Reduction of α,β- and α,ω-Dibromoalkanes at Silver Cathodes

Electrochemical reduction of halogenated organic pollutants via electrolysis at silver cathodes is a remediation strategy that has been undertaken by many laboratories, including our own, due to the catalytic capability of silver to cleave carbon–halogen bonds reductively.  However, due to the complexity of these polyhalogenated compounds, their reduction mechanisms are not well understood and have, in turn, inspired us to map the reductions of various alkyl halides at silver.

Past investigations of the electrochemical behavior of monohalides at silver cathodes have demonstrated that (a) identity of the halide, (b) position of the halide, and (c) dryness of the solvent–electrolyte [dimethylformamide (DMF) containing 0.050 M tetramethylammonium perchlorate (TMAP)] have an effect on the outcome of the reduction.¹  Rondinini et al.² reported that a second group with specific affinity for the silver surface increases the effects observed in cyclic voltammetry due to the structure of various alkyl halides.  On the basis of this dependence, Rondinini and co-workers suggest that, in the case of an α,ω-dihaloalkane, both halogens are coadsorbed at silver; thus, intramolecular coupling should be favored.  Our present work begins to illuminate how the relative positions of the halogens affect the reduction mechanisms of α,β- and α,ω-dibromoalkanes.

Cyclic Voltammetry.  Comparison of cyclic voltammograms for the reduction of 1,2-dibromohexane (1) and 1,6-dibromohexane (2) in DMF–0.050 M TMAP at a scan rate of 0.1 V s^–1 reveals a single cathodic peak for each compound  (Figure 1).  Interestingly, the cathodic peak observed for reduction of 1 is shifted 0.68 V less negative and the peak current is approximately one-half of that observed for reduction of 2.  Second, the cathodic peak for 2 is observed close to the cathodic peak potentials observed for 1- and 2-bromohexane.   For both 1 and 2, a plot of peak current versus concentration for solutions between 0.5 and 20 mM at a scan rate of 0.1 V s^–1 show behavior anticipated for a typical irreversible reaction; however, closer inspection reveals that at lower concentrations the current function (i_p/(C₀^*ν^1/2)) increases, which can be attributed to an escalation of the relative contribution from reduction of adsorbed reactant.  Cathodic peak currents obtained for reduction of 0.5, 2.0, and 8.0 mM solutions of 1 and 2 are linear with respect to the square root of the scan rate for moderate scan rates (0.05 to 1 V s^–1); at higher scan rates (>10 V s^–1), the peak currents no longer increase. 

Controlled-Potential (Bulk) Electrolyses.  Bulk reduction of 1 and 2 at silver electrodes in DMF–0.050 M TMAP reveals that product distribution and reaction mechanism are dependent on the relative positions of the two bromine atoms.  Reductions of 1 exhibit a second spike in the current–time (i–t) curve; on the other hand, reduction of 2 results in a single exponential decay in the i–t curve.  Furthermore, the sole product of reduction of 1 is 1-hexene, whereas reduction of 2 affords hexane as the major product.  Coulometric n values for the reduction of 1 indicate a two electron pathway; however, higher n values arise from electrolysis of 2.  Electrolyses performed in the presence of a 100-fold excess of deuterium oxide give no evidence for the presence of carbanions during the reduction of 1; in contrast, monodeuterated hexane was detected after reduction of 2 in the presence of deuterium oxide.  No monobrominated species were detected when 1 was electrolyzed exhaustively in the absence or presence of an excess of a proton donor (hexafluoroisopropanol or water).  Moreover, no monobrominated product was detected when electrolysis of 1 was interrupted.

References

 

1. Strawsine, L. M.; Mubarak, M. S.; Peters, D. G. J. Electrochem. Soc. 2013, 160, G3030–G3037.

2. Rondinini, S.; Mussini, P. R.; Muttini, P.; Sello, G. Electrochim. Acta  2001, 46, 3245–3258.