An intramolecular Mannich type reaction of ortho-amino aromatic azo dye and its detection effect for formaldehyde

A series of aromatic primary amine dyes were designed and applied for the modification of silk fibmin with Mannich-type reaction in the previous studies. Most aromatic primary amine dyes achieved the expected results with the participation of formaldehyde. However, those azo dyes with ortho-amino aromatic structure lost their colour and did not have any modification on silk fibmin under the same conditions of treatment. Therefore, the decolorization phenomenon of ortho-aniline aromatic azo dyes was analyzed using FT-IR, ESI-MS, NMR and Gaussian computation and the results indicated that an intramolecular Mannich reaction occurred between ortho-amino aromatic azo dyes and formaldehyde. In addition, a secondary effect of reduction of dyes by formaldehyde took place, in which the conjugated chromogenic system of the dye molecules was destroyed, producing benzimidazole, o-phenylenediamine and nitrogen-containing heterocyclic substances with polymethyl substitution. This decolorization of dye was studied as a detection or monitoring method for formaldehyde, and it showed advantages of sensitive response, clear visual indication, and no volatilization of organic solvent.