Electrochemical reductive cleavage of carbon-halogen bonds in 5-bromo-1,3-dichloro-2-iodobenzene

Article abstract of DOI:10.1016/j.tet.2004.09.020

The electrochemical reduction of carbon-halogen bonds in 5-bromo-1,3-dichloro-2-iodobenzene follows quadratic activation-driving force relationship except in one of the carbon-chlorine bonds. The variation of the transfer coefficient with the electrode potential has been estimated using the voltammetric data coupled with the convolution analysis. The standard potentials pertaining to the reduction of carbon-halogen bonds are evaluated using the Marcus theory of outer sphere electron transfer.

Full text of DOI:10.1016/j.tet.2004.09.020

Tetrahedron 60 (2004) 10967–10972
Electrochemical reductive cleavage of carbon–halogen bonds
in 5-bromo-1,3-dichloro-2-iodobenzene
*
M. Arun Prasad and M. V. Sangaranarayanan
Department of Chemistry, Indian Institute of Technology Madras, Chennai 600036, India
Received 28 June 2004; revised 23 August 2004; accepted 9 September 2004
Available online 8 October 2004
Abstract—The electrochemical reduction of carbon–halogen bonds in 5-bromo-1,3-dichloro-2-iodobenzene follows quadratic activation–
driving force relationship except in one of the carbon–chlorine bonds. The variation of the transfer coefficient with the electrode potential has
been estimated using the voltammetric data coupled with the convolution analysis. The standard potentials pertaining to the reduction of
carbon–halogen bonds are evaluated using the Marcus theory of outer sphere electron transfer.
q 2004 Elsevier Ltd. All rights reserved.
1. Introduction
An elegant method for the estimation of standard potentials
of irreversible systems obeying a quadratic activation–
driving force relationship was demonstrated in the dis-
sociative reduction of perbenzoates.3 The above method-
ology, making use of linear variation of transfer coefficient
(a) with the electrode potential (E), was adopted in the
electrochemical reduction of several organic4–6 and bio-
logically relevant molecules.7 However, studies in this
direction on the stepwise reductive cleavage reactions
involving rapid decomposition of the anion radicals
(especially aromatic compounds) are limited.
Electrochemical single electron transfer reactions constitute
a frontier area of research1 and the kinetics of these
reactions can be analyzed using the Marcus–Hush theory of
outer sphere electron transfer assuming the validity of the
Born–Oppenheimer approximation. Electron transfer to an
organic molecule (RX) is often accompanied by bond
cleavage leading to a radical (R%) and an anion (XK)
occurring either in a stepwise manner (reaction 1) or a single
elementary step (reaction 2).
0
RX þ eK#RX$K ðERX=RX
RX$K/R$ CXK
Þ
(1a)
(1b)
(2)
$K
In this communication, we report the standard potentials of
the irreversible reduction of aromatic carbon–halogen bonds
in 5-bromo-1,3-dichloro-2-iodobenzene using the quadratic
activation–driving force relationship. Convolution potential
sweep voltammetry is the main tool for investigating the
reaction kinetics, since, in contrast to homogeneous redox
catalysis, the convolution approach allows one to obtain
extensive data on the logarithmic electron transfer rate
constant (ln kET ) versus electrode potential (E) variation. It
has to be emphasized here that the present approach fails
for systems involving linear variation of ln kET with E
(Butler–Volmer kinetics)—an example of which also is
demonstrated in the present study.
RX þ eK/R$ þ XK ðERX=R þX
Application of general electrochemical techniques yields
the transfer coefficient and the forward electron transfer rate
Þ
0
$
K
constant for the above reactions, but not the information
`
0
about the standard potential (E ) vis-a-vis the standard rate
constant; since the dissociative electron transfer reactions
are completely irreversible, E0 values can not be obtained
directly (for example, by cyclic voltammetry). However, the
kinetic analysis of homogeneous redox catalysis of the
electrochemical reduction leads to the determination of E0.2
2. Experimental
Keywords: 5-Bromo-1,3-dichloro-2-iodobenzene; Marcus theory; Stepwise
mechanism; Convolution analysis; Transfer coefficient; Outer-sphere
electron transfer.
The voltammetric studies were carried out in a single
compartment electrochemical cell thermostated at 298 K,
using the Bio Analytical Systems (BAS) 100A Electro-
chemical workstation. The working electrode was a glassy
* Corresponding author. Tel.: C91 22578269; fax: C91 22570545;
e-mail: mvs@chem.iitm.ac.in
0040–4020/$ - see front matter q 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2004.09.020

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M. A. Prasad, M. V. Sangaranarayanan / Tetrahedron 60 (2004) 10967–10972
carbon disc (BAS) of 3 mm diameter while platinum foil
(2 cm2) served as the counter electrode. The working
electrode was polished with the alumina slurry (BAS) and
ultrasonically rinsed prior to use. The electrochemical
pretreatment was carried out in the background solution
using several cycles at 0.05–1 V sK1 in a wide potential
range. Tetra n-butyl ammonium bromide (nBu4NBr)
(Flu0ka) was the supporting electrolyte and used as received.
N,N -dimethyl formamide (DMF) was initially distilled
from anhydrous copper sulfate, then the distillate was again
distilled from calcium hydride under reduced pressure and
of the radical anion may be viewed as an intramolecular
dissociative single-electron transfer from the p* orbital to
the s* orbital of the carbon–halogen bond.11 Interestingly,
the neutral radicals are easier to reduce than the parent
halobenzenes and immediately undergo a second electron
transfer to form RK. However, the characteristic features of
the reduction wave of halobenzene are solely governed by
the kinetics of the first electron transfer. RK abstracts a
proton either from the solvent or the supporting electrolyte
to give the hydrocarbon RH and it was observed that several
halobenzenes and other aromatic halides upon electrolysis
yielded 100% of RH.10 A recent investigation, involving in
situ electrochemical-NMR spectroscopy,12 has revealed that
the aryl anion abstracts a proton preferably from the solvent
rather than the supporting electrolyte. It is worth mentioning
here that the hydrocarbon RH is reducible and, in fact, a
second wave is observed in some cases before the
background discharge.10 However, in most cases, the
reduction wave of RH is suppressed by the background
discharge current of the supporting electrolyte.
˚
stored over 4 A molecular sieves. Silver/silver ion (1 mM)
electrode (BAS) was used as the quasi-reference electrode
which was subsequently calibrated with the ferrocene/
ferrocenium couple under identical conditions of solvent
and supporting electrolyte. The background subtracted
voltammograms were analyzed by the convolution
approach, the experimental and computational details of
which have been described earlier.8 NMR spectra were
recorded on Bruker Avance 400 spectrometer. UV–Vis
spectrum was obtained with the Ocean Optics UV/Vis
spectrometer.
Figure 1 shows the cyclic voltammogram pertaining to the
reduction of 5-bromo-1,3-dichloro-2-iodobenzene at the
glassy carbon electrode in DMF containing 0.1 M nBu4NBr
as the supporting electrolyte. The reduction waves a, b, c
and d represent, respectively, the two-electron reduction of
carbon–iodine (C–I), carbon–bromine (C–Br) and two
carbon–chlorine (C–Cl (1) and C–Cl (2)) bonds. This
assignment follows from the fact that the carbon–halogen
bonds are susceptible to reduction in the order of:
C–IOC–BrOC–ClOC–F. Each wave represents the hydro-
genolysis of a carbon–halogen bond finally leading to the
formation of benzene and hence an overall consumption of
eight electrons in a single voltammetric cycle. The
voltammogram B shows the reduction waves of 1,3-dichloro-
benzene which corresponds to the peaks c and d of the
voltammogram A. The p* level of 5-bromo-1,3-dichloro-2-
iodobenzene being comparatively lower than the monosub-
stituted benzene is proved by the fact that the reduction
potential of carbon–iodine bond (wave a) is ca. 336 mV
more positive than that of iodobenzene. A similar behaviour
also arises for the carbon–bromine bond, the reduction
potential of which is ca. 486 mV more positive than that of
bromobenzene. However, the peak potential of wave d
corresponds to that of chlorobenzene.10 All the waves
5-Bromo-1,3-dichloro-2-iodobenzene was synthesized by
the following procedure: 2,6-dichloroaniline was bromi-
nated by passing the vapours of bromine into a solution of
2,6-dichloroaniline in hydrochloric acid. The solid
4-bromo-2,6-dichloroaniline was filtered and purified by
column chromatography (silica gel). 4-Bromo-2,6-dichloro-
aniline was then diazotized in hydrochloric acid (6 M)
using aqueous sodium nitrite (6 M) and the resulting
solution was slowly added to aqueous potassium iodide
(5 M). When no gas was evolved, the crude product was
filtered, washed with aqueous sodium hydroxide, sub-
sequently with sodium metabisulphite and finally with
water. The residue was purified by column chromatography
(silica gel) using hexane as the eluent to give 5-bromo-1,
3-dichloro-2-iodobenzene as white solid: Mp 67.8–68.7 8C
(literature:9 67.5–68.2 8C); 1H NMR (400 MHz, CDCl3,
TMS as internal standard) d 7.48 (2H, s); 13C NMR
(100 MHz, CDCl3) d 102.41, 122.36, 129.89, 141.33;
UV–Vis (CHCl3) broad band centered at 300 nm. The
compound was crystallized in hexane (colorless crystals) for
the electrochemical studies and the crystal structure
confirms the identity of the compound†.
remain irreversible even at a scan rate of 2000 mV sK1
,
indicating that the life time of the radical anion is less than
10K4 s. The peak currents of the waves a, b, c and d are
proportional to the square root of the sweep rate. The
transfer coefficient of the reduction of carbon–halogen bond
can be calculated from the peak width measurements (Eq. 3)
and the values are listed for various carbon–halogen bonds
in Table 1.
3. Results and discussion
Halobenzenes (RX) undergo irreversible electron transfers
at the electrode surface and are capable of hosting
transitorily the incoming electron in their p* orbitals
leading to the radical anions (RX%K) (reaction 1a). The
radical anions readily undergo decomposition with a first
order rate constant greater than 104 sK1 to neutral radicals
(R%) and halide ions (XK) (reaction 1b).10 The dissociation
1:856RT
1
a Z
(3)
F
ðEP=2 KEPÞ
The a values close to or greater than 0.5 are expected for
stepwise mechanism. However, this is not an absolute
criterion3 and our systematic study has revealed that the
reduction of carbon–iodine bond, for which the a value is
less than 0.5, indeed follows a stepwise mechanism.13
† Crystallographic data have been deposited with the Cambridge Crystal-
lographic Data Centre as supplementary publication numbers CCDC
231991. Copies of the data can be obtained, free of charge, on application
to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (Fax: C44(0)-1223-
336033 or e-mail: deposit@ccdc.cam.ac.uk).

M. A. Prasad, M. V. Sangaranarayanan / Tetrahedron 60 (2004) 10967–10972
10969
Figure 1. Cyclic voltammograms of (A) 5-bromo-1,3-dichloro-2-iodobenzene and (B) 1,3 dichlorobenzene in DMF/0.1 M nBu4NBr at glassy carbon
electrode. Scan rate: 200 mV sK1; temperature: 298 K.
3.1. Convolution analysis
dichloro-2-iodobenzene at a scan rate of 200 mV sK1. In the
case of closely spaced waves IL was obtained at a potential
at which the minimum occurs in the plot of derivative of the
convolution current at the plateau region. The logarithmic
analysis of the convolution current in conjunction with the
voltammetric current yields the heterogeneous electron
transfer rate constant14 as in Eq. 6.
In order to analyze the thermodynamic and kinetic
behaviour of the electron transfer, it is essential to deduce
the electron transfer rate constant as a function of potential.
In this respect, convolution voltammetry is a powerful
electrochemical tool since it employs all the data points of
the voltammetric wave rather than the peak characteristics
alone.3 The convolution current (I) is related to the actual
current (i) through the convolution integral14 (Eq. 4).
IL KIðtÞ
ln kET Z ln D1=2 Kln
(6)
iðtÞ
ð
t
Figure 3 depicts the variation of ln kET with E at various
scan rates for the reduction of C–I, C–Br, C–Cl (1) and C–Cl
(2) bonds. In the cases of C–I, C–Br and C–Cl (2), the
variations are parabolic, obeying the quadratic activation
(DG*)–driving force (DG0) relationship (Eq. 7).
1
pffiffiffi
p
iðuÞ
I Z
1=2 du
(4)
0 ðt KuÞ
The plot between I versus E is sigmoidal in shape with the
plateau being reached when the applied potential is
sufficiently negative. Under this condition, I reaches its
limiting value IL defined as in Eq. 5
ðDG0Þ2 DG0
DGꢀ Z
ꢀ C
CDGꢀ0
(7)
16DG0
2
IL Z nFAD1=2Cb
(5)
However, in the reduction of C–Cl (1) bond, the variation of
ln kET with E is linear. In this case the transfer coefficient is
constant, conforming to Butler–Volmer kinetics (a obtained
from the slope of the plot (Eq. 9) equals that obtained from
where D is the diffusion coefficient and Cb, the bulk
concentration. Figure 2 shows the convolution potential
sweep voltammogram of the reduction of 5-bromo-1,3-
Table 1. Electrochemical reduction behaviour of carbon–halogen bonds in 5-bromo-1,3-dichloro-2-iodobenzene
ER0X=RX$K in mV
versus SCE
ER0X=RX$K for halobenzenes in mV
versus SCE from Ref. 17
Carbon–halogen bond
Transfer coefficient from peak
width measurements
Transfer coefficient
from the Eq. 9
C–I
C–Br
C–Cl (1)
C–Cl (2)
0.331
0.451
0.659
0.604
—
—
0.667
—
K1401
K2124
—
K2240 (iodobenzene)
K2440 (bromobenzene)
—
K2856
K2780 (chlorobenzene)

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M. A. Prasad, M. V. Sangaranarayanan / Tetrahedron 60 (2004) 10967–10972
Figure 2. Convolution potential sweep voltammogram of background subtracted voltammetric curve for the reduction of 5-bromo-1,3-dichloro-2-iodobenzene
in DMF containing 0.1 M nBu4NBr at the glassy carbon electrode. Scan rate: 200 mV sK1; temperature: 298 K.
Figure 3. Potential dependence of logarithmic heterogeneous electron transfer rate constant for the reduction of (a) carbon–iodine bond (b) carbon–bromine
bond (c) carbon–chlorine (1) bond and d) carbon–chlorine (2) bond at various scan rates.

M. A. Prasad, M. V. Sangaranarayanan / Tetrahedron 60 (2004) 10967–10972
10971
the peak width measurements (Table 1)). It may be argued
that a parabola with a small curvature would also fit the
experimental ln kET versus E plot for the reduction of C–Cl
(1) bond. However, this possibility can be excluded, since
the ln kET versus E plot is too linear to be accounted for
within the purview of a stepwise mechanism. Furthermore,
a is too large to be accounted for by the theory within the
framework of a concerted pathway.
models for outer sphere or dissociative electron transfers
predict that a should be 0.5 at zero driving force15,16
(DG0ZF(EKE0)Z0). From the linear a versus E variation,
ER0X=RX$K of the reduction of respective carbon–halogen
bonds can be estimated as the potential at which a becomes
0.5.4 Table 1 shows ER0X=RX$K values for the reduction of C–I,
C–Br and C–Cl (2) bonds obtained using the above
methodology. Table 1 also provides ER0X=RX$K values for
the reduction of iodobenzene, bromobenzene and chloro-
benzene for comparison.10,17 The standard potentials of the
reduction of bromobenzene and chlorobenzene were
determined through the kinetic analysis of homogeneous
redox catalysis of the electrochemical reduction by Saveant
et al.10 and that of the reduction of iodobenzene was
obtained in an approximate way using the standard free
energy of anion radical cleavage.17
3.2. Standard potential
The rate of change of activation energy (DG*) with respect
to the driving force (DG0), yields the transfer coefficient as
Eq. 8
vDGꢀ
vDG0
DG0
8DGꢀ0
a Z
Z 0:5 C
(8)
The standard potentials of the reduction of C–I and C–Br
are 839 and 316 mV more positive than those for the
reduction of iodobenzene and bromobenzene, respectively
(Table 1). This is consistent with the fact that the energy
of the p* orbital of the ring increases with the elimination
of each halogen, viz. lowest at the first wave and highest
at the fourth wave, which is reflected in the standard
potentials of the reduction of respective carbon–halogen
bonds. Even though a is substantially lower in the
reduction of C–I, the energy of the p* orbital of the
ring is low enough to trap the unpaired electron before it
Experimentally a is estimated from the derivative of ln kET
versus E plot using Eq. 9
RT d ln kET
a ZK
(9)
F
dE
wherein the symbols R, T and F assume the usual
significance. Figure 4 shows the variation of a with E at
various scan rates for the reduction of C–I, C–Br and C–Cl
(2) bonds. Since the variation of ln kET with E is parabolic, a
varies linearly with E. As implied by Eq. 8, most theoretical
Figure 4. Variation of apparent transfer coefficient with electrode potential at various scan rates for the reduction of (a) carbon–iodine bond (b) carbon–
bromine bond and (c) carbon–chlorine (2) bond.

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M. A. Prasad, M. V. Sangaranarayanan / Tetrahedron 60 (2004) 10967–10972
dissociatively reduces the C–I bond in a successive step.13
A difference between the ER0X=RX$K for the reduction of
C–Cl (2) and that for the reduction of chlorobenzene may
be attributed to the different supporting electrolyte
(nBu4NI) and working electrode (mercury) employed in
the earlier study.10 Further, in our estimation of standard
potentials, double layer corrections have not been applied;
however it has been demonstrated18 that the standard
potential calculations carried out incorporating the double
layer effects amounts to a maximum difference of only
0.06–0.07 V. This fact is particularly significant, since
good experimental data are obtained from the glassy
carbon electrode—the double layer properties of which
are unknown.
found, in the online version, at doi:10.1016/j.tet.2004.09.
020
References and notes
1. Costentin, C.; Robert, M.; Saveant, J. M. J. Am. Chem. Soc.
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1750–1751.
4. Summary
The electrochemical reduction of 5-bromo-1,3-dichloro-2-
iodobenzene results in four irreversible voltammetric
waves consuming eight electrons in a single cycle. The
reduction of C–I, C–Br and C–Cl (2) bonds lead to
parabolic ln KET versus E plots obeying quadratic
activation–driving force relationship. The analysis
employing the Marcus theory of outer sphere electron
transfer in conjunction with the convolution approach
yields standard potentials of the reduction of carbon–
halogen bonds in 5-bromo-1,3-dichloro-2-iodobenzene
except in one of the carbon–chlorine bonds wherein the
reduction follows Butler–Volmer kinetics.
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Acknowledgements
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M.A.P. thanks the CSIR, India for the award of a Senior
Research Fellowship.
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Supplementary data
18. Antonello, S.; Musumeci, M.; Wayner, D. D. M.; Maran, F.
J. Am. Chem. Soc. 1997, 119, 9541–9549.
Supplementary data associated with this article can be