Azo Dyes — Textile Industry

Azo coupling reactions between diazonium salts and phenols/arylamines produce brightly coloured azo dyes (–N=N– chromophore). These are the largest class of synthetic dyes worldwide, used in textiles, leather, and food colouring. Example: Congo Red, Methyl Orange, Sunset Yellow.

SMILES of Methyl Orange azo group region: CN(C)c1ccc(/N=N/c2ccc(S(=O)(=O)[O-])cc2)cc1 (simplified)

Pharmaceuticals

• Sulfa drugs (sulfonamide antibiotics) are synthesised via diazotization of p-aminobenzenesulfonamide intermediates
• Procaine (local anaesthetic) contains a para-amino group on an aromatic ring, derived from reduction of nitro precursors
• Serotonin, dopamine, adrenaline — biogenic amines are neurotransmitters; understanding amine chemistry underpins pharmacology

Dye Intermediates

• Aniline (Nc1ccccc1) is one of the world's most important industrial chemicals; produced by reduction of nitrobenzene; used to make rubber chemicals, dyes, polyurethanes
• Indigo (blue jeans dye) synthesis involves aniline derivatives

Agricultural Chemistry

• Many herbicides and pesticides contain amine functional groups (e.g., alachlor, atrazine) — their water solubility and biological activity are related to the amine basicity

Diagnostics

• The ninhydrin test (detects primary amines and amino acids) is used in forensic fingerprint detection and amino acid analysis

Explosives

• TNT (trinitrotoluene) is made from toluene; reducing one –$NO_{2}$ gives an amine intermediate used in dye synthesis
• PETN, RDX contain –$NO_{2}$ groups that can be reduced to amines under controlled conditions