Direct Reduction of 1-Bromo-6-Chlorohexane and 1-Chloro-6-Iodohexane at Silver Electrodes in Dimethylformamide
Authors: Rose John Andrew 
Published: February 2020

In recent years, electrochemical reduction of halogenated environmental pollutants has been investigated; this research includes well known pollutants (such as chlorofluorocarbons (CFCs) and 4,4′-(2,2,2-trichloroethane-1,1-diyl)bis(chlorobenzene) (DDT)), persistent organic pollutants (such as decabromodiphenyl ether), and disinfection by-products (such as haloacetic acids).1  In addition, the use of silver cathodes has proven to be advantageous due to their catalytic behavior in the reduction of carbon–halogen bonds.  Studies have shown that the identity of the halogen and the structure of the halogenated compound influence the resulting products observed in controlled-potential electrolysis (CPE).2  Recently, the electrochemical behavior of 1,2-dibromohexane and 1,6-dibromohexane has been investigated,3 but polyhalogenated compounds in which the identities of the halogens differ has not yet been investigated with silver cathodes.

     In an attempt to gain a better understanding of the reduction mechanism of polyhalogenated organic compounds, we have explored in this study the electrochemical reductions of 1-bromo-6-chlorohexane and 1-bromo-6-chlorohexane.  Cyclic voltammograms for reduction of 1-bromo-6-chlorohexane in dimethylformamide (DMF) containing 0.050 M tetra-n-butylammonium tetrafluoroborate (TBABF4) show a single cathodic peak at –1.45 V vs. a cadmium amalgam reference electrode.4  In comparison, voltammograms for the reduction of 1-chloro-6-iodohexane in DMF–0.050 M TBABF4 display two cathodic peaks at –0.83 and –1.53 V vs. a cadmium amalgam reference electrode.  Controlled-potential (bulk) electrolyses were conducted to reduce 1-bromo-6-chlorohexane in DMF–0.050 M TBABF4 at a silver gauze cathode; calculated n values indicate that the mechanism includes a two-electron cleavage of the carbon–bromine bond to afford 1-chlorohexane as the major product, along with cyclohexane, n-hexane, 1-hexene, and 1,6-hexadiene as minor species.  In contrast, bulk electrolyses of 1-chloro-6-iodohexane indicate that reduction at a potential (–0.95 V) corresponding to the first voltammetric peak is a one-electron process that leads to significant production of a dimer (1,12-dichlorododecane) together with smaller amounts of 1-chlorohexane.  On the other hand, electrolysis at a potential (–1.60 V) corresponding to the second voltammetric peak yields completely dehalogenated products in addition to 1,12-dichlorododecane and 1-chlorohexane, via a combination of one- and two-electron processes.

References:

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This reference electrode consists of a saturated cadmium–mercury amalgam in contact with DMF saturated with both sodium chloride and cadmium chloride; its potential is –0.76 V vs. SCE at 25°C. 

Figure 1