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 ΔHf°(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 (ΔGrxn° ≈ -32 kcal/mol), followed closely by reductive dechlorination (Δ Grxn° ≈ -27 kcal/mol), dehydrochlorination (ΔGrxn° ≈ -27 kcal/mol), and nucleophilic substitution by OH- (ΔGrxn° ≈ -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 (ΔGrxn° ≈ +15 kcal/mol) and was predicted to occur only at relatively high pH values. This result suggests that reduction by natural attenuation is unlikely.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry