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, CH ₂Cl-CH₂-CH₂Cl, C^•H ₂-CHCl-CH₂Cl, CH₂Cl-C^•H-CH ₂Cl, CH₂=CCl-CH₂Cl, cis-CHCl=CH-CH ₂Cl, trans-CHCl=CH-CH₂Cl, CH₂=CH-CH ₂Cl, 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)-CH ₂Cl. 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 (Δ _Grxn^° ≈ -27 kcal/mol), dehydrochlorination (Δ_Grxn^° ≈ -27 kcal/mol), and nucleophilic substitution by OH^- (Δ_Grxn^° ≈ -25 kcal/mol). For both reduction reactions studied, it was found that the first electron-transfer step, yielding the intermediate C^•H ₂-CHCl-CH₂Cl and the CH₂Cl-C ^•H-CH₂Cl species, was not favorable in the standard state (Δ_Grxn^° ≈ +15 kcal/mol) and was predicted to occur only at relatively high pH values. This result suggests that reduction by natural attenuation is unlikely.