By the entrance of amino or hydroxyl groups into the molecule dyestuffs are formed.
Benzo-orthoxazines, -metoxazines and -paroxazines are known: dibenzoparoxazine or phenoxazine is the parent of a valuable series of dyestuffs; dibenzoparathiazine or thiodiphenylamine is important from the same aspect.
Benzo-ortho-diazines exist in two structural forms, cinnolin and phthalazine; benzo-meta-diazines are known as quinazolines; benzo-para-diazines are termed quinoxalines; the dibenzo-compounds are named phenazines, this last group including many valuable dyestuffs - indulines, safranines, &c. In addition to the types of compounds enumerated above we may also notice purin, tropine and the terpenes.
At the same time, however, all dyestuffs are coloured substances.
The ortho-compounds condense to azimido benzenes, the metacompounds yield azo-dyestuffs, and the para-compounds yield bis-diazo compounds of the type XN2ï¿½C6H4ï¿½N2X.