values and product distributions arising from bulk electrolyses of 5.0 mM solutions of 1 and 2 depend on the positions of the bromine atoms: (a) for 1, n was 2.13 and 1-hexene was the only product; (b) for 2, n was 2.12 and a mixture of products was obtained [1-hexene (21%), n-hexane (37%), 1,5-hexadiene (22%), 5-hexen-1-ol (9%), and a trace of n-dodecane]. When 2 was electrolyzed in the presence of a proton or deuteron donor (2,2,2-trifluoroethanol or D₂O), the n value and the amount of n-hexane increased, whereas 1-hexene and 1,5-hexadiene decreased in yield. We conclude that reduction of 1 follows a concerted mechanism, but that reduction of 2 proceeds via carbanionic intermediates.

Full text of DOI:10.1016/j.electacta.2015.10.164

Electrochimica Acta 186 (2015) 369–376
Contents lists available at ScienceDirect
Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Direct Reduction of 1,2- and 1,6-Dibromohexane at Silver Cathodes in
Dimethylformamide
Erin T. Martina, Lauren M. Strawsinea, Mohammad S. Mubarakb, Dennis G. Petersa,
*
a
Department of Chemistry, Indiana University, Bloomington, Indiana 47405, USA
b
Department of Chemistry, The University of Jordan, Amman 11942, Jordan
A R T I C L E I N F O
A B S T R A C T
Article history:
Received 19 August 2015
Received in revised form 8 October 2015
Accepted 26 October 2015
Available online 30 October 2015
Electrochemical reduction of 1,2-dibromohexane (1) and 1,6-dibromohexane (2) at silver cathodes in
dimethylformamide (DMF) containing tetramethylammonium perchlorate (TMAP) has been investigated
with the aid of cyclic voltammetry and controlled-potential electrolysis. Cyclic voltammograms for
reduction of 1 and 2 both exhibit a single irreversible cathodic peak associated with reduction of carbon–
bromine bonds; however, the cathodic peak potential (ꢀ0.33 V) for 1 is significantly less negative than
that (ꢀ1.00 V) for 2, and the peak current for 1 is approximately half of that for 2. Cyclic voltammograms
for 0.5–20.0 mM solutions of 1 and 2, separately, show that the parameter Ipc/C* increases as the
concentration (C*) decreases; this trend is likely due to a combination of adsorption phenomena and a
Keywords:
Carbon–bromine bond cleavage
Electrochemical reduction
Silver cathodes
potential-dependent transfer coefficient (a). Coulometric n values and product distributions arising from
bulk electrolyses of 5.0 mM solutions of 1 and 2 depend on the positions of the bromine atoms: (a) for 1, n
was 2.13 and 1-hexene was the only product; (b) for 2, n was 2.12 and a mixture of products was obtained
[1-hexene (21%), n-hexane (37%), 1,5-hexadiene (22%), 5-hexen-1-ol (9%), and a trace of n-dodecane].
When 2 was electrolyzed in the presence of a proton or deuteron donor (2,2,2-trifluoroethanol or D2O),
the n value and the amount of n-hexane increased, whereas 1-hexene and 1,5-hexadiene decreased in
yield. We conclude that reduction of 1 follows a concerted mechanism, but that reduction of 2 proceeds
via carbanionic intermediates.
Concerted mechanism
Carbanion intermediates
ã 2015 Elsevier Ltd. All rights reserved.
1. Introduction
Since approximately the year 2000, many studies have revealed
the catalytic ability of silver as a cathode material for electro-
Our interest in mechanisms for the reduction of polyhalo-
genated organic compounds at silver cathodes stems in part from
ongoing investigations of the electrochemical remediation of
halogenated organic pollutants [1]. Employing silver cathodes has
become popular, owing to the fact that such electrodes promote
the facile reductive cleavage of carbon–halogen bonds—a process
often occurring at potentials considerably less negative at silver
than at cathodes such as glassy carbon or mercury. In the present
work, by examining the electrochemical behavior of 1,2- and 1,6-
dibromohexane, we have sought to develop a fuller understanding
of the mechanisms involved in the reduction of polyhalogenated
organic compounds at silver cathodes. Historically, the electro-
chemical reduction of a wide variety of halogenated organic
compounds. In this introductory section of our report, and to keep
the final listing of all references to a manageable size, we have
opted to cite just a relatively small number of the large population
of previous publications that are relevant to the present work, but
yet provide readers with useful starting points to identify main
contributors to this field and to locate the larger body of papers in
this area of research.
Elucidation of the mechanism for the reduction of benzyl
chloride at silver in acetonitrile containing tetraethylammonium
perchlorate has been accomplished by Amatore and co-workers,
who employed electrochemical surface-enhanced Raman spec-
troscopy [3] and density functional theory [4] to identify adsorbed
benzyl chloride, benzyl radical, and benzyl anion; in subsequent
research [5], these investigators used digital simulations to explain
the features and trends seen in cyclic voltammograms for
reduction of benzyl chloride, especially with respect to adsorption
phenomena.
chemical reduction of vicinal and
a,v-dihaloalkanes has been
studied extensively at mercury and glassy carbon cathodes, and the
literature in these areas has been recently reviewed [2].
* Corresponding author. Tel.: +1 812 855 9671; fax: +1 812 855 8300.
E-mail address: peters@indiana.edu (D.G. Peters).
http://dx.doi.org/10.1016/j.electacta.2015.10.164
0013-4686/ã 2015 Elsevier Ltd. All rights reserved.

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E.T. Martin et al. / Electrochimica Acta 186 (2015) 369–376
In addition, there have been studies that reflect on how the
identity and structure of halogenated organic compound
trifluoroethanol (Fluka, ꢁ99%), diethyl ether (EMD, absolute,
a
anhydrous), sodium bicarbonate (Macron, 99.7%), and sodium
sulfate (EMD, anhydrous). Dimethylformamide (DMF, Omnisolv,
99.99%,) was used as solvent for electrochemical experiments.
Tetramethylammonium perchlorate (TMAP, GFS Chemicals, re-
agent grade), employed as supporting electrolyte, was recrystal-
lized twice from a mixture of ethanol and water, then stored in a
vacuum oven at 80 ꢂC for 48 h to remove traces of water. Deaeration
of all solutions was accomplished with the aid of zero-grade argon
(Air Products).
influence its direct reduction at silver. In an investigation of the
electrochemical behavior of haloadamantanes at silver, Rondinini
et al. [6] reported that the position of the halogen atom determines
the yield of dimeric product obtained from a bulk electrolysis. In
recent work [7] in which the reduction of primary, secondary, and
tertiary alkyl monohalides at silver cathodes was probed by means
of cyclic voltammetry and controlled-potential electrolysis, our
laboratory found that products arising from reduction of iodides
are highly dependent on the position of the halogen atom, whereas
no such trend was observed for the corresponding bromides and
chlorides.
2.2. Cells, electrodes, instrumentation, and procedures
Electrochemical reduction of halogenated environmental pol-
lutants at silver cathodes has become an increasingly active area of
research. Recent examples of such investigations include the
reduction of volatile organic halides (such as polyhalogenated
methanes and ethanes) [8–12], pesticides (such as lindane and
DDT) [13,14], flame retardants (such as decabromodiphenyl ether
and hexabromocyclododecane) [15,16], and freons (such as CFC-
113) [17–20].
A final category of studies concerns a variety of fundamental
mechanistic aspects of the reduction of halogenated organic
compounds at silver cathodes. A particularly interesting publica-
tion by Simonet [21] deals with the electrochemical behavior of
1,3-dibromopropane at silver in dimethylformamide containing
tetra-n-butylammonium salts; it was proposed that reduction of
this compound entails a two-electron process to afford a biradical
intermediate that affords cyclopropane and a polymer. Other work
by the group of Simonet pertains to the reductive homocoupling of
alkyl bromides and iodides at silver–palladium and silver cathodes
[22,23]. An ongoing series of papers from the groups of Gennaro
and Isse has focused on important topics pertaining to the use of
silver cathodes for the electrochemical reduction of organic
halides: (a) catalytic efficiency of silver for reduction of benzyl
halides and the adsorption of starting materials and products [24];
(b) mechanism of dissociative electron transfer to organic
chlorides [25–27]; (c) activation of carbon–halogen bonds toward
carboxylation at silver cathodes [28–30]; and (d) solvent effects
[31].
Cyclic voltammetry was performed in a previously described
cell [32] fitted with a silver (3.0-mm diameter, Alfa Aesar, 99.9%)
working electrode; the geometric area of the planar, circular
electrode was 0.071 cm2. At the start of each series of experiments
and between successive experiments, the working electrode was
polished with 0.05-mm alumina paste (Buehler) on a rotary
Master-Tex polishing pad (Buehler), and washed thoroughly with
DMF in an ultrasonic bath. A coil of platinum wire served as the
auxiliary (counter) electrode, and the reference electrode con-
sisted of a cadmium-saturated mercury amalgam in contact with
DMF saturated with both sodium chloride and cadmium chloride;
all potentials in this paper are given with respect to this reference
electrode, which has a potential of –0.76 V versus the aqueous
saturated calomel electrode (SCE) at 25 ꢂC [33–35]. To carry out
cyclic voltammetry experiments, a Princeton Applied Research
Corporation (PARC) model 2273 or 273A potentiostat with a
PowerSuite1 software package was employed, and data were
processed with the aid of OriginPro 2015 software.
Controlled-potential (bulk) electrolyses were performed with
the aid of a PARC model 173 potentiostat. A locally written
LabView program was used for data collection, and the acquired
data were processed with OriginPro 2015 software. A two-
compartment (divided) cell described elsewhere [36], utilized for
bulk electrolysis, consisted of a carbon rod auxiliary anode in DMF
containing TMAP inside the anode compartment that was
separated from the cathode compartment by a sintered-glass
disk backed by a methyl cellulose–solvent–electrolyte plug. For
bulk electrolyses performed in this cell, the cathode compartment
contained 20 mL. As described above, a cadmium-saturated
mercury amalgam reference electrode was utilized. Working
electrodes, each with an estimated surface area of 45 cm2 [37],
were fabricated from silver gauze, woven from 0.356-mm-
diameter wire, with an additional single wire for the electrical
lead purchased from Alfa Aesar (99.9%). Due to the volatility of
products arising from reduction of the dibromohexanes, sparging
of argon through the cathode compartment to remove oxygen
was terminated as soon as the background current reached a
baseline level at the chosen potential, whereupon the substrate
was immediately injected into the cell. Prior to each electrolysis,
the silver working cathode was cleaned by ultrasonication for at
least 30 min in an aqueous sodium bicarbonate paste, then rinsed
thoroughly with distilled water, and dried for 20 min in an oven at
180 ꢂC.
In the present work, we have employed cyclic voltammetry and
controlled-potential electrolysis to examine the direct electro-
chemical reductions of 1,2-dibromohexane (1) and 1,6-dibromo-
hexane (2) at silver cathodes in dimethylformamide (DMF)
containing tetramethylammonium perchlorate (TMAP). Products
arising 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). On
the basis of our findings, mechanistic schemes for the reduction of
1 and 2 are proposed.
2. Experimental
2.1. Reagents
Each of the following chemicals (with purity given in
parentheses) was purchased from Sigma Aldrich, or from a
different commercial source when indicated, and was used as
received: 1,2-dibromohexane (98%), 1,6-dibromohexane (Acros
Organics, 98%), 1-bromohexane (98%), 2-bromohexane (Columbia
Organic Chemicals, 99%), n-dodecane (99 + %), n-hexane (EMD,
96%), 1-hexene (Alfa Aesar, 98%), 1,5-hexadiene (98%), 5-hexen-1-
ol (99%), 1-hexanol (J. T. Baker, purified), dimethylformamide-d7
(DMF-d7, 99.5 atom % D), deuterium oxide (D2O, 99.9 atom % D),
1,1,1,3,3,3-hexafluoro-2-propanol (HFIP, Matrix Scientific, 99%),
methyl-3-oxobutanoate (methyl acetoacetate, 99%), 2,2,2-
2.3. Separation, identification, and quantitation of products
All product identification was achieved by means of gas
chromatography–mass spectrometry (GC–MS); an Agilent
6890N gas chromatograph, fitted with a 30 m ꢃ 0.32 mm capillary
column (J & W Scientific) with a DB-5 stationary phase consisting
of 5% phenylpolysiloxane and 95% methylpolysiloxane, was used in
tandem with an Agilent 5973 inert mass-selective detector
operating in electron ionization mode (70 eV).

E.T. Martin et al. / Electrochimica Acta 186 (2015) 369–376
371
Prior to a bulk electrolysis, known amounts of an internal
standard (n-heptane) were injected into the catholyte. At the end
of each controlled-potential (bulk) electrolysis, the catholyte was
partitioned three times between diethyl ether and cold brine. Then
the ether phase was dried over anhydrous sodium sulfate and
concentrated by means of rotary evaporation. Products were
separated and quantitated by use of an Agilent 7890A gas
chromatograph, equipped with a flame-ionization detector; n-
dodecane was quantitated with the DB-5 column described above,
whereas the more volatile species (1,5-hexadiene, 1-hexene, n-
hexane, 5-hexen-1-ol, and 1-hexanol) were quantitated with a
To understand better the enhanced reduction of 1 and 2 at a
silver cathode, separate cyclic voltammograms were recorded,
under otherwise identical conditions, for each species at a glassy
carbon electrode. Cathodic peak potentials (Epc) are more negative
with glassy carbon, being ꢀ1.27 and ꢀ1.65 V, respectively, for
separate 2.0 mM solutions of 1 and 2. A comparison of Epc values
found with glassy carbon and with silver reveals that a larger
positive shift is observed for reduction of 1 (0.94 V) than for
reduction of 2 (0.65 V). Unlike the results seen with a silver
cathode, the cathodic peak currents (Ipc) for reduction of the
compounds at glassy carbon are similar (55 and 63 mA for 1 and 2,
12.5-m (including a 2.5-m particle trap) ꢃ 0.53-mm ꢃ 20-
m
m PLOT
respectively). Thus, when the cathodic peak currents for each
species are compared for glassy carbon and silver, the difference in
fused-silica capillary column (Varian). Using a procedure devel-
oped in earlier research [38], we determined the product
distributions tabulated in this report from gas chromatographic
response factors and peak areas measured with the aid of Agilent
ChemStation software. To determine response factors, a mock cell
that contained anticipated concentrations of products, along with
appropriate amounts of the internal standard, was prepared and
allowed to stir (without application of a potential) for the same
length of time as an actual bulk electrolysis.
Ipc for 1 is almost negligible (13
mA), whereas Ipc for reduction of 2
at silver is almost double that for glassy carbon (127 versus 63
mA).
To probe these reductions further, cyclic voltammograms for
reduction of separate solutions of 1 and 2 at concentrations
ranging from 0.5 to 20.0 mM were recorded at 0.100 V sꢀ1
.
Experimentally, as the bulk concentration (C*) of each substrate
was increased, two trends were observed: (a) the cathodic peak
potential (Epc) for reduction of 2 showed a larger shift toward more
negative values than that for reduction of 1 and (b) the peak
3. Results and discussion
current (Ipc) increased more dramatically for 2 (31
m
A mMꢀ1) than
for 1 (23
m
A mMꢀ1). Further examination of Ipc as a function of C*
3.1. Cyclic voltammetric behavior of 1,2-dibromohexane (1) and 1,6-
dibromohexane (2)
revealed, as shown in Fig. 2, that the parameter Ipc/C* increases for
both 1 and 2 as C* is lowered. According to Wopschall and Shain
[39], such an observation suggests adsorption of each compound.
On the other hand [40], the trends observed in Fig. 2 could
signify a potential-dependent variation of the transfer coefficient
Shown in Fig. 1 are cyclic voltammograms recorded at
0.100 V sꢀ1 for reduction of separate 2.0 mM solutions of 1,2-
dibromohexane (1) and 1,6-dibromohexane (2) at a silver cathode
in DMF containing 0.050 M TMAP. For 1, a single irreversible
cathodic peak is seen at ꢀ0.33 V, whereas the cathodic peak for 2 is
observed at ꢀ1.00 V. A noteworthy feature of Fig. 1 is that the
(a
) for reduction of each substrate. For scan rates between 0.05 and
1.00 V sꢀ1, we calculated
from the slope of a plot of cathodic peak
potential (Epc) versus the logarithm of the scan rate (log ) for 0.5,
2.0, and 8.0 mM solutions, where = –1.15(RT/F)( log Epc); this
a
n
a
D
n/D
cathodic peak current (Ipc) for 2 (127
m
A) is almost double the
approach has been previously employed by Isse and co-workers
[41] in studies of the reduction of organic halides at silver cathodes.
Transfer coefficients of 0.37, 0.27, and 0.15 were obtained,
respectively, for concentrations of 0.5, 2.0, and 8.0 mM 1;
value observed for 1 (68 A). Under the same experimental
m
conditions, separate cyclic voltammograms for reduction of 1- and
2-bromohexane exhibit single irreversible cathodic peaks at
ꢀ1.23 and ꢀ1.09 V, respectively, with corresponding cathodic
estimated errors in
procedure was followed to determine values of
and 8.0 mM solutions of 2 ( = 0.17, 0.12, and 0.08, respectively).
These results support the conclusion that for the reduction of 1
a
are no more than 0.07. An identical
peak currents of 86 and 115
m
A.
a
for 0.5, 2.0,
a
a
and 2 is both potential- and concentration-dependent. A potential-
Fig. 1. Cyclic voltammograms recorded at 0.100 V sꢀ1 for reduction of 2.0 mM
solutions of 1,2-dibromohexane (1, solid curve) and 1,6-dibromohexane (2, dotted
curve) at a silver cathode (area = 0.071 cm2) in oxygen-free DMF–0.050 M TMAP.
Potentials are given with respect to a reference electrode consisting of a cadmium-
saturated mercury amalgam, in contact with DMF saturated with both cadmium
chloride and sodium chloride; this electrode has a potential of ꢀ0.76 V versus an
aqueous saturated calomel electrode (SCE) at 25 ꢂC. For 1 the scan goes from +0.25
to ꢀ0.75 to +0.25 V; for 2 the scan goes from –0.05 to ꢀ1.8 to ꢀ0.05 V.
Fig. 2. Cathodic peak current as a function of initial concentration,i.e., (Ipc/C*), for
0.5–20.0 mM solutions of 1,2-dibromohexane (1, solid circles) and 1,6-dibromo-
hexane (2, solid squares) obtained at a fixed scan rate of 0.100 V sꢀ1 at a silver
cathode (area = 0.071 cm2) in oxygen-free DMF–0.050 M TMAP.

372
E.T. Martin et al. / Electrochimica Acta 186 (2015) 369–376
dependent
a
has been observed by Corrigan and Evans [42] for the
passed (Q). A plot of average charge as a function of initial
concentration, i.e., Q/C*, versus the initial concentration (Fig. 3)
shows that, for 1.0–20.0 mM 1, all values of Q/C* are approximately
reduction of tert-nitrobutane at mercury and platinum electrodes
in acetonitrile containing various tetraalkylammonium salts, and
this work was subsequently extended by Sanecki and Skitał [43].
125 m
C mMꢀ1 (with an experimental error of <12%); however, Q/C*
Due in part to the observed dependence of
a
on concentration,
for 0.5 mM 1 is unexpectedly high (161 ꢄ15
m
C mMꢀ1). On the
other hand, the same kind of plot for solutions of 2 between 1.0 and
8.0 mM shows a decrease in Q/C* from approximately 187 ꢄ23 to
along with possible complications arising from interaction of a
substrate with the silver surface, the prevalence of a concerted or
stepwise process cannot easily be distinguished on the basis of the
observed value of
of bromine substituents. We plan to investigate this issue in future
work with other related compounds.
98 ꢄ 7
132 ꢄ 4
Fig. 3, which supports Fig. 2, reveals that, for low concentrations of
either or 2, is not directly dependent on the initial
m
C mMꢀ1; however, for 14.0 mM 2, Q/C* increases to
a, especially because 1 and 2 each possess a pair
m
C mMꢀ1 and remains at that value up to 20.0 mM.
1
Q
To ascertain if values of
a
for 1 and 2 account for the trends seen
concentration and that cyclic voltammograms for 1 and 2 are
distinctive.
in Fig. 2, theoretical peak currents were calculated for 0.5, 2.0, and
8.0 mM solutions of 1 and 2 from the equation for an irreversible
electrochemical process [40]. These calculations utilized the
following parameters: (a) a diffusion coefficient of 7.60 ꢃ10ꢀ6
cm2 sꢀ1, which was estimated on the basis of relations taken from
work by Bard and Kojima [44] as well as by Wilke and Chang [45];
(b) the appropriate cathodic peak potential found experimentally
Plots of Ipc versus v1/2 obtained for reduction of 0.5, 2.0, and
8.0 mM solutions of 1 were linear for scan rates ranging from 0.05–
10 V sꢀ1. As the initial concentration of 1 was increased, the slope of
each line was elevated, which suggests typical diffusion behavior
for an irreversible electrochemical process. However, for scan rates
of 20–50 V sꢀ1, the plots showed limiting behavior, in that peak
at 0.100 V sꢀ1; and (c) our calculated
a
values. In all cases, the
currents no longer increased in proportion to scan rate (
n). In fact, a
theoretical peak current was lower than the experimentally
observed value. This difference might be due to use of a geometric
electrode area that (owing to electrode surface roughness) is too
small or an equation that is not suitable due to adsorption of the
substrate. On the basis of these calculations, the percentage
decrease in the theoretical values of Ipc/C* for 1 between 0.5 and
2.0 mM (21%) and between 2.0 and 8.0 mM (25%) is inflated with
respect to the experimentally calculated percentage decrease (7%
and 11%, respectively) obtainable from Fig. 2. Similarly, the
theoretical percentage decrease in Ipc/C* for 2 between 0.5 and
2.0 mM (18%) is higher than the experimental value (4%); on the
other hand, for concentrations of 2 between 2.0 and 8.0 mM, the
theoretical percentage decrease in Ipc/C* (18%) is in good
agreement with the experimental value (21%).
plot of peak current as a function of scan rate and initial
concentration, i.e., Ipc/(nC*), versus scan rate revealed that, beyond
10 V sꢀ1, all points converge into one horizontal line, which
suggests the dominance of adsorption phenomena at high scan
rates [39].
Plots similar to those described in the preceding paragraph
were made for reduction of 0.5, 2.0, and 8.0 mM 2 (Fig. 4). As for the
reduction of 1, the peak current (Ipc) for reduction of 2 varies
linearly with respect to v1/2 at slow scan rates; however, unlike the
cyclic voltammetric behavior of 1, the scan rate at which linearity is
lost depends on the initial concentration of 2. Furthermore, for
8.0 mM solutions of 2, recorded at scan rates between 20 and
50 V sꢀ1, Ipc decreases slightly at faster scan rates. A plot of Ipc/(
versus v for reduction of 2 is similar to that described above for the
nC*)
We integrated the area under each cathodic peak recorded at
reduction of 1, with the exception that the values of Ipc/(
typically greater for 2 than for 1.
nC*) are
0.100 V sꢀ1 for various concentrations of 1 and 2 to find the charge
Fig. 3. Average charge as a function of initial concentration (Q/C*) versus the initial concentration found from cyclic voltammograms for reduction of 1,2-dibromohexane (1,
bottom, solid circles) and 1,6-dibromohexane (2, top, solid squares), obtained at a fixed scan rate of 0.100 V sꢀ1 at a silver cathode (area = 0.071 cm2) in oxygen-free DMF–
0.050 M TMAP.

E.T. Martin et al. / Electrochimica Acta 186 (2015) 369–376
373
presence of
a 100-fold excess of D2O, no deuterium was
incorporated into the resulting 1-hexene. Additional electrolyses
of 1 were done in the presence of two different acids [1,1,1,3,3,3-
hexafluoro-2-propanol(pKaDMF = 18.8)andmethyl-3-oxobutanoate
(pKaDMF = 15.2)] [46,47], each added in a 100-fold excess over the
substrate; interestingly, neither 1- nor 2-bromohexane was found
as a product. All of our observations support a conclusion that the
reduction of 1 proceeds via a concerted mechanism, as described in
a review by Fry [48]. In the latter publication, it is reported that
open-chain vicinal dihalides undergo concerted reduction where
both halogens are anti to each other; for all vicinal dihalides, the
dihedral angle between the two carbon–halogen bonds determines
the ease of a concerted mechanism.
In contrast to the behavior of 1, Table 1 reveals that electrolyses
of 5.0 and 10.0 mM solutions of 2 resulted in n values of 2.12 and
2.14, respectively, and the major products were 1-hexene, n-
hexane, 1,5-hexadiene, and 5-hexen-1-ol, along with small
quantities of n-dodecane. Several electrolyses of 2 were carried
out at potentials more negative than ꢀ1.20 V, but the resulting
product distributions did not differ significantly from those shown
in Table 1. To gain insight into the mechanisms involved in the
formation of these products, reduction of 2 was carried out in the
presence of a 100-fold excess of D2O; for this set of experiments,
the average n value was 2.49 and the product distribution
consisted of 1-hexene (15%), n-hexane (50%), 1,5-hexadiene
(16%), and 5-hexen-1-ol (9%) as well as traces of n-dodecane.
Note that, for electrolyses conducted in the presence of D2O, both
the amount of n-hexane and the coulometric n value increased,
whereas the yields of the alkenes decreased. In addition, we
employed GC–MS to assess the extent of deuterium incorporation
into the n-hexane and 1-hexene; data were corrected for the
natural isotopic abundances of 2H and 13C [49]. These experiments
revealed that 51% and 24% of the n-hexane was mono- and
dideuterated, respectively, whereas 41% of the 1-hexene was
deuterated. However, reduction of 2 in DMF-d7 containing 0.050 M
TMAP showed no evidence of deuterium incorporation into n-
hexane. These results indicate that electrolytic reduction of 2
Fig. 4. Peak current versus square root of scan rate obtained from cyclic
voltammograms recorded at 25 mV sꢀ1 to 10 V sꢀ1 for 0.5 (solid squares), 2.0 (solid
circles), and 8.0 (solid triangles) mM solutions of 1,6-dibromohexane (2) in oxygen-
free DMF–0.050 M TMAP.
Table 1
Coulometric data and product distributions for controlled-potential electrolyses of
1,2- and 1,6-dibromohexane at silver cathodes in DMF–0.050 M TMAP.
Substrate Concn. (mM) E (V)
nb
Product distribution (%)a
10 11 12
3
7
8
Total
1
2
2
2
2
5.0
5.0
10.0
5.0c
5.0e
ꢀ0.48 2.13 107
ꢀ1.20 2.12 21
ꢀ1.20 2.14 22
ꢀ1.20 2.49 15
107
90
37 22
39 18
50 16
9
7
9
1
1
87
TRd 90
ꢀ1.20 3.54
4
75
5
18 TRd 102
1 = 1,2-dibromohexane; 2 = 1,6-dibromohexane; 3 = 1-hexene; 7 = n-hexane; 8 = 1,5-
hexadiene; 10 = 5-hexen-1-ol; 11 = 1-hexanol; 12 = n-dodecane.
a
Yield expressed as percentage of substrate converted to product.
Average number of electrons per molecule of substrate.
500 mM D2O was present.
TR = trace.
involves carbanionic intermediates,
a conclusion previously
b
c
reached in our laboratory for the reduction of 2 at reticulated
glassy carbon cathodes [50]. To add more support for the
intermediacy of carbanions in the reduction of 2, we electrolyzed
d
e
50 mM 2,2,2-trifluoroethanol was present.
5.0 mM
2 in DMF–0.050 M TMAP containing 50 mM 2,2,2-
trifluoroethanol (pKaDMF = 24.1) [46,47]. Major observations were
as follows: (a) the n value increased dramatically to 3.54; (b) the
yield of n-hexane rose to 75%; (c) the quantities of the alkenes
decreased significantly; and (d) 1-hexanol was produced in 18%
yield. It is interesting that 1-hexanol was obtained as a significant
product when 2,2,2-trifluoroethanol was employed as a proton
donor, whereas no 1-hexanol was seen when D2O was added to the
solvent–electrolyte. We suggest that this difference stems from
the fact that the solvent–electrolyte system is less protic with
2,2,2-trifluoroethanol than with D2O. As a consequence, D2O can
effectively solvate any OH– that forms from deprotonation of water
by electrogenerated bases, thereby blocking the formation of
1-hexanol. On the other hand, 2,2,2-trifluoroethanol will proton-
ate electrogenerated carbanions to afford n-hexane and the
2,2,2-trifluoroethoxide ion, and the latter base should at least
be partially protonated by adventitious water to give a small
3.2. Controlled-potential (bulk) electrolyses of 1,2-dibromohexane (1)
and 1,6-dibromohexane (2)
Coulometric data and product distributions for controlled-
potential (bulk) electrolyses of separate solutions of 1 and 2 at
silver gauze electrodes in DMF containing 0.050 M TMAP are
shown in Table 1. Each entry corresponds to the average of at least
triplicate experiments. Product yields, which were reproducible to
ꢄ5% absolute, represent the amount of starting material incorpo-
rated into each species. Potentials were chosen to be approxi-
mately 200 mV more negative than the peak potential found by
means of cyclic voltammetry. As discussed in the following
paragraphs, the coulometric n values and products are dependent
on the positions of the two bromine atoms.
Reduction of 1 afforded an n value of 2.13, and 1-hexene was the
only product. In separate experiments, when 1 was reduced in the
Scheme 1. Mechanism for electrochemical reduction of 1,2-dibromohexane (1).

374
E.T. Martin et al. / Electrochimica Acta 186 (2015) 369–376
amount of hydroxide that will attack 2 (or transient 1-bromohex-
ane) to give 1-hexanol.
hydroxide ion. Next, in reaction (2), 5 accepts two electrons with
release of bromide ion, followed by protonation to yield n-hexane
(7). Alternatively, 2 can undergo a double E2 elimination with
hydroxide ions to produce 1,5-hexadiene (8) [Reaction (3)].
Similarly, 5 can experience an E2 elimination with hydroxide to
form 1-hexene (3) [Reaction (4)], or 5 can participate in an
SN2 reaction with hydroxide to afford 1-hexanol (11) [Reaction (6)].
On the other hand, as shown in reaction (5), 2 can undergo an
E2eliminationwithhydroxidetogive6-bromo-1-hexene(9)which,
via an SN2 reaction with hydroxide, leads to 5-hexen-1-ol (10).
Finally, n-dodecane can arise from the SN2 reaction between5 and 6
[Reaction (7)].
Note,asdiscussedearlier,thattheadditionofaprotondonor(2,2,2-
trifluoroethanol) minimizes hydroxide-promotedeliminations [Reac-
tions (3), (4), and (5)], thereby diminishing the yields of unsaturated
products, whereas the added proton donor greatly increases the yield
of n-hexane by protonating intermediates 4 and 6.
3.3. Mechanistic aspects of the electroreduction of 1,2-dibromohexane
(1)
Shown in Scheme
1 is the proposed mechanism for
electrochemical reduction of 1. We conclude that 1 undergoes a
concerted, two-electron reduction with subsequent loss of two
bromide ions to afford 1-hexene (3). Because we observed no
production of n-hexane, we rule out a conceivable alternate
process that involves sequential removal of two bromide ions and
formation of a carbanionic intermediate that is protonated to form
the alkane [48].
3.4. Mechanistic aspects of the electroreduction of 1,6-dibromohexane
(2)
Finally, as discussed earlier, studies by means of cyclic
voltammetry for different concentrations (C*) of 1 and 2 and for
different scan rates (v) have provided evidence for adsorption of
both compounds. However, this conclusion requires further
experimental and more quantitative confirmation that we plan
Depicted in Scheme 2 is the suggested mechanistic scheme for
reduction of 2. In reaction (1), 2 accepts a pair of electrons, with
expulsion of a bromide ion, to afford an intermediate (4) which is
protonated by adventitious water to give 1-bromohexane (5) and
Scheme 2. Mechanism for electrochemical reduction of 1,6-dibromohexane (2).

E.T. Martin et al. / Electrochimica Acta 186 (2015) 369–376
375
to address in future work. Consequently, in Schemes 1 and 2, we
have chosen intentionally not to indicate that any of the reactants
(or products) is adsorbed.
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4. Conclusions
Electrochemical reductions of 1,2-dibromohexane (1) and 1,6-
dibromohexane (2) at silver cathodes in DMF containing 0.050 M
TMAP proceed via two different mechanisms. Reduction of 1 leads
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both bromine atoms by uptake of two electrons. On the other hand,
electrolytic reduction of 2 affords a mixture of products (1-hexene,
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traces of n-dodecane), all of which are proposed to arise from
carbanionic intermediates. For the reduction of 2, addition of D2O
leads to significant incorporation of deuterium into n-hexane and
1-hexene. Moreover, the presence of a potent proton donor (2,2,2-
trifluoroethanol) during the reduction of 2 significantly increases
both the coulometric n value and the amount of n-hexane, whereas
the yields of the various alkenes decrease substantially.
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M.S.M. acknowledges financial support from The University of
Jordan during his sabbatical year (2014–2015) at Indiana
University.
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