Sulfur trioxide, electrophilic aromatic substitution

In the preceding section, benzene reacted with cations to form substituted benzene derivatives. The cations of interest include Br+, C1+, the nitronium ion, and sulfur trioxide or sulfuric acid, which react as electrophiles. In principle, benzene may react with any cation, including carbocations, once that cation is formed. Carbocations are generated by several different methods they react with nucleophiles, as described for reactions of alkenes with acids such as HX (Chapter 10, Section 10.2) and for S l reactions (Chapter 11, Section 11.4). If benzene reacts with a carbocation, a new carbon-carbon bond is formed, and electrophilic aromatic substitution will give an arene. The reaction of benzene and its derivatives with carbocations is generically called the Friedel-Crafts reaction, after the work of French chemist Charles Friedel (France 1832-1899) and his American protege, James M. Crafts (1839-1917). The reaction takes two fundamental forms Friedel-Crafts alkylation and Friedel-Crafts acylation. Both variations will be discussed, beginning with the alkylation reaction. 

Aromatic sulfonation, like nitration, balogenation, alkylation, and acylation, is a typical electrophilic substitution reaction. Sulfonation, however, differs from these other reactions in two marked respects it is reversible, and reaction temperature can, in certain cases, have an important influence on the position of the entering group, as shown on p. 344. These characteristics have tended to complicate studies of the reaction mechanism and rate of sulfonation and to render difficult the drawing of general conclusions. Other factors having the same effect are the tendency of sulfur trioxide to form a complex with the sulfonic acids and the pronounced tendency of all Lubs, pp. 534ff. 

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