Strongest H-Bond

[justadream]

Mcat-review

"The more polar a bond is, the stronger the hydrogen bond. The H-F bond is the most polar, followed by the H-O bond, and lastly the H-N bond."

I remember TBR saying the strongest H-bond is formed from am N-H with an Oxygen.

Can someone explain why TBR (I presume) is correct while Mcat-review is wrong?

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[The Brown Knight]

For H bonds you have to take into account both the strength of the partial positive charge on the hydrogen (as well as the strength of the partial negative charge on the heteroatom) AND the DENSITY of H-bonds in a solution.
The N-H (presumably from ammonia) will donate 3 H-bonds per molecule and O has a strong partial negative charge and can accept 2 H-bonds on its lone pairs. Though I don't know if this would make the STRONGEST bond.
But this does explain why HF, despite being more polar than water, has a lower boiling point than water.

 

[justadream]

@The Brown Knight

So the argument is that H-F has only one X-H bond whereas N can have up to 3.

 

[The Brown Knight]

The argument is sort of in a gray area: while F has high electronegativity it can only donate one H-bond, and while N has lower electronegativity it can donate 3 H-bonds

 

[justadream]

@The Brown Knight

What about a primary amine (NH2)? Is two H-bonds enough to overcome being weaker?

 

[The Brown Knight]

Again in a bit of gray area:
First, you lose the ability to donate 3 H-bonds. You want to donate as many H bonds as possible and accept as many is possible - this is what I meant by H-bond DENSITY. So a primary amine will have a lower H-bond density than ammonia.
But, a primary amine is probably more basic than ammonia due to alkyl substituent; but IMO it's probably not as good as water in the partial negative charge.

 

[AmishExpressWay]

There are too many factors to take into account here to make any real decision. Unless you run experiments in real life, that is.

For example, H-F has one REALLY polar bond, while NH3 has 3 weaker (but still really polarized) polar bonds. The question here becomes one of quantity vs quality.

How many hydrogen bonds can you fit in a certain volume of the specific molecule and how strong each individual hydrogen bond is decides the hydrogen bonding potential of the molecule.

#ChemSwag. Do wish I had some verbal swag though 🙁.