Boiling points: Alkanes VS Alkenes

Question

Hello! This is my first time on Wyzant and I guess I'll be trying it out now. Anyways, I have a question: Why do alkanes generally have a higher boiling points than alkenes?

I have tried googling, but I just ended up confusing my head with these long stack exchange answers :O Hopefully someone can help me get things straight

Answer 1

The boiling point of a substance is the temperature at which the vapor pressure of the solution is equal to the atmospheric question. In turn, the vapor pressure is dependent on the intermolecular forces (IMFs) holding the molecules of the liquid together. So, the stronger these forces, the higher the boiling point.

So, we want to look at the intermolecular forces holding an alkane together and compare it to those holding an alkene together. Remember, we're looking at INTERmolecular forces, not INTRAmolecular forces.

Both alkanes and alkenes are nonpolar and will have only dispersion forces. The alkane will have greater dispersion forces because for the same carbon length (e.g. comparing hexane to hexene), the alkane will have more electrons and a greater area over which to spread those electrons. This will increase the dispersion force and so we would expect the alkane to have a slightly higher boiling point than the alkene with the same number of carbon atoms.

Hope that makes sense.