adipic acid

Adipic acid
IUPAC name	hexanedioic acid
Other names	butane-1,4-dicarboxylic acid
Identifiers
CAS number	124-04-9 Y
PubChem	196
EC-number	204-673-3
SMILES	O=C(O)CCCCC(=O)O
InChI	1/C6H10O4/c7-5(8)3-1-2-4-6(9)10/h1-4H2,(H,7,8)(H,9,10)
InChI key	WNLRTRBMVRJNCN-UHFFFAOYAY
ChemSpider ID	191
Properties
Molecular formula	C6H10O4
Molar mass	146.14 g mol−1
Appearance	White crystals
Density	1.36 g/cm3
Melting point	152.1 °C, 425 K, 306 °F
Boiling point	337.5 °C, 611 K, 640 °F
Solubility in water	fairly soluble[1]
Acidity (pKa)	4.43, 5.41
Hazards
MSDS	External MSDS
EU Index	607-144-00-9
EU classification	Irritant (Xi)
R-phrases	R36
NFPA 704	1 2 0
Flash point	196 °C
Autoignition temperature	422 ºC
Related compounds
Related dicarboxylic acids	glutaric acid pimelic acid
Related compounds	hexanoic acid adipic acid dihydrazide
Y (what is this?)  (verify) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Adipic acid is the organic compound with the formula (CH₂)₄(COOH)₂. From the 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.^[2]

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.^[2]

Related processes start from cyclohexanol, which is obtained from the hydrogenation of phenol.^[2]

Alternative methods of production

Several methods have been developed by carbonylation of butadiene. For example, the hydrocarboxylation proceeds as follows:^[2]

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.^[3] 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.

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. ^[4]

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 LD50 of 3600 mg/kg for oral ingestion by rats.^[2]

References

Appendix

• E-number E355.
• U.S. FDA citations - GRAS (21 CFR 184.1009), Indirect additive (21 CFR 175.300, 21 CFR 175.320, 21 CFR 176.170, 21 CFR 176.180, 21 CFR 177.1200, 21 CFR 177.1390 , 21 CFR 177.1500, 21 CFR 177.1630 , 21 CFR 177.1680, 21 CFR 177.2420, 21 CFR 177.2600)
• European Union Citations - Decision 1999/217/EC - Flavoing Substance; Directive 95/2/EC, Annex IV - Permitted Food Additive; 2002/72/EC, 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 (OECD)
• adipic acid on chemicalland
• Solutia Inc product website