When comparing alkynes, alkenes, and alkanes of similar molecular weight and structure(unbranched), I am pretty sure alkynes will have the highest boiling point due to the fact of increased electron density from your pi orbitals. It is more "polarizable" so you have an increased dipole-dipole interaction between molecules thus increasing bp. I think that sounds right, what do you think?
I kinda thought so, but after looking for 2 hours on the net, no good answers, so I graphed alkane vs terminal alkene and other stuff.. (looked up the freaking BP manually...grrr....) and these are the conclusions...
1. Boiling boints of cis alkenes are higher than trans because of increased ability to stack and therefore increased symmetry
2. Boiling points of terminal alkenes
are LESS than those of their alkane counterparts (like ethene vs ethane)
3. Cyclic counterparts (e.g. cylcohexane) have higher boiling pointes because they can pack together better.
4. Terminal alkenes have lower BP than corresponding alkanes, BUT, nonterminal alkenes have a higher BP slightly (these will be cis and trans, with cis generally being higher)
5. Alkynes will have a higher BP than alkenes or alkanes.
So, to sum it up, here is the order, for corresponding alkanes and -kenes and -kynes...
cycloalkanes>alkynes> cis alkenes>trans alkenes>alkanes>terminal alkenes
I just need to post the graph now...