Monoazo (Monohydrazone) pigments based on acetoacetanilides

The monoazoacetoacetanilide series of pigments, traditionally known as Hansa Yellows, are long-established products that entered the market in the early twentieth century. They are mostly inexpensive products offering bright colors of moderate intensity covering the entire yellow area of the spectrum, good lightfastness, but inferior solvent resistance. The technical properties of the pigments may be explained by their molecular structures, which adopt the ketohydrazone tautomeric form, and their crystal structures. Their good lightfastness is attributed mainly to intramolecular hydrogen-bonding, while their generally inferior fastness to organic solvents is explained by the relatively weak intermolecular interactions in the crystal structure. The monoazoacetanilide pigments are synthesized by the traditional two-stage process of diazotization of a primary aromatic amine, followed by an azo coupling reaction of the resulting diazonium salt with an acetoacetanilide coupling component. Their main use is in decorative paints, although a few products are suitable for printing inks.