There exists azo-hydazone tautomerism for azo dyes

which consists of two concepts. Both azo form and hydrazone form are a pair of isomers

but exhibit different performance due to their structures. They can transform into each other under appropriate conditions. To confirm their isomeric structure is the base for a study of the relationship between the structure and properties. There are two misleading concepts prevailing in dyestuff industrial and academic circles. The former insists that azo form and hydrazone form co-exist with no difference between these two isomers

whilst the latter believes that they can transform into each other at any moment. An investigation into the tautomers transformation was conducted and the relationship between their structures and properties was discussed. Azo form and hydrazone form of an azo compoun possess different UV-Vs spectra

chemical properties and fastness. Usually it takes one of the tautomers as the stably-existed status

which determines its color and dyeing performance. Azo form or hydrazone form only can be transformed into each other under specified conditions

which depended on their structures and environmental conditions

such as temperature

solvents

pH of media

etc. The confirmation methods of these tautomeric structure by IR

Raman

NMR

MS spectra and X ray differential spectra were introduced.