Stability of the Allyl Radical Resonance Revisited

An orbital view of the allyl radical. The p orbital on the central carbon can overlap equally well with a p orbital on either neighboring carbon because the structure is electronically symmetrical. 

Since the allyl radical is electronically symmetrical, it can be drawn in either of two resonance forms—with the unpaired electron on the left and the double bond on the right, or with the unpaired electron on the right and the double bond on the left. Neither structure is correct by itself the true structure of the allyl radical is a resonance hybrid of the two. (You might want to review Sections 2.4-2.6 if you need to brush up on resonance.) As noted in Section 2.5, the greater the number of resonance forms, the greater the stability of a compound. An allyl radical, with two resonance forms, is therefore more stable than a typical alkyl radical, which has only a single structure. 

A spin surface of the ally radical locates the position of the unpaired electron and shows that It is equally shared between the two terminal carbons. 

In addition to its effect on stability, delocalization of the unpaired electron in the allyl radical has other chemical consequences. Since the unpaired electron is delocalized over both ends of the -n orbital system., reaction with Br2 can occur at either end. As a result, allylic bromination of an unsym-metrical alkene often leads to a mixture of products. For example, bromination of l-octene gives a mixture of 3-bromo-l-octene and 1 -bromo-2-, octene. The two products are not formed in equal amounts, however, because the intermediate allylic radical is not symmetrical and reaction at the two ends is not equally likely. Reaction at the less hindered, primary end 1b favored. 

The products of allylic bromination reactions are particularly useful for conversion into dienes by dehydrohalogenation with base. Cyclohexene can be converted into 1,3-cyclohexadiene, for example. 

We can thus expand the stability ordering to include vinylic and allylic radicals. 

Predicting the Product ofan Aiiyiic Bromination Reaction 

What products would you expect from reaction of 4,4-dimethylcyclohexene with NBS  

Draw the alkene reactant, and identify the allylic positions. In this case, there are two different allylic positions we ll label them A and B. Now abstract an allylic hydrogen 

Problem 10.5 Draw three resonance forms for the cyciohexadienyl radical. 

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