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Electrochemical Study of Diphenyl Ether Derivatives Used As Herbicides

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Electrochemical Study of Diphenyl Ether Derivatives Used As Herbicides SAGE-Hindawi Access to Research International Journal of Electrochemistry Volume 2011, Article ID 904570, 6 pages doi:10.4061/2011/904570 Research Article Electrochemical Study of Diphenyl Ether Derivatives Used as Herbicides Amira Zaouak,1 Fatma Matoussi,2 and Mohamed Dachraoui1 1 Laboratoire de Chimie Analytique et d’Electrochimie, D´epartement de Chimie, Facult´e des Sciences de Tunis, Campus Universitaire, El Manar 2, Tunis 2092, Tunisia 2 D´epartement de G´enie Chimique et Biologique, Institut National des Sciences Appliqu´ees et de Technologie, Centre Urbain Nord, BP 676, Tunis cedex 1080, Tunisia Correspondence should be addressed to Fatma Matoussi, [email protected] Received 21 October 2010; Revised 24 February 2011; Accepted 25 February 2011 Academic Editor: Miloslav Pravda Copyright © 2011 Amira Zaouak et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. 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. 1. Introduction NDPE derivatives. This has not been described in the litera- ture. A detailed study is performed in the case of aclonifen The nitro diphenyl ethers (NDPEs) are commonly used using cyclic voltammetry and exhaustive electrolysis. The as herbicides. The environmental contamination with these results allow proposing a mechanistic scheme for its anodic toxic molecules incited a number of studies concerning their oxidation. Furthermore, the anodic behaviour of aclonifen is occurrence and determination in various matrixes such as analysed by means of differential pulse voltammetry (DPV), environmental waters and food by means of chromatography which is shown to be a sensitive method permitting a rapid and related techniques [1–15]. The number of electro- determination of the concerned herbicides. chemical studies concerning the NDPE derivatives are less numerous, even if the electrochemical approach is known to be effective both for analysis and destroying of various toxic 2. Experimental compounds [16–20]. This paper reports the electrochemical behaviour of a 2.1. Electrochemical. The electrochemical measurements are series of p-nitro diphenyl ether derivatives: aclonifen, bifen- made in acetonitrile in the presence of tetrabutylammonium ox, acifluorfen, nitrofen, and 4-nitro diphenyl ether parent tetrafluoroborate (NBu4BF4) 0.1 M as the supporting elec- compound. Scheme 1 presents the chemical structures of trolyte. the selected molecules which are considered as multi-target The voltammetric study is performed with a Voltalab 80 herbicides. analyser from radiometer. The working electrode is a 3 mm The corresponding chemical structures cumulate various diameter glassy carbon disk. functions presuming some electroactivity. Thus, the nitro The exhaustive electrolyses are carried out at a constant aromatic functional group is reducible with a characteristic potential located on the first wave. They are performed with cathodic wave. This has been observed in protic and aprotic a PTJ 35-2 potentiostat and an IG5 integrator both from media for bifenox and nitrofen [21, 22]. The present investi- Tacussel. The separated cell is equipped with a platinum grid gation is focused on the anodic behaviour of the concerned (4 cm2) and a platinum wire, respectively, as the working and 2 International Journal of Electrochemistry O OCH3 NH2 NO Cl NO2 O2N Cl 2 O O O Cl Aclonifen 4-nitro diphenyl ether Bifenox OOH NO F C Cl 2 3 NO2 O O Cl Cl Acifluorfen Nitrofen Scheme 1: Names and chemical structures of the studied nitro diphenyl ether derivatives. the auxiliary electrode. The reference is a saturated calomel electrode (SCE). The electrolysis solutions are evaporated 5 then extracted with chloroform before chromatographic analysis. 0 The differential pulse voltammetry is performed with a A) − POL 150 radiometer analyser associated with MDE 150 stand µ 5 ( equipped with a glassy carbon disk as the working electrode. i −10 2.2. HPLC/MS. The electrolysis products are separated and × analysed by HPLC with a C-18 column (150 mm 4.6 mm −15 and 5 μm particle size) at 40◦C. The mobile phase is a mixture of water/acetonitrile (40/60). A DAD detector is used. The −1.6 −1.4 −1.2 −1 −0.8 −0.6 −0.4 −0.2 0 0.2 mass spectrometric analysis is performed with an Agilent E (V) 1100 MSD triple-quadrupole in the APCI negative mode. Nitrogen is the nebulizer and the collision gas. The analysis Figure 1: Reduction voltammogram of aclonifen. Acetonitrile, ◦ −1 parameters are fixed as follows: T = 400 C; gas flow: NBu4BF4 0.1 M, C = 0.5mM, v = 100 mV s , glassy carbon disk 12 L/min; nebulizer gas pressure: 50 psi; capillary voltage: (d = 3 mm), reference: SCE. 5000 V; corona current: 4 μA. 2.3. Chemical. The studied compounds: aclonifen, bifenox, (ii) FTIR spectrum:wavenumber(cm−1): 2970–2930 (C– acifluorfen, nitrofen, 4-nitro diphenyl ether, and the sup- H); 1444 (N=N); 1405, 1376 (C–NO2); 1200–1250 porting electrolyte tetrabutylammonium tetrafluoroborate (C–N); 1039, 1060 (C–O); 751 (C–Cl). are from Fluka. The solvent acetonitrile (HPLC quality) is purchased from Panreac. (iii) UV-visible spectrum: λmax = 299 nm; 498 nm (>2300 mol−1Lcm−1). Aclonifen Oxidation Product. Characterisation of the main product of the electrochemical oxidation of aclonifen is achieved in the electrolysis solution using HPLC/mass spec- 3. Results and Discussion trometry coupling, ultraviolet-visible absorption, and FTIR spectrophotometry. 3.1. Voltammetric Study (i) Mass spectrum: m/z = 524 (100%), 526 (70%), 528 3.1.1. Cathodic Reduction. The voltammetric study shows (10%); 390 (5%); 249 (3%). The molecular cluster that all compounds except acifluorfen have similar cathodic is composed of peaks at m/z = 524, 526 and behaviours. A single reversible peak is obtained as observed 528. This result perfectly fits the empirical formula in Figure 1, illustrating the reduction voltammogram of C24H14Cl2N4O6 containing two chlorine atoms and aclonifen. This is attributed to the formation of a stable 35 leads to the isotopic distribution: C24H14Cl2N4O6, anion radical at the concerned time scale. The reduction peak 35 37 37 C24H14Cl ClN4O6,andC24H14Cl2N4O6. characteristics are given in Table 1. International Journal of Electrochemistry 3 10 Table 2: Oxidation of nitro diphenyl ether derivatives. Acetonitrile, C = v = −1 0 NBu4BF4 0.1 M, 1mM, 100 mV s , glassy carbon disk (d = 3 mm), reference: SCE. −10 −20 Compounds Epa (V) ne − A) 30 Aclonifen 1.714 2.2 µ ( i −40 Nitrofen 1.924 4.2 −50 Bifenox 2.237 4.3 −60 4 nitro DPE 1.649 4.3 −70 Acifluorfen 2.531 3.3 Epa: anodic peak potential; −1.8 −1.6 −1.4 −1.2 −1 −0.8 −0.6 −0.4 −0.2 0 0.2 ne: number of exchanged electrons E (V) Figure 2: Reduction voltammogram of acifluorfen. Acetonitrile, via a complex mechanism. Several electron transfers and C = v = −1 NBu4BF4 0.1 M, 2mM, 100 mV s , glassy carbon disk associated chemical reactions are involved. In fact, further (d = 3 mm), reference: SCE. information is required for clarifying the anodic process. This concerns especially the oxidation products. Table 1: Reduction of nitro diphenyl ether derivatives. Acetonitrile, In the following, we report the more detailed investiga- C = v = −1 NBu4BF4 0.1 M, 1mM, 100 mV s , glassy carbon disk tion concerning the electrochemical oxidation of aclonifen. d ( = 3 mm), reference: SCE. The study is performed at the voltammetric and exhaustive electrolysis scales. Compounds Epc (V) ΔEp (mV) As observed in Figure 3, the oxidation voltammogram Aclonifen −1.165 0.062 of aclonifen exhibits two anodic peaks. The first one is Bifenox −1.031 0.057 well defined and the corresponding number of electrons − nitrofen 1.446 0.063 exchanged per mole is close to 2. 4NitroDPE −1.606 0.061 Furthermore, the peak potential variation as a function − ∗ Acifluorfen 1.384 of the scan rate gives a linear Ep = f (log v)graph,the Epc: cathodic peak potential; ΔEp: width of the reversible. cathodic peak. slope is approximately 30 mV (Figure 4). We notice that the ∗ irreversible peak. concentration does not affect the Ep value. These results are consistent with an ECE type mechanism, the chemical step being a first-order deactivation of the cation radical ensuing It appears that the electrochemical reversibility is not the first electron transfer. affected by the substitution of the nitroaromatic ring by various groups such as Cl, NH ,COH, and COOCH , 2 2 3 3.2. Exhaustive Electrolysis. The exhaustive electrolyses of whereas in the same time, the reduction potential is notice- aclonifen are carried out at a constant potential (E = ably modified. 1.7 V/SCE) located at the first anodic peak. They are ff Acifluorfen behaves in a di erent manner as shown in stopped after the consumption of 2 F/mole. The HPLC- Figure 2. The cyclic voltammogram involves two cathodic mass spectrometric analysis of the electrolysis solution shows peaks, both irreversible. The first one is a “plateau”-shaped that the major oxidation product is a dimeric compound monoelectronic wave. The second peak corresponds to the deriving from the starting material. The molecular weight exchange of approximately 3 electrons. Thus, the presence is around 524. Furthermore, the mass spectrum given in of an acidic group neighbouring the nitro functional group the experimental section shows a molecular cluster at m/z: induces very probably an intramolecular deactivation of the 524 (100%), 526 (70%), and 528 (13%) compatible with the anion radical.
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