Electrophilic Attack on Derivatives of Benzene | Organic Chemistry 2
Electrophilic Substitution of Substituted Benzenes
Substituents on benzene can be grouped into:
- activators (electron donors) ⇒ second electrophilic attack to the ortho and para positions
- deactivators (electron acceptors) ⇒ second electrophilic attack to the meta positions
Induction and Resonance = 2 main effects for the activation or the deactivation
Activated Benzene:
D ⇒ Donor:
- NH2, OH (strongly activating)
- NHCOR, OR (moderately activating)
- alkyl, aryl (weakly activating)
- halogen
Deactivated Benzene:
A ⇒ Acceptor:
- N+R3, NO2, CF3, CN, SO3H (strongly deactivating)
- COH, COR, COOH, COOR, CONH2 (moderately deactivating)
Selectivity:
- the position of attack is controlled by the strongest activator (and then by steric effects).
- para product is preferred over ortho product because of steric effects.
Synthetic Strategies
Changing the sense of the directing power of substituents:
Blocking / Deblocking a position:
Moderation of Strong Activators ⇒ change the selectivity of the Electrophilic Attack: