Talk:Hydrogen bond

From Academic Kids

Lone pairs and HBr

Someone help me. I need to find a substance that is highly vicous due to Hydrogen bonding. Email me at emailaddress@rock.com

I think most people tend to think of charge as not able to be less than the fundamental electronic charge, but as often much more (as in "Do not enter! High Voltage!"). So I think it's misleading and/or unhelpful to call partial charges "strong" without mentioning that they are indeed partial or providing other guidance as to how strong "strong" is.

Doesn't HBr have three lone pairs of electrons, not two? I don't find the article's description of why HBr has weaker hydrogen bonding that compelling; theoretically an individual HBr molecule could form four hydrogen bonds, right? The catch is that in a pure HBr solution, there are only as many H molecules as HBr molecules so the total number of bonds to be formed is limited by the H molecules, leaving each molecule with a total of two...

A hydrogen bond only forms when the H atom is attached to either an F,O, or N atom, and is "sufficiently close" to another H atom also attached to an F,O or N atom. The reason is because the F, O and N atoms are very electronegative and their nuclei are very small and (and so the charge density is relatively high). The high electronegativity atom draws the electron away from the hydrogen atom, leaving the proton relatively exposed. The proton (slightly positive in charge) can now approach an electronegative atom, and form the "hydrogen bond". It has to approach an F,O or N atom, because (you can think of it this way) their nuclei are about the same size as the exposed proton, which means the hydrogen bond formed will be a good fit. This is sort of a hand-wavy explanation, but the essentials are all here. Yes, the hydrogen atom does participate in other intermolecular forces, but they aren't anywhere as strong as what we typically think as "hydrogen bonding" - as such, they aren't as important. HappyCamper 05:17, 28 Mar 2005 (UTC)

• Also, the Lewis dot diagram for HBr does indeed indicate that Br has 3 lone pairs. However, it is important to recognize that these "lone pairs" are only used as a heuristic to understand the organizational structure that is at the heart of chemistry. In fact, quantum mechanical calculations have shown (for example, the water molecule), there are actually no "bunny ears" sticking out from the O atom which are often used to represent the two lone pairs there. The chemistry of water just behaves as if it did, and so using lone pairs to designate this should be understood as strictly a tool. HappyCamper 05:17, 28 Mar 2005 (UTC)
• The "catch" that you mentioned has doesn't have much to do with the lack of hydrogen bonding for HBr. It has to do with the fact that the Br atom is very big. Even if the proton from H can hydrogen bond to it, the charge would be spread out over such a large area that the resulting bond would be very very weak. Keep in mind, however that the hydrogen bonds will form and break very frequently if the temperature is high enough. You might be interested to know, for example, that HF forms hydrogen bonds, and in fact, it is possible for HF to form rings of 5 molecules, all bonded together with hydrogen bonds! Granted, the bonds will break and spontaneously form other structures. HappyCamper 05:17, 28 Mar 2005 (UTC)
• And yes, I agree with your first paragraph, but I think in this context it isn't necessary to introduce the complication behind how electronegativities are derived. HappyCamper 05:17, 28 Mar 2005 (UTC)

Proposal for Clarification

I think we should mention on the page somewhere that the hydrogen bond is not necessarily intermolecular. It can be intramolecular as well. Consider the compound H2NCH2CH2CHO for example (1-aminopropanal). The H atom attached to the N atom can hydrogen bond to the aldehyde end! HappyCamper 05:17, 28 Mar 2005 (UTC)

Add relative hydrogen bond strengths

Can someone look up in a table the range relative strengths of hydrogen bonds in these configurations? HappyCamper 05:17, 28 Mar 2005 (UTC)

  F-H ..... H-F
  F-H ..... H-O-R
  F-H ..... H-N-R
R-O-H ..... H-O-R
R-O-H ..... H-N-R
R-N-H ..... H-N-R