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 Δf (C3H5O2Cl, l) = −(534.6 ± 1.1) kJ·mol-1, by rotating-bomb combustion calorimetry. The corresponding enthalpy of vaporization, Δvap (C3H5O2Cl) = (64.9 ± 0.5) kJ·mol-1, was also obtained from vapor pressure versus temperature measurements by the transpiration method, leading to Δf (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 Δf [CH(CH3)COOH, g] = −(293 ± 3) kJ·mol-1, from which DH°[H−CH(CH3)COOH] = 380.7 ± 3.9 kJ·mol-1 and DH°[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, XCH2Cl, XCH(CH3)Cl, XCHCH2, and XC6H5 (X = H, Cl). The order DH°(C−H) > DH°(C−Cl) is observed for the carboxylic acids and all other RX compounds. Comparison of DH°[X−CH(CH3)COOH] and DH°[X−CH2COOH] (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.