Electrophilic Aromatic Substitution in Arylamines

Arylamines contain two functional groups, the amine group and the aromatic ring they are difunctional compounds. The reactivity of the amine group is affected by its aryl substituent, and the reactivity of the ring is affected by its amine substituent. The same electron delocalization that reduces the basicity and the nucleophilicity of an arylamine nitrogen increases the electron density in the aromatic ring and makes arylamines extremely reactive toward electrophilic aromatic substitution. 

The reactivity of arylamines was noted in Section 12.12, where it was pointed out that —rffl2 —NHR, and —NR2 are ortho, para-directing and exceedingly powerful activating groups. These substituents are such powerful activators that electrophilic aromatic substitution is only rarely performed directly on arylamines. 

Direct nitration of aniline and other arylamines, for example, is difhcult to carry out and is accompanied by oxidation that leads to the formation of dark-colored tars. As a solution to this problem it is standard practice to first protect the amino group by acylation with either acetyl chloride or acetic anhydride. 

Amide resonance within the A-acetyl group competes with delocalization of the nitrogen lone pair into the ring. 

Protecting the amino group of an arylamine in this way moderates its reactivity and permits nitration of the ring to be achieved. The acetamido group is activating toward electrophilic aromatic substitution and is ortho, para-directing. 

The net effect of the sequence protect-nitrate-deprotect is the same as if the substrate had been nitrated directly. Because direct nitration is impossible, however, the indirect route is the only practical method. 

Outline syntheses of each of the following from aniline and any necessary organic or inorganic reagents. 

Newman projections showing the conformations leading to (a) 1-butene, and (b) trans-2-butene by Hofmann elimination of sec-butyltrimethylammonium hydroxide. The major product is 1-butene. 

---