The general formula for a saturated fatty acid is CnH2nO2
then 2n = 2 × 16 = 32
molecular formula of the saturated fatty acid is C16H32O2
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).
Now, let's compare the stucture of a saturated fatty acid with the structure of a 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 C16H(32-2)O2 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
then 2n-2 = (2 × 16) -2 = 32 - 2 = 30
molecular formula of the monounsaturated fatty acid is C16H30O2
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).
Now, 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
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:
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:
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.
Below is a summary of some properties of fatty acids:
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