Conjugated unsaturated systems allyl radical

The new p orbital of the allyl radical is conjugated with those of the double bond => the allyl radical is a conjugated unsaturated system. 

Figure 13.3 The n molecular orbitals of the allyl cation. The allyl cation, like the allyl radical, is a conjugated unsaturated system. The shapes of molecular orbitals for the allyl cation calculated using quantum mechanical principles are shown alongside the schematic orbitals. 

Benzylic radicals and benzylic cations are conjugated unsaturated systems and hoth are unusually stable. They have approximately the same stabilities as allybc radicals (Section 10.8) and allylic cations (Section 13-4). This exceptional stability of benzylic radicals and cations can be explained by resonance theory. In the case of each entity, resonance structures can be written that place either the unpaired electron (in the case of the radical) or the positive charge (in the case of the cation) on an ortho or para carbon of the ring (see the following structures). Thus resonance delocalizes the unpaired electron or the charge, and this delocabzation causes the radical or cation to be highly stabibzed. 

The interesting behavior shown mainly by (V)-EPTM was not confirmed by terpolymers containing 5,6-dimethylene-2-norbomene (X)19,94 or 5-(2 -vinyl-3 -butenyl)-2-norbomene (XT), both being model compounds of triene (V). In fact, the first EPTM contains two conjugated double bonds in obligatory dsoid conformation which were found5 very reactive toward methyl free radicals. The second EPTM has one bis-allylic hydrogen atom in the unsaturated system ... 

The chemistries of the benzyiic and allylic positions are very similar. Intermediate carbocations, free radicals and carbanions formed at these positions are stabilized by delocalization with the adjacent ir system, the benzene ring in the case of the benzyiic position. Another aspect of arene chemistry is the enhanced stability of unsaturated arenes having double bonds conjugated with the benzene ring. This property is akin to the stability of conjugated di- and polyenes. 

Pertubation interactions between FMOs have been utilized to predetermine the regioselectivity of allylic and benzylic hydrogen-abstraction reactions. Thus, it is shown that, in unsaturated compounds containing two non-conjugated and non-equivalent r-systems, one system contributes mainly to the HOMO and the other to the HOMO-1. The important FMO interaction in these free-radical hydrogen-abstraction processes is SOMO-HOMO. Analysis of four representative examples corroborates empirical evidence and previously published studies/ ... 

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