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

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journal contribution
posted on 2010-11-25, 00:00 authored by Eric J. Bylaska, Kurt R. Glaesemann, Andrew R. Felmy, Monica Vasiliu, David A. Dixon, Paul G. Tratnyek
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 ΔGS(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, CH2−CHCl−CH2Cl, CH2Cl−CH−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 (ΔGrxn° ≈ −32 kcal/mol), followed closely by reductive dechlorination (ΔGrxn° ≈ −27 kcal/mol), dehydrochlorination (ΔGrxn° ≈ −27 kcal/mol), and nucleophilic substitution by OHGrxn° ≈ −25 kcal/mol). For both reduction reactions studied, it was found that the first electron-transfer step, yielding the intermediate CH2−CHCl−CH2Cl and the CH2Cl−CH−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.