Elsevier

Electrochimica Acta

Volume 218, 10 November 2016, Pages 311-317
Electrochimica Acta

Direct Reduction of 1-Bromo-6-chlorohexane and 1-Chloro-6-iodohexane at Silver Cathodes in Dimethylformamide

https://doi.org/10.1016/j.electacta.2016.09.066Get rights and content

Abstract

Cyclic voltammetry and controlled-potential (bulk) electrolyses have been employed to probe the electrochemical reductions of 1-bromo-6-chlorohexane and 1‐chloro-6-iodohexane at silver cathodes in dimethylformamide (DMF) containing 0.050 M tetra-n-butylammonium tetrafluoroborate (TBABF4). A cyclic voltammogram for reduction of 1-bromo-6-chlorohexane shows a single major irreversible cathodic peak, whereas reduction of 1-chloro-6-iodohexane gives rise to a pair of irreversible cathodic peaks. Controlled-potential (bulk) electrolyses of 1-bromo-6-chlorohexane at a silver gauze cathode reveal that the process involves a two-electron cleavage of the carbon–bromine bond to afford 1-chlorohexane as the major product, along with 6-chloro-1-hexene, n‐hexane, 1‐hexene, and 1,5-hexadiene as minor species. In contrast, bulk electrolyses of 1-chloro-6-iodohexane indicate that the first voltammetric peak corresponds to a one-electron process, leading to production of a dimer (1,12-dichlorododecane) together with 1-chlorohexane and 6-chloro-1-hexene as well as 1‐hexene and 1,5-hexadiene in trace amounts. At potentials corresponding to the second cathodic peak, reduction of 1-chloro-6-iodohexane is a mixture of one- and two-electron steps that yields the same set of products, but in different proportions. Mechanistic schemes are proposed to explain the electrochemical behavior of both 1‐bromo-6-chlorohexane and 1-chloro-6-iodohexane.

Introduction

In a recent publication [1] from our laboratory, the electrochemical reduction of a pair of dibromoalkanes, namely 1,2- and 1,6-dibromohexane, at silver cathodes in dimethylformamide was examined. Cyclic voltammograms for each compound exhibit a single irreversible peak, but the cathodic peak potentials are significantly different. Moreover, for identical concentrations of each compound, the cathodic peak current for 1,2-dibromohexane is one half of that for 1,6-dibromohexane, an observation directly related to the number of electrons associated with reduction of each species and thus with the mechanism for reduction of each substrate. Whereas only one product (1-hexene) is formed via bulk electrolysis of 1,2-dibromohexane, a mixture of five different species (1‐hexene, n-hexane, 1,5-hexadiene, 5-hexen-1-ol, and n‐dodecane) can arise from the controlled-potential reduction of 1,6-dibromohexane. In hindsight, it is surprising that the electrochemical behavior of no other dihalohexane at a silver cathode has been investigated. However, in relatively recent work, Simonet [2] has probed the electrochemical reduction of 1,3-dibromopropane at silver cathodes in dimethylformamide containing tetra-n-butylammonium salts; this compound was found to undergo two-electron reduction to afford a diradical that undergoes polymerization, as well as cyclization to yield cyclopropane. Aside from these two publications, we are not aware of any other work that deals with reduction of α,ω‐dihaloalkanes at a silver cathode.

In much earlier work, our laboratory carried out a series of studies [3], [4], [5], [6], [7] that involved the direct reduction of a family of α,ω-dihaloalkanes (ranging from compounds with n‐propyl to n-decyl moieties and with identical or dissimilar halogens) at carbon cathodes in dimethylformamide. Whereas bulk electrolyses of various α,ω-dihalopropanes led almost exclusively to the formation of cyclopropane [3], the α,ω‐dihalobutanes and α,ω-dihalopentanes afforded the corresponding cycloalkanes in modest yield [4], [5]. On the other hand, preparative-scale electrolyses of α,ω-dihalogenated hexanes, octanes, and decanes produced no carbocyclic products, but only mixtures of straight-chain alkanes and alkenes, along with some dimeric species arising from radical-coupling reactions [6], [7].

In the last two decades, there has been considerable interest in the use of silver cathodes to investigate electron-transfer events centered on the electrochemical reduction of carbon–halogen bonds. Investigations have focused on (a) mechanistic features of these processes, (b) applications in the field of electrosynthesis, and (c) uses of electrochemical reductions at silver electrodes for the dehalogenation of environmental pollutants. These studies have revealed the catalytic ability of silver as a cathode for the reductive cleavage of carbon–halogen bonds at much less negative potentials than for other electrodes such as glassy carbon or mercury [8]. Without being encyclopedic, but hoping to provide some guidance to interested readers, we summarize briefly in the next paragraph some recent literature that identifies both important contributors to and applications in this expanding field of research.

Utilizing surface-enhanced Raman spectroscopy [9], density functional theory [10], and digital simulation [11], Amatore and co-workers investigated the mechanism for reduction of benzyl chloride at a silver cathode in acetonitrile, particularly with respect to the existence and detection of adsorbed benzyl chloride as well as benzyl radicals and benzyl anions. Rondinini et al. [12] studied the direct reduction of haloadamantanes at silver to assess how the position of the halogen moiety affects the yield of the dimer formed via controlled-potential (bulk) electrolysis. More recently, the same laboratory has prepared silver nanoparticles and employed them as composite-supported catalysts for the reduction of chloroform to methane in an aqueous medium [13]. In other work by the group of Simonet [14], [15], the behavior of alkyl iodides at silver electrodes was probed, and it was observed that homo-dimerization is the dominant process. A number of publications related to the reduction of halogenated organic compounds at silver cathodes by Isse, Gennaro, and their co-workers include investigations of the reduction of benzyl halides [16], the mechanism of dissociative electron transfer to organic chlorides [17], [18], [19], and the carboxylation of activated carbon–halogen bonds [20], [21], [22]. In our laboratory, a recent study of the reduction of an assortment of primary, secondary, and tertiary alkyl monohalides at silver cathodes was conducted [23]. In addition, electrochemical reduction (remediation) of some well-known environmentally hazardous and halogenated pollutants at silver cathodes has been examined; these compounds include freons such as CFC-113 [24], [25], [26], [27], pesticides such as lindane [28] and DDT [29], and flame retardants such as decabromodiphenyl ether [30] and hexabromocyclododecane [31].

In the present study, we have sought to extend our knowledge of the electrochemical reduction of α,ω-dihaloalkanes by utilizing cyclic voltammetry and controlled-potential (bulk) electrolysis to investigate the direct electrochemical reductions of 1-bromo-6-chlorohexane (1) and 1-chloro-6-iodohexane (2) at silver cathodes in dimethylformamide (DMF) containing tetra-n-butylammonium tetrafluoroborate (TBABF4) as the supporting electrolyte. Products resulting from bulk electrolyses of 1 and 2 have been separated, identified, and quantitated with the aid of gas chromatography (GC) and gas chromatography–mass spectrometry (GC–MS). Utilizing the information acquired, we propose mechanistic schemes for the reduction of 1 and 2.

Section snippets

Reagents

Each of the following chemicals was purchased and used as received unless otherwise indicated, their purities and commercial sources being given in parentheses: 1‐bromo-6-chlorohexane (1) (95%, Sigma Aldrich), 1-chloro-6-iodohexane (2) (96%, Sigma Aldrich), 1,12-dichlorododecane (96 +%, Acros Organics), n-hexadecane (99%, Sigma Aldrich), n-heptane (99%, Mallinckrodt), n-hexane (96%, EMD), 1-hexene (98%, Alfa Aesar), 1,5-hexadiene (98%, Sigma Aldrich), deuterium oxide (D2O, 99.9 atom % D, Sigma

Cyclic voltammetric behavior of 1-bromo-6-chlorohexane (1) and 1-chloro-6-iodohexane (2)

Fig. 1 depicts a cyclic voltammogram recorded at 100 mV s−1 for the reduction of a 5.0 mM solution of 1-bromo-6-chlorohexane (1) at a silver electrode in DMF containing 0.050 M TBABF4. One major irreversible cathodic peak is observed at −1.45 V, and there is a small (possibly reversible) peak on the rising portion of the large peak. Shown in Fig. 2 is a cyclic voltammogram recorded at 100 mV s−1 for the reduction of a 5.0 mM solution of 1-chloro-6-iodohexane (2) at a silver electrode in DMF containing

Conclusions and future research

Cyclic voltammetry and controlled-potential (bulk) electrolysis have been employed to investigate the direct electrochemical reductions of 1-bromo-6-chlorohexane (1) and 1-chloro-6-iodohexane (2) at silver cathodes in dimethylformamide (DMF) containing 0.050 M tetra-n-butylammonium tetrafluoroborate (TBABF4). A cyclic voltammogram for the reduction of 1 displays one major irreversible cathodic peak, whereas two irreversible cathodic peaks are seen in a cyclic voltammogram for reduction of 2.

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