Natural Colorants—Quinoid, Naphthoquinoid, and Anthraquinoid Dyes

In this chapter, natural dyes that contain a quinoid group are overviewed. The classification of the dyes has been made according to their basic chemical structure into benzoquinone dyes, naphthoquinone dyes, and anthraquinone dyes. Representative examples for such dyes are carthamine (CI Natural Red 26), which contains benzoquinone groups; juglone and lawsone, which are naphthoquinone dyes; and alizarin (1,2-dihydroxy-9,10-anthraquinone). A considerable number of these dyes have been used since historical times and are still in use for selected applications at present. Examples of such crops are green walnut shell (juglone), henna (lawsone), and madder (alizarin). Due to the high number of plant sources and the great variety of substances that are extracted as a complex mixture, representatives that are still of importance have been selected and will be discussed as examples for this class of natural dyes.