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What substance that is liquid at STP has the lowest freezing/melting point? I think it's either 2-methylpentane or 3-methylpentane, but I get different numbers from different sources.

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    – andselisk
    Commented Apr 27, 2022 at 16:51

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After a bit of random searching through small molecules, I found this very large dataset with 28 645 experimentally measured melting points. Sorting them by increasing melting point and checking the first few entries individually, it seems that N,N-diethylmethylamine holds the record, with a melting point of −196 °C (!) and a boiling point of 63 to 65 °C.

I also mentioned another possibility for very low melting liquid substances at room temperature, namely ionic liquids. Even though the species present are much larger and have ionic interactions, presumably the ions pack so extremely poorly in the solid that freezing is very unfavourable. Aldrich already offers multigram quantities of ionic liquids melting at −88 °C, and ongoing research offers even lower melting alternatives, such as the ones in Reiter et al. [1] which purportedly do not crystallize until −150 °C. This limit may well be improved upon in the next few years.

If mixtures are allowed as well as pure substances, then a combination of small organic molecules or ionic liquids will very likely reach even lower temperatures than the examples above without freezing.

Reference

  1. Reiter, J.; Jeremias, S.; Paillard, E.; Winter, M.; Passerini, S. Fluorosulfonyl-(Trifluoromethanesulfonyl)Imide Ionic Liquids with Enhanced Asymmetry. Phys. Chem. Chem. Phys. 2013, 15 (7), 2565. DOI: 10.1039/c2cp43066e.
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  • $\begingroup$ Great answer. Quick unrelated side question: why is "N,N-diethylmethylamine" not called "N-ethyl-N-methyl-ethylamine"? (And you are right that e.g. Sigma-Aldrich uses the same name as you)...is that a colloquialism or the bona fide correct name? $\endgroup$
    – Curt F.
    Commented Mar 8, 2015 at 5:08
  • $\begingroup$ @CurtF. I'm not sure, I think the second is formally more correct (IUPAC actually would recommend N-ethyl-N-methylethan-1-amine) but the first is just more concise and easier to pronounce. $\endgroup$ Commented Mar 8, 2015 at 11:09
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    $\begingroup$ The true melting point for diethylmethylamine is yet unknown; the original reference for this "melting point" value is J. Org. Chem. 22, 840-843 (1957), which states that the material became a glass at liquid nitrogen temperature. Also of interest is that the same exact data points are given for isopropyldimethylamine and diisopropylmethylamine, but have apparently never been misconstrued as melting points for those compounds! $\endgroup$ Commented May 25, 2022 at 15:49
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    $\begingroup$ When dealing with such a huge dataset it's hard to avoid the small fraction of incorrect reports. The -196C=77K value is suspicious since it is the boiling point of liquid nitrogen. Wikipedia claims 130K as the melting point (or is this the glass point?) which is a bit more reasonable but still very impressive! $\endgroup$ Commented Oct 24, 2022 at 22:27

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