Energetics of C-F, C-Cl, C-Br, and C-I Bonds in 2-Haloethanols. Enthalpies of Formation of XCH2CH2OH (X = F, Cl, Br, I) Compounds and of the 2-Hydroxyethyl Radical

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• 作者：Carlos E. S. Bernardes ; Manuel E. Minas da Piedade ; Lu&iacute ; sa M. P. F. Amaral ; Ana I. M. C. L. Ferreira ; Manuel A. V. Ribeiro da Silva ; Herm&iacute ; nio P. Diogo ; Benedito J. Costa Cabral
• 刊名：Journal of Physical Chemistry A
• 出版年：2007
• 出版时间：March 8, 2007
• 年：2007
• 卷：111
• 期：9
• 页码：1713 - 1720
• 全文大小：108K
• 年卷期：v.111,no.9(March 8, 2007)
• ISSN：1520-5215

文摘

The energetics of the C-F, C-Cl, C-Br, and C-I bonds in 2-haloethanols was investigated by using acombination of experimental and theoretical methods. The standard molar enthalpies of formation of 2-chloro-,2-bromo-, and 2-iodoethanol, at 298.15 K, were determined as fchars/Delta.gif" BORDER=0 >_fournals/jpcafh/111/i09/eqn/jp0675678e10001.gif">(ClCH₂CH₂OH, l) = -315.5 ± 0.7kJ·mol^-1, fchars/Delta.gif" BORDER=0 >_fournals/jpcafh/111/i09/eqn/jp0675678e10002.gif">(BrCH₂CH₂OH, l) = -275.8 ± 0.6 kJ·mol^-1, fchars/Delta.gif" BORDER=0 >_fournals/jpcafh/111/i09/eqn/jp0675678e10003.gif">(ICH₂CH₂OH, l) = -207.3 ± 0.7kJ·mol^-1, by rotating-bomb combustion calorimetry. The corresponding standard molar enthalpies ofvaporization, fchars/Delta.gif" BORDER=0 >_vapournals/jpcafh/111/i09/eqn/jp0675678e10004.gif">(ClCH₂CH₂OH) = 48.32 ± 0.37 kJ·mol^-1, fchars/Delta.gif" BORDER=0 >_vapournals/jpcafh/111/i09/eqn/jp0675678e10005.gif">(BrCH₂CH₂OH) = 54.08 ± 0.40kJ·mol^-1, and fchars/Delta.gif" BORDER=0 >_vapournals/jpcafh/111/i09/eqn/jp0675678e10006.gif">(ICH₂CH₂OH) = 57.03 ± 0.20 kJ·mol^-1 were also obtained by Calvet-dropmicrocalorimetry. The condensed phase and vaporization enthalpy data lead to fchars/Delta.gif" BORDER=0 >_fournals/jpcafh/111/i09/eqn/jp0675678e10007.gif">(ClCH₂CH₂OH, g) =-267.2 ± 0.8 kJ·mol^-1, fchars/Delta.gif" BORDER=0 >_fournals/jpcafh/111/i09/eqn/jp0675678e10008.gif">(BrCH₂CH₂OH, g) = -221.7 ± 0.7 kJ·mol^-1, and fchars/Delta.gif" BORDER=0 >_fournals/jpcafh/111/i09/eqn/jp0675678e10009.gif">(ICH₂CH₂OH, g) =-150.3 ± 0.7 kJ·mol^-1. These values, together with the enthalpy of selected isodesmic and isogyric gas-phase reactions predicted by density functional theory (B3LYP/cc-pVTZ) and CBS-QB3 calculations wereused to derive the enthalpies of formation of gaseous 2-fluoroethanol, fchars/Delta.gif" BORDER=0 >_fournals/jpcafh/111/i09/eqn/jp0675678e10010.gif">(FCH₂CH₂OH, g) = -423.6 ±5.0 kJ·mol^-1, and of the 2-hydroxyethyl radical, fchars/Delta.gif" BORDER=0 >_fournals/jpcafh/111/i09/eqn/jp0675678e10011.gif">(CH₂CH₂OH, g) = -28.7 ± 8.0 kJ·mol^-1. Theobtained thermochemical data led to the following carbon-halogen bond dissociation enthalpies: DH^o(X-CH₂CH₂OH) = 474.4 ± 9.4 kJ·mol^-1 (X = F), 359.9 ± 8.0 kJ·mol^-1 (X = Cl), 305.0 ± 8.0 kJ·mol^-1 (X =Br), 228.7 ± 8.1 kJ·mol^-1 (X = I). These values were compared with the corresponding C-X bond dissociationenthalpies in XCH₂COOH, XCH₃, XC₂H₅, XCH=CH₂, and XC₆H₅. In view of this comparison thecomputational methods mentioned above were also used to obtain fchars/Delta.gif" BORDER=0 >_fournals/jpcafh/111/i09/eqn/jp0675678e10012.gif">(FCH₂COOH, g) = -594.0 ± 5.0kJ·mol^-1 from which DH^o(F-CH₂COOH) = 435.4 ± 5.4 kJ·mol^-1. The order DH^o(C-F) > DH^o(C-Cl) >DH^o(C-Br) > DH^o(C-I) is observed for the haloalcohols and all other RX compounds. It is finally concludedthat the major qualitative trends exhibited by the C-X bond dissociation enthalpies for the series of compoundsstudied in this work can be predicted by Pauling's electrostatic-covalent model.