Omega Fatty Acids Chemistry Tutorial

Key Concepts

• A fatty acid is a long chain carboxylic acid.⁽¹⁾
• Essential fatty acids are those that must be ingested because the body requires them but cannot synthesise them.
• Essential fatty acids are unsaturated fatty acids.
• Only two fatty acids are thought to be essential for humans:

  (i) alpha-linolenic acid (an omega−3 fatty acid)

  (ii) linoleic acid (an omega−6 fatty acid)

• The omega classification for fatty acids tells us the location of the first double bond in the hydrocarbon chain.

  The "omega" carbon atom ( C^ω) refers to the last carbon atom on the hydrocarbon chain.

  O
  	\\
  	C	-Cα-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-	Cω
  	/
  HO

  (i) omega−3 (ω−3) refers to the third carbon atom counting from the end of the chain.

  (ii) omega−6 (ω−6) refers to the sixth carbon atom counting from the end of the chain.

• omega−3 and omega−6 fatty acids are found in:

  (i) fish

  (ii) vegetable oils such as olive and canola oil

  (iii) seeds such as chia, pumpkin and sunflower seeds

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Omega Classification of Fatty Acids

Fatty acids are long chain carboxylic acids.

Fatty acids contain the COOH (carboxyl) functional group at the end of a long hydrocarbon chain.

Fatty acids can be classified as:

• saturated (only single bonds between carbon atoms)

or,

• unsaturated (containing one or more double bonds between carbon atoms).

Some examples of saturated and unsaturated fatty acids are give below:

Using the IUPAC recommendations for naming carboxylic acids, Chemists number the carbon chain of a fatty acid starting from the carbon of carboxyl, COOH, functional group as 1.
The structural formula for palmitoleic acid is shown below with the carbons numbered from 1 to 16 according to IUPAC recommendations:

and we note that the double bond occurs between carbons 9 and 10 in the hydrocarbon chain.

Chemists classify palmitoleic acid as:

• a carboxylic acid (it has the COOH functional group)
• unsaturated (it has a C=C)
• monounsaturated (it has only ONE C=C)
• a fatty acid (long chain carboxylic acid)

There is another way we could classify fatty acids which is based on the location of the double bond.
Biologists, especially biochemists, commonly classify fatty acids this way.
To do this they label the carbon adjacent to the carboxyl group the alpha carbon, α carbon, (alpha being the first letter of the Greek alphabet), and the last carbon atom in the hydrocarbon chain as the omega carbon atom, ω carbon (omega being the last letter of the Greek alphabet) as shown on the structural formula of palmitoleic acid below:

Then they count back from the omega carbon atom (omega−1, or, ω−1) towards the alpha carbon atom until they hit upon the first double-bonded carbon atom as shown below:

Palmitoleic acid, shown above, would then be classified as an omega−7 (ω−7) fatty acid.
The double bond occurs between the 7^th and 8^th carbon atoms as counted from the end of the hydrocarbon chain.

Consider oleic acid, another monounsaturated fatty acid:

Oleic acid would be classified as an omega−9 (ω−9) fatty acid.

Consider linoleic acid, a polyunsaturated fatty acid (it has two lots of C=C):

Linoleic acid would be classified as an omega−6 (ω−6) fatty acid.

Consider linolenic acid, another polyunsaturated fatty acid (it has 3 lots of C=C):

Linolenic acid is an omega−3 (ω−3) fatty acid.

Consider arachidonic acid, another polyunsaturated fatty acid (it has 4 lots of C=C):

Arachidonic acid is an omega−6 (ω−6) fatty acid.

This omega classification can only be given to unsaturated fatty acids, that is, the fatty acid MUST contain one or more double bonds between carbon atoms.

The omega classification does not tell us how many double bonds there are, nor does it tell us how long the chain is, it only tells us the location of the first double bond as counted from the end of the hydrocarbon chain.

Omega−3 Fatty Acids and Omega−6 Fatty Acids

The human body lacks the ability to introduce a double bond between carbons 9 and 10 and beyond, as counted from the carboxyl functional group (IUPAC nomenclature for chemistry), so fatty acids with double bonds beyond the ninth carbon atom are essential fatty acids because we must eat foods containing them.
Two essential fatty acids are linoleic acid and alpha-linolenic acid (α-linolenic acid).

α-linolenic acid, C₁₈H₃₀O₂, is an example of an omega−3 fatty acid (ω−3 fatty acid).
According to IUPAC nomenclature rules, the carbon of the carboxyl group is given the number 1, the carbon atom next to that is 2, and so on and so forth until the final carbon atom is numbered 18.
Numbering according to IUPAC rules is shown as blue numbers on the skeletal structural formula shown below:

The carbon chain is 18 carbon atoms long, which, according to IUPAC nomenclature makes the parent carbon chain "octadeca".
The location of each of the three (tri) double bonds (en) are on carbons 9, 12 and 15, that is 9,12,15-trien
The functional group is COOH, or a carboxyl group, and since there are no other functional groups present this is named as an alkenoic acid, with the oic acid suffix
The completed IUPAC name⁽²⁾ is octadeca-9,12,15-trienoic acid or 9,12,15-octadecatrienoic acid
α-linolenic acid is an omega-3 fatty acid because, when counted from the last carbon atom in the chain (omega carbon atom, 1), the first C=C encounted occurs at the third carbon atom along from the omega carbon atom:

Linoleic acid, C₁₈H₃₂O₂, on the other hand, is an example of omega-6 fatty acid (ω-6 fatty acid).
According to IUPAC nomenclature rules, the carbon of the carboxyl group is given the number 1, the carbon atom next to that is 2, and so on and so forth until the final carbon atom is numbered 18.
Numbering according to IUPAC rules is shown as blue numbers on the skeletal structural formula shown below:

The carbon chain is 18 carbon atoms long, which, according to IUPAC nomenclature makes the parent carbon chain "octadeca".
The location of each of the two (di) double bonds (en) are on carbons 9 and 12, that is 9,12-dien
The functional group is COOH, or a carboxyl group, and since there are no other functional groups present this is named as an alkenoic acid, with the oic acid suffix
The completed IUPAC name⁽³⁾ is octadeca-9,12-dienoic acid or 9,12-octadecadienoic acid
Linoleic acid is an example of an omega-6 fatty acid because, when counted from the omega carbon atom (last carbon atom of the chain), the first C=C occurs at the sixth carbon atom in the chain as shown below:

Note that the double bonds in the carbon skeleton of omega−3 and omega−6 fatty acids introduces kinks in the 3-dimensional arrangement. These kinks prevent the fatty acid molecules from packing together closely, reducing the strength of the intermolecular forces acting between them, thereby reducing their melting points (as compared to a saturated fatty acid with the same hydrocarbon chain length). For this reason, we find that omega−3 and omega−6 fatty acids are liquids (oils) rather than solids (fats) at room temperature and pressure.

These omega−3 and omega−6 fatty acids are found in such foods as:

• fish oil
• vegetable oils such as olive oil and canola oil
• seeds such as chia, pumpkin and sunflower seeds