I have noted a little trend in the boiling points of alkynes, alkenes and alkanes. Upon searching for the explanation, I have chanced upon an answer written by M.A.R. (Alkane, Alkene, Alkyne boiling point). In his answer, he explains the boiling point trend which goes in the order alkyne > alkane > alkene by invoking the concept of polarisability of the electron cloud. This makes perfect sense as polarisability is linked to the strength of intermolecular forces and the strength of such forces determines the boiling points of the substances in the liquid state.

However, my point of contention is his explanation of how alkynes are more polarisable due to their more electron-dense triple bond and how alkanes are more polarisable than alkenes because they have two more electrons.

This leads me to wonder what is meant by polarisability. Evidently, I have learnt it in school and I know perfectly well what is meant by it (Refer to https://en.wikipedia.org/wiki/Fajans%27_rules). However, it seems to me that M.A.R.'s "polarisability" is completely different from what I perceive it to be.

M.A.R. says that the more electron-dense triple bond is more polarisable. However, there are so many electrons packed in that small space. That is analogous to a small anion and evidently, small anions are very much not very polarisable.

M.A.R. then seems to contradict himself by not subscribing to the same logic in his explanation of the case of the boiling points of alkanes vs. alkenes. By his previous logic used in the case of alkynes, the double bond being more electron-dense than the single bond would mean that the polarisability of alkenes is more than that of alkanes. Instead, he uses another reasoning that the alkenes have 2 less electrons than the alkanes.

Thus, this leads me to question: Is M.A.R.'s use of "polarisability" correct in a chemistry sense?

On a side note, I would have rationalised this boiling trend using electronegativity and bond dipoles because atoms with sp-hybridised orbitals are more electronegative than sp2 and subsequently sp3. However, this is also flawed because I would not be able to explain why alkane > alkene.

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    $\begingroup$ I'm not entirely convinced by the explanation you read, but at the same time I'm not keen on speculating on the reasons why there is such a trend. I think the definition of polarisability is perfectly fine (mathematically it is described with a matrix). The issue is probably more with how the molecular structure links to polarisability. $\endgroup$ – orthocresol Sep 16 '17 at 12:26
  • $\begingroup$ @orthocresol Indeed, this is just speculation and all we are proposing, be it electronegativity and dipoles or polarisability, are all hypotheses. However, I just want to point out that M.A.R.'s answer seems to be wrong in some areas so that they can be corrected and improved upon. This is so as to prevent users of the site from being misled. $\endgroup$ – Tan Yong Boon Sep 16 '17 at 13:18
  • $\begingroup$ Yeah. You should probably drop him a comment on his answer, since not everybody will see this. $\endgroup$ – orthocresol Sep 16 '17 at 13:18

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