# Adipic acid

Adipic acid or hexanedioic acid is the organic compound with the formula (CH$_2$)$_4$(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 by oxidation of a mixture of cyclohexanone and cyclohexanol, which is called KA oil, an abbreviation of ketone-alcohol oil. Nitric acid is the oxidant. The pathway is multistep. Early in the reaction, the cyclohexanol is converted to the ketone, releasing nitrous acid:
    HOCH(CH$_2$)$_5$ + HNO$_3$ → O=C(CH$_2$)$_5$ + HNO$_2$ + H$_2$O
The cyclohexanone is then nitrosated, setting the stage for the scission of the C-C bond:
    HNO$_2$ + HNO$_3$ → [NO$^+$][NO$_3$]$^−$ + H$_2$O
    O=C(CH$_2$)$_5$ + NO$^+$ → O=C(CHNO)(CH$_2$)$_4$ + 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:
    CH$_2$=CH−CH=CH$_2$ + 2 CO + 2 H$_2$O → HO$_2$C(CH$_2$)$_4$CO$_2$H
Another method is oxidative cleavage of cyclohexene using hydrogen peroxide. The waste product is water.
Auguste Laurent discovered adipic acid in 1837 by oxidation of various fats with nitric acid via sebacic acid and gave it the current name because of that (ultimately from Latin adeps, adipis – "animal fat"; cf. adipose tissue).

### Reactions

Adipic acid is a dibasic acid (it has two acidic groups). The pK$_a$ 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 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.

## Safety

Adipic acid, like most carboxylic acids, is a mild skin irritant. It is mildly toxic, with a median lethal dose of 3600 mg/kg for oral ingestion by rats.

## Environmental

The production of adipic acid is linked to emissions of N $_2$O, a potent greenhouse gas and cause of stratospheric ozone depletion. At adipic acid producers DuPont and Rhodia (now Invista and Solvay, respectively), processes have been implemented to catalytically convert the nitrous oxide to innocuous products:
    2 N$_2$O → 2 N$_2$ + O$_2$

## Adipate salts and esters

The anionic (HO$_2$C(CH$_2$)$_4$CO$_2$$^−$) and dianionic ($^−$O$_2$C(CH$_2$)$_4$CO$_2$$^−$) forms of adipic acid are referred to as adipates. An adipate compound is a carboxylate salt or ester of the acid.
Some adipate salts are used as acidity regulators, including:
* Sodium adipate (E number E356)
* Potassium adipate (E357)
Some adipate esters are used as plasticizers, including:
* Bis(2-ethylhexyl) adipate
* Dioctyl adipate
* Dimethyl adipate