two fatty acids: Arachidic acid and arachidonic acid. Arachidic acid is a saturated fatty acid with a straight 20-carbon-long chain. Arachidonic acid is an unsaturated fatty acid with 20-carbon-long chain containing four cis- carbon-carbon double bonds, and thus curved back onto itself

Question

While similar in name, they have different physical properties. For example, arachidic acid, 1, has a higher boiling point (328°C) than arachidonic acid, 2 (169°C). What is the main reason the boiling point of arachidic acid, 1, is so much higher than that of arachidonic acid, 2?

A) Acid 1 has a longer hydrocarbon tail than Acid 2, and thus a higher interaction energy.
B) Acid 1 has a shorter hydrocarbon tail than Acid 2, and thus a higher interaction energy.
C) Acid 1 has a higher molecular weight, and thus larger interaction energy than than Acid 2
D) Acid 1 has a higher molecular weight, and thus smaller interaction energy than than Acid 2
E) Acid 1 has a straight hydrocarbon tail and thus a higher interaction energy than than Acid 2, which has a bent tail.

My answer is A

oobleck · Accepted Answer

I think I'd go with E. google turned up the following explanation:

Note that as a group, the unsaturated fatty acids have lower melting points than the saturated fatty acids.

The reason for this phenomenon can be found by a careful consideration of molecular geometries. The tetrahedral bond angles on carbon results in a molecular geometry for saturated fatty acids that is relatively linear although with zigzags. See graphic on the left.

This molecular structure allows many fatty acid molecules to be rather closely "stacked" together. As a result, close intermolecular interactions result in relatively high melting points.

On the other hand, the introduction of one or more double bonds in the hydrocarbon chain in unsaturated fatty acids results in one or more "bends" in the molecule. The geometry of the double bond is almost always a cis configuration in natural fatty acids. These molecules do not "stack" very well. The intermolecular interactions are much weaker than saturated molecules. As a result, the melting points are much lower for unsaturated fatty acids.