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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 (NO2+).

Fig.1
Nitration of benzene.

NO2+ is generated when water is eliminated from protonated nitric acid. Pure nitric acid contains only a small amount of NO2+. 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 NO2+ in nitrating acid than is obtainable in pure nitric acid. Other strong acids, such as HF and BF3, also promote nitration with nitric acid.

Fig.2
Generation of NO2+ by protonation of 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 (NO2+), which is availabe as nitronium tetrafluoroborate (NO2+BF4), is yet more effective in nitration.

The nitration of aromatic compounds with a subsequent reduction of the nitro group yields the corresponding aminobenzene derivatives.

Fig.3
Nitration and the subsequent nitro group's reduction yields aminobenzene.

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.

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