TBR Concept Question?

[deleted927298]

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IN the TBR Orgo book, it says that the Hydrogen bond formed from amine's lone pairs bonding with the protic H of an alcohol is stronger than the H-bond formed b/w alcohol's oxygen lone pairs and the protic H of an amine. Why is this when H-bonds from OH's are stronger than H-bonds from amines?

 

[IsleyOfTheNorth]

Edit: I was able to confirm it, but I still don't know why. (Paywall)
https://onlinelibrary.wiley.com/doi/pdf/10.1002/chem.19960021206

Edit 2: My first idea was wrong. It turns out that it is because Oxygen is MORE electronegative that it shares less of that negativity with the hydrogen. Nitrogen "gives up" more electron density, resulting in a stronger bond.

See the discussion here:
Strongest Hydrogen Bond?

 

[deleted927298]

Yes, I agree. But the concept of why the Hydrogen bond formed from amine's lone pairs bonding with the protic H of an alcohol is stronger than the H-bond formed b/w alcohol's oxygen lone pairs and the protic H of an amine is still confusing? So does this mean that the H-bond formed from binding the lone pairs in alcohol's O to the protic H of FH is stronger than the H-bond b/w F's lone pairs and OH's protic H?

 

[IsleyOfTheNorth]

Think of it this way.

If you had two nitrogen atoms with lone pairs fighting over to create an h-bond with a proton (covalently connect to some R group) it would look like this:
N:--H--:N

Both nitrogens would "give up" the same amount of electron density and there would be no clear winner.

But now, imagine nitrogen fighting with oxygen over the same H:
N:--H---:O

Here nitrogen is "willing" to give up more of its electron density to hydrogen than oxygen will. Oxygen likes to hoard its electron density. Nitrogen can't. (Because it has fewer protons, it can't hold the electrons as well.) So hydrogen gets a bigger share of the electron density by sharing with nitrogen instead of oxygen. i.e. Hydrogen is on more equal footing with nitrogen.

The same is true for fluoride but even more so because it is less willing to share electron density than oxygen.

 

[BerkReviewTeach]

In the same way that a strong acid reacts better with a strong base than a weak acid reacts with a weak base, a strong lone pair donor will form a stronger H-bond with a more protic H than a weak lone pair donor will form a weaker H-bond with a less protic H. The concept of H-bonding is essentially the same as the concept behind Lewis Acid-Base chemistry.