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Explain mechanism of first step of general electrophilic substitution reaction for benzene. |
Answer» Solution :In first step of ELECTROPHILIC reaction of BENZENE, the destruction of stable `pi`-electron cloud of benzene ring is occur which can form intermediate product.  Intermediate product is known as `SIGMA`-complex or arenium ION. Due to breaking of `sigma`- bond is carried out in this step and also for the formaiton of `sigma`- complex more energy is required and hence, this step is slower in nature. One carbon of `sigma`-complex is `sp^(3)` HYBRIDIZED, so it is not an aromatic compound, it is less stable although it has proven its presence. It is proven that resonance is possible in intermediate `sigma`-complex. A, B, C are the resonance structure of ` sigma`-complex and (D) is its hybrid form. Rate is slow for the formation of intermediate carbocation or `sigma`-complex or arenium ion and in this reaction bond is broken down and form electrophile, so this reaction mechainsm is of "Electrophilic" type. 
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