| Abstract
| - The energetics of the C−F, C−Cl, C−Br, and C−I bonds in 2-haloethanols was investigated by using acombination of experimental and theoretical methods. The standard molar enthalpies of formation of 2-chloro-,2-bromo-, and 2-iodoethanol, at 298.15 K, were determined as Δf(ClCH2CH2OH, l) = −315.5 ± 0.7kJ·mol-1, Δf(BrCH2CH2OH, l) = −275.8 ± 0.6 kJ·mol-1, Δf(ICH2CH2OH, l) = −207.3 ± 0.7kJ·mol-1, by rotating-bomb combustion calorimetry. The corresponding standard molar enthalpies ofvaporization, Δvap(ClCH2CH2OH) = 48.32 ± 0.37 kJ·mol-1, Δvap(BrCH2CH2OH) = 54.08 ± 0.40kJ·mol-1, and Δvap(ICH2CH2OH) = 57.03 ± 0.20 kJ·mol-1 were also obtained by Calvet-dropmicrocalorimetry. The condensed phase and vaporization enthalpy data lead to Δf(ClCH2CH2OH, g) =−267.2 ± 0.8 kJ·mol-1, Δf(BrCH2CH2OH, g) = −221.7 ± 0.7 kJ·mol-1, and Δf(ICH2CH2OH, g) =−150.3 ± 0.7 kJ·mol-1. These values, together with the enthalpy of selected isodesmic and isogyric gas-phase reactions predicted by density functional theory (B3LYP/cc-pVTZ) and CBS-QB3 calculations wereused to derive the enthalpies of formation of gaseous 2-fluoroethanol, Δf(FCH2CH2OH, g) = −423.6 ±5.0 kJ·mol-1, and of the 2-hydroxyethyl radical, Δf(CH2CH2OH, g) = −28.7 ± 8.0 kJ·mol-1. Theobtained thermochemical data led to the following carbon−halogen bond dissociation enthalpies: DHo(X−CH2CH2OH) = 474.4 ± 9.4 kJ·mol-1 (X = F), 359.9 ± 8.0 kJ·mol-1 (X = Cl), 305.0 ± 8.0 kJ·mol-1 (X =Br), 228.7 ± 8.1 kJ·mol-1 (X = I). These values were compared with the corresponding C−X bond dissociationenthalpies in XCH2COOH, XCH3, XC2H5, XCHCH2, and XC6H5. In view of this comparison thecomputational methods mentioned above were also used to obtain Δf(FCH2COOH, g) = −594.0 ± 5.0kJ·mol-1 from which DHo(F−CH2COOH) = 435.4 ± 5.4 kJ·mol-1. The order DHo(C−F) > DHo(C−Cl) >DHo(C−Br) > DHo(C−I) is observed for the haloalcohols and all other RX compounds. It is finally concludedthat the major qualitative trends exhibited by the C−X bond dissociation enthalpies for the series of compoundsstudied in this work can be predicted by Pauling's electrostatic-covalent model.
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