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I learnt about intramolecular hydrogen bonding today, which occurs between molecules such as ortho-nitrophenol. What I was told is that in case of intramolecular bonding, the molecules separate from each other, opposite of what happens during intermolecular hydrogen bonding.

I don't understand why is this. Why would attraction within a molecule cause separation of the molecules? There is obviously some intermolecular hydrogen bonding present in this case, but I understand that this might be negligible. Still, why should the molecules spread away from each other?

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Molecules with inter-molecular hydrogen bonds tend to associate with one another, while molecules with intra-molecular hydrogen bonds tend to associate with themselves.

Ortho- and para-nitrophenol are often used as examples of molecules with intra- and inter-molecular hydrogen bonds respectively (see picture).

ortho/para-nitrophenol and their hydrogen bonds

Because of their molecular structures the ortho isomer can form an intramolecular hydrogen bond with itself, whereas the para isomer can only hydrogen bond with another molecule of p-nitophenol. One would say that the para isomer associates with other p-nitrophenol molecules through hydrogen bonding, whereas the ortho isomer associates with itself through hydrogen bonding. Because the para isomer associates with other molecules of p-nitrophenol through hydrogen bonding, it takes more energy to break up this association and cause the molecule to melt or boil. On the other hand, the ortho isomer melts and boils at lower temperatures because there are fewer intermolecular forces (such as the hydrogen bonding) holding the ortho molecules together.

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  • $\begingroup$ Would it be possible to calculate the boiling/melting point of substances in an ideal situation where no hydrogen bonding existed? $\endgroup$ – Gummy bears Jul 10 '14 at 12:32
  • $\begingroup$ There is no simple method to do this that I am aware of. There are software packages that attempt to estimate various physical properties of compounds. I'm not sure how accurate or available they are - I think research groups are still trying to improve\refine the packages. $\endgroup$ – ron Jul 10 '14 at 13:22
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    $\begingroup$ What I mean to say is: is the boiling point of intramolecular hydrogen bonded substances less than that if there was no hydrogen bonding present? $\endgroup$ – Gummy bears Jul 10 '14 at 14:31
  • $\begingroup$ If all hydrogen bonds, both inter- and intra-molecular, were removed, then (I think) generally, compounds that had inter-molecular hydrogen bonds would now boil lower and compounds that had intra-molecular hydrogen bonds would now boil higher. $\endgroup$ – ron Jul 10 '14 at 15:00
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    $\begingroup$ Yes, there is still some intermolecular attraction, but less than there would be without the H-bonding. I suspect that the H-bonding reduces the dipolar nature of the groups involved (the NO2 and OH groups in this case) and thereby lessens intermolecular electrostatic attractive forces that dipoles can create. $\endgroup$ – ron Jul 10 '14 at 15:52

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