• All activating substituents and weakly deactivating halogens are ortho-para directors, and all substituents that are more deactivating are meta directors. When substituted benzene undergoes an electrophilic substitution reaction, an ortho-substituted carbocation, a meta-substituted carbocation, and a para-substituted carbocation can be formed. The relative stabilities of the three carbocations determine the preferred pathway of the reaction.
  
  
• The methoxy substituent (an activating group), for example, donates electron into the ring and stabilize the ortho- and para-substituted carbocations as shown.  Therefore, the most stable carbocation is obtained by directing the incoming group to the ortho and para positions. Thus, any substituent that donates electrons is an ortho-para director.

• In contrast, the ammonuium ion substituent (a deactivating group), for example, withdraws electron from the ring and destabilize the ortho- and para-substituted carbocations as shown.  Therefore, the most stable carbocation is obtained by directing the incoming group to the meta position. Thus, any substituent that withdraws electrons is a meta director.

  

• In the following examples, the methoxy group and ethyl group are activating substituents which preferably direct the incoming electrophile to ortho and para position. These substituted benzenes undergo electrophilic aromatic substitution faster than benzene.

  

• A methoxy group is so strongly activating group so that anisole quickly brominates in water without a catalyst. In the presence of excess bromine, this reaction proceeds to give the tribromide substituted product.

  

Text Books

• C. M. Loudon, Organic Chemistry, Oxford University Press, 2002, New Delhi.
• R. R. Morrison, R. N. Boyd and S. K. Bhattacharjee, Organic Chemistry, Dorling Kindersley (India) Pvt. Ltd, 2011, New Delhi.