Ellison_BDEs - Bond Dissociation Energies of Organic Molecules STEPHEN J BLANKSBY AND G BARNEY ELLISON Department of Chemistry University of

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Bond Dissociation Energies of Organic Molecules STEPHEN J. BLANKSBY* AND G. BARNEY ELLISON* ,‡ Department of Chemistry, University of Wollongong, NSW, 2522, Australia, and Department of Chemistry & Biochemistry, University of Colorado, Boulder, Colorado 80309-0215 Received August 6, 2002 ABSTRACT In this Account we have compiled a list of reliable bond energies that are based on a set of critically evaluated experiments. A brief description of the three most important experimental techniques for measuring bond energies is provided. We demonstrate how these experimental data can be applied to yield the heats of formation of organic radicals and the bond enthalpies of more than 100 representative organic molecules. Introduction The making and breaking of bonds is the basis of all chemical transformation. A sound knowledge of the energies required to break bonds and the energies released upon their formation is fundamental to understanding chemical processes. 1 The energy required for homolytic bond cleavage at 298 K corresponds to the enthalpy of reaction 1, Δ rxn H 298 (1), which is by definition 2 the bond dissociation enthalpy of the molecule AB, DH 298 (AB): Conversely, if the radicals A and B recombine to form the molecule AB, then thermal energy equivalent to the bond dissociation enthalpy is released, according to the first law of thermodynamics. Using these ideas, it is possible to determine the energetics of a wide range of simple but important reactions involving the exchange of a single bond. This is achieved by subtracting the energy gained from the bond formed from the energy required to break the initial bond. For example, consider the energetics of reaction 2, where a single carbon - hydrogen bond is broken and a hydrogen - chlorine bond is formed: Table 1 provides a set of experimentally determined bond enthalpies: typical values range from 60 to 130 kcal mol - 1 . Using the tabulated values for the carbon - hydrogen bond enthalpy of ethane, DH 298 (CH 3 CH 2 - H) ) 101.1 ( 0.4 kcal mol - 1 , and the bond enthalpy of hydrochloric acid, DH 298 (HCl) ) 103.15 ( 0.03 kcal mol - 1 , the forward reaction is determined to be exothermic since Δ rxn H 298 (2) ) DH 298 (CH 3 CH 2 - H) - DH 298 (HCl) )- 2.1 ( 0.4 kcal mol - 1 ; the reverse reaction is endothermic, Δ rxn H 298 ( - 2) )+ 2.1 ( 0.4 kcal mol - 1 . This simple scheme allows the elucidation of precise thermochemistry for a broad range of chemical reactions for which experimental bond en- thalpies are available. One must be cautious, however. In reactions where multiple bonds are broken, the bond enthalpy of a particular bond can be changed dramatically by the cleavage of ancillary bonds within the molecule. That is, once the first bond is broken, the remaining bond enthalpies are often altered. Several examples of this behavior will be discussed. Thermochemistry The bond dissociation energy for a species, AB, at room temperature is the bond enthalpy , DH 298 (AB). By definition, it is the reaction enthalpy of the bond homolysis reaction 1, Δ rxn H 298 (1), and thus depends exclusively on the relative enthalpies of formation of reactant and product states:
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This note was uploaded on 07/25/2008 for the course CEM 850 taught by Professor Borhan during the Fall '05 term at Michigan State University.

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Ellison_BDEs - Bond Dissociation Energies of Organic Molecules STEPHEN J BLANKSBY AND G BARNEY ELLISON Department of Chemistry University of

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