Energetics of the C−Cl Bond in CH3CH(Cl)COOH. Enthalpy of Formation of (S)-(−)-2-Chloropropionic Acid and of the 1-Carboxyethyl Radical

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Molecules, spectroscopy, kinetics, environment, & general theory, 2002-10, Vol.106 (42), p.9855-9861
• Main Authors: Lagoa, Ana L. C, Diogo, Hermínio P, Minas da Piedade, Manuel E, Amaral, Luísa M. P. F, Guedes, Rita C, Costa Cabral, Benedito J, Kulikov, Dmitry V, Verevkin, Sergey P, Siedler, Michael, Epple, Matthias
• Format: Article
• Language: English
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/jp020412p

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.