Mechanism
While steric effects are a consideration, the major contribution of EWGs is achieved by utilizing the nature of conjugated systems (specifically the ease through which mesomeric effects travel through such systems) to create regions of positive charge within the resonance contributors. For example, in nitrobenzene the resonance structures have positive charges around the ring system:
The resulting resonance hybrid, now possessing δ+ charges in the ortho- and para- positions repels approaching electrophiles increasing the relative success of attack in the meta position.
The selectivities observed with EDGs and EWGs were first described in 1892 and have been known as the Crum Brown-Gibson Rule.
Read more about this topic: Deactivating Groups
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