Bond dissociation energies, some values

The moral is that we should try to avoid using the bond energies in Table 4-3 as a measure of AH0 for the dissociation of just one bond in a polyatomic molecule. For this we need what are called bond-dissociation energies, some of which are given in Table 4-6. The values given have been selected to emphasize... 

From this value and known C—H bond dissociation energies, pK values can be calculated. The electrochemical measurements can be made on halides or on alkyl-lithium compounds. This type of approach has some significant uncertainties but nevertheless can provide a least a semiquantitative estimate of acidities of very weakly acidic hydrocarbons. The pK for isobutane obtained in this way is 71. The necessary electrochemical measurements cannot be made directly for methane, but an extrapolation from toluene and diphenylmethane leads to a range of 52-62 for the pK of methane. ... 

The radical stabilization provided by various functional groups results in reduced bond dissociation energies for bonds to the stabilized radical center. Some bond dissociation energy values are given in Table 12.6. As an example of the effect of substituents on bond dissociation energies, it can be seen that the primary C—H bonds in acetonitrile (86 kcal/mol) and acetone (92kcal/mol) are significantly weaker than a primaiy C—H... 

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

The one-electron reduction potentials, (E°) for the phenoxyl-phenolate and phenoxyl-phenol couples in water (pH 2-13.5) have been measured by kinetic [pulse radiolysis (41)] and electrochemical methods (cyclic voltammetry). Table I summarizes some important results (41-50). The effect of substituents in the para position relative to the OH group has been studied in some detail. Methyl, methoxy, and hydroxy substituents decrease the redox potentials making the phe-noxyls more easily accessible while acetyls and carboxyls increase these values (42). Merenyi and co-workers (49) found a linear Hammett plot of log K = E°l0.059 versus Op values of substituents (the inductive Hammett parameter) in the 4 position, where E° in volts is the one-electron reduction potential of 4-substituted phenoxyls. They also reported the bond dissociation energies, D(O-H) (and electron affinities), of these phenols that span the range 75.5 kcal mol 1 for 4-amino-... 

Table I summarizes the values recommended here and indicates the magnitude of their uncertainties. These values have many implications outside the chemistry of phenyl benzoate itself. For example, some bond dissociation energies (kcal. per mole) in other species are D(Ph—COPh) = 85 D(PhCO—COPh) = 62 D(PhC02—COPh) = 77 D(PhO—C02Ph) = 62 D(PhCO—NH2) = 84.

Alternatively, for many classes of compounds the heats of formation can be estimated through additivity of bond properties or group additivity rules [32], Let s take a simple example using the additivity of bond properties to estimate the heat of formation of some species A. Suppose we know (1) the heat of formation of a related compound ABR, where B is the atom to which A is bonded and R is the rest of the molecule, (2) the heat of formation of BR, and (3) that for a series of other molecules in which a A-B bond occurs the A-B bond-dissociation-energy is nearly constant, and we assign it the value B.D.E.(A-B) J. Now consider the endothermic reaction... 

Unfortunately, there were two errors in Heicklen and Knight s analysis. Subsequent work66 has shown that their quantum yields were too low by a factor of two. Second, at the lowest pressures used (0.6 torr) some of the excited mercury will disappear by fluorescence, thus giving lower quantum yields than would otherwise have been obtained. Both of these errors would give too large a value for E0 it should be smaller than 2.2 kcal/mole. Consequently, the double bond dissociation energy in C3Fe should be between 112.7 and 114.9 kcal/mole. 

R. Walsh, Acc. Chem. Res., 14, 246 (1981). Bond Dissociation Energy Values in Silicon-Containing Compounds and Some of Their Implications. 

In order to test the point-charge method experimentally measured dissociation energy and interatomic distance are required for each chemical bond. Dissociation energies for most homonuclear diatomic molecules have been measured spectroscopically and/or thermochemically. Interatomic distances for a large number of these are also known. However, for a large number of, especially metallic diatomic molecules, equilibrium interatomic distances have not been measured spectroscopically. In order to include these elements in the sample it is noted that for those metals with measured re, it is found to be related, on average, to 5, the distance of closest approach in the metal, by re = 0.78(5. On this assumption reference values of interatomic distance (d) become available for virtually all elements, as shown in the data appendix. In some special cases well-characterized dimetal bond lengths have also been taken into account for final assessment of interatomic distance. 

The RSE is calculated here as the difference between the homolytic C-C bond dissociation energy in ethane (5) and a symmetric hydrocarbon 6 resulting from dimerization of the substituted radical 2. By definition the C-C bonds cleaved in this process are unpolarized and, baring some strongly repulsive steric effects in symmetric dimer 6, the complications in the interpretation of substituent effects are thus avoided. Since two substituted radicals are formed in the process, the reaction enthalpy for the process shown in Equation 5.5 contains the substituent effect on radical stability twice. The actual RSE value is therefore only half of the reaction enthalpy for reaction 5.5 as expressed in Equation 5.6. 

Numerical values for a number of bond dissociation energies are given in this chapter. It is intended to be complete as far as directly determined dissociation energies are concerned some dissociation energies deduced thermochemically from heats of formation of radicals or atoms are included, often for purposes of comparison with directly determined values, but the list is not intended to be exhaustive or critical in this respect, and the reader can supply the missing values by simple arithmetic. Often older or less reliable estimates of dissociation energies are not discussed. 

Gharnley, Skinner, and Smith 4 have deduced the average bond dissociation energy D(B - OR) in the esters of boric acid B(OR)3. The heats of formation of four alkyl borates were determined from their heats of hydrolysis. From these, together with some subsidiary data, they found the following values of 5(B-0) in R3BO3 5(B-O) =5(RO-H) x kcal, where x=4 l, 4 3, 5 O5... 

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