Role of sigma and pi complexes aromatic Electrophilic substitution

Answer ::

The formation of the sigma complex in electrophilic aromatic substitution has a higheractivation energythan the formation of a carbocation in electrophilic addition to analkene. Therefore, the rates of electrophilic aromatic substitution reactions are slower than the rates ofelectrophilic addition reactionsto alkenes for the same electrophile. For example,brominereacts instantly with alkenes, butdoes not reactat all with benzene except in the presence of a strong Lewis acid catalyst.

In the faster second step of theelectrophilic substitutionmechanism, the proton bound to the sp3-hybridized ring carbon atom leaves, restoring the aromatic π system. A nucleophile, acting as a base, extracts the leaving proton.

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