quinone

Quinone is a class of organic compounds characterized by a fully conjugated cyclic dione structure, typically derived from the oxidation of hydroquinones. The parent compound is benzoquinone, specifically 1,4-benzoquinone (p-benzoquinone). Quinones span a range of fused and singly substituted systems, including naphthoquinones (two fused rings with carbonyls at positions 1 and 4) and anthraquinones (three rings with two carbonyl groups), as well as various polycyclic derivatives.

Common examples include p-benzoquinone, 1,4-naphthoquinone, juglone (5-hydroxy-1,4-naphthoquinone), anthraquinone, and ubiquinone (coenzyme Q). Ubiquinone is a redox-active quinone with a long isoprenoid side chain that participates in mitochondrial electron transport; phylloquinone (vitamin K1) and menaquinone (vitamin K2) are naphthoquinones essential for blood clotting.

Chemical properties and reactivity: Quinones are electrophilic at their carbonyl carbons and function as electron acceptors. They readily undergo two-electron redox processes, interconverting with their corresponding hydroquinones in biological and chemical redox cycles. This redox behavior underpins their roles in respiration and photosynthesis, as well as in dye chemistry and polymer redox systems. Quinones can participate in nucleophilic additions to the carbonyls and in various cycloaddition reactions typical of conjugated diones.

Occurrence and uses: In biology, quinones such as ubiquinone and vitamin K family members participate in electron transport and coagulation cascades, respectively. Industrial applications include dyes, pigments, antioxidants, and redox-active materials for batteries and electrochemical devices. Some quinones have pharmacological activity, as seen with certain naphthoquinone-based drugs.

Synthesis: Quinones are commonly prepared by oxidation of the corresponding hydroquinones, using oxidants such as dichromate, silver oxide, or molecular oxygen, often under catalytic or aerobic conditions. Safety and environmental considerations vary with substituents and specific compounds.