hydrogen bonding

[browncomputer]

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is hydrogen bonding only limited to F, O and N? or can other elements hydrogen bond as well but very very very weakly (like P, S, Se, Si, C and such?)
thanks!

 

[JMMTB]

Yep. F, O, and N only. The Hydrogen bond donor (OH, NH2...) must have a Hydrogen whereas the acceptor does not need to have one (carbonyls, ethers) but can still have a hydrogen (OH, NH2). But either way F, O, and N.

Also steric hinderance can inhibit hydrogen bonding like in the case of triphenylmethanol. An IR shows a narrow, sharp peak instead of a classic broad OH peak.

Hydrogen bonding is FON! (and keeps everything alive)

 

[quicheduty]

Hydrogen bonding is FON! (and keeps everything alive)

+1 +1 +1
Honestly, ever since I learned that adage my life has been significantly improved (lol)

 

[browncomputer]

okay because there was a question that asked which of the following has the weakest hydrogen bond: HF, H2O, H2S, or HCl. I put H2O since Sulfur and chlorine do not hydrogen bond but the answer was H2S since sulfur made very weak h-bonds. so should the answer have been H2O then?

and thank you for that IR tip!

 

[NextStepTutor_1]

In this case, you should use process of elimination--you know that HF and H2O both have very strong hydrogen bonds, so those two answer choices are out. Then, you are left with H2S and HCl. Among the two, Sulfur is the least electronegative, so it will pull electrons away from the hydrogen the least--creating the weakest hydrogen bond.

 

[gettheleadout]

okay because there was a question that asked which of the following has the weakest hydrogen bond: HF, H2O, H2S, or HCl. I put H2O since Sulfur and chlorine do not hydrogen bond but the answer was H2S since sulfur made very weak h-bonds. so should the answer have been H2O then?

and thank you for that IR tip!

I'm guessing this wasn't an AAMC material question? In any case hydrogen bonds involving chlorine and sulfur do occur but are typically not discussed at the level of MCAT chemistry.

In this case, you should use process of elimination--you know that HF and H2O both have very strong hydrogen bonds, so those two answer choices are out. Then, you are left with H2S and HCl. Among the two, Sulfur is the least electronegative, so it will pull electrons away from the hydrogen the least--creating the weakest hydrogen bond.

My problem with this advice, and really with this question, is that there isn't a clear expectation of the knowledge level expected on the MCAT. Typical general chemistry courses discuss only hydrogen bonds involving fluorine, oxygen, and nitrogen, and all review material I've seen (TBR, TPR, Kaplan, EK) has recommended approaching such questions from a level that does indeed limit hydrogen bonding to these hydrogen bond acceptors. The AAMC PS content outline does not specify regarding this matter. In any case I can imagine two approaches to the question at hand:

1) Functioning on the level typically considered to be expected by the MCAT, the examinee rules out H2S and HCl as species for which hydrogen bonding is not present intermolecularly. The examinee then selects H2O as the species with the weakest hydrogen bonds forming as a result of the weaker dipole moment in the H–O bonds compared to the H–F bonds in HF.

2) Functioning with a more expanded knowledge base, the examinee considers all answer choices to represent species in which hydrogen bonding occurs, and selects H2S as the species for which the weakest hydrogen bonds form as a result of the weakest dipole moment across the H–S bond as compared to the other species offered as choices.

It's quite problematic.