Analysis of C-F bond cleavages in methylfluorobenzoates-Fragmentation and dimerization of anion radicals using convolution potential sweep voltammetry

Article abstract of DOI:10.1016/j.electacta.2009.10.047

The electrochemical reduction of methylfluorobenzoates at glassy carbon electrodes is analyzed using the convolution potential sweep voltammetry (CPSV). The stabilization of the radical anion due to the electron-withdrawing group is shown to lead to intra-molecular stepwise dissociative electron transfer. While methyl 2-fluorobenzoate (ortho isomer) follows EC mechanism, the methyl 4-fluorobenzoate (para-isomer) undergoes electro-dimerization prior to C-F bond cleavage. The first order rate constant for the EC mechanism and the dimerization rate constant for the electro-dimerization are deduced from the classical as well as convolution potential sweep voltammetry. A plausible mechanism of dimerization is suggested. The influence of the electron-withdrawing groups is illustrated by comparing the reduction behaviour of 4-fluorobenzonitrile. The potential energy surfaces and electron density mapping employing Gaussian 03 calculations provide further support for the validation of the mechanism pertaining to C-F bond cleavages.

Full text of DOI:10.1016/j.electacta.2009.10.047

Electrochimica Acta 55 (2010) 1664–1669
Contents lists available at ScienceDirect
Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Analysis of C–F bond cleavages in methylfluorobenzoates—Fragmentation and
dimerization of anion radicals using convolution potential sweep voltammetry
A. Muthukrishnan, M.V. Sangaranarayanan ,1
∗
Department of Chemistry, Indian Institute of Technology Madras, Chennai-600036, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 24 June 2009
Received in revised form 19 October 2009
Accepted 19 October 2009
Available online 29 October 2009
The electrochemical reduction of methylfluorobenzoates at glassy carbon electrodes is analyzed using
the convolution potential sweep voltammetry (CPSV). The stabilization of the radical anion due to the
electron-withdrawing group is shown to lead to intra-molecular stepwise dissociative electron trans-
fer. While methyl 2-fluorobenzoate (ortho isomer) follows EC mechanism, the methyl 4-fluorobenzoate
(para-isomer) undergoes electro-dimerization prior to C–F bond cleavage. The first order rate constant
for the EC mechanism and the dimerization rate constant for the electro-dimerization are deduced from
the classical as well as convolution potential sweep voltammetry. A plausible mechanism of dimerization
is suggested. The influence of the electron-withdrawing groups is illustrated by comparing the reduction
behaviour of 4-fluorobenzonitrile. The potential energy surfaces and electron density mapping employ-
ing Gaussian 03 calculations provide further support for the validation of the mechanism pertaining to
C–F bond cleavages.
Keywords:
Methylfluorobenzoates
Convolution potential sweep voltammetry
Intra-molecular electron transfer
EC-mechanism
Ion-radical interactions
©
2009 Elsevier Ltd. All rights reserved.
1
. Introduction
C–F bond cleavages, a proper choice of the compounds is warranted
since the para-substituted fluorocompounds may dimerize rather
than undergoing simple reduction. Further, all methylhaloben-
zoates (excluding the fluoro) undergo Ullmann type of coupling on
reactive nickel surfaces [15], sacrificial anodes such as zinc, iron,
aluminium, etc. [16,17]. It is hence of interest to investigate the
electron transfer processes in methylfluorobenzoates. In aromatic
fluorocompounds, the radical anions dissociate to form the neu-
tral radicals and corresponding ions. The neutral radicals lead to
defluorinated compounds, upon abstraction of a proton. [18]. In
the case of substrates undergoing dimerization, the logarithmic
analysis of the convoluted current provides the dimerization rates
and for acetophenone, the dimerization rate constant has been
The reductive cleavage of C–X bonds (where X = F, Cl, Br, I, O,
S, etc.) constitutes a fascinating area of research in the study of
electron transfer reactions in view of the crucial role played by the
functional group, dielectric properties of the solvent and magnitude
of the driving force [1–3]. While there are compounds which follow
solely stepwise or concerted electron transfer pathways, there exist
cases where the mechanism itself depends upon the driving force
and a transition from one mechanism to the other is feasible [4–8].
The C–F bond cleavage reactions in aromatic substrates is ren-
dered difficult on account of the high activation energy barriers
and hence their study has been restricted to a smaller number of
aromatic compounds. The reductive de-fluorination of 1,3 difluo-
robenzene [9], pentafluoronitrobenzene [10] on mercury electrode
and fluoromethylarenes [11] on glassy carbon electrode deserves
mention in this context. Alternately, the C–F bond reductions can
be accomplished with the help of suitable redox catalysts such as
anthracene, phenanthrene, benzonitrile, chrysene, etc. [12]. A dif-
ferent strategy for carrying out the reduction of C–F bonds consists
in introducing electron-withdrawing groups in the aromatic ring
e.g. fluorobenzonitrile [13,14]. Although electron-withdrawing
groups such as –CN, –NO , –COOCH , etc. are expected to facilitate
5
−1
−1
deduced as 4 × 10 mol l s by Saveant and Tessier [19]. Since
the C–CN bond breaking (instead of C–F bond cleavage) occurs in
the case of meta-fluorobenzonitrile [13], the ortho and para isomers
of methylfluorobenzoates are chosen in the present study.
The convolution potential sweep voltammetry (CPSV) is a valu-
able technique since (i) the entire data points of the voltammogram
can be made use of and (ii) no a priori assumption of the govern-
ing rate laws is required. While the linear free energy relation is
the basis of the Butler–Volmer equation for simple electron trans-
fer processes, the kinetics of dissociative electron transfer reactions
is customarily investigated using the quadratic activation-driving
force relation via the Marcus–Hush formalism [20]. In this com-
munication, we investigate the mechanism of C–F bond cleavages
in methyl 2-fluorobenzoate (OFMB) and methyl 4-fluorobenzoate
(PFMB) using CPSV and density functional calculations.
2
3
∗ Corresponding author. Tel.: +91 44 22574209; fax: +91 44 22570545.
E-mail address: sangara@iitm.ac.in (M.V. Sangaranarayanan).
ISE member.
1
0
013-4686/$ – see front matter © 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2009.10.047

A. Muthukrishnan, M.V. Sangaranarayanan / Electrochimica Acta 55 (2010) 1664–1669
1665
Fig. 1. Cyclic voltammograms of (a) OFMB (at 50 mV s 1), (b) PFMB (at 50 mV s−1), (c) 4,4 -biphenyldicarboxylicdimethylester (at 100 mV s ) and (d) is the multi-cycle
−
ꢀ
−1
voltammogram of PFMB depicting the reversibility of anodic peaks. The supporting electrolyte is 0.1 M TBAP. The current refers to the background-subtracted values.
1
2
. Methodology
and FTIR studies: H NMR (400 MHz, CDCl ) ı 3.95 (6H, s), 7.69 (4H,
3
d), 8.13 (4H, d); FTIR data: the C–O stretching frequency (di-ester
−
1
ꢀ
−1
2.1. Experimental
at 1721 cm and 4,4 -biphenyldicarboxylic acid at 1677 cm ).
Acetonitrile (HPLC grade, SRL, India,) was distilled over anhy-
2.3. Density functional calculations
drous calcium hydride protected by 3 Å molecular sieves in argon
atmosphere before each experiment. The supporting electrolyte
tetrabutylammonium perchlorate (TBAP) (Sigma–Aldrich, elec-
trochemical grade) was used without further purification. The
reactants methyl 2-fluorobenzoate and methyl 4-fluorobenzoate
The quantum chemical calculations were carried out using
Gaussian 03 [21] with the help of Becke three-parameter hybrid
exchange correlation of Lee, Yang and Parr (B3LYP) and the 6-
311++g(d,p) basis set wherein the Polarizable Continuum Model
(PCM) was employed for the solvent. The deduced molecular orbital
coefficients were employed for constructing the Singly Occupied
Molecular Orbitals (SOMO) pertaining to the neutral species and
the radical anions.
(
Sigma–Aldrich) were used as received.
The cyclic voltammetric experiments were carried out using the
electrochemical workstation (CH Instruments 660A), in a single-
compartment electrochemical cell thermostatted at 298 K. The
working electrode was a glassy carbon (GC) electrode of 3 mm
diameter (CH Instruments, USA) polished with the alumina slurry
and sonicated before use. While the Saturated Calomel Electrode
3. Results and discussion
(
SCE) was the reference electrode, Pt wire served as the counter
3.1. Cyclic voltammetric studies
electrode. All the potentials are reported with respect to SCE. The
solution resistance was measured before each experiment and
compensated leaving the residual resistance of ∼35 ꢀ.
A typical cyclic voltammogram of methyl 2-fluorobenzoate
OFMB) at the scan rate of 0.05 V s is shown in Fig. 1a while that
−1
(
pertaining to methyl 4-fluorobenzoate (PFMB) is depicted in Fig. 1b.
In the case of OFMB, a single irreversible cathodic peak at −2.183 V
is noticed. For PFMB, an irreversible cathodic peak at −2.298 V
appears along with two additional anodic peaks. The cyclic voltam-
mograms of the two isomers being significantly different from each
other, it is instructive to analyze then separately.
ꢀ
2
.2. Preparation of 4,4 -biphenyldicarboxylic dimethylester
ꢀ
In order to prepare the 4,4 -biphenyldicarboxylic dimethylester,
0.5 g of the corresponding acid was dissolved using excess volume
of thionyl chloride and was refluxed under moisture-free condi-
tions until the reaction mixture became homogeneous. The reaction
◦
mixture was cooled to 0 C and 15 ml of methanol was slowly
3.2. Reduction of methyl 2-fluorobenzoate
added. The whole solution was poured into ice cold water and
the extraction was carried out with ethyl acetate, followed by (i)
drying with anhydrous Na SO and (ii) evaporation of the solvent.
The compound (4,4 -biphenyldicarboxylic dimethylester) was re-
crystallized from mixture (25 ml) of ethanol and dichloromethane.
The characterization of the compound was carried out using NMR
In order to obtain more insights into the mechanism of electron
transfer process in OFMB, the influence of the scan rate needs to be
analyzed in a systematic manner. For this purpose, low scan rates
(in the range 0.01–0.05 V s ) as well as high scan rates (>10 V s
employed. Further, the experimental data at high scan rates indicate
2
4
ꢀ
−
1
−1
)

1
666
A. Muthukrishnan, M.V. Sangaranarayanan / Electrochimica Acta 55 (2010) 1664–1669
Table 1
Parameters derived from the cyclic voltammetric data.
viz. [27]
ꢁ
ꢂ
RT
E = E + F ln
I − I
0
l
Parameters
Methyl 2-fluorobenzoate
Methyl 4-fluorobenzoate
(3)
I
E0 vs SCE/V
−2.190
−2.277
Thus, the intercept of the plot of ln[I − I]/I vs E at high scan rates
˛
D/cm
0.81
0.74
l
2
−1
−5
−5
6
0
s
0.83 × 10
2.50 × 10
region (Fig. 3a) provides E as −2.190 V. This value is in agreement
−
1
−1
−1
with E0 estimated as −2.157 V from the arithmetic mean of anodic
Rate constants
10.87 s
1.38 × 10 mol l s
0
and cathodic peak potentials, viz. E = (Ep,a + Ep,c)/2, at high scan
rates. The slope deduced from the plot of Fig. 3a is 28 mV for scan
−
1
−1
a reversible (diffusion-controlled) behaviour while a kinetically
controlled behaviour was noticed at lower scan rates. The reversible
peak occurring at high scan rates is indicative of a stable radical
anion and thus the cyclic voltammogram depicted in Fig. 1a implies
rates ranging from 10 V s to 50 V s and is consistent with the
theoretical value of 25.6 mV for the systems under diffusion control.
0
The availability of the standard reduction potential (E ) can then be
exploited to deduce the value of the uni-molecular rate constant (k)
as shown in Eq. (4). The limiting convolution current (I ) is related
to the voltammetric current (i) and the convolution current (I) as
k
•
−
•
−
l
the occurrence of EC mechanism (RX + e ꢀ RX −→ R + X ). The
peak potential shifts cathodically with increase in the scan rate
and the slope of ıEp,c vs ı log(v) plot i.e. ıEp,c/ı log(v) is estimated as
[
27]
ꢃ
ꢄ
2
5 mV consistent with the anticipated value of 29.6 mV for a first
RT
RT
I − I
0
l
E = E
+
ln k + F ln
(4)
order chemical reaction. It is hence inferred that the electrochem-
ical reduction occurs via the first order elimination of fluoride ion
and that the decomposition of radical anion is the rate determining
step [22,23]. The transfer coefficient is deduced from peak width
2F
i
−
1
At scan rates in the range of 0.01–0.05 V s , the slope of ln (I − I)/i
l
vs E plot is 29 mV(Fig. 4a), in gross agreement with the theoretical
value of 25.6 mV for systems solely under kinetic control. The rate
constant k follows as 10.87 s for the above range of scan rates;
(
Ep/2 − Ep) value and equals 0.81 (Table 1).
−1
the dimensionless kinetic parameter ꢂ is given by
3
.2.1. Convolution potential sweep voltammetry
The classical cyclic voltammetric analysis due to Nicholson and
RT k
ꢂ =
(5)
F v
Shain [24] is valuable since tabular compilations and working
curves have been provided as diagnostic criteria for different types
of electron transfer processes. However, when a wide range of scan
rates is employed, the CPSV is more preferred since a priori assump-
tion of kinetic laws is no longer required. The convolution current (I)
is related to the voltammetric current (i) through the convolution
integral [25] as
v being the scan rate employed. Thus, the value of log(ꢂ) ranges
from 0.77 to 0.92 for OFMB and as shown elsewhere [23], for
systems solely under kinetic control, log(ꢂ) should be greater
than 0.28. We may recall that the reductive cleavage of 1-chloro-
2,4-dinitrobenzene [28] follows EC mechanism and the reversible
behaviour was noticed at high scan rates in the cyclic voltammetric
8
−1
analysis. Further, the rate constants ranging from 1.6 × 10 s to
ꢀ
t
−3 −1
4
.0 × 10
s
have been reported for reductive cleavage of various
I(t) = √1
i(u)
du
(1)
1
/2
organic compounds such as halobenzophenone, halobenzonitrile,
halonitrobenzene, etc. [29].
ꢁ
0
(t − u)
It is of interest to enquire whether the above rate constants per-
taining to the EC mechanism of OFMB can also be deduced from the
classical analysis of Nicholson and Shain (case VI of Ref. [24]). The
analysis of [24,30] provides a tabular compilation of the dimension-
less current function ␹(at) (where a = nFv/RT) as well as a working
curve for the ratio between the anodic and cathodic peak currents
The convoluted current estimated using Lawson–Maloy algorithm
26] is depicted in Fig. 2a. A typical sigmoidal curve is noticed with
a plateau at large negative potentials. This represents the limiting
convolution current
[
b√
Il = nFAC
D
(2)
(ip,a/ip,c) as a function of (k␶), where k denotes the rate constant,
where Cb is the bulk concentration of OFMB (1 mM), D being the
ꢃ being the time elapsed from the peak potential to the switch-
−5
2
−1
corresponding diffusion coefficient (0.83 × 10 cm s ), A is the
ing potential. The mean value of the rate constants estimated from
2
−1
surface area of the electrode (7.065 mm ) and ‘n’ denotes the num-
Table XI of Nicholson and Shain analysis in the scan rates >10 V s
0
−1
ber of electrons involved (n = 2). In order to estimate E , the regime
provides the rate constant k as 13.18 s , in gross agreement with
the value of 10.87 s deduced from CPSV. Furthermore, when the
−
1
where the diffusion control occurs (high scan rates) is made use of,
Fig. 2. The convoluted current for the reduction of (a) OFMB and (b) PFMB at high scan rates. The scan rates are 20 V s 1, 30 V s−1, 40 V s and 50 V s
−1
.
−
−1

A. Muthukrishnan, M.V. Sangaranarayanan / Electrochimica Acta 55 (2010) 1664–1669
1667
−
1
Fig. 3. Estimation of the standard electrode potentials from the convoluted current at high scan rates for (a) OFMB and (b) PFMB. The scan rates are as follows: 10 V s (+),
−
1
−1
−1
2
0 V s (ꢁ), 40 V s (ꢁ) and 50 V s (♦).
−
1
−1
Fig. 4. The dependence of the convoluted current on potential deduced from the data at low scan rates for (a) OFMB and (b) PFMB. The scan rates are 10 mV s (+), 20 mV s
−
1
−1
−1
(ꢁ), 30 mV s (*), 40 mV s (ꢁ) and 50 mV s (♦).
ratio k/a is large (i.e. low scan rates), it is possible to estimate the
rate constants from the shift of the peak potential using equation
dence of the peak potential on scan rates. If the ratio (ip,a/ip,c) is
independent of the concentration, it is inferred that the dimeriza-
tion occurs via the coupling of the neutral radicals. On the other
hand, the decrease of the peak current ratio with increase in the
concentration is suggestive of the mechanism as the coupling of
radical anions. Table 2 provides the dependence of the peak cur-
rent ratio on the reactant concentration wherein the decrease of
ip,a/ip,c is noticed, thus suggesting the coupling of radical anions
as the most likely method of dimer formation. This inference is
consistent with the mechanism of dimerization occurring via the
coupling of (i) 4-fluorobenzonitrile radical anions on platinum as
postulated by Houser et al. [13] and (ii) pentafluoronitrobenzene
radical anions on glassy carbon by Andrieux et al. [10]. The origin
(
83) of Nicholson and Shain [24], viz.
ꢅ
ꢆ ꢇ
RT
nF
k
a
Ep = E −
0.78 − ln
(6)
0
Employing the experimental peak potentials at the scan rates rang-
−
1
−1
ing from 0.01 V s to 0.05 V s in the above equation leads to the
−1
mean rate constant as 11.65 s . Thus, the rate constants deduced
from the ratio of the peak currents, shift of the peak potential with
scan rates and CPSV analysis are consistent with each other.
3.3. Reduction of methyl 4-fluorobenzoate (PFMB)
In the case of PFMB, two additional anodic reversible peaks
Table 2
The ratio between anodic and cathodic peak currents as
a function of the concentration of PMFB at the scan rate
of 0.1 V s and switching potential of −2.6 V.
appear in the cyclic voltammogram (Fig. 1b and d) which may
be attributed to the formation of dimers subsequent to the C–F
bond cleavage. In order to confirm the origin of the anodic
−
1
ꢀ
Cb/mM
ip,a/ip,c
peaks, the cyclic voltammogram of the authentic sample of 4,4 -
biphenyldicarboxylic dimethylester (prepared as mentioned in
Section 2.1) was also obtained as shown in Fig. 1c. The occurrence
of the two anodic peaks in the cyclic voltammogram of the authen-
tic sample validates the dimerization hypothesis. The dimerization
mechanism may occur via two plausible methods as given below.
In order to distinguish between the paths A and B, there exist
two diagnostic criteria, viz. (i) the variation of the peak current
ratio with the concentration of the reactant and (ii) the depen-
0.99
1.96
0.398
0.347
0.313
0.304
0.291
0.287
0.289
0.287
0.287
2
3
4
5
.91
.85
.76
.66
6.54
7
8
.41
.26

1
668
A. Muthukrishnan, M.V. Sangaranarayanan / Electrochimica Acta 55 (2010) 1664–1669
Fig. 5. The representation of SOMO for (a) OFMB and (b) PFMB. The molecular orbital coefficients are obtained using the isovalue of 0.05 units.
6
−1
−1
and the corresponding
(
k ) is deduced as 1.38 × 10 mol l s
d
dimensionless parameter (ꢂ ) as 2.95.
d
3.5. Estimation of bond lengths
In dissociative electron transfer reactions, two different types
of mechanism are customarily encountered, viz. stepwise and con-
certed. The stepwise pathway refers to the formation of a stable
radical anion subsequent to electron transfer. In order to infer the
stepwise mechanism, bond lengths and partial atomic charges2
were calculated using Gaussian 03 version with the help of B3LYP
and 6-311++g(d,p) as the basis set. The Polarizable Continuum
Model (PCM) was employed for the solvent. Fig. 5 depicts the SOMO
of OFMB and PFMB radical anions. It is seen that the SOMO are
present above the ester group and benzene ring. This implies that
the electron is first transferred to the ␲* orbital of the reactants and
subsequently to ␴* orbital of the C–F bond.
Scheme 1. The two possible mechanisms for dimerization of PMFB.
of the concentration dependence is attributed to the second order
kinetics arising in the second step of Scheme 1(A) given above.
The variation of the peak potential with scan rates also provides
a diagnostic criterion for distinguishing between the two pathways
of dimerization. In this case, the following equation depicting the
variation Ep with log(v) has been postulated as
The computed potential energy surfaces in vacuum for the
anionic radicals, exhibit two minima [32] indicating respectively
the presence of (i) ␲-anion radical and (ii) ␴-anion radical as
shown in Fig. 6. The first and deeper minimum denotes the strong
stabilization of ␲-anion radical. The second minimum arises as
a consequence of the ion-radical interactions. It is anticipated
that electron-withdrawing groups will enhance the ion-radical
interaction [33–36] on account of the attraction of the partial pos-
∂
Ep
1
1 + b
RT
F
=
ln(10)
(7)
∂
log(v)
wherein b refers to the order of the reaction. If the pathway as sug-
gested by (A) occurs, b = 2 and hence ıEp/ı log(v) should be 19.7 mV.
The experimental value for PFMB is 18 mV, thus supporting the
mechanism A wherein the rate determining step is the second order
dimerization of radical anions. The transfer coefficient follows as
−
itive charge (on carbon), towards the negatively charged F . From
the investigation on the reduction of haloacetonitriles, Cardinale
et al. [33] demonstrated that the ion-radical interaction energy
−
−
−
0
.74 from peak width measurements.
decreases from Cl to I . In general, the compounds having F as the
functional group should have higher ion-radical interactions. In the
present study, both the isomers possess the electron-withdrawing
carbonyl group. In vacuum phase calculation, both the compounds
exhibit two minima and the ion-radical interaction energy typically
varies from 0.6 eV to 0.8 eV.
3
.4. Standard reduction potentials and dimerization rate
constants
The potential dependent convoluted current is depicted in
Fig. 2b. As in the earlier case, the standard reduction potential (E )
follows from the high scan rate data as −2.277 V (Fig. 3b) and is in
agreement with that obtained from the cathodic and anodic peak
potentials (−2.279 V). Further, the standard reduction potentials of
OFMB and PFMB are comparable to that of ethyl 3-fluorobenzoate
0
In the presence of the solvent (acetonitrile), the minimum per-
taining to ␴-anion complex has vanished, thus indicating that the
ion-radical interaction energy is absent in the polar medium. How-
ever, both the fragments (neutral radicals and fluoride ions) and
the ␲-anion complex become stable in the polar medium as can
be seen from the corresponding minimum in the potential energy
calculations. The spike appearing in the potential energy may be
due to the high electronegativity of fluorine atoms.
[
31].
In order to estimate the dimerization rate constant (k ) the fol-
d
lowing equation for the electrode potential [19]
ꢃ
ꢄ
ꢁ
ꢂ
RT
E = E + 3 ln
2k
RT
I − I
0
d
l
√
+ F ln
(8)
F
3FA
D
i2/3
2
In the case of stepwise mechanism, the cleaving bond length of the radical anion
is employed wherein the dimensionless kinetic parameter is
will not be significantly different from that of the reactant. From the quantum chem-
ical calculations using Gaussian 03, it is found that the bond lengths of (a) OFMB
neutral: 1.35 Å and anioic radical: 1.38 Å) and (b) PFMB (neutral: 1.34 Å and anionic
radical: 1.39 Å) are nearly the same. The Mulliken charge analysis yields the partial
charge of fluorine in the radical anionic forms as −0.25 (OFMB) and −0.28 (PFMB).
Hence the bond is not broken after the electron transfer.
defined as ꢂd = (RT/F)(k C /v). The diffusion coefficient (D) for
d
b
(
−
5
2
−1
PFMB is estimated as 2.50 × 10 cm s using Eq. (2), Using the
0
above-mentioned value of E and the intercept from the plot of
2/3
ln[(I − I)/i ] vs E of Fig. 4b, a typical dimerization rate constant
l

A. Muthukrishnan, M.V. Sangaranarayanan / Electrochimica Acta 55 (2010) 1664–1669
1669
Fig. 6. (a) Potential energy diagram pertaining to radical anions of (a) OFMB and (b) PFMB. The dashed lines denote the surfaces in vacuum while the solid line refers to the
energy surfaces in the solvent (acetonitrile).
3.6. Effect of electron withdrawing groups in dimerization
References
reaction
[
[
[
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scan rates, the C–F bond gets reduced yielding only one irreversible
peak while an increase in the scan rates shifts the cathodic peak
potential to more negative values. When the reduction potential
reaches the value pertaining to that of –CN bonds at higher scan
rates (∼30 V s ), the –CN gets reduced first while the C–F cleavage
occurs leading to the subsequent dimerization. Consequently, 4-
fluorobenzonitrile dimerizes only at very high scan rates in contrast
to the dimerization of PFMB even at low scan rates. The reduction
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bonds. Thus, the reduction of the ester group occurs first, followed
by the C–F bond cleavage due to the intra-molecular electron trans-
fer at low scan rates although the reversible behaviour is noticed
at higher scan rates.
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8
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4
. Conclusions
The mechanism of C–F bond cleavage in methylfluorobenzoates
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The helpful comments of the reviewers are gratefully acknowl-
edged. M.A. thanks CSIR for the award of a senior research
fellowship. This work was supported by the DST and CSIR.