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Topic: Hydrogen Bond  (Read 4834 times)

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Offline AlbertoA

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Hydrogen Bond
« on: August 19, 2011, 08:29:58 PM »
Can the molecules of HCl can form a Hydrogen bond? and Why?
PLEASE don't say answers like "HB can only exist in X-H -- X where X is O, F or N"
I've read a new definition in a book, i don't remember the author, but it bassically says that and HB is formed when an electronegative atom is bonded with a H, so the H will have a partial positive cahrge allowing HB with the other electronegative atom... I think I read it in "Inorganic chemistry" by Housecroft
Also, does C-H --- O exists?

Offline Schrödinger

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Re: Hydrogen Bond
« Reply #1 on: August 20, 2011, 12:37:14 AM »
Let me answer your question about C-H---O first : There is no hydrogen bonding here. For H-bonding, as you have probably already read, you need a partial positive charge on the H. But the electronegativity difference between C and H is too less for any kind of substantial partial positive charge development on H. Hence the negatively charged (partial) oxygen cannot get hydrogen bonded to the C-H hydrogen.

This charge separation is substantial in the case of F, N and O. i.e, if the H is bonded to these. So, F-H---F, O-H---O, N-H---N, O-H---F, O-H---N, (and all the other permutations) are considered to be hydrogen bonded ones.

HCl is not hydrogen bonded, although there is a good amount of dipole-dipole attraction. Due to the size of Cl, this interaction is weaker than the usual H bonds.

Hope my explanation was clear.
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Offline AlbertoA

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Re: Hydrogen Bond
« Reply #2 on: August 20, 2011, 12:48:43 PM »
Let me answer your question about C-H---O first : There is no hydrogen bonding here. For H-bonding, as you have probably already read, you need a partial positive charge on the H. But the electronegativity difference between C and H is too less for any kind of substantial partial positive charge development on H. Hence the negatively charged (partial) oxygen cannot get hydrogen bonded to the C-H hydrogen.

This charge separation is substantial in the case of F, N and O. i.e, if the H is bonded to these. So, F-H---F, O-H---O, N-H---N, O-H---F, O-H---N, (and all the other permutations) are considered to be hydrogen bonded ones.

HCl is not hydrogen bonded, although there is a good amount of dipole-dipole attraction. Due to the size of Cl, this interaction is weaker than the usual H bonds.

Hope my explanation was clear.

Even if the Cl is more electronegative than N? how does size matter? because.. the  partial negative charge would be spread out in a larger surface?


Offline Schrödinger

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Re: Hydrogen Bond
« Reply #3 on: August 20, 2011, 01:58:32 PM »
Yes. Charge density is important.
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Offline Yakimikku

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Re: Hydrogen Bond
« Reply #4 on: August 22, 2011, 02:54:14 PM »
Let me answer your question about C-H---O first : There is no hydrogen bonding here. For H-bonding, as you have probably already read, you need a partial positive charge on the H. But the electronegativity difference between C and H is too less for any kind of substantial partial positive charge development on H. Hence the negatively charged (partial) oxygen cannot get hydrogen bonded to the C-H hydrogen.

This is true, but I'd like to point out that C-H---X hydrogen bonds do exist in cases where the substituents on the carbon are sufficiently electron withdrawing. :)

Offline Schrödinger

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Re: Hydrogen Bond
« Reply #5 on: August 26, 2011, 01:40:16 PM »
You mean something like F-C-H---F? F being electronegative.... ?
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Offline Yakimikku

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Re: Hydrogen Bond
« Reply #6 on: August 30, 2011, 09:19:13 PM »
You mean something like F-C-H---F? F being electronegative.... ?

I would hesitate to generalize it in that way. Maybe C-H---X where the substituents on the C drawn significant electron density off the H, and X has sufficient electron density to accept the hydrogen bond.

Chloroform can be involved in hydrogen bonding as a C-H hydrogen bond donor for example. Other examples are out there too. I did a quick search on ACS and found a example (http://dx.doi.org/10.1021/cg060806w) where CC-H---N hydrogen bonding creates a two-dimensional network.

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