Reactions of Aromatic Compounds - Examples of Ar-SE Reaction
Nitration of Aromatic Compounds
The nitration of benzene is one of the most thoroughly investigated electrophilic aromatic substitutions. The nitrating agent is the nitronium ion ().
is generated when water is eliminated from protonated nitric acid. Pure nitric acid contains only a small amount of . As a result, nitration with pure nitric acid proceeds only very slowly. However, strongly activated aromatic compounds may also be nitrated with nitric acid in a reasonable period of time. Nevertheless, a mixture of concentrated nitric acid and concentrated sulfuric acid, called nitrating acid, is usually applied to the nitration of aromatic compounds. Sulfuric acid is a stronger acid than nitric acid. Thus, protonation of nitric acid by sulfuric acid leads to a larger amount of in nitrating acid than is obtainable in pure nitric acid. Other strong acids, such as and , also promote nitration with nitric acid.
Due to its strong -M and -I effect, the nitro group is a strongly deactivating substituent. The introduction of a second and, in particular, a third nitro group through electrophilic aromatic substitution therefore requires extremely drastic reaction conditions, such as the application of hot nitrating acid or a mixture of white fuming nitric acid and concentrated sulfuric acid. The direct application of the nitronium ion (), which is availabe as nitronium tetrafluoroborate (), is yet more effective in nitration.
The nitration of aromatic compounds with a subsequent reduction of the nitro group yields the corresponding aminobenzene derivatives.
The direct introduction of an amino group to aromatic compounds through electrophilic aromatic substitution is impossible.
Aminobenzene derivatives are important starting products for the synthesis of diazonium salts, as diazonium salts can be converted into a variety of aromatic compounds (e.g. Sandmeyer reaction and Schiemann reaction) that are not easily available by other synthetic pathways.