Adipic acid is the organic compound with the formula4(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.
Preparation and reactivity 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.
Alternative methods of production 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 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. Historically, adipic acid was prepared by
oxidation of various fats, thus the name. Reactions
Adipic acid is a dibasic acid. Its pKa's 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 The great 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. In medicine
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. In foods
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.
Safety 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. Environmental
The production of adipic acid is linked to emissions of N
2O, a potent greenhouse gas and cause of stratospheric ozone depletion.
References
Appendix E-number E355.
U.S. FDA citations - GRAS, Indirect additive European Union Citations - Decision 1999EC
- Flavoing Substance; Directive 95EC, Annex IV - Permitted Food Additive; 2002EC, Annex
A - Authorized monomer for Food Contact Plastics External links
International Chemical Safety Card 0369 SIDS Initial Assessment Report for Adipic
acid from the Organisation for Economic Co-operation and Development
adipic acid on chemicalland Solutia Inc product website