chapter  12
13 Pages

Nucleophilic aromatic substitution

BySir Richard Norman, J.M. Coxon

Although benzene is moderately reactive towards electrophiles, it is inert to nucleophiles. The example of nucleophilic substitution involves the displacement of hydride ion. This is a relatively high-energy species, and the reaction would be unfavourable but for the fact that an oxidizing agent, nitrobenzene itself, is present which removes the hydride ion. The six-membered nitrogen-containing heteroaromatic compounds are activated towards nucleophiles. The four halobenzenes are, like alkenyl halides, very inert to nucleophiles in normal conditions. Benzynes are too unstable to be isolated and react with any nucleophile which is present, e.g. when generated in liquid ammonia they react to give amines and when formed from organometallic compounds they react with second molecule of the organometallic compound. The fusion of aromatic sulfonates with caustic alkali at high temperatures gives phenols by displacement of sulfite ion. The Bucherer reaction is reversible, and by careful control of the relative amounts of ammonia and water it may be used to convert amines into phenols.