Electrochemical Reduction of 5-Chloro-2-(2,4-Dichlorophenoxy)Phenol (Triclosan) in Dimethylformamide
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Journal of Electroanalytical Chemistry 638 (2010) 100–108 Contents lists available at ScienceDirect Journal of Electroanalytical Chemistry journal homepage: www.elsevier.com/locate/jelechem Electrochemical reduction of 5-chloro-2-(2,4-dichlorophenoxy)phenol (triclosan) in dimethylformamide Kyle N. Knust a, Matthew P. Foley a, Mohammad S. Mubarak b, Sonja Skljarevski a, Krishnan Raghavachari a, Dennis G. Peters a,* a Department of Chemistry, Indiana University, Bloomington, IN 47405, USA b Department of Chemistry, The University of Jordan, Amman 11942, Jordan article info abstract Article history: Cyclic voltammetry and controlled-potential (bulk) electrolysis have been used to investigate the direct Received 30 July 2009 reduction of 5-chloro-2-(2,4-dichlorophenoxy)phenol (triclosan, 1) in dimethylformamide (DMF) con- Received in revised form 5 October 2009 taining tetra-n-butylammonium tetrafluoroborate (TBABF4). Cyclic voltammograms for reduction of 1 Accepted 7 October 2009 at glassy carbon electrodes exhibit three irreversible peaks attributed to successive reductive cleavage Available online 25 October 2009 of the three aryl carbon–chlorine bonds. Bulk electrolyses of 1 at reticulated vitreous carbon cathodes held at a potential between the first and second cathodic peaks afford only 5-chloro-2-(4-chlorophen- Keywords: oxy)phenol; however, at a more negative potential, a mixture of 5-chloro-2-(4-chlorophenoxy)phenol, Triclosan 5-chloro-2-phenoxyphenol, and 2-phenoxyphenol is obtained. A scheme consisting of electron-transfer 5-Chloro-2-(2,4-dichlorophenoxy)phenol Catalytic reduction steps and accompanying chemical reactions is proposed that follows the classic mechanism for the reduc- Nickel(I) complexes tion of aryl halides. To provide support for this mechanism, theoretical calculations based on density functional theory have been performed to model the electronic structures of 1 and the likely intermedi- ates formed via its electroreduction. Catalytic reduction of 1 by nickel(I) salen and nickel(I) diethylsalen, each mediator electrogenerated at a glassy carbon electrode in DMF – 0.10 M TBABF4, has been explored with the aid of cyclic voltammetry and controlled-potential electrolysis. Ó 2009 Elsevier B.V. All rights reserved. 1. Introduction There have been reports that exposure to triclosan causes skin irri- tation, photoallergic contact dermatitis, and immunotoxic and First used in 1972 as a component of surgical scrub for aseptic neurotoxic reactions in humans. Substantial levels of triclosan have hospital procedures, triclosan or 5-chloro-2-(2,4-dichlorophen- been found in human milk and fatty tissues. Furthermore, the Envi- oxy)phenol (1) is a pervasive, broad-spectrum antibacterial disin- ronmental Protection Agency of the United States has indicated fectant found in a number of common household products, that carcinogenic dioxins might arise as an impurity in the manu- including cosmetics, deodorants, mouthwashes, soaps, and tooth- facture of triclosan, and it has been found that dioxins can be pastes, and it is infused in consumer items such as bedding, kitchen formed when triclosan-contaminated natural waters are exposed utensils, socks, toys, and trash bags. to sunlight. Accordingly, there is interest in the degradative dechlorination of triclosan. Cl OH Over the past decade, there has been a number of publications [1–11] pertaining to the electrochemical behavior of triclosan. O Pemberton and Hart [1] used a screen-printed carbon electrode for the analytical determination of triclosan in toothpastes and mouthwashes; in an aqueous medium at pH 10, electrooxidation of triclosan gives a single wave that involves a one-electron trans- fer. Safavi et al. [2,3] investigated the electrochemical behavior of Cl Cl triclosan at mercury cathodes. Using cyclic voltammetry and con- trolled-potential electrolysis, Wang and co-workers [4] investi- 1 gated the direct reduction of triclosan at carbon cathodes in aqueous buffer solutions and in methanol or dimethylformamide * Corresponding author. Tel.: +1 812 855 9671; fax: +1 812 855 8300. containing tetra-n-butylammonium hydroxide; two products, E-mail address: [email protected] (D.G. Peters). chlorobenzene and 2-phenoxyphenol, were identified. Two papers 0022-0728/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jelechem.2009.10.012 K.N. Knust et al. / Journal of Electroanalytical Chemistry 638 (2010) 100–108 101 [5,6] have dealt with the oxidation of triclosan at boron-doped dia- used as a standard for the quantitation of 2-phenoxyphenol, was mond surfaces. A surfactant-coated electrode has been evaluated acquired from Acros Organics. Each of the following chemicals, by Raghupathy and co-workers [7] for the oxidative detection of used without additional purification, was obtained from the Al- triclosan in aqueous media. Sirés et al. [8] examined the electro- drich Chemical Company: n-hexadecane (99%), 4-chloro-2- Fenton oxidation of triclosan in an aqueous medium of pH 3 and methoxyphenol (technical grade), copper(II) carbonate (basic, re- in an acetonitrile–water mixture; in the latter solvent, mixtures agent grade), 1-bromo-4-chlorobenzene (99%), bromobenzene of 2,4-dichlorophenol, 4-chlorocatechol, chlorohydroquinone, (99%), aluminum chloride (anhydrous powder, 99.99%), ethylene- chloro-p-benzoquinone, and carboxylic acids (such as maleic, oxa- diamine (99+%), 1-(2-hydroxyphenyl)propan-1-one (97%), lic, formic, and acetic acid) were obtained. Strategies have been re- nickel(II) acetate tetrahydrate (98%), and [[2,20-[1,2-ethanediylbis- ported for the oxidative measurement of triclosan in an aqueous (nitrilomethylidyne)]bis[phenolato]]-N,N0,O,O0]nickel(II) (hereafter medium at an indium-doped tin oxide electrode coated with a film referred to as nickel(II) salen, 98%). Dimethylformamide (DMF, composed of carbon nanoparticles and poly(diallyldimethylammo- ACS grade, 99.8%) from Fisher Scientific was utilized without fur- nium chloride) (PDDAC) [9] and at a glassy carbon electrode with a ther purification as the solvent for all electrochemical experiments. cellulose film embedded with PDDAC [10]. Cyclic voltammograms Tetra-n-butylammonium tetrafluoroborate (TBABF4, >98%), from for the oxidation of triclosan adsorbed onto carbon nanoparticles GFS Chemicals, Inc., was employed as supporting electrolyte and immobilized on a glassy carbon surface exhibit a single peak attrib- was stored in a vacuum oven at 70–80 °C to remove traces of water uted to a one-electron process to form a phenoxy radical that present. All deaeration procedures were accomplished with zero- undergoes polymerization [11]. grade argon (Air Products). During the same period of time as the preceding references, there have been papers [12–17] dealing with the electrochemical 2.2. Cells, electrodes, instrumentation, and procedures degradation of chlorinated phenols. As part of an analytical detection protocol, Saby and co-workers [12] have investigated Information concerning the cell for cyclic voltammetry is avail- the electrocatalytic oxidation of mono-, di-, tri-, tetra-, and penta- able in an earlier paper [23]. We constructed a circular, planar 2 chlorophenols, and Rodgers et al. [13] have employed PbO2, IrO2, working electrode with an area of 0.071 cm for cyclic voltamme- and SnO2 anodes in an aqueous medium to dechlorinate a similar try by press-fitting a short length of glassy carbon rod (Grade GC- set of compounds. Cyclic voltammetry has been used by Ezˇerskis 20, 3.0-mm-diameter, Tokai Electrode Manufacturing Company, and Jusys [14] to probe the oxidation of chlorinated phenols at Tokyo, Japan) into a Kel-F shroud. Before each cyclic voltammo- platinum and gold electrodes in aqueous sodium hydroxide solu- gram was recorded, the working electrode was cleaned with an tions, and Cheng and co-workers [15,16] utilized a variety of cath- aqueous suspension of 0.05-lm alumina on a Master-Tex (Buehler) ode materials to study the hydrodehalogenation of chlorinated polishing pad, after which the electrode was rinsed ultrasonically phenols. Finally, in a very recent report, Wang and Wang [17] have in DMF and wiped dry before being inserted into the electrochem- examined the electrochemical degradation of 2,4-dichlorophenol ical cell. To acquire cyclic voltammograms, we employed a Faraday in a diaphragm cell consisting of a palladium-modified gas-diffu- MP scanning potentiostat (model F02A, Obbligato Objectives Inc.) sion cathode, a titanium–iridium oxide–ruthenium oxide anode, together with a companion program installed on a personal and an aqueous sodium sulfate electrolyte solution; although the computer. primary products are 4-chlorophenol, 2-chlorophenol, and phenol, Details pertaining to the cell, instrumentation, and procedure these species are subsequently converted into a mixture of other for controlled-potential electrolysis are presented in a previous compounds (hydroquinone, benzoquinone, and aliphatic carbox- publication [24]. Working electrodes for controlled-potential elec- ylic acids). trolysis were reticulated vitreous carbon disks (approximately As part of an ongoing program in our laboratory, we have 4 mm in thickness, 2.4 cm in diameter, and 200 cm2 in surface already studied the direct and cobalt(I) salen-catalyzed electro- area) that were cut from commercially available material (RVC chemical dechlorination of several environmentally deleterious 2X1-100S, Energy Research and Generation, Inc., Oakland, CA). Pro- substances—1,1,2-trichloro-1,2,2-trifluoroethane (CFC-113)