Quinone(redirected from quinoid)
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any one of a class of cyclic diketones in which both carbonyl (C=O) groups are in a system of conjugated double bonds. Quinones are obtained in the form of colored crystals, which have a sharp odor. For example, para-benzoquinone (I), with a melting point of 116°C, and 1,4-naphthoquinone (II), with a melting point of 128.5°C, are yellow, while ortho-benzoquinone (III), with a melting point of 70°–80°C (with decomposition), and anthraquinone are red (seeANTHRAQUINONE).
Various pigments in nature are derivatives of quinones; for example, muscapharin, the dye substance of the fly agaric, is a derivative of para-benzoquinone. Quinones are readily reduced to yield diatomic phenols [for example, in industry, hydroquinone is obtained from (I) in this way]. They actively enter in diene synthesis,
as well as form molecular complexes with phenols. They are used in analytical chemistry.
Quinones are obtained by several methods, including the oxidation of the aromatic hydrocarbons or of their hydroxy or amino derivatives; for example, (I) is obtained by the oxidation of aniline, while (II) and anthraquinone are obtained by the oxidation of naphthalene and anthracene, respectively. Quinones and their derivatives are intermediate products in the production of dyes (seeANTHRAQUINONE DYES). They are also used as fungicides, insecticides, and tanning agents. 2-Methyl-1,4-naphthoquinone is a vitamin of the vitamin K group.