18.13 : Effetto orientante dei sostituenti: gruppi meta-orientanti

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.