Pictures of the Day CH310M/CH318M Fall 2007

11-12-07
Allyl Cations and Radicals
Allyl cations are stabilized by resonance with the adjacent pi bond. The resonance distributes the positive charge (blue color) to the two carbons on the ends. Because the charge is spread out so well in an allyl cation, it is even more stable than the t-butyl cation, which is stabilized by hyperconjugation with three methyl groups. This is important for reactions in which carbocations are involved such as acid-catalyzed addition of water and H-X reactions with alkenes.
Allyl radicals are also stabilized by resonance with the adjacent pi bond. The resonance distributes the unpaired electron to the two carbons on the ends. In the above picture, the computer has calculated the location of the unpaired electron density, and the blue color indicates the highest concentration of unpaired electron density. Because the unpaired electron density is spread out so well in an allyl radical, it is even more stable than the t-butyl radical, which is stabilized by hyperconjugation with three methyl groups. This is important for free radical halogenation chein reactions. Because radicals adjacent to double bonds form the fastest of all, this type of reaction predominates in molecules with double bonds. The halogen ends up attaching to the carbon that gives the most stable alkene (most substituted) when there is a difference. This is referred to as allylic halogenation.