Allylic radical, molecular orbital stability

Stabilized allyl radical will be stabilized further if substituents are introduced. This stabilization occurs to different degrees in the ground state and the transition structure for rotation. In the ground state the substituent acts on a delocalized radical. Its influence on this state should be smaller than in the transition structure, where it acts on a localized radical. In the transition state the double bond and the atom with the unpaired electron are decoupled, i.e. in the simple Hiickel molecular orbital picture, the electron is localized in an orbital perpendicular to the jt(- c bond. 

In molecular orbital terms, the stability of the allyl radical is due to the fact that the unpaired electron is delocalized, or spread out, over an extended 7T orbital network rather than localized at only one site, as shown by the computer-generated MO in Fig 10.3. This delocalization is particularly apparent in the so-called spin density surface in Figure 10.4, which shows the calculated location, of the unpaired electron. The two terminal carbons share the unpaired electron equally. 

The Stability of the Allyl Radical 13.3A Molecular Orbital Description of the Allyl Radical... 

Conjugated compounds undergo a variety of reactions, many of which involve intermediates that retain some of the resonance stabilization of the conjugated system. Common intermediates include allylic systems, particularly allylic cations and radicals. Allylic cations and radicals are stabilized by delocalization. First, we consider some reactions involving allylic cations and radicals, then (Section 15-8) we derive the molecular orbital picture of their bonding. 

In one study, various substituted allyl radicals were generated by sulfenate photolysis, and it was shown that coupling was controlled both by steric and by frontier molecular orbital considerations [153]. p-Scission is favored by a-substitution and the stability of the putative alkyl radical. A particularly clever device used in another study was the thermal equilibrium between allylic sulfoxides and sulfenates used to generate allyloxy and other C3H5O radicals [154]. 

An explanation of the stability of the allyl radical can be approached in two ways in terms of molecular orbital theory and in terms of resonance theory (Section 1.8). As we shall see soon, both approaches give us equivalent descriptions of the allyl radical. The molecular orbital approach is easier to visualize, so we shall begin with it. (As preparation for this section, it may help the reader to review the molecular orbital theory given in Sections 1.11 and 1.13.)... 

Just as with allyl and benzyl cations and anions, allyl and benzyl radicals are stabilized by delocalization. The crucial molecular orbital, the singly occupied molecular orbital (SOMO), is essentially the same as the MO that is doubly occupied for the respective anions (see Figures 1.21 B and 1.22). 

Benzylic carbocations, radicals, and anions resemble their allylic counterparts in being conjugated systems stabilized by electron delocalization. This delocalization is describable in resonance, valence bond, and molecular orbital terms. 

In Summary Allylic radicals, cations, and anions are unusually stable. In Lewis terms, this stabilization is readily explained by electron delocalization. In a molecular-orbital description, the three interacting p orbitals form three new molecular orbitals One is considerably lower in energy than the p level, another one stays the same, and a third is higher in energy. Because only the first two are populated with electrons, the total it energy of the system is lowered. 

Any electron in the MO of an allyl system has the same energy as an electron in a 2p atomic orbital. Therefore, it makes no contribution to the net stability of the molecule, and it does not destabilize the n system either. That is why it is called a nonbonding orbital. Figure 11.8 shows the electron configurations of the n orbitals of the carbocation, radical, and carbanion. Figure 11.7b shows the molecular orbitals of the allyl carbocation. In this case, the nonbonding orbital,, is not occupied. 

This phenomenon, observed also for allyl- and benzylsilanes, > is attributed to hyperconjugative stabilization of the aminium radicals by interaction of the half-vacant nitrogen nonbonding p-orbital with the carbon-silicon o-orbital. a-Silyl substitution also causes a significant decrease in the oxidation potentials of ethers by as much as 0.9 This effect is strongly dependent on molecular geometry,... 

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