Лако-красочные материалы — производство

Almost without exception, azo dyes are made by diazotization of a primary aro­matic amine followed by coupling of the resultant diazonium salt with an elec­tron-rich nucleophile. The diazotization reaction is carried out by treating the pri­mary aromatic amine with nitrous acid, normally generated in situ with hydro­chloric acid and sodium nitrite. The nitrous acid nitrosates the amine to generate the N-nitroso compound, which tautomerises to the diazo hydroxide [Eq. (1)].

H

Ar—N ^ Ar—N (1)

N=0 " N-OH

Protonation of the hydroxy group followed by the elimination of water gener­ates the resonance-stabilised diazonium salt [Eq. (2)].

Ar—N

•Л

N

For weakly basic amines, i. e., those containing several electron-withdrawing groups, nitrosyl sulfuric acid (NO+HSO 4-) is used as the nitrosating agent in sul­furic acid, which may be mixed with phosphoric, acetic, or propionic acid.

A diazonium salt is a weak electrophile and hence reacts only with highly electron rich species such as amino and hydroxy compounds. Even hydroxy com­pounds must be ionized for reaction to occur. Consequently, hydroxy compounds such as phenols and naphthols are coupled in an alkaline medium (pH >p^a of phenol or naphthol; typically pH 7-11), whereas aromatic amines such as N, N — dialkylamines are coupled in a slightly acid medium, typically pH 1-5. This pro­vides optimum stability for the diazonium salt (stable in acid) without deactivat­ing the nucleophile (protonation of the amine).