Overview

Adipic acid, also known as hexanedioic acid, is a white crystalline organic compound (C6H11O4), is derived from the oxidations of various fats. This organic compound is the most important synthetic dicarboxylic acid produced annually. It has a slight acidic taste and a very faint odor. Also, it is less soluble in water and is soluble in alcohol and acetone. Adipic acid is used as precursor in manufacture of nylon. It is also used in chemical industry to produce other polymers, detergents, plasticizers and coatings.

Esters of adipic acid which are known as adipates are used as softeners in plastic and as synthetic grease base oils. Adipic acid causes serious damage to eyes and is mildly toxic so it should be handled carefully. This acid produced mild to no skin irritation, whereas its powdered form may irritate the mucous membranes of the lungs and nose of humans.

Adipic acid is produced through a two-stage process. The first stage of manufacturing usually involves the oxidation of cyclohexane to form cyclohexanone/cyclohexanol mixture. The second stage entails oxidizing this mixture with nitric acid to produce adipic acid. N2O is produced as a byproduct during the nitric acid oxidation stage and is potentially emitted in the waste gas stream during which N2O is generated in the second stage.

Manufacturing process

Adipic acid is produced through a two-stage process. The first stage of manufacturing usually involves the oxidation of cyclohexane to form cyclohexanone/cyclohexanol mixture. The second stage entails oxidizing this mixture with nitric acid to produce adipic acid. N₂O is produced as a byproduct during the nitric acid oxidation stage and is potentially emitted in the waste gas stream during which N₂O is generated in the second stage.

Applications

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 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. As an acidulant in baking powders, it avoids the undesirable hygroscopic properties of tartaric acid. Adipic acid, rare in nature, does occur naturally in beets, but this is not an economical source for commerce compared to industrial synthesis.