Most Stable Hydrogen Bonding

[Coltuna]

Hello all,

Working through BR Organic passages (and failing) and am having trouble grasping a concept for a specific question. It reads:

The STRONGEST hydrogen bond is formed between:
A. the lone pair of O and hydrogen bonded to O
B. the lone pair of N and hydrogen bonded to O
C. the lone pair of O and hydrogen bonded to N
D. the long pair of N and hydrogen bonded to N.

I thought the answer should be A due to the relative partial negative charge of Oxygen and partial positive charge of hydrogen due to electronegativity; however, they're saying the answer is B due to the increased basicity of the Nitrogen (less electronegative than oxygen) and partial positive charge of hydrogen bonding to O.

Basically is A wrong because the lone pair on Oxygen could just as easily "yank" that hydrogen off the other Oxygen as it could share it?

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[aldol16]

Well, you have to two things here. The H-bond donor has to push the proton away from it, i.e. hold it less tightly, in order for it to be a good H-bond. On the other side, the H-bond acceptor has to want to donate its electron pair to the proton to form the H-bond. So for the first condition, you want a very electronegative element that's donating the H-bond. For the second, you want an electropositive element that will easily push away its lone pair. So you can rule out C and D because the H-bond donor is weaker than A or B. Then you can eliminate A because O is more electronegative than N and therefore will not want to push its lone pairs towards the proton as much as N.

 

[Coltuna]

So basically more electronegative atoms hold the proton less tightly and are more willing to "allow" it to hydrogen bond while the less electronegative atom is more willing to allow its electrons to participate in the hydrogen bonding?

 

[aldol16]

So basically more electronegative atoms hold the proton less tightly and are more willing to "allow" it to hydrogen bond while the less electronegative atom is more willing to allow its electrons to participate in the hydrogen bonding?

Yes.