Hybrid orbitals, radical configuration

Ethylene has the well-known classical >2/1 structure with a barrier to rotation. The next in complexity of the simple hydrides is the methyl radical CH3. The obvious (sp2) planar arrangement can only accommodate six of the seven valence electrons. The electronic configuration of this molecule can therefore not be described in terms of either atomic wave functions or hybrid orbitals. An alternative approach is to view the structure of the methyl radical as a reduced-symmetry form, derived from the structure of methane, to be considered next. 

All the acyl radicals have nucleophilicities between that of a primary and a secondary alkyl radical i.e, the acetyl radical is more nucleophilic than the ethyl radical, but the benzoyl radical is much less nucleophilic than the benzyl radical. The different polarizability of these last two radicals, due to their different configuration, was considered an important factor of this behavior. An incipient positive charge in a transition state similar to a charge-transfer complex (27) can be stabilized in the benzyl radical 32) by the aromatic orbitals, but not in the benzoyl radical, in which the unpaired electron occupies a hybrid orbital (33). 

The ff-t5Tpe ethoxycarbonyl radical is on the contrary less nucleophilic than the acetyl radical (Table 29) in this Ccise the unpaired electron occupies a hybrid orbital and the incipient positive charge in the transition state cannot be stabilized by the lone-pair electron of the alkoxy group, as with the alkoxyalkyl radical, so that only the inductive effect is working and a clean reduction of nucleophilicity is observed. The remarkable fact is therefore that the same substituent, an a-alkoxy group, produces opposite polar effects depending on the electronic configuration of the carbon-centered radical. 

The possibility of orbital reorganization in ion-radicals should be taken into account. Thus, organic derivatives of three-valence phosphorus exist in the sp hybrid state and an electron is removed from one of these orbitals. The cation-radical formed can retain the initial orbital configuration, but can also convert into the sp d hybrid state. In the latter case, one additional orbital of the phosphorus atom becomes accessible for the reactant attack. If Y—H bond in the reactant is weak, the reaction R3P+ -f YH proceeds with the participation of an sp frontal orbital according to the radical mechanism. Thiols are typical reactants with the weak Y—H bonds. If anions (A ) react with RjP, the vacant sp d orbital of the phosphorus appears to be a target. Scheme 3.14 illustrates the orbital pictures and the reaction directions. 

What is the reason for the ordering in stability of alkyl radicals To answer this question, we need to inspect the alkyl radical structure more closely. Consider the structure of the methyl radical, formed by removal of a hydrogen atom from methane. Spectroscopic measurements have shown that the methyl radical, and probably other alkyl radicals, adopts a nearly planar configuration, best described by sp hybridization (Figure 3-2). The unpaired electron occupies the remaining p orbital perpendicular to the molecular plane. 

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