Quinones are a class of organic compounds consisting of a six-carbon cyclic dione structure containing two double-bonds, such as p-benzoquinone, o-benzoquinone. Having the properties of open-chain dicarboxylic ketones, quinones can take part in addition reactions and reduction reactions. But they lack the properties of the aromatic compounds. Quinoid structures and their color are closely related, and therefore most of the quinone compounds are colored substances. Quinones also include naphthoquinone , phenanthrenequinone and anthraquinones in a broad way.
Quinones is a class of compounds comprising quinones and compounds easily converted into structures with characteristics of quinones, as well as compounds closely related with the quinones in biosynthesis, often found in nature as pigment in animals, plants, microbes. The main active ingredients of many important traditional Chinese herbs such as rhubarb, cassia, senna, comfrey, giant knotweed, Polygonum, aloe vera have quinone compounds.
According to their different chemical structures, quinone compounds can be divided into benzene quinone , naphthalene quinones , anthraquinones and quinones. These quinones transfer electrons in redox reactions. And some have anti-bacterial or anti-tumor activity, such as juglone, 5-Hydroxy-2-Methyl-1,4-Naphthoquinon, lapachone. And some are the main active ingredients of Chinese herbal medicines, such as shikonin and isoalkannin class in puccoon which have the hemostatic, anti-inflammatory, anti-viral and anti-tumor activity.
If there is no phenolic hydroxyl group in the molecule, quinones are nearly colorless. With auxochromes such as phenolic hydroxyl groups, quinones will show a certain color such as yellow, orange, reddish brown, purple and so on. The more auxochromes there are, the deeper the color is. The color of anthracene derivatives ranges mostly from yellow to orange-red. Natural quinones are mostly colored crystals. Benzoquinones and naphthoquinones are found mostly in free form; and anthraquinones exist in binding form as glycosides in traditional Chinese medicine, the most of which are difficult to obtain good crystallinity. Most of free anthracenes and quinone derivatives are sublimable. Small-molecule benzoquinones and naphthoquinones are volatile and distillable along with the water vapor. Free anthracenes and quinone derivatives can be dissolved in methanol, ethanol, ethyl acetate, ethyl ether, benzene, chloroform and other organic solvents, slightly soluble or insoluble in water. After combined into glycosides, with the increased polarity, quinones can be easily dissolved in methanol, ethanol, and soluble in hot water, but slightly soluble in cold water, and almost insoluble in benzene, ether, chloroform and other non-polar organic solvents.