Three-Electron, Two-Orbital Interaction

Figure 3.8. Three-electron, two-orbital interaction (a) odd electron in higher orbital, moderate stabilization (b) odd electron in the lower orbital, large stabilization and the possibility of electron...

As oximes and hydrazones are not good electron acceptors, the mechanism for these reactions most likely involves an intermediate ketyl radical anion, which adds to the C=N double bond. A three-electron-two-orbital interaction involving the nitrogen-centred radical with the lone pair of the adjacent... 

Substituent Effects and Reactivity. If the SOMO is relatively low in energy, the principal interaction with other molecules will be with the occupied MOs (three-electron, two-orbital type, Figure 3.8). In this case the radical is described as electrophilic. If the SOMO is relatively high in energy, the principal interaction with other molecules may be with the unoccupied MOs (one-electron, two-orbital type, Figure 3.10). In this case the radical is described as nucleophilic. Substituents on the radical center will affect the electrophilicity or nucleophilicity of free radicals, as shown below. 

As in the case for alkene additions, if the SOMO of the radical is relatively high in energy, such as is the case for alkyl radicals, the principal interaction with the abstractable X-H bond will be with its unoccupied a MO (one-electron-two-orbital type), and such a radical would be considered nucleophilic. If the SOMO is relatively low in energy, such as is the case for perfluoroalkyl radicals, the principal interaction with the abstractable X-H bond will be with its occupied a MO (three-electron-two-orbital type), and the radical is considered electrophilic. Either way, a good match-up in polarities in an H-atom transition state will give rise to beneficial transition state charge-transfer interaction [130,136,137]. 

A free radical is stabilized by an X substituent (Figure 13a) through a two-orbital, three-electron 7r-type interaction. The nucleophilicity of the radical is greatly increased. The X -substituted free radicals are more easily oxidized. The n effects of X substitution are somewhat augmented by the inductive effect of the electronegative X in stabilizing the radical. 

The valence bond model constructs hybrid orbitals which contain various fractions of the character of the pure component orbitals. These hybrid orbitals are constructed such that they possess the correct spatial characteristics for the formation of bonds. The bonding is treated in terms of localised two-electron two-centre interactions between atoms. As applied to first-row transition metals, the valence bond approach considers that the 45, 4p and 3d orbitals are all available for bonding. To obtain an octahedral complex, two 3d, the 45 and the three 4p metal orbitals are mixed to give six spatially-equivalent directed cfisp3 hybrid orbitals, which are oriented with electron density along the principal Cartesian axes (Fig. 1-9). 

---