Electrophilic Substitution of Benzene Derivatives | Organic Chemistry 2

The electrophilic attacks of benzene derivatives are studied in this chapter: electrophilic substitution of substituted benzenes, activating and deactivating substituents, meta- ortho- para- directing substituents, synthesis strategies

Electrophilic Substitution of Substituted Benzenes

Inductive effect and resonance are the 2 main effects explaining the donor or acceptor character of a substituent. The substituents of benzene can be grouped into:

- activating group (electron donors) ⇒ electrophilic attack occurs at the ortho and para positions of the substituent
- deactivating group (electron acceptors) ⇒ electrophilic attack occurs to the meta positions of the substituent

 

Activated Benzene:

 

D ⇒ Donor:
- NH2, OH (strongly activating)
- NHCOR, OR (moderately activating)
- alkyl, aryl (weakly activating)


Selectivity:

The position of attack is controlled by the strongest activating substituent and then by steric effects. Because of the sterics, the para product will be preferred over ortho product

 

Deactivated Benzene:

 

A ⇒ Acceptor:
- N+R3, NO2, CF3, CN, SO3H (strongly deactivating)
- COH, COR, COOH, COOR, CONH2 (moderately deactivating)

 

Deactivating groups are generally meta directing, with the exception of halides which are deactivating but ortho/para directing

Synthesis Strategies

Change the position of the electrophilic attack:
 

 

Lock / Unlock a position on benzene:
 


 

 

Moderate the donor character of a substituent: