One might think that something as simple as the melting and boiling points of an organic compound would be pretty well established. But I’m finding different figures from different sources, so I’m trying to find numbers that can be taken as reliable. Specifically the m.p. and b.p. for two related hydrocarbons: 2-methylpentane and 3-methylpentane. The melting points in particular seem to be all over the place. My interest in this is that if the given numbers are correct, they would indicate a remarkably low melting point for a compound of their molecular weight, and an astonishingly wide liquid range. That’s why I’m not sure I can trust the data I’ve found online.
You’re finding different values using Google, or using traditional scientific sources? If the latter, that would be odd. But if the former, not.
Try the NIST Webbook. I get:
2-methylpentane: Tm = 120K, Tb = 334K
3-methylpentane: Tm = 155K, Tb = 336K
For comparison n-hexane: Tm = 178K, Tb = 342K
I’m not really seeing any strangeness here. The liquid range is pretty typical, and the more compact shape of the branched hydrocarbons produces the typical slight reduction in melting and boiling points associated with a lower average London force between the molecules.
The CRC Handbook of Chemistry and Physics Internet Version 2005 <http://www.hbcpnetbase.com> agrees with those except for claiming that the Tm of 3-methylpentane is -162.9 C =~111K. I’d sorta like to know if 2-methylpentane or 3-methylpentane has the lower Tm
That is a little odd. The first thing I would do, however, is check a more recent CRC. Could be a typo in that edition. I can’t think of any theoretical reasons to pick one over the other, unfortunately.
It might be quite difficult, experimentally, to separate the isomers well enough to get good melting point data. After all, if we believe the boiling point data, the two boiling points differ by only 2 degrees. Going to take a lot of theoretical plates to fractionate that. You can use crystallization, it seems, but that’s at cryogenic temperatures, which makes it rather a pain. And the motivation is probably generally low – it’s hard to think of an application where 3-methylpentane will work but 2-methylpentane won’t, and vice versa. The start of a chemical synthesis would count, but it’s unusual to start a synthesis from an alkane, i.e. without any functional group with which to work.
It’s kind of an interesting case, actually. May I ask why you want to know?
Awhile back when we were going through the worst winter in thirty years, I decided to accumulate a list of various physical/chemical milestones between the freezing point of water and the liquifacation point of oxygen. I knew that ethanol freezes at a pretty low temperature, but I wondered: what substance that’s liquid at STP has the lowest freezing point? The lowest references I could find were 2- and 3-methyl pentane, but the different cites for 3- meant I couldn’t be sure.
I recently posted this question on a chemistry forum and got back a great answer: a chemical known by various formula names which I will abbreviate here as Diethylmethylamine (C2H5)2(CH3)N boils at 63°C and freezes at an astounding -196°C. The latter is approximately the temperature of liquid nitrogen; and unlike liquid nitrogen it wouldn’t form a vapor barrier if something at room temperature was immersed in it. That’ would be one damn cold dunking. 
IANA Chemistry-anything, but 250C of liquid range for something even that complex sounds pretty amazing. An outlier for sure. Thanks for sharing.
I just noticed something: diethylmethylamine and 3-methylpentane are very similar in configuration: both consist of two ethyl groups and one methyl group bonded to a central atom, nitrogen and carbon respectively. Maybe that configuration promotes low freezing points? I wonder if there’s an analogous chemical with boron as the central atom.
Closest I could find was triethylborane, which doesn’t have a particularly low melting point. It’s highly pyrophoric so not exactly something you’d play with. 
Yeah, you’re pretty much guaranteed to meet the green fireball now and then when working with alkyl boranes.
Chemspider lists a bp for Et2MeB, but I don’t see an mp.Diethyl(methyl)borane | C5H13B