Additivity of Inductive Substituent Effects

The effects of multiple substituents in a benzene ring are additive leading to a global a value of Eo, and the reactivity can be expressed as in Equation (5). 

Additivity is only valid for meta and para substituents (X) in structure 1 and also for substituents X in different benzene rings equally disposed in structure 2 relative to the reaction centre Y The combined o value for I is (ctx, + + Qx,) and that for (2) is (ox, + + x, + + x, + x, 

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