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In the article “Boiling Points of the Family of Small Molecules, CHwFxClyBrz: How Are They Related to Molecular Mass?”, Laing gave a useful examination of the boiling points of small molecules versus molecular mass (1). However, a molecule escaping from a liquid is not closely analogous to “a satellite breaking free from the earth’s gravitational field” with the requirement of “a minimum escape velocity”, such that the required kinetic energy is proportional to the mass of the satellite at that escape velocity. The difference is that all of the mass of the satellite is acted upon by the restraining force (or curved space–time) of gravity, while the mass of the boiling molecule is practically irrelevant to the crucial van der Waals, London, or mutual-polarization forces in the liquid. Thus germanium tetrachloride, hexafluorobenzene, pentacarbonylruthenium, and tetrapropyltin do not boil at appreciably higher temperatures than carbon tetrachloride, benzene, pentacarbonyliron, or tetrapropylmethane, respectively, where, in most cases, the additional potentially polarizing and polarizable electrons of the heavier molecules are buried in the center. Much more information is available elsewhere (2) and in references therein. The boiling points of a great variety of substances are predicted there. Note that highly polar molecules are explicitly excluded, and that transitional-element compounds, for example, fit the published correlation quite well, excepting only WCl6. Literature Cited- Laing, M. J. Chem. Educ. 2001, 78, 1544.
- Rich, R. L. J. Chem. Educ. 1995, 72, 9–12; on using polarizabilty to predict boiling points.
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