Electrochemical Study of Diphenyl Ether Derivatives Used as Herbicides

The electrochemical behaviour of five nitro diphenyl ethers used as herbicides is investigated in acetonitrile. A detailed study by cyclic voltammetry and exhaustive electrolysis is carried out for the anodic oxidation of 2-Chloro-6-nitro-3-phenoxyaniline (aclonifen) and shows that the major oxidation product is a dimeric compound. A mechanistic scheme involving a coupling process is postulated for the electrochemical oxidation of this compound. Furthermore, the use of differential pulse voltammetry on a glassy carbon electrode permits the selective determination of aclonifen. The limit of detection is 0.6 μg/mL.

This paper reports the electrochemical behaviour of a series of p-nitro diphenyl ether derivatives: aclonifen, bifenox, acifluorfen, nitrofen, and 4-nitro diphenyl ether parent compound.Scheme 1 presents the chemical structures of the selected molecules which are considered as multi-target herbicides.
The corresponding chemical structures cumulate various functions presuming some electroactivity.Thus, the nitro aromatic functional group is reducible with a characteristic cathodic wave.This has been observed in protic and aprotic media for bifenox and nitrofen [21,22].The present investigation is focused on the anodic behaviour of the concerned NDPE derivatives.This has not been described in the literature.A detailed study is performed in the case of aclonifen using cyclic voltammetry and exhaustive electrolysis.The results allow proposing a mechanistic scheme for its anodic oxidation.Furthermore, the anodic behaviour of aclonifen is analysed by means of differential pulse voltammetry (DPV), which is shown to be a sensitive method permitting a rapid determination of the concerned herbicides.The voltammetric study is performed with a Voltalab 80 analyser from radiometer.The working electrode is a 3 mm diameter glassy carbon disk.

Experimental
The exhaustive electrolyses are carried out at a constant potential located on the first wave.They are performed with a PTJ 35-2 potentiostat and an IG5 integrator both from Tacussel.The separated cell is equipped with a platinum grid (4 cm 2 ) and a platinum wire, respectively, as the working and  (iii) UV-visible spectrum: λ max = 299 nm; 498 nm (>2300 mol −1 L cm −1 ).

Cathodic Reduction.
The voltammetric study shows that all compounds except acifluorfen have similar cathodic behaviours.A single reversible peak is obtained as observed in Figure 1, illustrating the reduction voltammogram of aclonifen.This is attributed to the formation of a stable anion radical at the concerned time scale.The reduction peak characteristics are given in Table 1.It appears that the electrochemical reversibility is not affected by the substitution of the nitroaromatic ring by various groups such as Cl, NH 2 , CO 2 H, and COOCH 3 , whereas in the same time, the reduction potential is noticeably modified.
Acifluorfen behaves in a different manner as shown in Figure 2. The cyclic voltammogram involves two cathodic peaks, both irreversible.The first one is a "plateau"-shaped monoelectronic wave.The second peak corresponds to the exchange of approximately 3 electrons.Thus, the presence of an acidic group neighbouring the nitro functional group induces very probably an intramolecular deactivation of the anion radical.

Anodic Oxidation.
The oxidation voltammograms of the studied compounds have a rather similar pattern.The obtained anodic peaks do not show any reversibility on the reverse scan.The corresponding data are given in Table 2.They show that the studied diphenyl ether derivatives are oxidized at potentials superior to 1650 mV/SCE.In addition, for all compounds, excepted aclonifen, the current peak is high and implies the exchange of an important number of electrons.The corresponding n e values are obtained by calculating the current ratio i Pa /i Pc , i Pa , and i Pc are, respectively, the oxidation and reversible reduction peak currents.The obtained high values suggest that the anodic oxidation of diphenyl ether derivatives proceeds This concerns especially the oxidation products.
In the following, we report the more detailed investigation concerning the electrochemical oxidation of aclonifen.The study is performed at the voltammetric and exhaustive electrolysis scales.
As observed in Figure 3, the oxidation voltammogram of aclonifen exhibits two anodic peaks.The first one is well defined and the corresponding number of electrons exchanged per mole is close to 2.
Furthermore, the peak potential variation as a function of the scan rate gives a linear E p = f (log v) graph, the slope is approximately 30 mV (Figure 4).We notice that the concentration does not affect the E p value.These results are consistent with an ECE type mechanism, the chemical step being a first-order deactivation of the cation radical ensuing the first electron transfer.

Exhaustive Electrolysis.
The exhaustive electrolyses of aclonifen are carried out at a constant potential (E = 1.7 V/SCE) located at the first anodic peak.They are stopped after the consumption of 2 F/mole.The HPLCmass spectrometric analysis of the electrolysis solution shows that the major oxidation product is a dimeric compound deriving from the starting material.The molecular weight is around 524. Furthermore, the mass spectrum given in the experimental section shows a molecular cluster at m/z: 524 (100%), 526 (70%), and 528 (13%) compatible with the presence of two chlorine atoms.The peak at m/z = 249 is probably due to the elimination of a nitrogen molecule from the molecular ion of the azo compound which derives from aclonifen (Scheme 2).
The proposed chemical structure is supported by the spectrometric analysis in UV-visible range.Actually, a significant absorption is observed at 498 nm, which may be attributed favourably to a nitrogen double bond of a substituted azobenzene.Moreover, when comparing the FTIR spectrum of the electrolysis solution to that of aclonifen, there is a new band located at 1444 cm −1 which can be attributed to the N=N.These results suggest that dimerization is the predominant pathway in the anodic oxidation of aclonifen.It is worth noticing that this type of oxidative coupling has been described in the literature for  aromatic amine such as aniline and its derivatives, which are oxidized in the well-known polyaniline conducting polymer.
For aclonifen, the ultimate oxidation product being an azo dimeric compound, the amino group is likely the preferred site for electron uptake and coupling.This explains why

Analysis of Aclonifen Using Differential Pulse Voltammetry (DPV)
The results of the above electrochemical study of several herbicides can be used for their electrochemical determination.The present section concerns the determination of aclonifen by differential pulse voltammetry.The DPV analysis is preferred to cyclic voltammetry for quantification, since the developed peaks are better defined.The DPV anodic voltammogram on a glassy carbon electrode of aclonifen in the presence of bifenox is given in Figure 5. Thus, a selective determination of aclonifen is possible in natural matrix containing both compounds.Figure 6 illustrates the DPV voltammograms obtained with aclonifen concentrations comprised in the range: 1-7 μg/mL.
The calibration plot of the peak current versus the concentration is linear (R 2 = 0.998).The limit of detection (LOD) and the limit of quantification (LOQ) calculated according to the XTP 90-210 standard are, respectively, 0.6 and 0.8 μg/mL.

Conclusion
The electrochemical behaviour of five nitro diphenyl ether derivatives used as herbicides is investigated in acetonitrile on a glassy carbon electrode.On one hand, the cathodic reduction leads to the corresponding anion radical which is stable at the voltammetric scale, except acifluorfen which gives an irreversible cathodic peak.This is due probably to an intramolecular protonation of the nitroaromatic anion radical by the neighbouring acidic group.On the other hand, the studied herbicides give irreversible oxidation peaks.The number of exchanged electrons is superior to 2 except for aclonifen.A detailed study carried out for this compound shows that the predominant oxidation process involves a coupling step.Indeed the major oxidation product is an azo dimeric compound.A mechanistic scheme accounting for this result is proposed.
Furthermore, this electrochemical study shows that differential pulse voltammetry on a glassy carbon electrode is effective in the selective determination of aclonifen in the presence of bifenox.The corresponding limits of detection and quantification are obtained.

2. 1 .
Electrochemical.The electrochemical measurements are made in acetonitrile in the presence of tetrabutylammonium tetrafluoroborate (NBu 4 BF 4 ) 0.1 M as the supporting electrolyte.

Scheme 2 :
Scheme 2: chemical structure of the major oxidation product of aclonifen.