Ambiguous Hydrogen Bonding Questions

[tuhtles]

When questions refer to hydrogen bonding, what do they mean exactly? I mean, what consists of "H bonding"? I get confused because technically esters can accept H bonds because they have a lone pair, but they are aprotic and are unable to donate H bonds. So overall, can we say that esters CAN participate in hydrogen bonding? When questions refer to H bonding do they mean that the compound must be BOTH H bond donor AND acceptor to be considered to be able to hydrogen bond?

I've come across this ambiguity in a couple of questions from different resources and each consider H bonding differently. i.e one case considers a lone pair as being able to H bond and the other resource doesn't refer to this.

An example.... (SPOILER ALERT: QUESTION FROM KAPLAN FL 3R!!)

Initially I picked C (the correct answer) but then thought about this ambiguity and picked D because I thought that the tertiary amine has a lone pair and *technically* can participate in H-bonding.. so it should have at least some adherence... I also didn't thing steric hinderance would be in issue

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[enervate]

Is there an explanation? Those lone pairs on malachite green can definitely be locked up in resonance. Other than that, those nitrogen atoms should be able to hydrogen bond with water or, in this case, cellulose.

 

[The_Sunny_Doc]

Hmm, there's a diminished tendency for aromatic amines to form H bonds. I probably would have picked the answer you did.

Would like to hear the explanation they give you if you've got it.

 

[tuhtles]

This is the explanation:

"Hydrogen bonding occurs between OH groups and NH groups in any combination. Since the glucose monomer in cellulose has OH groups, cellulose should be capable of hydrogen bonding to either alizarin, with its two OH groups, or to aniline yellow, with its aromatic amine func- tionality. Malachite green, on the other hand, has no acidic protons available for hydrogen bond- ing due to the methylation of the nitrogen atoms."

What do you guys think?

And good point with the resonance part! That would decrease the availability of the lone pair, which is the essence behind why pyrimidine is basic and pyrrole is not.

 

[enervate]

This is the explanation:

"Hydrogen bonding occurs between OH groups and NH groups in any combination. Since the glucose monomer in cellulose has OH groups, cellulose should be capable of hydrogen bonding to either alizarin, with its two OH groups, or to aniline yellow, with its aromatic amine func- tionality. Malachite green, on the other hand, has no acidic protons available for hydrogen bond- ing due to the methylation of the nitrogen atoms."

What do you guys think?

Seems like someone at Kaplan slipped up. The explanation acknowledges that cellulose has OH groups but states that malachite green wouldn't bond to cellulose because there are no "acidic protons available." Clearly there are; they're just on cellulose not malachite green.

 

[The_Sunny_Doc]

Seems like someone at Kaplan slipped up. The explanation acknowledges that cellulose has OH groups but states that malachite green wouldn't bond to cellulose because there are no "acidic protons available." Clearly there are; they're just on cellulose not malachite green.

 

[tuhtles]

Thanks for your responses. Glad to hear you guys would have picked the answer I did.

But on a general note, when questions ask for whether or not something will hydrogen bond, should we interpret it as anything that is either a hydrogen bond acceptor or hydrogen bond donor as being able to hydrogen bond? Or do tests usually see hydrogen bonding compound as strictly being both hydrogen bond acceptor and hydrogen donor? I thought it was the former but then questions like this Kaplan one throw me off

Thanks!

 

[The_Sunny_Doc]

Thanks for your responses. Glad to hear you guys would have picked the answer I did.

But on a general note, when questions ask for whether or not something will hydrogen bond, should we interpret it as anything that is either a hydrogen bond acceptor or hydrogen bond donor as being able to hydrogen bond? Or do tests usually see hydrogen bonding compound as strictly being both hydrogen bond acceptor and hydrogen donor? I thought it was the former but then questions like this Kaplan one throw me off

Thanks!

Best guess is that they'll ask about H bonding indirectly, for example...

"Which compound has the highest boiling point?"
"Which compound will be found in the distillate?"
"Which compound will elute through the GC apparatus first?"
"Which compound is miscible with __?"

or

"Which solvent favors carbocation formation?"

If they wanted you to know about the diff between H bond acceptor and H bond donors they could ask something about whether the compound can hydrogen bond with itself. It can only H bond with itself if it is both a donor and acceptor, so tertiary amines and ethers would be wrong answers.

Just speculation.

One semi-related MCAT tip from EK Orgo is that if a question asks about a solvent "the answer is always ether."

 

[Heday]

Thanks for your responses. Glad to hear you guys would have picked the answer I did.

But on a general note, when questions ask for whether or not something will hydrogen bond, should we interpret it as anything that is either a hydrogen bond acceptor or hydrogen bond donor as being able to hydrogen bond? Or do tests usually see hydrogen bonding compound as strictly being both hydrogen bond acceptor and hydrogen donor? I thought it was the former but then questions like this Kaplan one throw me off

Thanks!

I actually would've picked C. Of all the practice FLs I've taken, it seems to me that AAMC wants you to says that hydrogen bonding requires for the molecules to have a H bonded to an electronegative atoms (Nitrogen, Oxygen, or Fluorine). Therefore, if the molecule has a H bonded to N, O, or F, then it hydrogen bonds... if it doesn't, then no hydrogen bonding.

Although someone mentioned that cellulose may have acidic protons, when we mention hydrogen bonding, we want both molecules in question to have H bonded to N, O, or F. It's really a simple rule and you'll end up spending less time thinking about the question.

Of course, this is my opinion from about six AAMC exams... it's your decision to use my rational for H-bonding questions.

 

[Bah9p8]

I actually would've picked C. Of all the practice FLs I've taken, it seems to me that AAMC wants you to says that hydrogen bonding requires for the molecules to have a H bonded to an electronegative atoms (Nitrogen, Oxygen, or Fluorine). Therefore, if the molecule has a H bonded to N, O, or F, then it hydrogen bonds... if it doesn't, then no hydrogen bonding.

Although someone mentioned that cellulose may have acidic protons, when we mention hydrogen bonding, we want both molecules in question to have H bonded to N, O, or F. It's really a simple rule and you'll end up spending less time thinking about the question.

Of course, this is my opinion from about six AAMC exams... it's your decision to use my rational for H-bonding questions.

This is what I would look for with the MCAT in regards to hydrogen bonding, electronegative elements on both compounds with hydrogens attached. Hydrogen bonding in DNA double strands is also important.

 

[The_Sunny_Doc]

I actually would've picked C. Of all the practice FLs I've taken, it seems to me that AAMC wants you to says that hydrogen bonding requires for the molecules to have a H bonded to an electronegative atoms (Nitrogen, Oxygen, or Fluorine). Therefore, if the molecule has a H bonded to N, O, or F, then it hydrogen bonds... if it doesn't, then no hydrogen bonding.

Although someone mentioned that cellulose may have acidic protons, when we mention hydrogen bonding, we want both molecules in question to have H bonded to N, O, or F. It's really a simple rule and you'll end up spending less time thinking about the question.

Of course, this is my opinion from about six AAMC exams... it's your decision to use my rational for H-bonding questions.

Great tip. The simpler, the better! That seems to be the rule with a lot of concepts on the MCAT.