Delocalized Electrons Can Affect the Product of a Reaction

Our ability to correctly predict the product of an organic reaction often depends on recognizing when organic molecules have delocalized electrons. For example, the alkene in the following reaction has the same number of hydrogens on both of its carbons  

Therefore, the rule that tells us to add the electrophile to the s] carbon bonded to the most hydrogens predicts that approximately equal amounts of the two products will be formed. When the reaction is carried out, however, only one of the products is obtained. (Notice that the stability of the benzene ring prevents its double bonds from undergoing electrophilic addition reactions.) 

The rule leads us to an incorrect prediction of the reaction product because it does not take electron delocalization into consideration. It presumes that both carbocation intermediates are equally stable since they are both secondary carbocations. It does not take into account that one intermediate is a secondary alkyl carbocation, whereas the other is a secondary benzylic cation. Because the secondary benzylic cation is stabilized by electron delocalization, it is formed more readily. The difference in the rates of formation of the two carbocations is sufficient to cause only one product to be obtained. 

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For another example of how delocalized electrons can affect the product of a reaction, we will compare the products formed when isolated dienes (dienes that have only localized electrons) undergo electrophilic addition reactions to the products formed when conjugated dienes (dienes that have delocalized electrons) undergo the same reactions. 

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