Effect of substituents on electrophilic aromatic substitution

Substituents already bonded to an aromatic ring influence both the rate of electrophilic substitution and the position of any further substitution. The effect of a particular substituent can be predicted by a consideration of the relative stability of the first-formed arenium cation, formation of which constitutes the rate-lintiting step. In general, substituents that are electron releasing activate the ring to further substitution - they help to stabilize the arenium ion. Substituents that are electron withdrawing destabilize the arenium ion, therefore, are deactivating and hinder further substitution. 

For the position of further substitution, we also need to consider resonance forms of the arenium ion. 

Halogens (a withdrawers, n donors) ortho and para directors ----F -------Cl --------Br -------1 

Stabilization of the arenium ion through electron-donating effects is typical of alkyl substituents. This is not strictly an inductive effect (see Section 4.3.3), 

Groups that are particularly strong electron releasers do not achieve this by an inductive effect, but they have heteroatoms with lone pair electrons that are able to stabilize resonance structures by transferring the charge to the heteroatom, i.e. an electron-releasing resonance effect. An amino group is typical of this type of substituent. 

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TABLE 15.2 EFFECT OF SUBSTITUENTS ON ELECTROPHILIC AROMATIC SUBSTITUTION ... 

Effect of Substituents on Electrophilic Aromatic Substitution Ortho-Para Directors Meta Directors... 

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