Adipic acid or hexanedioic acid is the organic compound with the formula (CH2)4(COOH)2. From an industrial perspective, it is the most important dicarboxylic acid: about 2.5 billion kilograms of this white crystalline powder are produced annually, mainly as a precursor for the production of nylon. Adipic acid otherwise rarely occurs in nature, but it is known as manufactured E number food additive E355. Salts and esters of adipic acid are known as adipates. Preparation and reactivity Adipic acid is produced from a mixture of cyclohexanone and cyclohexanol called KA oil, the abbreviation of ketone-alcohol oil. The KA oil is oxidized with nitric acid to give adipic acid, via a multistep pathway. Early in the reaction, the cyclohexanol is converted to the ketone, releasing nitrous acid: HOC6H11 + HNO3 → OC(CH2)5 + HNO2 + H2O Among its many reactions, the cyclohexanone is nitrosated, setting the stage for the scission of the C-C bond: HNO2 + HNO3 → NO+NO3− + H2O OC6H10 + NO+ → OC6H9-2-NO + H+ Side products of the method include glutaric and succinic acids. Nitrous oxide is produced in about one to one mole ratio to the adipic acid, as well, via the intermediacy of a nitrolic acid. Related processes start from cyclohexanol, which is obtained from the hydrogenation of phenol. Alternative methods of production Several methods have been developed by carbonylation of butadiene. For example, the hydrocarboxylation proceeds as follows: CH2=CH−CH=CH2 + 2 CO + 2 H2O → HO2C(CH2)4CO2H Another method is oxidative cleavage of cyclohexene using hydrogen peroxide. The waste product is water. Historically, adipic acid was prepared by oxidation of various fats, thus the name (ultimately from Latin adeps, adipis – "animal fat"; cf. adipose tissue). Reactions Adipic acid is a dibasic acid (it has two acidic groups). The pKa values for their successive deprotonations are 4.41 and 5.41. With the carboxylate groups separated by four methylene groups, adipic acid is suited for intramolecular condensation reactions. Upon treatment with barium hydroxide at elevated temperatures, it undergoes ketonization to give cyclopentanone. Uses About 60% of the 2.5 billion kg of adipic acid produced annually is used as monomer for the production of nylon by a polycondensation reaction with hexamethylene diamine forming nylon 66. Other major applications also involve polymers; it is a monomer for production of polyurethane and its esters are plasticizers, especially in PVC. In medicine Adipic acid has been incorporated into controlled-release formulation matrix