Electron-donating substituentsIn 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.
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.
Electron-withdrawing substituentsWhen you synthesize disubstituted benzenes, and the first added group is an electron-withdrawing substituent, this group usually directs incoming electrophiles to the meta position (shown in the first figure).
So, for benzenes substituted with electron-withdrawing groups, the cation resulting from meta substitution is more stable than the cation resulting from either ortho or para substitution.
The main point to remember here is that electron-donating groups direct substitution to the ortho and para positions, while pi electron-withdrawing groups direct substitution to the meta position.To clarify what is meant by electron-donating and electron-withdrawing substituents: Any substituent whose first atom (the one that's attached to the benzene ring) has a lone pair will be a pi electron donor to the phenyl ring, as shown in the resonance structure in the next figure.
The only exceptions are the halogens, which are not terribly good pi donors. They deactivate the ring as a result of being highly electronegative groups, pulling electrons away from the benzene ring toward themselves, making the ring less nucleophilic. But even though halogens are ring deactivators, they're still ortho-para directors.
Pi-withdrawing groups (such as NO2 groups, carbonyl groups, CN, and so on) pull electrons away from the ring and deactivate it, making the ring less nucleophilic. Pi electron-withdrawers are thus ring deactivators. A deactivator means that the reaction of benzenes substituted with these substituents will be slower than the reaction of unsubstituted benzene. Pi electron-withdrawing substituents are meta directors. The table outlines the nature of different substituents.
|Halogens (F, Cl, Br, I)
|—COR (COOH, COOR, CHO, and so on)