Homolytic absolute

Energies for a selection of homolytic bond dissociation reactions of two-heavy-atom hydrides are provided in Table 6-2. These have been drawn from a larger collection found in Appendix A6 (Tables A6-1 to A6-8). A summary of mean absolute deviations from G3 calculations (based on the full collection) is provided in Table 6-3. 

Table 6-3 Mean Absolute Deviations from G3 Calculations in Energies of Homolytic Bond Dissociation Reactions... 

With due attention to the noted failures, it is evident that relative homolytic bond dissociation energies, unlike absolute homolytic bond dissociation energies, can be reasonably well described with simple and practical models. 

Overall, the performance of Hartree-Fock models is very poor. In most cases, activation energies are overestimated by large amounts. This is not surprising in view of previous comparisons involving homolytic bond dissociation energies (see Table 6-2), which were too small. In terms of mean absolute deviation from the standard (MP2/6-311+G ) calculations, STO-3G yields the poorest results and 3-2IG the best results. 6-3IG and 6-311+G models provide nearly identical activation energies (just as they did for transition-... 

In homolytic substitution reactions, the 2-position of thiophene is the preferred site of attack. This is easily rationalized in terms of frontier orbital theory (B-76MI31401). Because of symmetry, both HOMO and LUMO of thiophene have the same absolute values for the coefficients (as shown in 216). Thus it is immaterial whether the [SOMO (radical)-HOMO (thiophene)] or the [SOMO (radical)-LUMO (thiophene)] interaction determines the site of attack only the 2-position is the point at which the radical would attack. The same conclusion is iso reached by consideration of product development control (74AHC(16)123). Attack at the 2-position would result in a transition state with an allylic radical, which would be stabilized to a greater extent than the one arising from attack at position 3 (Scheme 57). 

Stability in chemistry is not an absolute, but a relative concept. Let us consider the standard heats of reaction AH0 of the homolytic dissociation reaction R—H —> R + H. It reflects, on the one hand, the strength of this C—H bond and, on the other hand, the stability of the radical R produced. So the dissociation enthalpy of the R—H bond depends in many ways on the structure of R. But it is not possible to tell clearly whether this is due to an effect on the bond energy of the broken R—H bond and/or an effect on the stability of the radical R that is formed. 

The enthalpies, KH, of the homolytic cleavage of the central Si-Si bond in disilane dimers a can be calculated from temperature-dependent ESR experiments, using Eq. 3, in which C is the concentration of the radical, T is the absolute temperature and A is a. constant. The change in the concentration of the radicals as a function of temperature could be followed by EPR as shown in Fig. 6a for radical 3b. The concentrations of the thermally generated radicals (2b and 3b) were determined by calibration of the height of the EPR signal of the silyl radicals in comparison with a 3 X 10 M toluene solution of TEMPO (2,2,6,6-tetramethyl-piperidinooxy). 

Homolytic fission of the chiral ammonium ylide (143) generates two diastereomeric radical pairs which evolve by two different routes (a) cage recombination leads to the two diastereomeric ketoamines (161) and (162), where the migrating center has retained its absolute stereochemistry (b) radical escape gives achiral free radicals that combine to yield racemic products (Scheme 35). 

The electrical conductivity a of liquid sulfur increases with temperature except near the viscosity maximum of ca. 170 °C where a minimum of the conductivity is observed. Above 200 °C the plot of log a vs 1/T was found by several authors to be linear but the slopes of these linear relationships as well as the absolute conductivities vary considerably [118-122]. On the assumption that the conductivity at these temperatures is intrinsic, values of about 1.6 eV were derived for the activation energy at high temperatures (up to 900 °C) [121, 122], an energy which is much higher than the activation energy for the formation of free spins by homolytic bond dissociation (see above). 

Oxidation of diethyl a-benzylmalonate (25) by Mn(III) acetate in acetic acid at 70 °C in the presence of mono- or disubstituted alkynes leads to dihydronaphthalene derivatives (26) in moderate to good yields (equation 33). A mechanistic scheme involving the formation of the corresponding malonyl radical, its addition to a triple bond and intramolecular homolytic aromatic substitution of the vinyl radical adducts is discussed. Absolute rate constants, obtained from competitive studies, for the addition of a-benzylmalonyl radicals to a variety of alkynes cover few orders of magnitude e.g. the rate constants at 60 °C are 3x10 and 1 x 10 s for 4-octyne and phenylacetylene respectively. 

Bravo A, Bjprsvik HR, Fontana F, Liguori L, Mele A, Minisci F (1997) New methods of free-radical perfluoroalkylation of aromatics and alkenes. Absolute rate constants and partial rate factors for the homolytic aromatic substitution by n-perfluorobutyl radiceil. J Org Chem 62 7128-7136... 

---