18.17: 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.