Anilinium hydrogen sulfate

Initially, an arylamine does not react as an activated aromatic compound with sulfuric acid but as a base instead anilinium hydrogen sulfates are produced (Figure 5.18). However, when the latter are heated, they decompose to give the starting materials reversibly. Only via the very small equilibrium amounts of free amine and free sulfuric acid does one observe the slow formation of the substitution product, an aromatic aminosulfonic acid. This aminosulfonic acid is a zwitterion, like an a-aminocarboxylic acid. 

Fig. 5.18. Sulfonation of aniline. Anilinium hydrogen sulfate, which is initially formed from the substrate and concentrated sulfuric acid, reacts via the minimal equilibrium amount of free aniline its amino groups directs the electrophile to the para-position.

Aniline is usually sulfonated by baking the salt, anilinium hydrogen sulfate, at 180-200 the chief product is the / -isomer. In this case we cannot discuss orientation on our usual basis of which isomer is formed faster. Sulfonation is... 

Ammonium salts contain a nitrogen atom with four bonds that has a positive charge. Four-bonded nitrogen atoms derived from amines are ammonium ions (if they re derived from aniline, they re anilinium ions). If the four bonds are all to carbon atoms, the nitrogen atom is quaternary. Salts contain a cation (named first) and an anion (named last). Typical anions include Cl (chloride), Bi" (bromide), HSO (hydrogen sulfate or bisulfate), and NOj-(nitrate). Figure 13-5 shows two examples of ammonium ions. 

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