"Omega-3s" refer to omega-3 fatty acids which are a type of long-chain, polyunsaturated carboxylic acid. Long-chain carboxylic acids are referred to as "fatty acids".
The three omega-3 fatty acids studied were:
- α-linolenic acid (ALA) which is found in plant seeds and oils
- eicosapentaenoic acid (EPA) which is found in fish, algae and phytoplankton
- docosahexaenoic acid (DHA) which is found in fish, algae and phytoplankton
The IUPAC name of α-linolenic acid is (9Z,12Z,15Z)-9,12,15-octadecatrienoic acid.
The carbon atom of the carboxyl functional group (COOH) is labelled as 1, the next carbon atom in the chain is 2, then 3, etc, up until we reach the last carbon in the chain, carbon 18. The parent hydrocarbon for this molecule is therefore octadecane, with a suffix added for the carboxyl functional group, so we have octadecanoic acid.
Along the way we find 3 (tri) double bonds (en) at carbons numbered 9, 12 and 15, so we modify the name of the carboxylic acid in one of two ways:
- 9,12,15-octadecatrienoic acid
- octadeca-9,12,15-trienoic acid
So why would (9Z,12Z,15Z)-9,12,15-octadecatrienoic acid be called an omega-3 acid?
There is an alternative naming "system" (not IUPAC) in which the first carbon atom is not labelled 1, but instead it is called "alpha" (α), and, the carbon atom at the end of the long hydrocarbon chain is called "omega" (ω). Then, the last carbon atom in the chain (omega) is labelled 1, and you start counting back towards the carboxyl functional group. If you do this, you will find that carbon 3 has a double on it, hence, (9Z,12Z,15Z)-9,12,15-octadecatrienoic acid is called an omega-3 fatty acid. The 3 indicates the position of the first double bond from the omega carbon atom.
The skeletal structural formula of eicosapentaenoic acid is shown below:
The IUPAC name for this molecule is (5Z,8Z,11Z,14Z,17Z)
There are 20 carbon atoms in the chain so the parent hydrocarbon chain is icosane, which is modified with a suffix because there is a carboxyl functional group, icosanoic acid.
There are 5 (pent) double bonds (en) located on carbons 5, 8, 11, 14 and 17, so the name of this molecule is either:
- 5,8,11,14,17-icosapentenoic acid
- icosa-5,8,11,14,17-pentenoic acid
Notice that, in the alternative naming "system" in which the last carbon atom in the chain is labelled "omega" and then you count backwards from this, we see that the first double bond encountered is on carbon 3, so this molecule is also referred to as an omega-3 fatty acid.
The skeletal structural formula below is that of docosahexaenoic acid:
This molecule has the IUPAC name (4Z,7Z,10Z,13Z,16Z,
Using the alternative numbering system, the first double bond occurs of the third carbon atom from the omega carbon atom (last carbon in the chain) so it is also classified as an omega-3 fatty acid.
While all three of these omega-3 fatty acids were shown to be effective in reducing the size of tumours in mice, however, higher doses of the plant-based α-linolenic acid was required to deliver the same impact as the omega-3 fatty acids found in fish (icosapentaenoic acid and docosahexaenoic acid)
Jiajie Liu, Salma A. Abdelmagid, Christopher J. Pinelli, Jennifer M. Monk, Danyelle M. Liddle, Lyn M. Hillyer, Barbora Hucik, Anjali Silva, Sanjeena Subedi, Geoffrey A. Wood, Lindsay E. Robinson, William J. Muller, David W.L. Ma. Marine fish oil is more potent than plant based n-3 polyunsaturated fatty acids in the prevention of mammary tumours. The Journal of Nutritional Biochemistry, 2017; DOI: 10.1016/j.jnutbio.2017.12.011
Introduction to naming organic molecules: http://www.ausetute.com.au/namctut1.html
Introduction to functional groups: http://www.ausetute.com.au/fungroup.html
Fatty acids: http://www.ausetute.com.au/fattyacid.html
Structure and properties of carboxylic acids: http://www.ausetute.com.au/carboxyl.html
Molecular formula: http://www.ausetute.com.au/molecularformula.html
2-Dimensional structural formula: http://www.ausetute.com.au/structural2D.html
Condensed structural formula: http://www.ausetute.com.au/condensedsf.html
Skeletal structural formula: http://www.ausetute.com.au/skeletal.html
Cis-trans isomers: http://www.ausetute.com.au/cistranso.html
Suggested Study Questions:
- What functional group is common to all fatty acids?
- Define the following terms as they are used in chemistry:
- Give the molecular formula for each of the three omega-3 acids in the article
- The structural formula given in the article are referred to as "skeletal". What does this mean in chemistry?
- Draw a 2-dimensional (full display) structural formula for each of the omega-3 acids in the article.
- On the structural formula of each of the three omega-3 acids circle the:
- carboxyl functional group in red
- double bonds in blue
- What features are common to three omega-3 acids in the article?
- In what ways do the the three omega-3 acids in the article differ?
- You have probable heard about omega-6 acids. How do you think an omega-6 acid will be similar to an omega-3 acid?
- How will an omega-6 acid be different from an omega-3 acid?
- What is meant by a cis isomer and a trans isomer?
- All the omega-3 fatty acids in the article are the cis isomers. Build a model, and draw, a trans isomer of one of the fatty acids.