Quinone

The quinones are a class of organic compounds that are formally 'derived from aromatic compounds by conversion of an even number of –CH= groups into –C(=O)– groups with any necessary rearrangement of double bonds', resulting in 'a fully conjugated cyclic dione structure'. The class includes some heterocyclic compounds.1,2-Benzoquinone1,4-Benzoquinone1,4-Naphthoquinone9,10-AnthraquinoneChloranilLawsonealizarinDDQ The quinones are a class of organic compounds that are formally 'derived from aromatic compounds by conversion of an even number of –CH= groups into –C(=O)– groups with any necessary rearrangement of double bonds', resulting in 'a fully conjugated cyclic dione structure'. The class includes some heterocyclic compounds. The archetypical member of the class is 1,4-benzoquinone or cyclohexadienedione, often called simply 'quinone' (thus the name of the class). Other important examples are 1,2-benzoquinone (ortho-quinone), 1,4-naphthoquinone and 9,10-anthraquinone. Quinones are oxidized derivatives of aromatic compounds and are often readily made from reactive aromatic compounds with electron-donating substituents such as phenols and catechols, which increase the nucleophilicity of the ring and contributes to the large redox potential needed to break aromaticity. (Quinones are conjugated but not aromatic). Quinones are electrophilic Michael acceptors stabilised by conjugation. Depending on the quinone and the site of reduction, reduction can either rearomatise the compound or break the conjugation. Conjugate addition nearly always breaks the conjugation. The term quinone is also used more generally for a large class of compounds formally derived from aromatic quinones through replacement of some hydrogen atoms by other atoms or radicals. A large scale industrial application of quinones is for the production of hydrogen peroxide. 2-Alkylanthraquinones are hydrogenated to the corresponding hydroquinones (quinizarins), which then transfer H2 to oxygen: In this way, several billion kilograms of H2O2 are produced annually. Derivatives of quinones are common in biologically active molecules. Some serve as electron acceptors in electron transport chains such as those in photosynthesis (plastoquinone, phylloquinone), and aerobic respiration (ubiquinone). Phylloquinone is also known as Vitamin K1 as it is used by animals to carboxylate certain proteins, which are involved in blood coagulation, bone formation, and other processes. Conversely, the toxicity of paracetamol is due to its metabolism to a quinone imine, which then reacts with liver proteins to cause liver failure. A natural example of the oxidation of hydroquinone to quinone is in the spray of bombardier beetles; hydroquinone is reacted with hydrogen peroxide to produce a fiery blast of steam, a strong deterrent in the animal world. The auto-oxidation of the neurotransmitter dopamine and its precursor L-Dopa generates the comparatively stable dopamine quinone which inhibits the functioning of dopamine transporter (DAT) and the TH enzyme leading to low mitochondrial ATP production.