Electrophilic aromatic substitution, acylation nitration, mechanism

Predict products and propose mechanisms for the common electrophilic aromatic substitutions halogenation, nitration, sulfonation, and Friedel-Crafts alkylation and acylation. Problems 17-44, 47, 48, 51, 59, G4, and 70... 

Aromatic compounds react mainly by electrophilic aromatic substitution, in which one or more ring hydrogens are replaced by various electrophiles. Typical reactions are chlorination, bromination, nitration, sulfonation, alkylation, and acylation (the last two are Friedel-Crafts reactions). The mechanism involves two steps addition of the electrophile to a ring carbon, to produce an intermediate benzenonium ion, followed by proton loss to again achieve the (now substituted) aromatic system. 

Benzene s aromaticity causes it to undergo electrophilic aromatic substitution reactions. The electrophilic addition reactions characteristic of alkenes and dienes would lead to much less stable nonaromatic addition products. The most common electrophilic aromatic substitution reactions are halogenation, nitration, sulfonation, and Friedel-Crafts acylation and alkylation. Once the electrophile is generated, all electrophilic aromatic substitution reactions take place by the same two-step mechanism (1) The aromatic compound reacts with an electrophile, forming a carbocation intermediate and (2) a base pulls off a proton from the carbon that... 

Isotope effects are also useful in providing insight into other aspects of the mechanisms of individual electrophilic aromatic substitution processes. In particular, since primary isotope effects are expected only when the breakdown of the rate-determining, the observation of a substantial kn/ko points to rate-determining deprotonation. Some typical isotope effects are summarized in Table 9.7. While isotope effects are rarely observed for nitration and halogenation, Friedel-Crafts acylation, sulfonation, nitrosation, and diazo coupling provide examples in which the rate of proton abstraction can control the rate of substitution. 

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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