Linoleic acid
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| Structural formula with the numbering of selected carbon atoms | ||||||||||||||||||||||
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| Surname | Linoleic acid | |||||||||||||||||||||
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| Molecular formula | C 18 H 32 O 2 | |||||||||||||||||||||
| Brief description |
colorless to yellowish liquid |
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| Molar mass | 280.45 g mol −1 | |||||||||||||||||||||
| Physical state |
liquid |
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| density |
0.902 g cm −3 (25 ° C) |
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| Melting point |
−7 ° C |
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| boiling point |
230 ° C (16 hPa) |
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| Refractive index |
1.467 (20 ° C) |
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| As far as possible and customary, SI units are used. Unless otherwise noted, the data given apply to standard conditions . Refractive index: Na-D line , 20 ° C | ||||||||||||||||||||||
Linoleic acid , scientifically ( cis , cis ) octadeca-9,12-dienoic acid, is a doubly unsaturated fatty acid with 18 carbon atoms (18: 2). Due to the position of its second double bond, it belongs to the group of omega-6 fatty acids and must be distinguished from linolenic acid . It is a so-called dienoic acid and an isolenic acid because the two double bonds are separated by a methylene group .
The name linoleic acid is derived from the Latin linum (Greek linon ) for flax (flax) and oleum "oil", which is why it was also known as linoleic acid or oleic acid until the 20th century.
Occurrence
Linoleic acid occurs as an ester chemically bound in many triglycerides , which are the main components of natural fatty oils . Grape seed oil with 58–78% and safflower oil ( safflower oil ) with 55–81% have the highest linoleic acid content of all vegetable oils. Also in hemp oil (around 50%), soybean oil (49–57%), cottonseed oil (45–58%), wheat germ oil (40–55%), corn germ oil (34–62%), sunflower oil (20–75%) and rapeseed oil (18–30%) there are high proportions; in contrast, there is olive oil with only 3–20% linoleic acid content.
Contrary to popular belief, these natural fats and oils do not contain free linoleic acid, but its glycerol ester .
Extraction and presentation
The fatty acids can be obtained from the triglycerides by alkaline saponification by boiling the corresponding fats or oils with alkalis . Since natural fats and oils always contain many different fatty acids, the resulting mixture is usually separated by distillation .
Commercially produced linoleic acid has a share of up to 67%, and it also contains a mixture of other saturated and unsaturated fatty acids, especially oleic acid .
properties
Pure linoleic acid is a colorless, oily and almost odorless liquid. It is sensitive to oxidation and ages in the air, turning yellow, which is due to the formation of hydroperoxides. The molar mass is 280.45 g mol −1. and the density 0.9 g · cm −3 . It has a melting point of −7 ° C and a boiling point of 230 ° C. The fatty acid is very poorly soluble in water, but good in many organic solvents. Chemically it is one of the hydrophobic carboxylic acids, however, is striking it deviates greatly from the other homologues pK S value of 7.9 (typically 4.75 to 4.95).
Biological importance
Linoleic acid, like α-linolenic acid , is an essential nutrient and must therefore be taken in with food. The dihomogammalinolenic acid (anti-inflammatory) and arachidonic acid (pro-inflammatory), which are important in inflammatory processes, are synthesized in the body from linoleic acid via the intermediate stage γ-linolenic acid (GLA) .
Linoleic acid is a regular component of human skin, especially the epidermis . The epidermal barrier, which is crucial for regulating the water balance - represented as a structure by the stratum corneum - consists of ceramides , free fatty acids and phospholipids . The ceramides are of particular importance for water regulation. The quantitatively most important ceramide is ceramide 1, the main component of which is linoleic acid. When applied externally, linoleic acid is able to
- To counteract external skin irritation (irritative contact dermatitis ),
- to counteract chronic photodamage to the skin,
- to recede stains that appear in the context of light-damaged aging skin.
However, due to the size of the molecule, the effect has only been proven on already damaged skin.
Linoleic acid is used in cosmetics without a specified maximum concentration .
proof
The detection and determination of the content of linoleic acid is usually carried out by gas chromatography of the methyl ester; In addition, the unsaturated isomers can be separated using silver nitrate thin-layer chromatography.
Technical use
Acylglycerols of linoleic acid and linolenic acid are used as an additive to varnish and other drying oils for coatings (lacquers, etc.).
See also
- Conjugated linoleic acids - different isomers of linoleic acid
Individual evidence
- ^ A b Walter Karrer : Constitution and occurrence of organic plant substances. Springer, 1958, ISBN 978-3-0348-6808-2 (reprint), p. 310.
- ↑ Entry on LINOLEIC ACID in the CosIng database of the EU Commission, accessed on July 2, 2020.
- ↑ a b c d e f Entry for CAS no. 60-33-3 in the GESTIS substance database of the IFA , accessed on December 8, 2016(JavaScript required) .
- ↑ David R. Lide (Ed.): CRC Handbook of Chemistry and Physics . 97th edition. (Internet version: 2016), CRC Press / Taylor and Francis, Boca Raton, FL, Physical Constants of Organic Compounds, pp. 3-338.
- ↑ David R. Lide (Ed.): CRC Handbook of Chemistry and Physics . 90th edition. (Internet version: 2010), CRC Press / Taylor and Francis, Boca Raton, FL, Dissociation Constants of Organic Acids and Bases, pp. 8-51.
- ↑ a b Hans-Dieter Belitz, Werner Grosch, Peter Schieberle: Textbook of food chemistry. 6th edition, Springer-Verlag, 2008, ISBN 978-3-540-73201-3 , p. 168.
- ↑ a b c d e entry on linoleic acid. In: Römpp Online . Georg Thieme Verlag, accessed on June 22, 2013.
- ↑ Linoleic acid data sheet from Sigma-Aldrich , accessed on May 29, 2011 ( PDF ).
- ^ Meyers Konversations-Lexikon, 4th edition from 1888–1890.
- ↑ Entry on safflower oil. In: Römpp Online . Georg Thieme Verlag, accessed on June 22, 2013.
- ↑ Entry on hemp oil. In: Römpp Online . Georg Thieme Verlag, accessed on June 22, 2013.
- ^ Siegfried Hauptmann : Organic Chemistry , 2nd reviewed edition, VEB Deutscher Verlag für Grundstoffindustrie, Leipzig 1985, ISBN 3-342-00280-8 , pp. 653-654.
- ↑ keyword "Linoleic Acid" In Hans Zoebelein (ed.): Dictionary of Renewable Resources. 2nd edition, Wiley-VCH, Weinheim and New York 1996, ISBN 3-527-30114-3 , p. 92.
- ↑ Kunio Yagi (Ed.): Lipid Peroxides in Biology and Medicine , Academic Press, New York, London, Paris, San Diego, San Francisco, Sao Paulo, Sydney Tokyo, Toronto 1982, ISBN 0-12-768050-0
- ↑ a b c d Jürgen Blecker: Chemistry for everyone . Compact Verlag, 2010, ISBN 3-8174-7856-9 , pp. 373 ( limited preview in Google Book search).
- ↑ B. Breuer, T. Stuhlfauth, HP Fock: Separation of fatty acids or methyl esters including positional and geometric isomers by alumina thin-layer chromatography. In: J. of Chromatogr. Science. 25, pp. 302-306 (1987).
