Adipic acid
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Adipic acid or hexanedioic acid is an organic compound with the chemical formula C6H10O4.<ref name="Myers">Template:Cite book</ref> It is a white crystalline powder at standard temperature and pressure.<ref name="Myers" /> From an industrial perspective, it is the most important dicarboxylic acid at about 2.5 billion kilograms produced annually, mainly as a precursor for the production of nylon.<ref name="Myers" /> Adipic acid otherwise rarely occurs in nature,<ref name="Ullmann">Template:Cite book</ref> 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,<ref name="Myers" /> 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:
The cyclohexanone is then nitrosated, setting the stage for the scission of the C-C bond:
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,<ref name=CatalysisToday2005>Template:Cite journal</ref> as well, via the intermediacy of a nitrolic acid.<ref name="Ullmann"/>
Related processes start from cyclohexanol, which is obtained from the hydrogenation of phenol.<ref name="Ullmann"/><ref>Template:OrgSynth</ref>
Alternative methods of production
Several methods have been developed by carbonylation of butadiene. For example, the hydrocarboxylation proceeds as follows:<ref name="Ullmann"/>
- CH2=CH−CH=CH2 + 2 CO + 2 H2O → HO2C(CH2)4CO2H
Another method is oxidative cleavage of cyclohexene using hydrogen peroxide.<ref>Template:Cite journal</ref> The waste product is water.
Auguste Laurent discovered adipic acid in 1837<ref>Template:Cite book</ref><ref>Template:Cite journal</ref> by oxidation of various fats with nitric acid via sebacic acid<ref>Template:Cite journal</ref> 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 pKa values for their successive deprotonations are 4.41 and 5.41.<ref name=Ullmanns>Template:Ullmann</ref>
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.<ref>Template:OrgSynth</ref>
Uses
About 60% of the 2.5 billion kg of adipic acid produced annually is used as monomer for the production of nylon<ref>{{#invoke:citation/CS1|citation |CitationClass=web }} PCI abstract for adipic acid</ref> 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.Template:Fact
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.<ref>Template:Citation</ref>
In foods
Small but significant amounts of adipic acid are used as a food ingredient as a flavorant and gelling aid.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> 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.<ref name="Adipic Acid">Template:Cite Merck Index</ref> Adipic acid, rare in nature, does occur naturally in beets, but this is not an economical source for commerce compared to industrial synthesis.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
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.<ref name="Ullmann"/>
Environmental
The production of adipic acid is linked to emissions of [[Nitrous oxide|Template:Chem]],<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> 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:<ref>Template:Cite book</ref>
- 2 N2O → 2 N2 + O2
Adipate salts and esters
The anionic (HO2C(CH2)4CO2−) and dianionic (−O2C(CH2)4CO2−) 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:
References
Appendix
- 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