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I'm interested in the melting/freezing points of organic compounds, but the online literature has data that's all over the map. I can understand that especially for isomers of alkanes, pure samples can be difficult to obtain; but as basic a property as melting point ought to be well-established and easy to reference.

For example, the m.p. listed for 3-methylpentane varies from -118 °C to -163 °C depending on source. That's an enormous spread; in fact, those two numbers reoccur frequently enough that I suspect that sources are quoting older sources possibly back to the early 20th century. And that's far from the only example I could cite.

Are there any reliable sources that have experimentally confirmed the values?

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    $\begingroup$ CRC handbook of chemistry and physics: citable and reliable. 97th edition lists 3-methylpentane on p. 3-386 with m.p. -162.89(0.05) °C; b.p. 63.3(0.5) °C. $\endgroup$
    – andselisk ♦
    Jan 29, 2019 at 23:40
  • $\begingroup$ PubChem and Wikipedia also have almost -163 °C. They aren't 100 % reliable but usually values should be OK. $\endgroup$
    – Mithoron
    Jan 30, 2019 at 1:29
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    $\begingroup$ @andselisk - my 56th edition CRC did not list the melting point. Sigh - perhaps time to finally upgrade... $\endgroup$
    – Jon Custer
    Jan 30, 2019 at 13:56
  • $\begingroup$ I think following open access melting point database will be helpful- pharmadigests.com/melting-point-database $\endgroup$
    – Areful Islam
    May 4 at 13:06

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You are absolutely right about suspecting the reliability of sources, based on their value spread. I think, I found what's going on with those internet (I assumed) sources. Yet, I know everybody agree with me that CRC handbook of chemistry and physics is The Bible for Physical Constants of Organic Compounds. Here is what I found on 2005 Internet Version of CRC handbook of chemistry and physics:

3-Methylpentane (CAS #: 96-14-0): physical form: liquid; Melting Point: $\pu{-162.90 °C}$; Boiling Point: $\pu{63.27 °C}$; and Density: $\pu{0.6598 g/cm^3}$ (pp. 3-398 of Ref.1)

Perfluoro-3-methylpentane (CAS #: 865-71-4): physical form: liquid; Melting Point: $\pu{-115 °C}$; Boiling Point: $\pu{58.4 °C}$; and Density: not listed (pp. 3-458 of Ref.1)

I think some reference sources may have mistaken with values of perfluoro-3-methylpentane as those of 3-methylpentane.

Nonetheless, evem PubChem listed two other values than $\pu{-162.90 °C}$ for melting point of 3-methylpentane (Ref.2): $\pu{-162.90 °C}$ from Lide, D.R., G.W.A. Milne (eds.). Handbook of Data on Organic Compounds. Volume I; 3rd ed.; CRC Press, Inc.: Boca Raton, FL. 1994., p. V4: 3865 (most trusted value since it is PEER REVIEWED - https://toxnet.nlm.nih.gov/cgi-bin/sis/search2/r?dbs+hsdb:@term+@rn+@rel+96-14-0); $\pu{-118 °C}$ from http://www.ilo.org/dyn/icsc/showcard.display?p_version=2&p_card_id=1263 (International Chemical Safety Cards (ICSC) are data sheets intended to provide essential safety and health information on chemicals in a clear and concise way); and third value of $\pu{-245 to -148 °F}$, basically, $\pu{-153.9 to -100 °C}$, from OSHA Occupational Chemical DB (http://www.osha.gov/chemicaldata/chemResult.html?RecNo=764). I can only speculate that they all listed wrong values because they got hits for some other substituted 3-methylpentanes with search keyword of 3-methylpentane. For example, CRC Handbook of Chemistry and Physics listed $\pu{118.27 °C}$ for boiling point of 3-ethyl-3-methylpentane (CAS #: 1067-08-9).

Yet, only CRC Handbook of Chemistry and Physics consistently listed $\pu{-162.90 °C}$ as the melting point of 3-methylpentane since 1994. Thus, I'd take that value at any time.

References:

  1. "Section 3: Physical Constants of Organic Compounds", in CRC Handbook of Chemistry and Physics, Internet Version 2005, David R. Lide, ed., http://www.hbcpnetbase.com, CRC Press, Boca Raton, FL, 2005 (pp. 3-398).
  2. https://pubchem.ncbi.nlm.nih.gov/compound/3-methylpentane#section=Melting-Point
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    $\begingroup$ It's a stretch from "many chemists regard the CRC handbook as a reliable source of data" to "I know everybody agree with me that". Still, good research... $\endgroup$
    – Buck Thorn ♦
    Jan 30, 2019 at 12:25

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