Using gas-phase molecular descriptors to predict dechlorination rates of chloroalkanes by zerovalent iron

Reductive dehalogenation of chlorinated compounds is the most important process occurring within the zerovalent iron (Fe⁰) barrier. The relative reaction rates of individual halocarbons with Fe⁰ can vary considerably. This variability has been the stimulus for using various chemical descriptors for a priori predictions of transformation rates via linear free-energy relationships (LFERs). Our objective was to determine the efficacy of four molecular descriptors to describe the transformation rates of three chloromethanes, three chloroethanes, and six chloropropanes by Fe⁰. This was accomplished by generating an internally consistent set of rate constants under controlled environmental conditions (16 °C, anaerobic) and regressing the surface-area normalized rate constants (k_SA) against (i) energy of the lowest unoccupied molecular orbital (E_LUMO); (ii) vertical attachment energies (VAE); (iii) thermal electron attachment rate constants; and (iv) the molar response from a commercial electron capture detector (ECD). Results showed good correlations between k_SA'S and all four descriptors (r²: 0.72-1.0), but a separate trend line was required for the chloromethanes and the chloroethanes/propanes. Given the availability and ease with which ECD response can be obtained, this physical measurement may provide a practical means of determining relative rates of reactivity of various halocarbons in permeable reactive iron barriers.