REACTIONS OF AROMATIC COMPOUNDS
When two or more substituents are present on an aromatic ring a combined effect is observed in subsequent reactions. In many cases it is easy to predict the effects of multiple substituent groups because the individual effects are mutually supporting of each other. In cases were there is a conflict in the directing effects of the substituent groups it can more difficult to predict what products will be produced.
When dealing with multiple substituents activating groups are generally stronger directors than deactivating groups.
1.Strong activating ortho, para-directors that stabilize the transition state through resonance. i.e. –OH, –OR
2.Activating ortho, para-directors. i.e. alkyl groups and halogens
3.Deactivating meta directors.
Nucleophilic substitution involves the attack of and electron rich group on the electron rich aromatic ring with subsequent lose of a leaving group and it’s electrons.
There are two mechanism that are seen with nucleophilic substitution reactions.
Addition-Elimination: requires strong electron withdrawing groups and a leaving group ortho or para to the electron withdrawing substituents.
Elimination-Addition: requires a good leaving group and the use of a very strong base or harsh reaction conditions(>200°C).