Aniline and its derivatives undergo aromatic substitution reactions with great ease; frequently, it is difficult to stop such a reaction until all available ortho and para positions have been substituted. This reactivity results from the great stability that the electron pair on nitrogen imparts to an adjacent positive charge (Fig. 4-19.). The reactivity of aniline found early industrial use in the recovery of bromine from the ocean, when bromine was still a rare element. Sea water contains about 0.015 percent bromine in the form of bromide ion. The bromide was oxidized to bromine by the addition of chlorine and then aniline was added. Even at this low concentration, bromine forms the insoluble solid 2,4,6-tribromoaniline; this compound can be easily isolated, so that the bromine can be recovered from it. Although this process has now been supplanted by less expensive methods, it illustrates the great reactivity of the aromatic ring in aniline toward electrophilic reagents like bromine (Fig. 11-5).
Figure 11-5. Aniline is so reactive toward aromatic substitution that the reaction in some cases is difficult to stop before trisubstitution has occurred.
If aniline is treated with acetic anhydride, it is converted to acetanilide (Fig. 11-6). The substituent on the ring is now an acetamido group, and is still an ortho, para director, but less strongly activating, so that mono substitution can be carried out easily. The acetyl group can then be removed by hydrolysis if desired ("Carboxylic Acids").
Figure 11-6. Aniline is converted to acetanilide by reaction with acetic anhydride. Acetanilide is less reactive than aniline, so mono substitution products are easily obtained.
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