Typical Electron-Withdrawing Groups Are ortho, meta-Directors Rather than meta-Directors in Electrophilic Aromatic Substitution

Electron-withdrawing groups are traditionally considered meta-directing in aromatic substitution reactions. However, when the pre-existing substituent is a π-acceptor, both experiment and calculation indicate that substantial amounts of ortho as well as meta substitution occur, with very little para reactivity. A simple perturbative MO argument rationalizes this finding. It is therefore suggested that these substituents are best understood as ortho, meta-directors, with a preference for meta, just as electron-donating groups are considered ortho, para-directors, with a preference for para.