Calculated One- and Two-Electron Reduction Potentials and Related Molecular Descriptors for Reduction of Alkyl and Vinyl Halides in Water

Abstract

One-and two-electron reduction potentials (E 1 and E 2 values) were calculated from published thermodynamic data for 39 halogenated C 1 and C 2 compounds, including many commonly encountered groundwater contaminants. Because reductive dehalogenation is an important pathway for their destruction under anaerobic conditions, information concerning the relevant reduction potentials may be useful for assessing the thermodynamic feasibility of a particular reaction, as well as in developing linear free energy relationships (LFERs) or other quantitative structure-activity relationships (QSARs) that may enable prediction of rates of transformation. E 1 values were calculated assuming a stoichiometry corresponding to dissociative electron transfer, which produces a carbon-centered radical and a halide ion. E 2 values were calculated for both hydrogenolysis and reductive β-elimination reactions. Uncertainties in the thermodynamic data for the organohalides under consideration may introduce substantial uncertainty in the resulting E 1 values. Hence, relationships between calculated E 1 values and various surrogate parameters were also investigated. E 1 values were correlated with lowest unoccupied molecular orbital (LUMO) energies and carbon-halogen homolytic bond dissociation energies (BDE values), which were computed via density functional theory. Correlations were also attempted between E 1 values and vertical attachment energies (VAE values), the latter representing experimental measures of the ease of reduction of a molecule to a radical anion in the gas phase. These alternate descriptors may provide a means for estimating E 1 . Additional studies will need to be undertaken to establish which descriptor best correlates with reactivity in environmental reductive dehalogenation.

Nanoscale materials for organohalide degradation via reduction pathways

“…In 2001, Roberts et al [149] demonstrated through theoretical calculations that the degradation of chlorinated ethylenes would undergo favorable thermodynamic pathways if two-electron transfer reductants were used relative to oneelectron transfer reductants. The calculations showed that two-electron reduction potentials for degradation of chlorinated ethylenes were more positive compared with oneelectron reduction potentials.…”

Abiotic Dehalogenation by Metals

“…It should be noted that different compounds may react via different mechanisms and with different reductants in the highly complex granular iron system. It has been clearly demonstrated that CT, which possesses very favorable one-and two-electron reduction potentials (0.085 and 0.673 V, respectively) (130), is reduced by Fe(II) sorbed to mineral surfaces, by magnetite, by conduction-band electrons, and by Fe(0) itself. It is reasonable to assume that all of these processes are occurring (at widely varying rates) in the granular iron system.…”

“…However, aliphatic halides containing two or fewer chlorine atoms are often unreactive with granular iron, although they may react readily with zinc or bimetallic reductants. It is somewhat surprising that DCM is largely unreactive with granular iron, while the chlorinated ethenes react rapidly with iron, given that the oneelectron reduction potential for dichloromethane is -0.428 V (standard hydrogen electrode), while the most favorable among the chloroethenes is -0.598 V for tetrachloroethene (130).…”

“…All the above reactions can lead to complicated pathways (e.g., Figure 9.1). The thermodynamics of reductive elimination tend to be more favorable than those of hydrogenolysis, due largely to the production of two heavily solvated chloride (or bromide) ions (130). Thus, it is not surprising that many vicinal dihalides react primarily via (see Table 9.2.).…”

“…The ability of granular iron to dechlorinate various substrates cannot be predicted a priori from the reduction potential of the substrate. The Fe(II)/Fe(0) couple has a redox potential of -0.447 V, suggesting that it is capable of reducing even monohalogenated methanes and ethanes, which have positive two-electron reduction potentials (130). However, aliphatic halides containing two or fewer chlorine atoms are often unreactive with granular iron, although they may react readily with zinc or bimetallic reductants.…”