Diarylide pigment
Diarylide pigments are organic compounds that are used as pigments in inks and related materials. They often are yellow or yellow-green. To some extent, these organic compounds have displaced cadmium sulfide from the market. Being pigments, these compounds exist as powders of low solubility in water. They are similar to the simpler monoazo pigments called arylide yellows.
Production and properties
The formation of diarylide pigments involves the reaction of doubly diazotized aromatic diamines with acetoacetanilide. By varying both of these components, several useful pigments have been produced. A related family of organic pigments are the simpler arylides, which arise from the coupling of mono-diazonium salts with the same coupling partners.The pigments' colors can range from yellow to yellow-green. One common diarylide yellow pigment is Pigment yellow 12. From the selection of diarylide yellow pigments shown below one can appreciate the subtle changes in the substituents. Worldwide production of organic pigments was estimated to be about 250,000 metric tons in 2006, with about 25%, or 62,500 t, being diarylide yellows.
Being pigments, these compounds have very low solubility, especially in water. Single crystals for X-ray crystallography can however be grown from hot solutions in organic solvents. For example, crystals of Pigment Yellow 13, 14, and 63 were grown by slow cooling 200 °C solutions of the pigments in 1,2,4-trichlorobenzene. These vigorous conditions also indicate the considerable thermal stability enjoyed by these compounds. X-ray crystallography, spectroscopy, and electronic structure calculations consistently show that these compounds exist as the keto-hydrazone tautomers, even though these structures are often depicted as diazo compounds.