The autoprotonation in reduction mechanism of pesticide ioxynil

Article abstract of DOI:10.1016/j.electacta.2010.01.094

The reduction mechanism of ioxynil (3,5-diiodo-4-hydroxy-benzonitrile) was studied in dimethylsulfoxide using the electrochemical methods (tast polarography, cyclic voltammetry and controlled potential electrolysis) combined with GC/MS identification of products. The reduction is accompanied by the cleavage of iodide yielding 3-iodo-4-hydroxybenzonitrile. Surprisingly, this process requires only one electron for the exhaustive electrolysis of the starting compound. We showed that the apparent one electron reduction observed in the aprotic solvent is due to the autoprotonation by another molecule of ioxynil. The overall one electron reduction (uptake of two electrons per two molecules of ioxynil) is changed in the presence of a strong proton donor to a two electron process per one molecule.

Full text of DOI:10.1016/j.electacta.2010.01.094

Electrochimica Acta 55 (2010) 8336–8340
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Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
The autoprotonation in reduction mechanism of pesticide ioxynil
Romana Sokolováa,∗,1, Magdaléna Hromadováa,1, Jirˇí Ludvíka,1, Lubomír Pospísˇila,1, Stefania Giannarellib
a J. Heyrovsky´ Institute of Physical Chemistry, v.v.i., Academy of Sciences of the Czech Republic, Dolejsˇkova 3, 18223 Prague, Czech Republic
b Department of Chemistry and Industrial Chemistry, University of Pisa, Via Risorgimento 35, 56100 Pisa, Italy
a r t i c l e i n f o
a b s t r a c t
Article history:
The reduction mechanism of ioxynil (3,5-diiodo-4-hydroxy-benzonitrile) was studied in dimethylsulfox-
ide using the electrochemical methods (tast polarography, cyclic voltammetry and controlled potential
electrolysis) combined with GC/MS identification of products. The reduction is accompanied by the cleav-
age of iodide yielding 3-iodo-4-hydroxybenzonitrile. Surprisingly, this process requires only one electron
for the exhaustive electrolysis of the starting compound. We showed that the apparent one electron
reduction observed in the aprotic solvent is due to the autoprotonation by another molecule of ioxynil.
The overall one electron reduction (uptake of two electrons per two molecules of ioxynil) is changed in
the presence of a strong proton donor to a two electron process per one molecule.
Received 23 November 2009
Received in revised form 21 January 2010
Accepted 22 January 2010
Available online 2 February 2010
Keywords:
Autoprotonation
Organic halides
Reduction
© 2010 Elsevier Ltd. All rights reserved.
Pesticides
Polarography
1. Introduction
yielding directly an aryl radical or in a stepwise mechanism going
through the formation of an anion radical as the intermediate (see
Ioxynil (3,5-diiodo-4-hydroxy-benzonitrile) belongs to the
group of benzonitrile herbicides, which inhibit the Photosystem
II receptor site of green plants [1,2]. Only a few papers in the
literature deal with the decomposition of these compounds. The
reductive dehalogenation is an important pathway for the degrada-
tion of this pesticide by Desulfitobacterium chlororespirans [3]. The
photohydrolysis of a carbon halogen bond leading to a monohalo-
genated dihydroxybenzonitrile was studied in the aqueous solution
by Malouki et al. [4]. The electrochemical reduction of three
3,5-dihalogeno-4-hydroxy-benzonitrile compounds in dimethyl-
sulfoxide was the subject of our previous report [5]. It was found
that these compounds are reduced to an anion radical followed by
the cleavage of the carbon–halogen bond. This interpretation was
based on the electrochemical detection of halide anions formed
in the process and by the detection of dehalogenated products by
GC–MS method.
Scheme 1) [13]. The overall reduction mechanism is influenced by
the presence of a proton donor, the solvent and substituents on the
aromatic ring.
The number of electrons consumed during the reductive
cleavage of halide anions from halogenoaromatics reported in
the literature is not consistent [21–28]. Jaworski et al. found
the one electron concerted process for 1-chloro-10-methyltri-
benzotriquinacene in acetonitrile and DMF, whereas a two electron
reduction involving DISP1 mechanism for the same compound in
benzonitrile was observed [23].
Coupled chemical reactions often involve protonation steps.
Protons are provided either by the added acids, by the solvent or
even by the parent molecule. The latter process, called autoproto-
nation, was indeed found in the reduction of phthalimides [29] and
hydroxyimines [30]. The overall stoichiometry of the reduction of
hydroxyimines involved the exchange of two thirds of an electron
per molecule. In this case the autoprotonation by the original reac-
tant occurs twice. One proton exchange involves the “father–son”
(starting molecule–anion radical) step and the second is the pro-
tonation of the resulting anion by the starting molecule. Another
example of the autoprotonation mechanism comes from the sub-
stituted acetophenones, where arylanion formed in the ECE/DISP
process reacts with starting molecule [31]. In the reduction mech-
anism of p-nitrobenzoic acid the self-protonation occurs not only
in the “father–son” step, but also through the protonation of ary-
lanion and other intermediates. The overall number of electrons
involved in the first reduction is 0.8 [32]. The complicated reduc-
The reductive cleavage of a series of organic halogenides, includ-
ing halogenoaromatics, benzyl halogenides and halogenoalkanes
has been extensively studied in the literature [6–20]. It has been
shown that the rate of cleavage of the halide ion from the anion
radical decreases in the order I > Br > Cl > F. The process of dissocia-
tive electron transfer (DET) may occur in a concerted mechanism
∗
Corresponding author.
E-mail address: sokolova@jh-inst.cas.cz (R. Sokolová).
ISE member.
1
0013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2010.01.094

R. Sokolová et al. / Electrochimica Acta 55 (2010) 8336–8340
8337
model PCL-848) and a data acquisition card (PCL-818) using 12-
bit precision. A three-electrode electrochemical cell was used with
an Ag|AgCl|1 M LiCl reference electrode (with potential 0.210 V vs.
SHE) separated from the test solution by a salt bridge. The working
electrode was a valve-operated static mercury electrode (SMDE2,
Laboratorní Prˇístroje, Prague) with an area 1.13 × 10−2 cm2 and
mechanically controlled drop time 2 s. The auxiliary electrode was
cylindrical platinum net. Oxygen was removed from the solution
by passing a stream of argon.
The reduction products of ioxynil were obtained by exhaustive
electrolysis of its 1.6 × 10−3 M solution in DMSO. The electrolysis
on the mercury pool was performed at the potential of the limiting
current of the first polarographic wave. A 6890 gas chromatograph
(Agilent Technologies) equipped with a quadrupole mass spectro-
metric detector (at 150 ◦C), model 5973N (electron impact 70 eV,
ion source 230 ◦C, interface temperature 280 ◦C), was used for
GC/MS analysis of the reduction products. The amount of 2 ␮l of the
electrolysed solution was injected into the GC/MS spectrometer.
The supporting electrolyte did not interfere with the analysis. The
chromatographic separation was performed on a 5% phenyl-95%
methylpolysiloxane HP-5MS chemical-bonded fused silica capil-
lary column (Hewlett-Packard) of 30 m length, 0.25 mm internal
diameter, and 0.25 ␮m film thickness. Helium of 99.995% purity
was used as a carrier gas. The initial temperature was 80 ◦C, then
a temperature increase of 4 ◦C min−1 up to 290 ◦C was applied, fol-
lowed by an isothermal period of 10 min. The injector (splitless
mode) was kept at 250 ◦C.
Scheme 1.
tion mechanism of halogenated pyrimidines in acetonitrile, which
involves the regeneration of the original molecule in a dispropor-
tionation (DISP) process, was studied in the absence and presence
of a proton donor. The authors found for 5-bromopyrimidine the
change in the number of electrons from 1.12 to 2.0 [27].
The reduction of halogenated aromatics follows Scheme 1. Since
this scheme yields both A• and AX•− species, two possible follow-
up reaction pathways should be considered. Radical A• can be
directly reduced to A− by the second electron
A• + e− ꢀ A−
(1)
Alternatively, a disproportionation step can participate in the
mechanism
A• + (AX)•− → A− + AX
(2)
Both reactions (1) and (2) finally yield the aryl anion A−. The
protonation of A− leads to the final product
3. Results and discussion
A
− + SH → AH + S−
(3)
We reported previously the reduction products of three 3,5-
dihalogeno-4-hydroxy-benzonitrile compounds [5]. The reduction
behaviour is influenced by the nature of the halogen atom on the
aromatic ring. The half-wave potentials of tast polarographic wave
shift towards more negative values in the order I > Br > Cl. Surpris-
ingly, the electric charge required for a complete electrolysis at
the potential of the first reduction corresponds to the consump-
tion of only one electron per molecule. It is probable that the
controversy between the consumed charge and formed products
originates from the participation of protonation reactions. For this
reason we scrutinised the present system in the presence of var-
ious proton donors. The apparent one electron reduction process
of ioxynil in aprotic media is changed in the presence of a strong
proton donor. The potassium tetraoxalate was chosen as the best
proton donor, because its first reduction wave occurs at −1.17 V,
which does not interfere with the first reduction wave of ioxynil.
Fig. 1 shows the tast polarogram of ioxynil in the presence of differ-
ent concentrations of potassium tetraoxalate. The limiting current
of polarographic wave increases with increasing concentration of
the proton donor. The height of the reduction wave reaches the lim-
iting value corresponding to two electrons at the ratio of ioxynil
and tetraoxalate equal to 1:2 (Fig. 2A). This implies that in the
presence of a sufficient proton donor concentration the mecha-
nism involves the uptake of two electrons and two protons. The
consumption of two electrons was confirmed by the controlled
potential electrolysis. With the increasing concentration of tetraox-
alate the half-wave potential (Fig. 2B) is shifted to more negative
values. This indicates that the rate determining step of the whole
process is an irreversible following chemical reaction involving
the protonation step. When the concentration ratio of ioxynil to
the potassium tetraoxalate reaches 1:2, the half-wave potential
becomes independent of the tetraoxalate concentration and attains
the limiting value E1/2 = −0.923 V. Thus the experimental observa-
tion indicates that the reduction has to involve two electrons and
two protons.
The overall stoichiometry involves two electrons per molecule
and the ratio between the starting compound and proton donor is
1:1
AX + 2e− + SH → AH + X− + S−
(4)
Such a reduction mechanism was found for halogenated ace-
tophenones [31]. The protonation can proceed also by the hydrogen
atom transfer (HAT) mechanism [12,17,28,29,33,34].
In this communication we report on the reduction of ioxynil in
dimethylsulfoxide solvent in the absence and presence of added
proton donor. We will demonstrate the presence of autoprotona-
tion in the reduction of ioxynil and its role in the observation of an
apparent one electron polarographic wave in our previous report
[5].
2. Experimental
2.1. Reagents
Ioxynil (3,5-diiodo-4-hydroxybenzonitrile) was purchased as
pesticide reference material from Dr. Ehrenstorfer, GmbH
a
(Augsburg, Germany). Tetrabutylammonium hexafluorophosphate
(Sigma) was used as a supporting electrolyte and dried before use.
Dimethylsulfoxide (DMSO, content of H2O < 0.005%, Aldrich) was
used as received. All reagents and chemicals were used without
any further purification.
2.2. Methods
Electrochemical measurements were done using an electro-
chemical system for cyclic voltammetry and tast polarography. It
consisted of a fast rise-time potentiostat and a lock-in amplifier
(Stanford Research, model SR830). The instruments were inter-
faced to a personal computer via an IEEE-interface card (AdvanTech,

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R. Sokolová et al. / Electrochimica Acta 55 (2010) 8336–8340
Fig. 1. Tast polarogram of 2.5 × 10−4 M ioxynil in 0.1 M TBAPF6 and DMSO at differ-
ent concentrations of potassium tetraoxalate: (a) 0, (b) 4.3 × 10−5 M, (c) 1.4 × 10−4 M,
(d) 2.9 × 10−4 M, (e) 4.3 × 10−4 M and (f) 7.0 × 10−4 M. Drop time was 1.5 s. Dotted
line represents the tast polarogram of the supporting electrolyte.
Fig. 2. The dependence of the limiting current (A) and the half-wave potential (B)
of the polarographic curves of 2.5 × 10−4 M ioxynil in 0.1 M TBAPF6 and DMSO on
the concentration of potassium tetraoxalate.
Under the conditions when no proton donor is present in the
solution we found that the reduction of ioxynil proceeds with the
consumption of one electron per molecule, which was obtained
by the exhaustive electrolysis at the potential of the first polaro-
graphic wave (see Fig. 3). It is likely that the hydroxyl group of the
starting molecule provides the necessary proton for the reduction
process. The height of the first reduction wave of ioxynil is lin-
early dependent on the concentration of the reactant. This excludes
the participation of a bimolecular reaction of the ‘father–son’ type.
We suppose that the reduction process follows Scheme 2. The rate
determining step is the cleavage of halogen anion (see Eq. (11)),
which was identified as one of the reduction products [5]. The peak
width of the first reduction wave measured by cyclic voltammetry
|Ep − Ep/2| = 72 mV gives the value of transfer coefficient 0.66 (cal-
Scheme 2.
Scheme 3.

R. Sokolová et al. / Electrochimica Acta 55 (2010) 8336–8340
8339
Fig. 3. Tast polarogram of 1.5 × 10−3 M ioxynil in 0.1 M TBAPF6 and DMSO. Drop time was 1.5 s.
culated from the peak width: |Ep − Ep/2| = 1.857 RT/˛F [35]). On the
formation of 3,5-diiodo-4-butoxybenzonitrile can be explained by
the reaction of phenolate anion with butyl presented in the sup-
porting electrolyte TBAPF6. The presence of butylated ioxynil and
dehalogenated product was found in the ratio roughly 1:1 (see
Fig. 4). This fact undoubtedly confirms the donation of proton from
the hydroxy group. The autoprotonation step in solution not con-
taining other proton donors was further confirmed by the addition
of KOH. The formation of phenolate from ioxynil in the presence
of KOH is accompanied by the release of H2O. It was necessary to
trap water by the activated molecular sieve, which was added to
the cell in order to drive the conversion of ioxynil to phenolate.
Indeed the addition of KOH and molecular sieve leads to a decrease
and finally a complete disappearance of the reduction wave of
ioxynil. We can conclude that the hydroxy group of ioxynil is a suf-
ficient supplier of protons for the reduction of half of the amount
of ioxynil. This is another confirmation of the autoprotonation in
voltammetric time scale the normalized current Ip/v1/2 decreases
with the increasing scan rate v (data not shown). The value of trans-
fer coefficient ˛ is higher than 0.5 indicating a stepwise cleavage
mechanism with the kinetic control by a chemical reaction (see
Scheme 1). Halogenated aromatic compounds are known to obey
such a stepwise mechanism [13]. The aryl radical formed by the
cleavage of halide anion undergoes immediately the second elec-
tron transfer in the overall ECE type reaction (Eq. (12)). The anion
AXH− is protonated by the starting molecule (Eq. (13)) and the
number of electrons is two per two molecules of ioxynil (Eq. (14)).
The hydrogen atom transfer (HAT) (Eqs. (15) and (16)) can com-
pete with the second electron transfer followed by the protonation
(Eqs. (12) and (13)) and this process has the same stoichiometry (Eq.
(17)). It is not generally easy to distinguish between ECE and HAT
type of mechanism. HAT is favored for lower rate of the halide ion
cleavage [34]. The experimental findings of the consumption of two
electrons per two molecules do not help us to distinguish between
these two pathways. On the contrary to a simple father–son proton
exchange, the mechanism in Scheme 2 indicates first the dehalo-
genation step and then the proton transfer from starting molecule
of ioxynil (AX2H) to a dehalogenated intermediate (AXH−). The
overall reduction mechanism indeed involves two molecules of
ioxynil as is shown for ioxynil in Scheme 3. The reduction wave
of dehalogenated product 3-iodo-4-hydroxybenzonitrile occurs at
−1.22 V (see second reduction wave in tast polarogram in Fig. 3).
The polarographic wave at −2.18 V belongs to the reduction of
hydroxybenzonitrile part of ioxynil [5]. Since the rate of the for-
mation of intermediates is so fast that we can see their reduction
waves even on the time scale of tast polarography (see Fig. 3), the
ECE mechanism followed by an autoprotonation step is the most
probable reaction pathway.
The GC–MS analysis of the products after exhaustive electrolysis
at potential E = −1.0 V also gives support to the autoprotona-
tion mechanism. Two main products were detected: 3-iodo-
4-hydroxybenzonitrile and 3,5-diiodo-4-butoxybenzonitrile. The
Fig. 4. Gas chromatogram of the solution of 3 × 10−3 M ioxynil in 0.1 M TBAPF6 and
DMSO after the exhaustive electrolysis at potential −1.0 V.

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the overall reduction mechanism of ioxynil. The half of the ioxynil
molecules provides the necessary protons for a two electron reduc-
tion of ioxynil as shown in Scheme 3. The two electron exhaustive
electrolysis in the presence of tetraoxalate yields exclusively 3-
iodo-4-hydroxybenzonitrile. No butylated ioxynil was found.
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4. Conclusions
The redox activity of halogenated hydroxybenzonitrile com-
pounds has been investigated in aprotic media. The polarographi-
cally and coulometrically observed one electron consumption per
one molecule of ioxynil is explained by the autoprotonation step
in its reduction mechanism. In effect, the process is a two electron
reduction involving only one half of the starting material. If the
source of the protons is other than the starting molecule, all ioxynil
is being converted to the dehalogenated product. The controlled
potential coulometry in the presence of the strong proton donor
confirms that two electrons are involved in the reduction process.
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Acknowledgements
This work was supported by the Grant Agency of the Czech
Republic (203/09/1607 and 203/08/1157) and Ministry of Educa-
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