Question

Do saturated and unsaturated fatty acids have high or low boiling points?

Answer 1

Think about it in terms of structures of the saturated and unsaturated fatty acids.

Saturated fatty acids have these polar head groups and long, straight hydrocarbon chains. These pack tightly - like packing together a bunch of pencils and wrapping an elastic band around them - with many intermolecular dipole and induced dipole interactions between adjacent fatty acids.

By contrast, the double bond (or bonds) in the unsaturated fatty acids introduces a rigid kink into the molecule. This would be like trying to pack together a bunch of broken pencils - the angled portion of the hydrocarbon tails can't pack tightly. They can't form anywhere near as many intermolecular interactions with each other, so the forces which hold them together are much weaker.

Now think of boiling as providing enough energy to disrupt the intermolecular bonds which hold the fatty acids together. When there are a lot of bonds to break - in the saturated fatty acids - you need a lot of energy. When there are not a lot of bonds to break - in the unsaturated fatty acids - you don't need as much energy. So the saturated fatty acids have higher boiling points and the unsaturated fatty acids have lower boiling points.

Always, always, always cast problems like these in terms of the physical interactions between molecules.

Answer 2

I said "intermolecular bonds." For the record, I'd like to change that to "intermolecular forces." Sorry for the confusion. :D

Answer 3

Thank you sooo much :)