Hydrogen Bond stength

[lexw]

Hey guys,

Can anyone explain how to determine the strengths of H-bonds. I know that F-H --- F bonds are the strongest, and the more electronegative leads to a stronger bond, but why is an O-H---N bond stronger than O-H---O bond?


thanks

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[Sammy1024]

Is the H bonded to both O and N?

 

[lexw]

yes the hydrogen is bonded to the oxygen and hydrogen bonded to nitrogen

 

[Sammy1024]

But hydrogen can only bond to one thing. Do you mean 2 different cases of OH and an NH?

 

[Teleologist]

I think he's wondering why the hydrogen "bond" - more like intermolecular association force - between OH and N and OH and O vary in strength - specifically, why is the former stronger, despite EN considerations (which would suggest otherwise).

OP: try considering the relative sizes of N and O; nitrogen is less EN than O, true, but nitrogen is also smaller than O. This may serve to increase the negative charge density on a nitrogen atom versus an oxygen atom.

 

[lexw]

OP: try considering the relative sizes of N and O; nitrogen is less EN than O, true, but nitrogen is also smaller than O. This may serve to increase the negative charge density on a nitrogen atom versus an oxygen atom.

isnt an O atom smaller than N when we're looking at atomic size? so the negative charge should be distributed over a larger volume. Does this mean that its 'easier' for the nitrogen to be an electron donor compared to an oxygen atom?

along these lines we would expect the strength of a H bond between N----- HF > O----- HF?

 

[Teleologist]

You're right, I goofed. Nitrogen is indeed larger than oxygen. I'm not sure if we can draw any conclusions though.

 

[syoung]

Yes. F is very small and electronegative, so when it gives up the proton, it is stable enough to balance the extra electron density
And N does this better than O because N is a better electron donor than O when it comes to picking up temporarily binding H.

• O−H…:N (29 kJ/mol or 6.9 kcal/mol)
• O−H…:O (21 kJ/mol or 5.0 kcal/mol)
• N−H…:N (13 kJ/mol or 3.1 kcal/mol)
• N−H…:O (8 kJ/mol or 1.9 kcal/mol)

Overall H-bonding among amines is weaker than in water (all H2O) because Nitrogen is less electronegative.

Sorry if it sounds slightly contradictory, but the solvent matters, as does the solute.