Free Energies for Degradation Reactions of 1,2,3-Trichloropropane from ab Initio Electronic Structure Theory

Electronic structure methods were used to calculate the gas and aqueous phase reaction energies for reductive dechlorination (i.e., hydrogenolysis), reductive β-elimination, dehydrochlorination, and nucleophilic substitution by OH⁻ of 1,2,3-trichloropropane. The thermochemical properties ΔH_f^°(298.15 K), S°(298.15 K, 1 bar), and ΔG_S(298.15 K, 1 bar) were calculated by using ab initio electronic structure calculations, isodesmic reactions schemes, gas-phase entropy estimates, and continuum solvation models for 1,2,3-trichloropropane and several likely degradation products: CH₃−CHCl−CH₂Cl, CH₂Cl−CH₂−CH₂Cl, C^•H₂−CHCl−CH₂Cl, CH₂Cl−C^•H−CH₂Cl, CH₂CCl−CH₂Cl, cis-CHClCH−CH₂Cl, trans-CHClCH−CH₂Cl, CH₂CH−CH₂Cl, CH₂Cl−CHCl−CH₂OH, CH₂Cl−CHOH−CH₂Cl, CH₂CCl−CH₂OH, CH₂COH−CH₂Cl, cis-CHOHCH−CH₂Cl, trans-CHOHCH−CH₂Cl, CH(O)−CH₂−CH₂Cl, and CH₃−C(O)−CH₂Cl. On the basis of these thermochemical estimates, together with a Fe(II)/Fe(III) chemical equilibrium model for natural reducing environments, all of the reactions studied were predicted to be very favorable in the standard state and under a wide range of pH conditions. The most favorable reaction was reductive β-elimination (ΔG_rxn^° ≈ −32 kcal/mol), followed closely by reductive dechlorination (ΔG_rxn^° ≈ −27 kcal/mol), dehydrochlorination (ΔG_rxn^° ≈ −27 kcal/mol), and nucleophilic substitution by OH⁻ (ΔG_rxn^° ≈ −25 kcal/mol). For both reduction reactions studied, it was found that the first electron-transfer step, yielding the intermediate C^•H₂−CHCl−CH₂Cl and the CH₂Cl−C^•H−CH₂Cl species, was not favorable in the standard state (ΔG_rxn^° ≈ +15 kcal/mol) and was predicted to occur only at relatively high pH values. This result suggests that reduction by natural attenuation is unlikely.