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.
18.13:
Directing Effect of Substituents: meta-Directing Groups
Substituents on the benzene ring that direct an incoming electrophile to undergo substitution at the meta position are called meta directors. All meta directors either have a 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 have resonance forms, the ortho and para intermediates have one unfavorable structure each due to repelling positive charges on adjacent atoms. However, adjacent atoms do not bear like charges in any of the resonance forms of the meta intermediates. Consequently, the ortho- and para-carbocation intermediates are less stable than the meta-carbocation intermediates. Therefore, substitution is preferred at the meta position.