15.5: Other Reactions of Aromatics

Nitrosation of aromatic amines

Aniline is a particularly important benzene derivative that can react with nitrous acid to produce a nitrosamine, followed by dehydration to give a diazonium salt. The reaction conditions are usually NaNO₂/H₂SO₄.

Diazonium salts are important synthetic intermediates because they are electrophilic – both at the aryl carbon atom and at the terminal nitrogen atom. For example, nucleophiles such as phenol react with diazonium salts to give deeply colored azo dyes.

However, diazonium salts also have nucleophilic aromatic substitution reactivity that occurs at the aryl C-N bond.

Some of these reactions have specific names:

1. Schiemann reaction – good way to make aryl fluorides

2. Sandmeyer reaction – good source of aryl chlorides, bromides, and nitriles

Other reactions of benzene derivaties:

Other reactions of aromatic rings that do not fit any category include:

a) Clemmensen reduction – converts an aromatic ketone into an alkyl group

b) Wolff-Kishner reduction – converts an aromatic ketone into an alkyl group

c) Oxidation – oxidizes benzylic methyl groups into carboxylic acids or methylenes into ketones

d) Birch reduction – reduces aromatic rings into 1,4-cyclohexadienes (electron-donating substituents will yield a product in which the substituent lies on a double bond, while an electron-withdrawing group will reduce so that the substituent is not directly attached to the double bond)

e) Reduction of a nitro group – reduces nitro groups to anilines

f) Acetylation of aniline – acetylated aniline will reduce the nucleophilicity of the aromatic ring so that reaction with an electrophile will only substitute once.