Energetics of the C-Cl bond in CH3CH(Cl)COOH.: Enthalpy of formation of (S)-(-)-2-chloropropionic acid and of the 1-carboxyethyl radical

The energetics of the C-Cl bond in 2-chloropropionic acid was investigated by using a combination of experimental and theoretical methods. The standard molar enthalpy of formation of liquid (S)-(-)-2-chloropropionic acid, at 298.15 K, was determined as Delta(f)H(m)(o)(C3H5O2Cl, 1) = -(534.6 +/- 1.1) kJ.mol(-1), by rotating-bomb combustion calorimetry. The corresponding enthalpy of vaporization, Delta(vap)H(m)(o)(C3H5O2Cl) = (64.9 +/- 0.5) kJ.mol(-1), was also obtained from vapor pressure versus temperature measurements by the transpiration method, leading to Delta(f)H(m)(o)(C3H5O2Cl, g) = -(469.7 +/- 1.2) kJ.mol(-1). This value, together with the enthalpy of the isodesmic and isogyric gas-phase reaction CH3CH(X)COOH(g) + C2H5(g) --> CH3CHCOOH(g) + C2H5X(g) (X = H, Cl) predicted by density functional theory calculations and other auxiliary data, was used to derive the enthalpy of formation of the gaseous 1-carboxyethyl radical as Delta(f)H(m)(o)[CH(CH3)COOH, g] = -(293 3) kJ.mol(-1), from which DHo[H-CH(CH3)COOH] = 380.7 +/- 3.9 kJ.mol(-1) and DHo[Cl-CH(CH3)COOH] = 298.0 +/- 3.2 kJ.mol(-1) were obtained. These values are compared with the corresponding C-H and C-Cl bond dissociation enthalpies in XCH2COOH, XCH3, XC2H5, XCH2-Cl, XCH(CH3)Cl, XCH=CH2, and XC6H5 (X = H, Cl). The order DHo(C-H-) > DHo(C-Cl) is observed for the carboxylic acids and all other RX compounds. Comparison of DHo[X-CH(CH3)COOH] and DHo[XCH2COOH] (X = H, Cl) indicates that the replacement of a hydrogen of the CH2 group of XCH2COOH by a methyl group leads to a decrease of both the C-H and C-Cl bond dissociation enthalpy. It is finally concluded that the major qualitative trends exhibited by the C-Cl bond dissociation enthalpies for the series of compounds addressed in this work can be predicted based on Pauling's electrostatic-covalent model.