Electrophilic Substitution Reactions of Substituted Benzenes

Halogens are deactivating groups, yet they are ortho, para-directors because the halogens are strongly electronegative, withdrawing electron density from a carbon atom through the σ-bond, and the halogens have nonbonding electrons that can donate electron density through π-bonding. If an electrophile reacts at the ortho or para position, the positive charge of the sigma complex is shared by the carbon atom bearing the halogen. The nonbonding electrons of the halogen can further delocalize the charge onto the halogen, giving a halonium ion structure. This resonance stabilization allows a halogen to be pi-donating, even though it is sigma-withdrawing.

• Reaction at the meta position gives a sigma complex whose positive charge is not delocalized onto the halogen-bearing carbon atom. Therefore, the meta intermediate is not stabilized by the halonium ion structure. Scheme 1 illustrates the preference for ortho and para substitution in the nitration of chlorobenzene.

  

• Ortho-para ratio differs with the size of the substituents. Nitration of toluene preferably gives ortho as the major product where the activating substituent is methyl group. Electrophilic substitution reaction of ethyl substituted benzene, however, gives ortho and para isomers equally. Bulky substituent such as tert-butyl benzene preferably gives para-isomer as the major product.

  

• Benzenes which are having a meta director (a deactivating group) on the ring, will be too unreactive to undergo either Friedel-Crafts alkylation or Friedel-Crafts Acylation.

  

• Aniline and N-substituted anilines also do not undergo Friedel-Crafts reactions because the lone pair on the amino group will form complex with the Lewis acid and converting the substituent into a deactivating meta director. Tertiary aromatic amines, however, can undergo electrophilic substitution because the tertiary amino group is a strong activator.

  

  • Phenols are highly reactive substrates for electrophilic aromatic substitution because of the presence of a strong activating group. So phenols can be alkylated or acylated using relatively weak Friedel-Crafts catalysts such as HF.

    

  • Phenoxide ions, generated by treating a phenol with sodium hydroxide, are even more reactive than phenols toward electrophilic aromatic substitution. It gives tribromosubstituted phenol when reacts with excess bromine and salicylic acid when reacts with carbon dioxide.