Resonance structures three atom “allyl” system

Remember that no resonance form has an independent existence A compound has characteristics of all its resonance forms at the same time, but it does not resonate among them. The p orbitals of all three carbon atoms must be parallel to have simultaneous pi bonding overlap between Cl and C2 and between C2 and C3. The geometric structure of the allyl system is shown in Figure 15-10. The allyl cation, the allyl radical, and the allyl anion all have this same geometric structure, differing only in the number of pi electrons. 

In allylic systems, favorable overlap of the p orbitals of the n system should require a coplanar arrangement of the three sp2 carbons and their five substituent atoms evidence that such a structure is indeed preferred comes, for example, from proton magnetic resonance observations that demonstrate barriers to bond rotation in the isomeric dimethylallyl ions 21, 22, and 23. These ions form stereo-specifically from the three dimethylcyclopropyl chlorides (Section 12.2), and barriers to rotation about the partial double bonds are sufficiently high to prevent interconversion at low temperature. At — 10°C, 21, the least stable isomer,... 

Allyl radical (Sections 10.8Aand 13.3) The radical formally related to propene by removal of a hydrogen atom from its methyl group. The two contributing resonance structures of the delocalized radical each include an unpaired electron on a carbon adjacent to the double bond, such that a p orbital on each of the three carbons overlaps to delocalize the radical to each end of the allyl system, in which the radical carbon is adjacent to a carbon-carbon double bond. 

Like an allylic carbocation, an allylic radical is resonance stabilized. However, in contrast to an allylic carbocation, which has two electrons in a delocalized 7i system, an allylic radical has three 7t electrons, one of which is unpaired. The unpaired electron is delocalized with an equal probability of being at either C-1 or C-3. The central carbon atom has no radical character. Lewis structures for the allylic... 

The eight atoms of the allyl radical lie in a plane, and all bond angles are approximately 120°. Each carbon atom is sp hybridized, and the three 2p orbitals participating in resonance delocalization of the radical are parallel to one another as shown in Figure 8.5. Like charged systems, in which a delocalized charge is more stable than a localized one, delocalized unpaired electron density leads to more stable structures than localized unpaired electron density. 

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