Linoleic acid

Linoleic acid
Names
Preferred IUPAC name
(9Z,12Z)-Octadeca-9,12-dienoic acid
Other names
cis,cis-9,12-Octadecadienoic acid
C18:2 (Lipid numbers)
Identifiers
3D model (JSmol)
3DMet
1727101
ChEBI
ChEMBL
ChemSpider
DrugBank
ECHA InfoCard 100.000.428
EC Number
  • 200-470-9
57557
KEGG
UNII
  • InChI=1S/C18H32O2/c1-2-3-4-5-6-7-8-9-10-11-12-13-14-15-16-17-18(19)20/h6-7,9-10H,2-5,8,11-17H2,1H3,(H,19,20)/b7-6-,10-9- Y
    Key: OYHQOLUKZRVURQ-HZJYTTRNSA-N Y
  • InChI=1/C18H32O2/c1-2-3-4-5-6-7-8-9-10-11-12-13-14-15-16-17-18(19)20/h6-7,9-10H,2-5,8,11-17H2,1H3,(H,19,20)/b7-6-,10-9-
    Key: OYHQOLUKZRVURQ-HZJYTTRNBX
  • CCCCC/C=C\C/C=C\CCCCCCCC(=O)O
Properties
C18H32O2
Molar mass 280.452 g·mol−1
Appearance Colorless oil
Density 0.9 g/cm3[1]
Melting point −12 °C (10 °F)[1]
−6.9 °C (19.6 °F)[2]
−5 °C (23 °F)[3]
Boiling point 229 °C (444 °F) at 16 mmHg[2]
230 °C (446 °F) at 21 mbar[3]
230 °C (446 °F) at 16 mmHg[1]
0.139 mg/L[3]
Vapor pressure 16 Torr at 229 °C
Hazards
NFPA 704 (fire diamond)
2
1
0
Flash point 112 °C (234 °F)[3]
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Y verify (what is YN ?)
Infobox references
Not to be confused with linolenic acid, alpha-Linolenic acid, or lipoic acid.

Linoleic acid is an organic compound with the formula COOH(CH2)7CH=CHCH2CH=CH(CH2)4CH3. Both alkene groups are cis. It is a fatty acid sometimes denoted 18:2 (n-6) or 18:2 cis-9,12. A linoleate is a salt or ester of this acid.[4]

Linoleic acid is a polyunsaturated omega-6 fatty acid. It is a colorless or white liquid that is virtually insoluble in water but soluble in many organic solvents.[2] It typically occurs in nature as a triglyceride (ester of glycerin) rather than as a free fatty acid.[5] It is one of two essential fatty acids for humans, who must obtain it through their diet.[6]

The word "linoleic" derives from the Latin linum "flax" + oleum "oil", reflecting the fact that it was first isolated from linseed oil.

History

In 1844, F. Sacc, working at the laboratory of Justus von Liebig, isolated linoleic acid from linseed oil.[7][8] In 1886, K. Peters determined the existence of two double bonds.[9] Its essential role in human diet was discovered by G. O. Burr and others in 1930.[10] Its chemical structure was determined by T.P. Hilditch and others in 1939, and it was synthesized by R. A. Raphael and F. Sondheimer in 1950.[11]

In physiology

The consumption of linoleic acid is vital to proper health, as it is an essential fatty acid.[12]

Metabolism and eicosanoids

Linoleic acid is a precursor to arachidonic acid (AA) with elongation and saturation.[13] AA is the precursor to some prostaglandins,[14] leukotrienes (LTA, LTB, LTC), and thromboxane (TXA).[15]

The metabolism of LA to AA begins with the conversion of LA into gamma-Linolenic acid (GLA), effected by Δ6desaturase.[16] GLA is converted to dihomo-γ-linolenic acid (DGLA), the immediate precursor to AA.

LA is also converted by various lipoxygenases, cyclooxygenases, cytochrome P450 enzymes (the CYP monooxygenases), and non-enzymatic autoxidation mechanisms to mono-hydroxyl products viz., 13-Hydroxyoctadecadienoic acid, and 9-Hydroxyoctadecadienoic acid; these two hydroxy metabolites are enzymatically oxidized to their keto metabolites, 13-oxo-octadecadienoic acid and 9-oxo-octadecdienoic acid. Certain cytochrome P450 enzymes, the CYP epoxygenases, catalyze oxidation of LA to epoxide products viz., its 12,13-epoxide, Vernolic acid, and its 9,10-epoxide, Coronaric acid. These linoleic acid products are implicated in human physiology and pathology.[17]

Uses and reactions

Linoleic acid is a component of quick-drying oils, which are useful in oil paints and varnishes. These applications exploit the lability of the doubly allylic C-H groups (-CH=CH-CH2-CH=CH-) toward oxygen in air (autoxidation). Addition of oxygen leads to crosslinking and formation of a stable film.[18]

Reduction of the carboxylic acid group of linoleic acid yields linoleyl alcohol.[19]

Linoleic acid is a surfactant with a critical micelle concentration of 1.5 x 10−4 M @ pH 7.5.

Linoleic acid has become increasingly popular in the beauty products industry because of its beneficial properties on the skin. Research points to linoleic acid's anti-inflammatory, acne reductive, skin-lightening and moisture retentive properties when applied topically on the skin.[20][21][22][23]

Linoleic acid is also used in some bar of soap products.

Dietary sources
Properties of vegetable oils[24][25]
TypeProcessing
treatment[26]		Saturated
fatty acids		Monounsaturated
fatty acids		Polyunsaturated
fatty acids		Smoke point
Total[24]Oleic
acid
(ω-9)		Total[24]α-Linolenic
acid
(ω-3)		Linoleic
acid
(ω-6)		ω-6:3
ratio
Avocado[27]11.670.652–66[28]13.5112.512.5:1250 °C (482 °F)[29]
Brazil nut[30]24.832.731.342.00.141.9419:1208 °C (406 °F)[31]
Canola[32]7.463.361.828.19.118.62:1238 °C (460 °F)[31]
Coconut[33]82.56.361.7175 °C (347 °F)[31]
Corn[34]12.927.627.354.715858:1232 °C (450 °F)[35]
Cottonseed[36]25.917.81951.915454:1216 °C (420 °F)[35]
Flaxseed/linseed[37]9.018.41867.853130.2:1107 °C (225 °F)
Grape seed 10.514.314.3  74.7–74.7very high216 °C (421 °F)[38]
Hemp seed[39]7.09.09.082.022.054.02.5:1166 °C (330 °F)[40]
Olive[41]13.873.071.310.50.79.814:1193 °C (380 °F)[31]
Palm[42]49.337.0409.30.29.145.5:1235 °C (455 °F)
Peanut[43]16.257.155.419.90.31819.6very high232 °C (450 °F)[35]
Rice bran oil2538.42.234.4[44]15.6232 °C (450 °F)[45]
High-oleic safflower oil[46]7.575.275.212.8012.8very high212 °C (414 °F)[31]
Sesame[47]?14.239.739.341.70.341.3138:1
Soybean[48]partially hydrogenated14.943.042.537.62.634.913.4:1
Soybean[49]15.622.822.657.77517.3:1238 °C (460 °F)[35]
Walnut oil[50]unrefined9.122.822.263.310.452.95:1160 °C (320 °F)[51]
Sunflower[52]8.9963.462.920.70.1620.5very high227 °C (440 °F)[35]
Cottonseed[53]hydrogenated93.61.50.60.20.31.5:1
Palm[54]hydrogenated88.25.70
The nutritional values are expressed as percent (%) by mass of total fat.

Other occurrences

Cockroaches release oleic and linoleic acid upon death, which discourages other roaches from entering the area. This is similar to the mechanism found in ants and bees, which release oleic acid upon death.[55]

Health effects

Consumption of linoleic acid has been associated with lowering the risk of cardiovascular disease and premature death.[56]

See also

References
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  2. William M. Haynes (2016). CRC Handbook of Chemistry and Physics (97th ed.). Boca Raton: CRC Press. pp. 3–338. ISBN 978-1-4987-5429-3.
  3. Record of CAS RN 60-33-3 in the GESTIS Substance Database of the Institute for Occupational Safety and Health
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  7. F. Sacc (1844) "Ueber das Leinöl, seine physicalischen und chemischen Eigenscharften und seine Oxydationsproducte". Liebig Annalen, volume 51, pages 213-230.
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  9. (1886), Monatsch., volume 7, pages 522-
  10. (1930: J Biol Chem, volume 86, pages 587-
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  16. Evidence suggests that infants must acquire Δ6desaturase breast milk. Breast-milk fed babies have higher concentrations of GLA than formula-fed babies, while formula-fed babies have elevated concentrations of LADavid F. Horrobin (1993). "Fatty acid metabolism in health and disease: the role of Δ-6-desaturase". American Journal of Clinical Nutrition. 57 (5 Suppl): 732S–7S. doi:10.1093/ajcn/57.5.732S. PMID 8386433.
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  22. Ando, H; Ryu, A; Hashimoto, A; Oka, M; Ichihashi, M (March 1998). "Linoleic acid and α-linolenic acid lightens ultraviolet-induced hyperpigmentation of the skin". Archives of Dermatological Research. 290 (7): 375–381. doi:10.1007/s004030050320. PMID 9749992. S2CID 23036319.
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