Bond dissociation energy data energies

Table II. Mean Bond Dissociation Energy Data for Some Early Transition Metal Complexesa and Estimated 5 Values for Thorium and Uranium.

As will be discussed in Chapter 13, calculated energies of one particular class of isodesmic reactions, so-called bond separation reactions, may be combined with experimental or high-quality calculated thermochemical data in order to lead directly to accurate heats of formation. These in turn can be used in whatever types of thermochemical comparisons are of interest. We start our assessment of isodesmic processes with bond separation reactions. Following this, we consider description of bond dissociation energies, hydrogenation energies and acid and base strengths in terms of isodesmic processes, that is, not as absolute quantities but expressed relative to standard compounds. 

Apply Create a graph using the bond-dissociation energy data in Table 8.2 and the bond-length data in Table 8.1. Describe the relationship between bond length and bond-dissociation energy. 

Based only on the bond dissociation energy data, use a AG° calculation to estimate if the following reaction may be reversible C-I (56), C-0 (91). Ignore all ionic bonds. All bond dissociation energies are in kilocalories per mole. 

The current, critical evaluation of the ideal gas thermodynamic data on the f1uoromethanes (8) and f1uoroethanes 9) will be particularly appropriate to the reduction of C-H bond dissociation energy data. Additional data on the enthalpy of formation of fluorine containing compounds will be recalculated (if necessary) to reflect the recent, direct determination of AH2(HF-nH20, 298) of Johnson, Smith and Hubbard (10). The data for the reference states, C(graphite), H2(g) and p2(g) are the same as those used by Rodgers et al. (8). 

G2 theory has been applied to many molecular systems and has in most cases been quite successful. It has been used to predict bond dissociation energies, ionization energies, electron affinities, appearance energies, proton affinities, and enthalpies of formation. This section describes several typical examples of the use of G2 theory to obtain thermochemical data. The reader is referred to several recent reviews for specific references and more details. 

Based on the bond dissociation energy data for the simple alkanes, we conclude that the C—H bond energy depends on whether the carbon atom is primary, secondary, or tertiary and not on the particular alkane in which it is found. The bond dissociation energy reflects the stabilities of the radical products, which increase in the same order as carbocations. 

Resonance theory can also account for the stability of the allyl radical. For example, to form an ethylene radical from ethylene requites a bond dissociation energy of 410 kj/mol (98 kcal/mol), whereas the bond dissociation energy to form an allyl radical from propylene requites 368 kj/mol (88 kcal/mol). This difference results entirely from resonance stabilization. The electron spin resonance spectmm of the allyl radical shows three, not four, types of hydrogen signals. The infrared spectmm shows one type, not two, of carbon—carbon bonds. These data imply the existence, at least on the time scale probed, of a symmetric molecule. The two equivalent resonance stmctures for the allyl radical are as follows ... 

Somewhat surprisingly perhaps, it has been found that [l.l.l]propellane is considerably less reactive than [2.2.1]propellane. Use the theoretically calculated enthalpy data below to estimate the bond dissociation energy of the central bond in each of the three propellanes shown. How might this explain the relative reactivity of the [1-1.1]- and [2.2. Ijpropellanes ... 

These data can be combined with ionization potential (IP) data according to the scheme below to determine bond dissociation energies (BDE). 

According to these data, which structural features provide stabilization of radial centers Determine the level of agreement between these data and the radical stabilization energies given in Table 12.7 if the standard C—H bond dissociation energy is taken to be 98.8 kcal/mol. (Compare the calculated and observed bond dissociation energies for the benzyl, allyl, and vinyl systems.)... 

What are the reasons for the observed reactivity order of alkane hydrogens toward radical chlorination A look at the bond dissociation energies given previously in Table 5.3 on page 156 hints at the answer. The data in Table 5.3 indicate that a tertiary C—H bond (390 kj/mol 93 kcal/mol) is weaker than a secondary C-H bond (401 kj/mol 96 kcal/mol), which is in turn weaker than a primary C H bond (420 kj/mol 100 kcal/mol). Since less energy is needed to break a tertiary C-H bond than to break a primary or secondary C-H bond, the resultant tertiary radical is more stable than a primary or secondary radical. 

Much fewer experiments are available in solution where the few reported data are generally more concerned about the effect of molecular structure than about bond dissociation energy. In simple shear, it is generally agreed that chain flexibility dominantly influences the rate of bond scission, with the most rigid polymers being the easiest to fracture [157]. The results are interpreted in terms of the presence of good and poor sequences in the chain conformation. 

Where no data exist, one wishes to be able to estimate thermochemical quantities. A simple and convenient method to do that is through the use of the method of group additivity developed by Benson and coworkers15,21 22. The earlier group values are revised here, and new group values calculated to allow extension of the method to sulfites and sulfates. In addition, a method based on the constancy of S—O bond dissociation energies is applied. 

TABLE 1. ESR data and bond dissociation energies of some sulfinyl radicals... 

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