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Today I learned about intramolecular hydrogen bonding, which occurs in molecules such as ortho-nitro-phenol.

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 this is. 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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I would recommend asking this question at Chemistry Stack Exchange, as it deals more directly with Chemistry instead of Physics. – Dave Coffman Jul 9 '14 at 17:02
@DaveCoffman Well it does have to do with physics as well. But if you insist, can you tell me how to migrate a question? – Gummy bears Jul 9 '14 at 17:03
Right, it does deal with physics, and I suppose that it would be OK here, but I would also ask it at Chemistry Stack Exchange. I have no idea how to migrate a question, but instead I would recommend just asking it in both places - it might be interesting to see the differences in answers given. – Dave Coffman Jul 9 '14 at 17:09
Cross-posting is not recommended unless the question does not get an answer at one of the two sites. I'd wait a few days before asking over at Chemistry.SE. – Kyle Kanos Jul 9 '14 at 18:36
Yes cross-posting is frowned upon. If you ever want to migrate a question, use a custom moderator flag. – Chris White Jul 10 '14 at 11:00

1 Answer 1

Hydrogen bonding arises when a chemical bond is polarised to one end of it has a slight positive charge and the other has a slight negative charge. In the case of o-nitrophenol it's mainly the OH bond that is polarised - the H atom has a slight positive charge and the O atom has a slight negative charge. The charge separation means the OH bond has an electric dipole.

Electric dipoles interact with each other, so the dipole on one o-nitrophenol molecule can attract the dipole on a neighbouring o-nitrophenol molecule. This creates the attractive force between the molecules known as a hydrogen bond. There is an extended description of this on this web site.

Just to complicate matters o-nitrophenol can also exhibit intra-molecular hydrogen bonding. This is because the OH and NO$_2$ groups are next door to each other, and the H on the OH group can interact with the O on the NO$_2$ group.

Response to comment:

Intramolecular hydrogen bonding will not cause molecules to repel each other, but it will reduce the attractive interactions between molecules. This is simply because in o-nitrophenol the OH group forms a hydrogen bond with the adjacent nitro group and that reduces its ability to form hydrogen bonds with other molecules. In p-nitrophenol the OH and nitro groups are on opposite sides of the molecule so there can't be any intramolecular hydrogen bonding and therefore the intermolecular hydrogen bonding is stronger. So o-nitrophenol has a melting point of 44ºC while p-nitro[henol has a melting point of 113ºC.

So intramolecular hydrogen bonding won't cause the molecules to separate, but it will make them bind together less strongly.

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Well yes, I understand this concept. However, I believe you missed out on a key point of my question. I was told that intramolecular hydrogen bonding causes the molecules to separate from each other, a reason why the melting point, etc. decrease in molecules with high levels of intramolecular HB. I don't understand why this is. Why would they separate from each other? – Gummy bears Jul 10 '14 at 12:29
@Gummybears: I've updated my answer to respond to your comment – John Rennie Jul 10 '14 at 14:17

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