Nonaromatic cyclohexadienyl carbocation

Experimental evidence indicates that electrophiles attack the it system of benzene to form a nonaromatic cyclohexadienyl carbocation known as an arenium ion. In showing this step, it is convenient to use Kekule structures, because these make it much easier to keep track of the it electrons ... 

This carbocation, called a cyclohexadienyl cation, arenium ion, or a-complex, then undergoes deprotonation to restore the aromaticity of the ring. If the Lewis base ( Y ) had acted as a nucleophile and bonded to carbon, the product would have been a nonaromatic cyclohexadiene derivative. Substitution occurs preferentially because a substantial driving force is present that favors rearomatization. 

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See also in sourсe #XX -- [ ]

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