Looking at the two alcohols, both have hydrogen bonding and are singly bonded throughout the molecule. However, propylene glycol has a larger electron cloud which to my knowledge would increase its London dispersion forces and thus its boiling point. I already know both molecules exhibit hydrogen bonding since they're both alcohols. However, it seems that ethylene glycol has a higher boiling point despite propylene glycol seemingly having stronger IMFs. I believe this has something to do with molecular shape but I can't seem to find the definitive answer explaining why ethylene glycol has a higher boiling point. I've used https://pubchem.ncbi.nlm.nih.gov/compound/Propylene-glycol#section=RTECS-Number and https://pubchem.ncbi.nlm.nih.gov/compound/1_2-Ethanediol#section=Odor for comparison of the two alcohols.
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$\begingroup$ "larger electron cloud which to my knowledge would increase its IMF" That sounds very wooly. IMF is a pointless catch-all term. Its about hydrogen bonds!! If you have them, everything else is largely irrelevant. $\endgroup$– KarlCommented Dec 20, 2020 at 7:49
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$\begingroup$ I would have other considerations but the fact that propylene glycol has a boiling point about 20 °C higher than that of etandiol should solve your issue. $\endgroup$– AlchimistaCommented Dec 20, 2020 at 9:45
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$\begingroup$ I'm not sure which sources say that propylene glycol has a higher boiling point, but if you've found such a source that'd be of great help! $\endgroup$– Samuel CCommented Dec 20, 2020 at 21:40
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1$\begingroup$ According to Merck, ethylene glycol has a b.p. of 195-198 °C at 1 bar, and propylene glycol has a b.p. of 187 °C at 1 bar. $\endgroup$– Nicolau Saker NetoCommented Dec 20, 2020 at 21:44
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This is all about linearity of the molecule and steric hindrance. Consider the linear structure of ethylene glycol versus the branched structure of propylene glycol. EG can "stack" more easily and therefore engage in more intermolecular bonding than PG. More attraction means more energy required to vaporize. On the other hand, 1,3 propane diol, which is the linear isomer of propylene glycol, has a higher boiling boiling point than EG at around 213C. To further illustrate - 1,4 butanediol (linear) has a higher boiling point than 1,3 butanediol (branched).
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$\begingroup$ Welcome to Chemistry SE! The addition of a reputable reference may help to improve how it is received. Also, commenting on the involvement of EG's terminal hydroxyls to forming intermolecular H bonds would help as well. $\endgroup$– z1273Commented Apr 13, 2021 at 12:03