Why is there a difference in melting points of stearic acid and oleic acid?

Answer 1

Acetic acid has a very small nonpolar end consisting of just a methyl group counteracted by a carboxylic acid. The carboxylic acid has the ability to deprotonate and become charged, hydrogen bond, and is quite polar. All of these factors contribute to its ability to interact readily with an aqueous solvent such as water through h-bond interactions.

Stearic acid, on the other hand, contains the same carboxylic acid functional group, however this group is far outweighed by the very, very long saturated nonpolar carbon chain. The aqueous solvent has nothing to interact with and therefore the stearic acid would rather interact via Van der Waals interactions with itself.

Answer 2

The differences in solubility of carboxylic acids acetic acid and benzoic acid should be very minimal, since both contain a carboxyl group. However, the differences that are there are because of size. The benzene ring on the benzoic acid is a bulky structure, making it more nonpolar and less soluble, and also more heavy and less soluble.

Answer 3

The difference in melting points is ONLY caused by the single cis-formed (unsaturated) double bond in the middle of the molecule (at the 9th C-atom) of oleic acid (cis-Δ9-Octadecenoic acid).

• Stearic acid (C18:0) Melting point 69.6 °C (343 K)
• Oleic acid(C18:1,cis-9) Melting point 13-14 °C (286 K)

This double bond is a 'structure mobility' reducing C=C (unsaturated) bond in oleic acid: it is far more rigid(fixed-angled of 120o) than the free rotatable C-C bonds (rotatable angle 105o).

Both structures can be viewed in 'Related links' just below this answer page.