8.2 Withdrawal of electrons from a benzene ring by resonance

Notice the difference in the electron densities of the benzene rings in the electrostatic potential maps for anisole and nitrobenzene.

The substituent already attached to benzene determines the location of the new substituent. There are two possibilities - a substituent will direct an incoming substituent either to the ortho and para positions or will direct an incoming substituent to the meta position. All activating substituents and the weakly deactivating halogens are ortho/para directors, and all substituents that are more deactivating than the halogens are meta directors.

1. All activating substituents direct an incoming electrophile to the ortho and para positions.

2. The weakly deactivating halogens also direct an incoming electrophile to the ortho and para positions.

3. All moderately deactivating and strongly deactivating substituents direct an incoming electrophile to the meta position.

To understand why a substituent directs an incoming electrophile to a particular position, we must look at the stability of the carbocation intermediate formed in the rate-determining step. When a substituted benzene undergoes an electrophilic substitution reaction, three different carocation intermediates can be formed. Which one is more likely to be formed depends on their relative stabilities. Comparing the relative stabilities of the three carbocations allow us to determine the preferred pathway of the reaction because the more stable the carbocation, the less energy required to make it.

The structure of the carocation intermediates formed from the reaction of an electrophile with toluene at the ortho, meta, and para positions.

The structure of the carbocation intermediates formed from the reaction of an electrophile with anisole at the ortho, meta, and para positions.

The structures of the carbocation intermediates formed from the reaction of an electrophile with protonated aniline at the ortho, meta, and para positions.