Preparation
Historically, adipic acid was prepared from various fats using oxidation. Currently adipic acid is produced from a mixture of cyclohexanol and cyclohexanone 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 → OC6H10 + 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.
Related processes start from cyclohexanol, which is obtained from the hydrogenation of phenol
Several methods have been developed by carbonylation of butadiene. For example, the hydrocarboxylation proceeds as follows:
- CH2=CHCH=CH2 + 2 CO + 2 H2O → HO2C(CH2)4CO2H
A method has been reported that utilizes principles of green chemistry in that water is the only by-product. Cyclohexene is oxidized with hydrogen peroxide using a tungstate-based catalyst and a phase transfer catalyst.The waste product is water.
Uses
By far the majority 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 6,6-nylon. Other major applications also involve polymers: it is a monomer for production of Polyurethane and its esters are plasticizers, especially in PVC.
Adipic acid has been incorporated into controlled-release formulation matrix tablets to obtain pH-independent release for both weakly basic and weakly acidic drugs. It has also been incorporated into the polymeric coating of hydrophilic monolithic systems to modulate the intragel pH, resulting in zero-order release of a hydrophilic drug. The disintegration at intestinal pH of the enteric polymer shellac has been reported to improve when adipic acid was used as a pore-forming agent without affecting release in the acidic media. Other controlled-release formulations have included adipic acid with the intention of obtaining a late-burst release profile.
Small but significant amounts of adipic acid are used as a food ingredient as a flavorant and gelling aid. It is used in some calcium carbonate antacids to make them tart.
Adipic acid, like most carboxylic acids, is a mild skin irritant. It is mildly toxic, with an LD50 of 3600 mg/kg for oral ingestion by rats
