the picture below): Attack occurs at the ortho and para positions, which have (partial) formal negative charges that indicate π electron excess at these positions. Another common argument, which makes identical predictions, considers the stabilization or destabilization by substituents of the Wheland intermediates resulting from electrophilic attack at the ortho/para or meta positions. The Hammond postulate then dictates that the relative transition state energies will reflect the differences in the ground state energies of the Wheland intermediates. (See electrophilic aromatic substitution for details of this argument.) The selectivities observed with EDGs and EWGs were first described in 1892 and have been known as the Crum Brown–Gibson rule. See also Electrophilic aromatic substitution References Functional groups