Adipic acid or hexanedioic acid is the organic compound with the chemical formula C6H10O4.
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:
The cyclohexanone is then nitrosated, setting the stage for the scission of the C-C bond:
Side products of the method include
glutaric acid and
. 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:
- CH2=CH−CH=CH2 + 2 CO + 2 H2O → HO2C(CH2)4CO2H
Another method is oxidative cleavage of cyclohexene using hydrogen peroxide.
Auguste Laurent discovered adipic acid in 1837
Reactions
Adipic acid is a
dibasic acid (it has two acidic groups). The pK
a values for their successive
are 4.41 and 5.41.
With the carboxylate groups separated by four , 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[ PCI abstract for adipic acid] 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
, 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
flavoring and
thickening agent.
It is used in some calcium carbonate
to make them
sourness. As an
acidulant in
, it avoids the undesirable
hygroscopic properties of
tartaric acid.
Adipic acid, rare in nature, does occur naturally in
, 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
Nitrous oxide,
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 N2O → 2 N2 + O2
Adipate salts and esters
The anionic (HO
2C(CH
2)
4CO
2−) and dianionic (
−O
2C(CH
2)
4CO
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:
Some adipate esters are used as , including:
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
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