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

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