Electronic Configurations of the Allyl Radical, Cation, and Anion

Electronic Configurations of the Allyl Radical, Cation, and Anion 

Comparison of the electronic structure of the allyl radical with the allyl cation and the allyl anion. The allyl cation has no electron in it2, leaving half a poative charge on each of Cl and C3. The allyl anion has two electrons in it2, giving half a negative charge to each of Cl and C3. 

The molecular orbital representation shows the allyl anion with a pair of electrons in 7T2, a nonbonding orbital. This picture is also consistent with the resonance forms shown earlier, with a lone pair of nonbonding electrons evenly divided between C1 and C3. 

Allylic halides and tosylates show enhanced reactivity toward nucleophilic displacement reactions by the Sn2 mechanism. For example, allyl bromide reacts with nucleophiles by the 8 2 mechanism about 40 times faster than n-propyl bromide. 

Allylic delocalization in the Sis 2 transition state. The transition state for the Sn2 reaction of allyl bromide with a nucleophile is stabilized by conjugation of the double bond with the p orbital that is momentarily present on the reacting carbon atom. The resulting overlap lowers the energy of the transition state, increasins the reaction rate. 

---