Aromatic Amines, Azo Compounds, and Dyes

Compounds with an amino group -NH₂ attached to a benzene ring are aromatic amines, with aniline as the representative example. The azo compounds synthesized from them are central to synthetic dyes.

Basic Knowledge

Aniline C₆H₅-NH₂: colorless oily liquid (turns brown in air), distinctive odor, weak base (K_b≈10^-10, weaker base than ammonia).
Synthesis: reduction of nitrobenzene C6H5-NO2 + 6[H] → C6H5-NH2 + 2H2O (Sn+HCl or Fe+HCl, followed by basification with NaOH to liberate aniline).
Main reactions:
① Salt formation: C6H5NH2 + HCl → C6H5NH3Cl (aniline hydrochloride, water-soluble)
② Acetylation: C6H5NH2 + (CH3CO)2O → C6H5-NH-COCH3 + CH3COOH (acetanilide, a historical antipyretic)
③ Oxidation with bleaching powder: aniline + bleaching powder (CaCl(ClO)) → reddish purple color (aniline detection)
④ Diazotization: C6H5NH2 + NaNO2 + 2HCl → C6H5-N2Cl + NaCl + 2H2O (at a low temperature of 5℃ or below, producing benzenediazonium chloride)
⑤ Diazo coupling: diazonium salt + phenoxide/aromatic amine → azo compound Ar-N=N-Ar' (the central structure of dyes). Example: C6H5-N2+ + C6H5-ONa → p-HO-C6H4-N=N-C6H5 + NaCl (p-hydroxyazobenzene, orange-red).
Main aromatic amines: aniline, p-aminobenzoic acid (PABA, UV filter and component of folate), p-aminophenol (photographic development), and sulfanilamide (sulfa drug, the first synthetic antibiotic).

Further Study (Principles and Applications)

Perkin's discovery of the synthetic dye mauve in 1856 happened accidentally from aniline and became the starting point of the modern chemical industry. Until then, dyes depended on natural sources such as indigo, madder, and cochineal; the emergence of synthetic dyes greatly advanced the textile industry. Azo dyes are the most widely used dye category, and their colors can be adjusted from orange-yellow to red to purple by changing the -N=N- bond and the electronic states of the rings.
Sulfa drugs: In 1932, Domagk discovered that Prontosil (an azo dye) suppressed bacterial infections, and sulfanilamide was later identified as the active substance. It inhibits folic acid synthesis and suppresses bacterial growth. As the only effective antibacterial treatment before penicillin, it greatly reduced wound infections during World War II and led to the 1939 Nobel Prize.
Aniline metabolism: Pathways include N-acetylation in the liver (N-acetylation genetic polymorphism), aromatic hydroxylation, and glucuronidation. Aniline derivatives can cause methemoglobinemia (Fe²⁺ → Fe³⁺, impairing O₂ transport) and are important in toxicology. Historically, an increase in bladder cancer among dye-industry workers (for example, due to 2-naphthylamine) is well known.

Points of Caution (Common Confusions and Frequent Errors)

① Diazotization must be carried out at 5℃ or below; at higher temperatures, the compound decomposes after becoming phenol. ② Aniline is a weak base and is slightly soluble in water, but its hydrochloride is water-soluble. ③ Acetanilide, produced by acetylation of aniline, was used as an antipyretic until the end of the 19th century, but was replaced by acetaminophen (Tylenol) because of liver toxicity (p-hydroxyacetanilide). ④ Do not confuse aniline + bleaching powder = reddish purple (aniline detection) with phenol + FeCl₃ = violet (phenol detection).

Practice

1. Write the complete process for synthesizing aniline from nitrobenzene, including equations and reagents.
2. Write the synthesis route from aniline hydrochloride to benzenediazonium chloride and then to p-hydroxyazobenzene.
3. Describe the characteristics of the bleaching-powder reaction used to detect aniline.