• 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 (Scheme 5).

  

  Scheme 5

• 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 (Scheme 6).

  

  Scheme 6

7.8 Nucleophilic Substitution

Aryl halides do not react with nucleophiles under the standard reaction conditions because the electron clouds of aryl ring repel the approach of a nucleophile. Nucleophiles can displace halide ions from aryl halides, if there are strong electron-withdrawing groups ortho or para to the halide. This class of reactions is called nucleophilic aromatic substitution reaction (Scheme 7).

Scheme 7

• Electron-withdrawing substituents such as nitro group make the ring reactive towards nucleophilic aromatic substitution but without at least one powerful electron-withdrawing group, the nucleophilic aromatic substitutions would be difficult. The mechanism of nucleophilic aromatic substitution cannot be the S_N2 mechanism because aryl halides cannot achieve the correct geometry for back-side approach of a nucleophile. The S_N1 mechanism also cannot be involved.
• Consider the reaction of 2,4-dinitrochlorobenzene with a nucleophile (Scheme 8). When a nucleophile attacks the carbon bearing the chlorine, a negatively charged sigma complex results. The negative charge is delocalized over the ortho and para carbons of the ring and further delocalized into the electron-withdrawing nitro groups. Loss of chloride from the sigma complex gives the nucleophilic substituted product.

  

  Scheme 8

• The leaving group ability of halogen in nucleophilic aromatic substitution reaction is following the order: F > Cl > Br > I. The incoming group should be a stronger base than the group that is being replaced (Scheme 9).

  

  Scheme 9