Adipic acid

Adipic acid or hexanedioic acid is the organic compound with the formula (CH2)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,[4] but it is known as manufactured E number food additive E355. Salts and esters of adipic acid are known as adipates.

Adipic acid
Names
Preferred IUPAC name
Hexanedioic acid
Other names
Adipic acid
Butane-1,4-dicarboxylic acid
Hexane-1,6-dioic acid
1,4-butanedicarboxylic acid
Identifiers
3D model (JSmol)
1209788
ChEBI
ChEMBL
ChemSpider
ECHA InfoCard 100.004.250
EC Number
  • 204-673-3
E number E355 (antioxidants, ...)
3166
KEGG
RTECS number
  • AU8400000
UNII
UN number 3077
  • InChI=1S/C6H10O4/c7-5(8)3-1-2-4-6(9)10/h1-4H2,(H,7,8)(H,9,10) Y
    Key: WNLRTRBMVRJNCN-UHFFFAOYSA-N Y
  • InChI=1/C6H10O4/c7-5(8)3-1-2-4-6(9)10/h1-4H2,(H,7,8)(H,9,10)
    Key: WNLRTRBMVRJNCN-UHFFFAOYAY
  • O=C(O)CCCCC(=O)O
  • C(CCC(=O)O)CC(=O)O
Properties
C6H10O4
Molar mass 146.142 g·mol−1
Appearance White crystals[1]
Monoclinic prisms[2]
Odor Odorless
Density 1.360 g/cm3
Melting point 152.1 °C (305.8 °F; 425.2 K)
Boiling point 337.5 °C (639.5 °F; 610.6 K)
14 g/L (10 °C)
24 g/L (25 °C)
1600 g/L (100 °C)
Solubility Very soluble in methanol, ethanol
soluble in acetone, acetic acid
slightly soluble in cyclohexane
negligible in benzene, petroleum ether
log P 0.08
Vapor pressure 0.097 hPa (18.5 °C) = 0.073 mmHg
Acidity (pKa) 4.43, 5.41
Conjugate base Adipate
Viscosity 4.54 cP (160 °C)
Structure
Monoclinic
Thermochemistry
−994.3 kJ/mol[3]
Hazards
GHS labelling:
Warning
H319
P264, P280, P305+P351+P338, P337+P313
NFPA 704 (fire diamond)
2
1
0
Flash point 196 °C (385 °F; 469 K)
422 °C (792 °F; 695 K)
Lethal dose or concentration (LD, LC):
3600 mg/kg (rat)
Safety data sheet (SDS) External MSDS
Related compounds
glutaric acid
pimelic acid
Related compounds
 hexanoic acid
adipic acid dihydrazide
hexanedioyl dichloride
hexanedinitrile
hexanediamide
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Infobox references

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(CH2)5 + HNO3 → O=C(CH2)5 + HNO2 + H2O

The cyclohexanone is then nitrosated, setting the stage for the scission of the C-C bond:

HNO2 + HNO3 → [NO+]NO3]− + H2O
O=C(CH2)5 + NO+ → O=C(CHNO)(CH2)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,[5] as well, via the intermediacy of a nitrolic acid.[4]

Related processes start from cyclohexanol, which is obtained from the hydrogenation of phenol.[4][6]

Alternative methods of production

Several methods have been developed by carbonylation of butadiene. For example, the hydrocarboxylation proceeds as follows:[4]

CH2=CH−CH=CH2 + 2 CO + 2 H2O → HO2C(CH2)4CO2H

Another method is oxidative cleavage of cyclohexene using hydrogen peroxide.[7] The waste product is water.

Historically, adipic acid was prepared by oxidation of various fats,[8] thus the name (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.[9]

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.[10]

Uses

About 60% of the 2.5 billion kg of adipic acid produced annually is used as monomer for the production of nylon[11] 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.[12]

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.[13]

In foods

Small but significant amounts of adipic acid are used as a food ingredient as a flavorant and gelling aid.[14] 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.[2] Adipic acid, rare in nature, does occur naturally in beets, but this is not an economical source for commerce compared to industrial synthesis.[15]

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.[4]

Environmental

The production of adipic acid is linked to emissions of N
2O,[16] 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:[17]

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:

  • Bis(2-ethylhexyl) adipate
  • Dioctyl adipate
  • Dimethyl adipate

References
  1. Mac Gillavry, C. H. (2010). "The crystal structure of adipic acid". Recueil des Travaux Chimiques des Pays-Bas. 60 (8): 605–617. doi:10.1002/recl.19410600805.
  2. "Adipic Acid". The Merck Index. Royal Society of Chemistry. 2013. Retrieved 2 March 2017.
  3. Haynes, W. M., ed. (2013). CRC Handbook of Chemistry and Physics (94th ed.). Boca Raton, Florida: CRC Press. ISBN 978-1-4665-7114-3.
  4. Musser, M. T. (2005). "Adipic Acid". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. doi:10.1002/14356007.a01_269. ISBN 3527306730.
  5. Parmon, V. N.; Panov, G. I.; Uriarte, A.; Noskov, A. S. (2005). "Nitrous oxide in oxidation chemistry and catalysis application and production". Catalysis Today. 100 (2005): 115–131. doi:10.1016/j.cattod.2004.12.012.
  6. Ellis, B. A. (1925). "Adipic Acid". Organic Syntheses. 5: 9; Collected Volumes, vol. 1, p. 560.
  7. Sato, K.; Aoki, M.; Noyori, R. (1998). "A "Green" route to adipic acid: direct oxidation of cyclohexenes with 30 percent hydrogen peroxide". Science. 281 (5383): 1646–47. Bibcode:1998Sci...281.1646S. doi:10.1126/science.281.5383.1646. PMID 9733504.
  8. Ince, Walter (1895). "Preparation of adipic acid and some of its derivatives". Journal of the Chemical Society, Transactions. 67: 155–159. doi:10.1039/CT8956700155.
  9. Cornils, Boy; Lappe, Peter (2000). "Dicarboxylic Acids, Aliphatic". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. doi:10.1002/14356007.a08_523. ISBN 3527306730.
  10. Thorpe, J. F.; Kon, G. A. R. (1925). "Cyclopentanone". Organic Syntheses. 5: 37; Collected Volumes, vol. 1, p. 192.
  11. "Adipic Acid". Archived from the original on 2015-05-18. Retrieved 2015-05-09. PCI abstract for adipic acid
  12. "Polyvinylchloride - an overview | ScienceDirect Topics". www.sciencedirect.com. Retrieved 2023-11-10.
  13. Roew, Raymond (2009), "Adipic Acid", Handbook of Pharmaceutical Excipients, pp. 11–12
  14. "Cherry Jell-O Nutrition Facts". Kraft Foods. Archived from the original on 24 September 2019. Retrieved 21 Mar 2012.
  15. American Chemical Society (9 February 2015). "Molecule of the Week: Adipic Acid".
  16. US EPA (12 August 2013). "U.S. Greenhouse Gas Inventory Report, Chapter 4. Industrial Processes" (PDF). Retrieved 2013-11-29.
  17. Reimer, R. A.; Slaten, C. S.; Seapan, M.; Koch, T. A.; Triner, V. G. (2000). "Adipic Acid Industry — N2O Abatement". Non-CO2 Greenhouse Gases: Scientific Understanding, Control and Implementation. Netherlands: Springer. pp. 347–358. doi:10.1007/978-94-015-9343-4_56. ISBN 978-94-015-9343-4.

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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