Homolytic Cleavage of a Bonds Involving C or

the energy difference of the final singly occupied orbitals must also be considered. How would be expected to change in the series C—C, C—N, C—O, and C—F 

Similarly, one may ask how A ° would be expected to change in the series C—F, C—Cl, C—Br, and C—I or the series HF, HCl, HBr, and HI. The situation is simpler for these series, since both the electronegativity and orbital size vary but have the same effect on the bond dissociation energy. Variation in orbital size is approximately pro- 

TABLE 4.1. Average Homolytic Bond Dissociation Energies (kJ/mol) of R—X 

There is ample evidence that a bonds do interact with other occupied orbitals and that cr orbitals interact with other empty orbitals. Evidence for the latter case is primarily in the form of chemical activation of C—H bonds by adjacent groups with low-lying empty or half-filled orbitals. The increased acidity of C—H bonds, that is, reactivity with Lewis bases, under these circumstances is discussed in Chapter 10. Several examples will serve to exemplify the interaction of cr bonds with adjacent filled orbitals. 

As discussed at the end of Chapter 3, one group orbital of a methyl or methylene group will always have the correct nodal characteristics to interact with an adjacent ti orbital or with an adjacent sp orbital in a tt fashion. The degree of interaction may be inferred from the energies of the orbitals, which may in turn be obtained by measurements of ionization potentials and application of Koopmans theorem. Thus, the methyl groups adjacent to the tt bond in (Z)-2-butene (ionization potential IP = 9.12 eV [63]) raise the energy of the tt orbital by 1.39 eV relative to that of ethylene (IP = 10.51 eV [87]). A similar effect is observed in cyclohexene [64]. 

The through-space interaction of the two n bonds of norbornadiene was presented in Chapter 3 as exemplifying a four-electron, two-orbital interaction. The interaction of the nonconjugated n bonds of 1,4-cyclohexadiene cannot be treated in the same way  

---

The results of several photolysis reactions of sulphur-containing rings can be rationalized by postulating this process. One example is the photolysis of lipoic acid (182) which yielded 185 (in water) or 186 (in methanol). The proposed mechanism is a homolytic scission of the S—S bond to the diradical 183 and migration of the tertiary hydrogen atom as a radical, to form the thionthiol 184 which reacts with the solvent. A similar mechanism which involves a primary homolytic cleavage of a C—S bond was assumed to occur in the photolysis of mercaptols . 

The commonest reactions involve the displacement of halide by hydroxide or cyanide ion to yield co-ordinated phenols or nitriles. Once again, the metal may play a variety of different functions. The polarisation of the C-Cl bond is the most obvious, but stabilisation of the product may be of equal importance, as could the involvement of a metal coordinated nucleophile. The availability of a one-electron redox inter-conversion between copper(n) and copper(i) also opens up the possibilities of radical mechanisms involving homolytic cleavage of the C-Cl bond. All of these different processes are known to be operative in various reaction conditions. In other cases, organocopper intermediates are thought to be involved. 

Homolytic cleavage of the C—M bond is the most common process for thermal or photolytic decomposition of alkyl and aryl derivatives of the transition metals. In terms of a molecular orbital scheme, the process involves transfer of an electron from a bonding orbital to a C—M antibonding orbital. Electronic factors which broaden the energy gap between these orbitals should stabilize the compound toward homolytic decomposition. 

In the presence of light, organic compounds containing a carboxylic group can extrude carbon dioxide. This process can occur either from the free acid or from the dissociated carboxylate form. In general, photodecarboxylation of non-dissociated acids involves direct homolytic cleavage of the C-C bond a to the carboxyl group. By contrast, in the carboxylates formal heterolytic and/or homolytic mechanisms can participate. Photoionization or photoinduced electron transfer (PET) reactions can also be involved (see Scheme l). - ... 

---