18.13 : Directing Effect of Substituents: meta-Directing Groups

Substituents on the benzene ring that can orient an incoming group to a position meta to itself during electrophilic substitution are meta directors.

All meta-directing groups are polar with a positive or a partial positive charge on the atom immediately neighboring the ring. Additionally, such atoms have no lone pairs.

These substituents withdraw the ring electrons through inductive and resonance effects.

To better understand the directing effects of these substituents, consider the addition of an electrophile on nitrobenzene to give the resonance contributors arising from the ortho, meta and para attack.

Ortho and para intermediates include one unfavorable structure with repelling positive charges on the adjacent atoms.

In comparison, in the meta intermediates, the positive charges are well separated with no adjacent atoms bearing like charges.

Thus, ortho and para carbocation intermediates are less stable than meta, preferring the substitution primarily at the meta position.

Substituents on the benzene ring that direct an incoming electrophile to undergo substitution at themeta position are calledmetadirectors. All metadirectors either havea positive charge on the atom directly bonded to the ring or a partial positive charge. These groups function by withdrawing electrons from the ring through inductive and resonance effects. Consider the carbocation intermediates formed upon the addition of an electrophile on nitrobenzene at the ortho, meta, and para positions. While all three haveresonance forms,the ortho and para intermediates haveone unfavorable structure eachdue to repelling positive charges onadjacent atoms. However, adjacent atoms do not bear like charges in any of the resonance forms of themetaintermediates. Consequently, the ortho- and para-carbocation intermediates are less stable than themeta-carbocation intermediates. Therefore, substitution ispreferred at the meta position.

Tags

Meta directing Substituents Benzene Ring Electrophilic Substitution Meta directors Positive Charge Inductive Effects Resonance Effects Carbocation Intermediates Ortho Meta Para Repelling Positive Charges Meta carbocation Meta substitution

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