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[MrShorty]

I know as ChemE's we don't always get too excited by theoretical/hypothetical discussions, but I found this an interesting revelation.

Ever since undergraduate Organic Chem, I'd always believed the general rule that, as the alkane molecule gets smaller, the melting point goes down. I won't bore you with the reasons, but I recently looked up the melting points of methane, ethane, propane, and butane, and found something interesting - the trend fails. Methane (91 K) and ethane (90 K) both have higher melting points than propane (85 K).

My colleague and I tried to think of an explanation, but couldn't come up with anything off the top of our heads. The author(s) of the wikipedia article on alkanes made an interesting observation. They noted that the trend for the odd numbered alkanes seems lower than the even numbered alkanes and attributed it to how effectively the molecules "packs" into the crystalline structure. So, maybe that has something to do with it.

Thanks for reading. If you have any interesting articles or explanations, I'd be interested to read them.

[kkala]

Interesting post about alkane properties. Mentioned wikipedia (http://en.wikipedia.org/wiki/Alkane) presents a relevant diagram to visually indicate melting point (along with boiling point) of first 14 alkanes. Melting point irregularity is strong for CH4 and C3H8; it gets smaller and smaller for higher alkanes.
Wikipedia also refers to alkane heat of combustion (see alsohttp://www.ausetute....u/usehydrc.html), reporting that it increases by about 650 kJ/mol for each CH2 added.