Benzyl group orbitals

The benzyl group acts in much the same way using the n system of the benzene ring for conjugation with the p orbital in the transition state. 

In addition to steric effects, there are other important substituent effects that influence both the rate and mechanism of nucleophilic substitution reactions. As we discussed on p. 302, the benzylic and allylic cations are stabilized by electron delocalization. It is therefore easy to understand why substitution reactions of the ionization type proceed more rapidly in these systems than in alkyl systems. Direct displacement reactions also take place particularly rapidly in benzylic and allylic systems for example, allyl chloride is 33 times more reactive than ethyl chloride toward iodide ion in acetone." These enhanced rates reflect stabilization of the Sjv2 TS through overlap of the /2-type orbital that develops at carbon." The tt systems of the allylic and benzylic groups provide extended conjugation. This conjugation can stabilize the TS, whether the substitution site has carbocation character and is electron poor or is electron rich as a result of a concerted Sjv2 mechanism. 

The ESR spectra of 63 and 65 do not provide information on whether or not the corresponding radical species carry an even number of benzyl groups attached elsewhere on the surface of the Cgg molecule. Looking at the high reactivity of radicals towards Cgo> it is very unlikely that only single radical adducts, such as RgCgo or RgCgg > are responsible for the observed ESR spectra. But very important information about the electronic and chemical properties of a coran-nulene substructure are extractable from these experiments. The orbitals with a... 

The allylic and benzylic groups provide even more delocalization due to further resonance stabilization. At the top end of the table are the relatively unstable phenyl and vinyl groups. Why are they so unstable In these two systems the unpaired electron is in an sp orbital that is perpendicular to the C—H bonding orbitals in neighboring CH groups. This decreases the opportunity for effective orbital interaction and delocalization. 

Group Orbitals of Representative tt Systems Benzene, Benzyl, and Allyl... 

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