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 Δ f H o m (C 3 H 5 O 2 Cl, 1) = -(534.6 ′ 1.1) kJ.mol - 1 , by rotating-bomb combustion calorimetry. The corresponding enthalpy of vaporization, Δ v a p H o m (C 3 H 5 O 2 Cl) = (64.9 ′ 0.5) kJ.mol - 1 , was also obtained from vapor pressure versus temperature measurements by the transpiration method, leading to Δ f H o m (C 3 H 5 O 2 Cl, g) = -(469.7 ′ 1.2) kJ.mol - 1 . This value, together with the enthalpy of the isodesmic and isogyric gas-phase reaction CH 3 CH(X)COOH(g) + C 2 H 5 (g) CH3CHCOOH(g) + C 2 H 5 X(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 Δ f H o m [CH(CH 3 )COOH, g] = -(293 ′ 3) kJ.mol - 1 , from which DH°[H-CH(CH 3 )COOH] = 380.7 ′ 3.9 kJ.mol - 1 and DH°[Cl-CH(CH 3 )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 XCH 2 COOH, XCH 3 , XC 2 H 5 , XCH 2 -Cl, XCH(CH 3 )Cl, XCH=CH 2 , and XC 6 H 5 (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(CH 3 )COOH] and DH°[X-CH 2 COOH] (X = H, Cl) indicates that the replacement of a hydrogen of the CH 2 group of XCH 2 COOH 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.