Bond-dissociation enthalpies

Chemistry is fundamentally about making and breaking bonds. Thus, in order to understand a reaction at its most basic level, one must account for how much energy 

Computational Organic Chemistry, Second Edition. Steven M. Bachrach 20t4 John Wiley Sons, Inc. Published 20t4 by John Wiley Sons, Inc. 

This is a homolytic cleavage, resulting in two radicals. Computation of the BDE requires simply computing the energies of each of the three species, the molecule AB and the two radicals A- and B-. 

TABLE 3.1 Bond Dissociation Enthalpy Real mol of Some Simple 

The Hartree-Fock (HF) method seriously underestimates the DDE for all of the examples in Table 3.1. This results from the inherent lesser electron correlation in the radicals than in the parent species, and electron correlation is completely neglected in the HF method. Inclusion of some electron correlation, as in the MP2 method, dramatically improves the estimated BDEs. In a stndy of the BDE of the C-H bond in 19 small molecules, Radom found that the mean absolute deviation (MAD) at the MP2/6-311+G(2df,p)//MP2/6-31G(d) level is 3.6 kcal mor. This error is generally larger than that associated with the experimental method, and so DPEs obtained using MP2 may be problematic. 

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A/i, the enthalpy of atomisation of chlorine, which is also half the bond dissociation enthalpy. 

The very low bond dissociation enthalpy of fluorine is an important factor contributing to the greater reactivity of fluorine. (This low energy may be due to repulsion between non-bonding electrons on the two adjacent fluorine atoms.) The higher hydration and lattice enthalpies of the fluoride ion are due to the smaller size of this ion. 

Electron affinity and hydration energy decrease with increasing atomic number of the halogen and in spite of the slight fall in bond dissociation enthalpy from chlorine to iodine the enthalpy changes in the reactions... 

Similar values have been obtained for AHffMesSi ) from two independent studies. The bond dissociation enthalpy DHfMeaSi-SiMea) = 332 +12 kJ moC was obtained from a kinetic study on the very low pressure pyrolysis of hexamethyldisilane and the enthalpy of formation of trimethylsilyl ion, AHf (MeaSi ) = 617.3 + 2.3kJmor, was determined using threshold photoelectron-photoion coincidence spectroscopy (TPEPICO). Both data are related to AHf°(Me3Si ). 

NMR line broadening is a suitable kinetic method for determining activation parameters for Co—C bond homolysis, and gave A//- values in the range 18-22.5 keal mol for a selection of Co(Por)R complexes containing secondary or tertiary alkyl groups.Bond dissociation enthalpies and entropies for several... 

K bond dissociation enthalpy, unless otherwise noted. 

A//298 is called the bond dissociation enthalpy or simply the bond enthalpy. Bond enthalpies are often also called bond energies because the small difference between the two values (ca. 2.5 kJ mol-1) can be ignored in many cases, particularly for polyatomic molecules. 

The nature of the bonding in this molecule has been the cause of considerable discussion. Its short length (112.8 pm) and its great strength (bond dissociation enthalpy 1072 kJ mol- ) are consistent with the usual triple-bond Lewis structure... 

These assumptions are consistent with the very large bond strength of the BF bond in BF3, which is larger than that of any other single bond. It has a bond dissociation enthalpy... 

Observation of the empirically linear equation 2 for the gas-phase enthalpy-of-formation data implies that AH(3) is constant for the series of compounds ML 20a b. AH°(3) can also be expressed in terms of the bond dissociation enthalpies (equation 5) by again using Scheme 1. 

TABLE 2. Bond dissociation enthalpies in organogermanium compounds (kJmol-1)... 

The value reported in the original paper is 4.2 kJ mol-1 smaller because the authors have used different auxiliary data to derive the equation relating the experimentally measured quantity with the bond dissociation enthalpy. 

Tributyltin hydride is a well known reducing agent46, in keeping with the low value of Sn—H bond dissociation enthalpy. This weakness of Sn—H bond has been used by Rathke... 

The only experimental values of bond dissociation enthalpies that have been reported in the literature are given in McMillen and Golden s review32d D(Me3Pb—Me) = 238 17 kJmol-1 and D(Et3Pb—Et) = 230 17 kJmol-1. These values and the data in Table 6 imply Ai °(PbMe3, g) = 227 18 kJmol-1 and AF °(PbEt3, g) = 221 18 kJmol-1. 

The main purpose of these final comments is to show a few general trends in the thermochemistry of Group 14 organometallic compounds, helped by some (hopefully) reliable values. And one of the trends is revealed by a rather usual plot1,2, in which the mean bond dissociation enthalpies of the species MR4 (i.e. one-fourth of the enthalpy required to break all the M—R bonds) are represented as a function of the enthalpy of formation of M in the gaseous state. As observed in Figure 4, for R = H and Me, D(M—H) and D(M—Me) increase with the enthalpy of formation (or sublimation) of M. It is noted, on the other hand, that the differences D(M—H) — D(M—Me) vary from 47.7 kJmol-1... 

FIGURE 4. Mean M—R bond dissociation enthalpies in MR4 compounds (M = element from Group 14 R = H or Me) versus the enthalpies of formation of gaseous M... 

The values +423, +480, +425 and +335 k mol 1 are referred to as the first, second, third and fourth bond dissociation enthalpies of methane, respectively. The bond dissociation enthalpy is the enthalpy required to break one mole of specific bonds in a specific molecule. 

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