How Electron-Donating Substituents Direct the Substitution on a Benzene Ring
In the synthesis of disubstituted benzenes, the cation resulting from ortho-para addition with electron donors will be more stable than the meta-derived cation. For example, if you brominate anisole, as shown in the first figure, you get substitution of the bromine at the ortho and para positions, but not at the meta position. This is because methoxy groups (OCH3) are pi electron donors, so they direct all incoming electrophile traffic into the ortho and para positions.
You can see why the methoxy group directs to the ortho and para positions by looking at the intermediate carbocation for both the para substitution and the meta addition, shown in the next figure.
With para substitution (and with ortho substitution), a much more stable intermediate carbocation is formed compared to the cation that’s formed when the substituent adds in the meta position. The intermediate carbocation that results from para substitution has four resonance structures, as shown in the second figure, with one of these resonance structures being particularly good because all valence octets on all atoms are filled. (This good resonance structure is boxed in the figure.) The carbocation resulting from meta substitution, on the other hand, has only three resonance structures, none of which have all atoms with filled valence octets. Recall the general rule that stability increases as the number of resonance structures increases. Thus, with electron donors on the aromatic ring, ortho-para products are selectively formed.