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Topic: Conditions required for Hydrogen Bonding  (Read 40860 times)

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Offline lokifenrir96

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Conditions required for Hydrogen Bonding
« on: April 11, 2012, 08:03:59 AM »
Hi, the 2 conditions given for hydrogen bonding are:

1) A hydrogen atom must be covalently bonded to O, N or F atom. I can understand this part, as O, N and F are highly electronegative and will leave the H with a positive dipole (more or less a H+ ion).

2) The other condition is that the atom with a lone pair which the H is hydrogen-bonded to must be O, N or F. I don't get this part, though. Why can't it be any atom with a lone pair? Is it because O, N and F are highly electronegative and so the lone pair will not be donated directly to the H but will be held on tightly by the O, N or F and form a bond based on electrostatic forces of attraction instead of covalent bond?

Thanks!

Offline AWK

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Re: Conditions required for Hydrogen Bonding
« Reply #1 on: April 11, 2012, 08:47:35 AM »
See IUPAC definition of hydrogen bond ("Both electronegative atoms are usually (but not neccesarily) from the first row of Periodic Table i.e., N, O or F")
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Offline JustinCh3m

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Re: Conditions required for Hydrogen Bonding
« Reply #2 on: April 11, 2012, 02:22:01 PM »
O, N, F are extremely electronegative.  It should also be pointed out that because H is so very small, it can "get in closer" making the +/- electrostatic hydrogen "bond" that much stronger.
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Offline blaisem

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Re: Conditions required for Hydrogen Bonding
« Reply #3 on: April 12, 2012, 09:37:45 AM »
The point is that you have a strong positive dipole in the hydrogen donor and a strong negative dipole in the hydrogen acceptor.  Highly electronegative atoms are best at making these strong negative dipoles.  You already answered this in your first point in order to explain the reason for the positive dipole on the hydrogen.  It follows that when the hydrogen has a strong positive dipole, its electronegative neighbor(s) has a corresponding negative dipole associated with it.  The same reason you need N,O,F for a strong positive dipole is why you need them for a strong negative dipole, too.

The unique feature of hydrogen bonding is, as already stated, the small size of the hydrogen atom.  If you look up Coulomb's law, you can see what role distance plays in these dipole-dipole interactions.

Offline cheese (MSW)

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Re: Conditions required for Hydrogen Bonding
« Reply #4 on: April 12, 2012, 11:28:27 AM »
There is a factor that is being overlooked here.  χN = 3.04; N forms strong H-bonds;
χCl = 3.16; Cl does not forms strong H-bonds. Why should this be?
It is now thought that strong H-bonds have some covalent character.
Certainly the symmetric [F-H-F]^- ion and super short [O-H-O] interactions are best
described in terms of 3c-2e covalent bonds.

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