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Fatty Acids

Key Concepts

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Fatty Acids: Structure and Formula

Fatty acids are long chain carboxylic acids, they contain the carboxyl, COOH, functional group.

The general formula for a fatty acid is R-COOH where R represents a long hydrocarbon chain.

If the hydrocarbon chain, R, contains only single bonds between the carbon atoms, the fatty acid is said to be saturated.

Saturated Fatty Acids
Name Condensed Structural Formula Molecular
formula
palmitic acid CH3-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-COOH

( CH3(CH2)14COOH )

C16H32O2
stearic acid CH3-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-COOH

( CH3(CH2)16COOH )

C18H36O2

The general formula for a saturated fatty acid is CnH2nO2

For example:

  • If n=16
    then 2n = 2 × 16 = 32
    molecular formula of the saturated fatty acid is C16H32O2
  • If n=18
    then 2n = 2 × 18 = 36
    molecular formula of the saturated fatty acid is C16H36O2

If the hydrocarbon chain, R, contains ONLY one double bond between carbon atoms (C=C), then the fatty acid is said to be monounsaturated (the prefix "mono" means one, "unsaturated" refers to the presence of a C=C or C≡C).

Monounsaturated Fatty Acids
Name Condensed Structural Formula Molecular
formula
palmitoleic acid CH3-CH2-CH2-CH2-CH2-CH2-CH=CH-CH2-CH2-CH2-CH2-CH2-CH2-CH2-COOH

( CH3(CH2)5CH=CH(CH2)7COOH )

C16H30O2
oleic acid CH3-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH=CH-CH2-CH2-CH2-CH2-CH2-CH2-CH2-COOH

( CH3(CH2)7CH=CH(CH2)7COOH )

C18H34O2

Compare the stuctures of a saturated and monounsaturated fatty acid.
In order to introduce ONE C=C into the saturated fatty acid known as palmitic acid (C16H32O2) to make the monounsaturated fatty acid known as palmitoleic acid, 2 atoms of hydrogen must be eliminated from the structure, and formula, of palmitic acid, so the molecular formula for palmitoelic acid is C16H32-2O2 which is C16H30O2

For fatty acids containing the same number of carbon atoms, the monounsaturated fatty acid formula contains 2 less hydrogen atoms than the formula for the saturated fatty acid.

General molecular formula for a monounsaturated fatty acid is CnH2n-2O2

For example:

  • If n=16
    then 2n-2 = (2 × 16) -2 = 32 - 2 = 30
    molecular formula of the monounsaturated fatty acid is C16H30O2
  • If n=18
    then 2n-2 = (2 × 18) -2 = 36 - 2 = 34
    molecular formula of the monounsaturated fatty acid is C18H34O2

If there is more than one C=C in the structure of the hydrocarbon chain of a fatty acid molecule, the fatty acid is said to be polyunsaturated (the prefix "poly" means many, "unsaturated" refers to the presence of C=C or C≡C).

Polyunsaturated Fatty Acids
Name Condensed Structural Formula Molecular
formula
linoleic acid CH3-CH2-CH2-CH2-CH2-CH=CH-CH2-CH=CH-CH2-CH2-CH2-CH2-CH2-CH2-CH2-COOH

( CH3(CH2)4CH=CHCH2CH=CH(CH2)7COOH )

C18H32O2
α-linolenic acid CH3-CH2-CH=CH-CH2-CH=CH-CH2-CH=CH-CH2-CH2-CH2-CH2-CH2-CH2-CH2-COOH

( CH3CH2CH=CHCH2CH=CHCH2CH=CH(CH2)7COOH )

C18H30O2

Compare the formula for the monounsaturated fatty acid known as oleic acid, C18H34O2 with the formula of the fatty acid containing 18 carbon atoms but 2 double bonds, linoleic acid, C18H32O2.
Increasing the number of double bonds by 1 reduces the number of hydrogen atoms in the structure (and formula) by 2.

In general, we can say that the molecular formula for a fatty acid will be CnH2n-2xO2
where n = total number of carbon atoms in the structure of the fatty acid
and x = number of double bonds between carbon atoms

For example:

  • stearic acid (saturated fatty acid)
    n = 18
    2n = 2 × 18 = 36
    x = 0
    2x = 2 × 0 = 0
    2n - 2x = 36 - 0 = 36
    CnH2n-2xO2 is C18H36O2
  • oleic acid (monounsaturated fatty acid, one C=C)
    n = 18
    2n = 2 × 18 = 36
    x = 1
    2x = 2 × 1 = 2
    2n - 2x = 36 - 2 = 34
    CnH2n-2xO2 is C18H34O2
  • linoleic acid (polyunsaturated fatty acid, two C=C)
    n = 18
    2n = 2 × 18 = 36
    x = 2
    2x = 2 × 2 = 4
    2n - 2x = 36 - 4 = 32
    CnH2n-2xO2 is C18H32O2
  • α-linolenic acid (polyunsaturated fatty acid, three C=C)
    n = 18
    2n = 2 × 18 = 36
    x = 3
    2x = 2 × 3 = 6
    2n - 2x = 36 - 6 = 30
    CnH2n-2xO2 is C18H30O2

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Properties of Fatty Acids

The physical properties of fatty acids, that is, melting point and solubility, is determined by the nature of the long hydrocarbon chain.
Fatty acids are non-polar molecules. Even though the carboxyl group is a polar functional group, the interactions between fatty acid molecules, and between fatty acid molecules and a solvent, are dominated by the nature of the long non-polar hydrocarbon chain.
Consider the saturated fatty acid known as palmitic acid, C16H32O2:

CH3-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2 -COOH
long non-polar hydrocarbon chain carboxylic acid functional group

In a sample of pure palmitic acid, any interactions between carboxylic acid functional groups would be minimal because practically all of intermolecular interactions occur between those long non-polar hydrocarbon chains, and these interactions are the weak intermolecular forces known as London forces or dispersion forces. However, because saturated fatty acids have a neat zig-zag structure they pack together well:

C C C C C C C C C C C C C C C C C C C C C C C C


and so there are lots of these interactions!
As the length of the saturated hydrocarbon chain increases, there are even more interactions between molecules!
When you studied the properties of carboxylic acids you would have noted that the melting point of alkanoic acids (saturated carboxylic acids) increases as the length of the non-polar hydrocarbon chain increases. As the length of the non-polar hydrocarbon increases, the extent of the weak van der Waals attraction (London or dispersion forces) between the molecules increases. This results in more energy being required to weaken the attraction between the molecules to melt the substance.
A saturated fatty acid is just a long chain alkanoic acid. As the length of the non-polar hydrocarbon chain increases, the melting point of the saturated fatty acid increases. Saturated fatty acids are expected to be solids at room temperature and pressure.

But what happens if the fatty acid molecule is unsaturated?
In naturally occuring fatty acids, introducing 1 or more double bonds into the long hydrocarbon chain causes the chain to "kink" (cis geometry). Oleic acid, a monounsaturated fatty acid, C18H34O2, with this kink is called cis-oleic acid and its skeletal structure is shown below:

As a result of this "kink", these molecules do not pack together as well so the extent of the weak intermolecular forces acting between the molecules is less. Therefore it requires less energy to melt an unsaturated fatty acid. The melting point of an unsaturated fatty acid is expected to be less than the melting point of a saturated fatty acid.

The long non-polar hydrocarbon chain of a fatty acid also determines its solubility in water and other solvents.
Polar water molecules are more strongly attracted to each other via hydrogen-bonds than they are to fatty acid molecules because the only interaction they can have with fatty acid molecules is via the weak intermolecular forces known as London forces or dispersion forces.
Therefore, fatty acids do not dissolve in water, fatty acids are said to be insoluble in water.

However, fatty acids will be soluble in non-polar solvents like hydrocarbons, fats and oils, where the non-polar hydrocarbon chains can interact with the non-polar solvent molecules via the weak intermolecular forces known as London forces or dispersion forces.

In terms of the chemical properties of fatty acids, we expect unsaturated fatty acids to undergo addition reactions, such as with halogens like iodine (I2), while saturated fatty acids can not undergo addition reactions.

Summary of the some properties of fatty acids:

Type General
Formula
Example Properties
Saturated
(no double bonds)
CnH2nO2
(n = no. carbon atoms)
CH3(CH2)14COOH
palmitic acid

CH3(CH2)16COOH
stearic acid

⚛ solids
⚛ unreactive
⚛ relatively non-polar
⚛ insoluble in water
⚛ more in animal fats

Monounsaturated
(1 double bond)
CnH2n-2O2
(n = no. carbon atoms)
CH3(CH2)7CH=CH(CH2)7COOH
oleic acid
⚛ softer than saturated fatty acid
⚛ reactive
⚛ relatively non-polar
⚛ insoluble in water

Polyunsaturated
(> 1 double bond)
CnH2n-2xO2
(n = no. carbon atoms
x = no. double bonds)
CH3(CH2)4CH=CHCH2CH=CH(CH2)7COOH
linoleic acid
⚛ likely to be oils
⚛ reactive
⚛ relatively non-polar
⚛ insoluble in water
⚛ more in vegetable oils

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Omega-3 Fatty Acids and Omega-6 Fatty Acids

Omega-3 fatty acids and omega-6 fatty acids are different classes of polyunsaturated fatty acids.

Chemists number the carbon chain of a fatty acid starting from the carbon of carboxyl, COOH, functional group as 1, using IUPAC rules for nomenclature.

Biologists, however, start counting from the last carbon atom in the chain.
The last letter of the greek alphabet is omega (ω) so biologists call the last carbon atom in the chain omega (ω), which is numbered as 1. Note that the first letter of the greek alphabet is alpha (α), so biologists refer to the carbon atom of the carboxyl functional group as the alpha carbon atom.

IUPAC
(chemistry)
16   15   14   13   12   11   10   9   8   7   6   5   4   3   2   1  
  CH3 - CH2 - CH = CH - CH2 - CH = CH - CH2 - CH = CH - CH2 - CH2 - CH2 - CH2 - CH2 - COOH  
biology 1
(ω)
  2   3   4   5   6   7   8   9   10   11   12   13   14   15   16
(α)
 

The fatty acid molecule above is classified as an omega-3 (ω-3) fatty acid because the first double bonded carbon atom occurs on the third carbon atom from the omega carbon atom (ω).

Using the same logic, an omega-6 fatty acid (ω-6 fatty acid) will be a polyunsaturated fatty acid in which the first double bond will occur on the sixth carbon atom along from the omega carbon atom (ω).
An example of an omega-6 fatty acid is shown below:

IUPAC
(chemistry)
18   17   16   15   14   13   12   11   10   9   8   7   6   5   4   3   2   1  
  CH3 - CH2 - CH2 - CH2 - CH2 - CH = CH - CH2 - CH = CH - CH2 - CH = CH - CH2 - CH2 - CH2 - CH2 - COOH  
biology 1
(ω)
  2   3   4   5   6   7   8   9   10   11   12   13   14   15   16   17   18
(α)
 

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-linoleic 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 name1 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 name2 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:

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1. It might also be noted that each of the double bonds has a cis geometry, or Z configuration in absolute geometry, so a better IUPAC name is (9Z,12Z,15Z)-octadeca-9,12,15-trienoic acid

2. It might also be noted that each of the double bonds has a cis geometry, or Z configuration in absolute geometry, so a better IUPAC name is (9Z,12Z)-octadeca-9,12-dienoic acid