Some examples of aromatic nitration

The introduction of one of the halogens onto the aromatic ring is an important synthetic procedure. Chlorine and bromine are reactive toward aromatic hydrocarbons, although catalysts are often needed to achieve desirable rates. Fluorine reacts too violently to be controlled. Iodine can effect substitution of only very reactive aromatic compounds, but synthetic methods involving oxidants for direct introduction of iodine into less reactive molecules have been developed. 

Chlorination often exhibits second-order kinetics in acetic acid solution. In nonpolar solvents, the reaction is catalyzed by such added reagents as hydrogen chloride and trifluoroacetic acid, ° and exhibits complex kinetics. This rate behavior has been interpreted in terms of acid-catalyzed cleavage of the Cl-Cl bond in a chlorine-substrate complex. Chlorination is much more rapid in polar than in nonpolar solvents. Bromination also tends to exhibit complex kinetics in acetic acid 

For preparative reactions, the catalytic effect of Lewis acids is often utilized. Zinc chloride or ferric chloride are used in chlorinations, and metallic iron, which 

Halogenations are also strongly catalyzed by certain other metal ions. A well-studied case is catalysis by mercuric ion. In solutions of halogen and mercuric carboxylate salts, the dominant halogenating agents is the acyl hypohalite. The 

Friedel-Crafts reactions are important methods for introducing carbon substituents on aromatic rings. The reactive intermediates are electrophilic carbon species. In some reactions, discrete carbonium ions or acylium ions are involved in other cases, however, the electrophile no doubt consists of the alkyl or acyl group still bonded to a potential leaving group, which is displaced in the substitution step. Whether discrete carbonium ions are involved depends primarily on the stability of the potential carbonium ion. 

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