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首页> 外文期刊>The journal of physical chemistry, A. Molecules, spectroscopy, kinetics, environment, & general theory >Energetics of the C-Cl Bond in CH_3CH(Cl)COOH. Enthalpy of Formation of (S)-(-)-2-Chloropropionic Acid and of the 1-Carboxyethyl Radical
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Energetics of the C-Cl Bond in CH_3CH(Cl)COOH. Enthalpy of Formation of (S)-(-)-2-Chloropropionic Acid and of the 1-Carboxyethyl Radical

机译：CH_3CH（Cl）COOH中C-Cl键的能级（S）-（-）-2-氯丙酸和1-羧乙基自由基的形成焓

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摘要

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_fH°_m(C_3H_5O_2Cl, l) = -(534.6 ± 1.1) kJ·mol~(-1), by rotating-bomb combusition calorimetry. The corresponding enthalpy of vaporization, Δ_(vap)H°_m(C_3H_5O_2Cl) = (64.9 ± 0.5) kJ·mol~(-1), was also obtained from vapor pressure versus temperature measurements by the transpiration method, leading to Δ_fH°_m(C_3H_5O_2Cl, g) = (469.7 ± 1.2) kJ·mol~(-1). This value, together with the enthalpy of the isodesmic and isogyric gas-phase reaction CH_3CH(X)COOH(g) + C_2H_5(g) → CH_3CHCOOH(g) + C_2H_5X(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_fH°_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_2COOH, XCH_3, XC_2H_5, XCH_2-Cl, XCH(CH_3)Cl, XCH = CH_2, and XC_6H_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_2COOH] (X = H, Cl) indicates that the replacement of a hydrogen of the CH_2 group of XCH_2COOH 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.
机译：通过结合实验和理论方法研究了2-氯丙酸中C-Cl键的能级。确定在298.15 K下形成液体（S）-（-）-2-氯丙酸的标准摩尔焓为Δ_fH°_m（C_3H_5O_2Cl，l）=-（534.6±1.1）kJ·mol〜（-1 ），通过旋转炸弹燃烧量热法。还通过蒸腾法从蒸汽压对温度的测量中获得了相应的汽化焓Δ_（vap）H°_m（C_3H_5O_2Cl）=（64.9±0.5）kJ·mol〜（-1），得出Δ_fH°_m （C_3H_5O_2Cl，g）＝（469.7±1.2）kJ·mol·（-1）。该值以及等密度和等回转相气相反应的焓CH_3CH（X）COOH（g）+ C_2H_5（g）→CH_3CHCOOH（g）+ C_2H_5X（g）（X = H，Cl）通过密度泛函函数预测理论计算和其他辅助数据，被用来推导出气态1-羧乙基自由基的形成焓，Δ_fH°_m [CH（CH_3）COOH，g] =-（293±3）kJ·mol〜（-1） ，由此得出DH°[H-CH（CH_3）COOH] = 380.7±3.9 kJ·mol〜（-1）和DH°[Cl-CH（CH_3）COOH] = 298.0±3.2 kJ·mol〜（-1）获得了。将这些值与XCH_2COOH，XCH_3，XC_2H_5，XCH_2-Cl，XCH（CH_3）Cl，XCH = CH_2和XC_6H_5中的相应C-H和C-Cl键解离焓进行比较（X = H，Cl）。对于羧酸和所有其他RX化合物，观察到DH°（C-H）> DH°（C-Cl）。 DH°[X-CH（CH_3）COOH]和DH°[X-CH_2COOH]（X = H，Cl）的比较表明，XCH_2COOH的CH_2基团的氢被甲基取代导致CH和C-Cl键的解离焓最终得出的结论是，可以基于鲍林的静电共价模型，预测本文涉及的一系列化合物的C-Cl键解离焓所表现出的主要质量趋势。

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