any of a class of organic compounds of the heterocyclic series characterized by a ring structure containing four atoms of carbon and two of nitrogen. The pyrazine ring is part of many polycyclic compounds of biological or industrial significance. The simplest member of the pyrazine family is pyrazine itself, a colourless, water-soluble solid with molecular formula C₄H₄N₂. Pyrazine is seldom prepared. Its hexahydro derivative, piperazine, was first made in 1888; several of its derivatives find use as chemical raw materials.

Other members of the pyrazine family are pteridines, alloxazines, and phenazines. Certain pigments, first isolated from butterfly wings in 1891, are pteridines, as are the folic acids (folic acid), which participate in a variety of essential chemical reactions in the body. Riboflavin (vitamin B₂), a growth-promoting factor, is an alloxazine compound. Among the large group of phenazine dyes, the best known are Perkin's mauve (the first commercial synthetic dye, discovered in 1856); aniline black; and indanthrene (or indanthrone), the first of the anthraquinone vat dyes.