Physics Stack Exchange is a question and answer site for active researchers, academics and students of physics. Join them; it only takes a minute:

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

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?

share|cite|improve this question
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

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.

share|cite|improve this answer
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

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.