Why stearic acid has more melting point than oleic acid?

Both stearic acid and oleic acid are C₁₈ fatty acids but differ in their unsaturation. Stearic is a saturated C₁₈ fatty acid whereas oleic acid is an unsaturated C₁₈ fatty acid.

Saturated fatty acids will show different physical properties compared with unsaturated fatty acids of similar chain length. One of the important property is melting point.

Two factors mainly influence melting point of fatty acids. One is molecular weight and another is unsaturation.

1. Molecular weight

As common with all hydrocarbons, melting point increases with molecular weight.

For example, stearic has a melting point of 69^oC whereas palmitic acid has 63^oC.

Stearic acid is a C₁₈ fatty acid and palmitic acid is C₁₆ fatty acid. Both are saturated fatty acids but stearic acid (C₁₈) has more molecular weight than palmitic acid (C₁₆) hence more melting point.

 

Molecular weight increases with number of carbons and hence chain length of the fatty acid. Therefore C₁₈ fatty acids will have more molecular weight than C₁₆ fatty acids.

As the chain length increases, the Van der Waals forces between fatty acid molecules increases thereby intermolecular forces increase. This results in an increase in the melting point.

2. Unsaturation

Now let’s consider second factor, unsaturation.

Again common with all hydrocarbons, melting point decreases with unsaturation.

For example, oleic acid, linoleic acid and linolenic acid all are unsaturated C18 fatty acids. Oleic acid melts at 13^oC, linoleic acid at -5^oC and linolenic acid at -11^oC.

Here the number of double bonds play important role.

Oleic acid has one double bond,

linoleic acid has two

and linolenic acid has three double bonds.

 

As the number of double bonds (unsaturation) increases, the molecule tends to lie in non-linear fashion and attains more spherical structure. Due to spherical, structure poly unsaturated fatty acids show less contact with other molecules hence less intermolecular interaction.

 

As the intermolecular interaction decreases, Van der Waals forces between molecules decrease and melting point decreases.