Industrial Organic Pigments, 4th Edition

2Hydrazone Pigments (Formerly Called Azo Pigments)

Hydrazone pigments carry a hydrazone function (NHN) between two sp²-hybridized C atoms. Formerly, hydrazone pigments were called ‘azo pigments’, because they were believed to contain an azo group (NN).

However, in recent decades dozens of single-crystal structure analyses as well as spectroscopic investigations revealed that all commercial ‘azo pigments’ do actually not contain an azo group, but exist as hydrazones instead [1, 2]. Therefore, all commercial ‘azo pigments’ should be called ‘hydrazone pigments’ (Figure 2.1).

Figure 2.1 Molecular structure of Pigment Yellow 74 as determined ...