Radical carbon atoms

To see why allylic radicals are so stable, look at the orbital picture in Figure 10.3. The radical carbon atom with an unpaired electron can adopt sp2 hybridization, placing the unpaired electron in a p orbital and giving a structure that is electronically symmetrical. The p orbital on the central carbon can therefore overlap equally well with a p orbital on either of the two neighboring carbons. 

Both are essentially planar, i.e. sp2 hybridised, at the radical carbon atom for only in this configuration is maximum p/n orbital overlap— with consequent stabilisation—possible. The stability of a radical increases as the extent of potential delocalisation increases thus Ph2CH- is more stable than PhCH2-, and Ph3C- (cf. p. 300) is a pretty stable radical. 

The shape of Ph3C- (49) is a matter of some interest as it has a bearing on the extent to which delocalisation of the unpaired electron, with consequent stabilisation, can occur. The radical carbon atom is certainly sp2 hybridised in (49), i.e. the bonds joining it to the three benzene nuclei all lie in the same plane but maximum stabilisation will only occur if all three benzene nuclei can be simultaneously coplanar (49a),... 

At the ends of the polymer chains and at the ends of the short oligomer units (see for example the trimer molecule of Table 1) a bond defect structure is expected. For the acetylene structure of the polymer chain this is a carbene —C— with two free valence electrons and in the case of the butatriene structure this is a radical carbon atom —C= with one free valence electron. In both rases there is a reactive chain end, which allows reaction of the chain with the neighbouring monomer molecules. These reactive structures and a possible nonreactive structure are listed in Table 1 as examples of the trimer molecules. 

X-ray analyses have shown that the C-O bond distance of a ketyl unit is generally around 1.30 A, which is longer than that of a free ketone (ca. 1.20 A) and shorter than that of an alkoxide (ca. 1.40 A). The radical carbon atom of a ketyl is still in a sp2-hybrid state. 

Because benzeneoxide has a conjugated but not localized radical carbon atom, O2 addition is another pathway toluene gives the following peroxide, which is linked within the ring ... 

The radical carbon atom forms a covalent bond with the oxygen atom attached to the carbon chain, which leads to the formation of a cyclic ether with the elimination of a hydroxy free radical. 

Radicals with three substituents (ligands) of the radical carbon atom, i.e. alkyl radicals, cf. Tables 3.1.1-3.1.5,... 

Radicals with three substituents (ligands) of the radical carbon atom, i.e. alkyl radicals (cf. tables 3.1.1---3.1.5). These alkyl radicals are classified into subgroups of acyclic (—CHj, —CH—, -CO, monocyclic, and polycyclic alkyl radicals. Further subdivision of the subgroups arranges the alkyl radicals into families with equal neighbour groups of the radical carbon. This subdivision is seen in the tables of contents and should be self-explanatory. 

The treatment may be extended to deal with the energy of alkyl radicals if one assumes that the single unpaired elect on on the radicalized carbon atom occupies of 2p7T molecular orbital and does not change the interaction of other electrons on other orbitals. As mentioned earlier this assumption is in accord with ESR-investi-gations [4]. In electronic terms the dissociation eijergy D of a bond Ri — R2 can then be formulated as... 

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