Tertiary amines & hydrogen bonds

[SaintJude]

Can someone please verify?

So tertiary amines (& ethers) can not hydrogen bond with each other, but they WILL hydrogen bond in an aqueous solutions (with water or alcohol...)

This is because hydrogen bonding occurs with an "F, O, or N attached to a Hydrogen"

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[syoung]

yep pretty much

 

[gettheleadout]

For hydrogen bonding to occur you need at least one species involved to have an H bonded to either an F, O, or N, because these highly electronegative atoms make the F/O/N-H bond very polar, giving H enough partial positive charge to exhibit the strong electrostatic interactions we call hydrogen bonds. These highly positive H's then electrostatically interact (hydrogen bond) with similar highly electronegative atoms (which will have great partial negative character) but these hydrogen bond "acceptors" need not be bonded to an H, as they will have partial negative character regardless. The partially positive H's are the hydrogen bond "donors."

 

[SaintJude]

It sounds like you know a lot about hydrogen bonding.

Have you ever heard the phrase "forming a hydrogen bond involves accepting an electron"? Does that mean that any F, O, N that forms a hydrogen bond is also acting as Lewis acid?

 

[MedPR]

It sounds like you know a lot about hydrogen bonding.

Have you ever heard the phrase "forming a hydrogen bond involves accepting an electron"? Does that mean that any F, O, N that forms a hydrogen bond is also acting as Lewis acid?

Lewis acids are electron pair acceptors. So, if only one electron is accepted, then it isn't a lewis acid.

Also, be careful about being nitpicky about H-bond accepting/donating. I don't remember which AAMC it was on, but there was a question that claimed esters were not able to hydrogen bond. Although h-bond was the obvious answer (by POE), it may lead you to believe that esters are not h-bond acceptors when they really are. I'm pretty sure I made a thread here just to verify that esters can hydrogen bond even though they can't donate a proton.

 

[syoung]

I think they can h-bond as long as it is in the presence of H2O or something that has a H+ to donate, esters cannot H-bond to itself.

 

[SaintJude]

Lewis acids are electron pair acceptors. So, if only one electron is accepted, then it isn't a lewis acid.

Also, be careful about being nitpicky about H-bond accepting/donating. I don't remember which AAMC it was on, but there was a question that claimed esters were not able to hydrogen bond. Although h-bond was the obvious answer (by POE), it may lead you to believe that esters are not h-bond acceptors when they really are. I'm pretty sure I made a thread here just to verify that esters can hydrogen bond even though they can't donate a proton.

thanks.Yes, this is why I made the thread.

Tertiary amine and esters cannot hydrogen bond [with each other]. The answer choices usually omit this clause to confuse test takers.

But they can form hydrogen bond with other molecules.

 

[gettheleadout]

It sounds like you know a lot about hydrogen bonding.

Have you ever heard the phrase "forming a hydrogen bond involves accepting an electron"? Does that mean that any F, O, N that forms a hydrogen bond is also acting as Lewis acid?

I'm not familiar with that phrase.

Lewis acids are electron pair acceptors. So, if only one electron is accepted, then it isn't a lewis acid.

Also, be careful about being nitpicky about H-bond accepting/donating. I don't remember which AAMC it was on, but there was a question that claimed esters were not able to hydrogen bond. Although h-bond was the obvious answer (by POE), it may lead you to believe that esters are not h-bond acceptors when they really are. I'm pretty sure I made a thread here just to verify that esters can hydrogen bond even though they can't donate a proton.

thanks.Yes, this is why I made the thread.

Tertiary amine and esters cannot hydrogen bond [with each other]. The answer choices usually omit this clause to confuse test takers.

But they can form hydrogen bond with other molecules.

Yeah, as long as the question is talking about single species (the ester or tertiary amine) then those answer choices would be incorrect because they can't H-bond with themselves. If the question is asking about a solution in which the ester/amine species is joined by a compound with an [F/O/N]-H bond then the ester/amine is most definitely involved as an H-bond acceptor.

 

[MedPR]

I'm not familiar with that phrase.



Yeah, as long as the question is talking about single species (the ester or tertiary amine) then those answer choices would be incorrect because they can't H-bond with themselves. If the question is asking about a solution in which the ester/amine species is joined by a compound with an [F/O/N]-H bond then the ester/amine is most definitely involved as an H-bond acceptor.

Well, the question I was referring to went something like this:

Which of the following reasons best explains why an aqueous solution of ethanol has a higher boiling point than an aqueous solution of compound X? (compound X is an ester)

A. something stuipd
B. something stupid
C. something stupid
D. Esters cannot form hydrogen bonds

The answer was D, even though esters can for hydrogen bonds in aqueous solution. I guess the fact that ethanol can h-bond with the water and with other ethanols makes the BP go up even more though.

 

[gettheleadout]

Well, the question I was referring to went something like this:

Which of the following reasons best explains why an aqueous solution of ethanol has a higher boiling point than an aqueous solution of compound X? (compound X is an ester)

A. something stuipd
B. something stupid
C. something stupid
D. Esters cannot form hydrogen bonds

The answer was D, even though esters can for hydrogen bonds in aqueous solution. I guess the fact that ethanol can h-bond with the water and with other ethanols makes the BP go up even more though.

Well that's definitely incorrect. Were I asked to choose between several compounds the one which would have the highest boiling point/lowest vapor pressure and were ethanol and some ester both choices I would definitely assume the ability to ethanol to H-bond both ways would make it superior to the ester. In this case, however, the ester in aqueous solution is going to be forming a completely normal H-bond with some water molecule's H. The AAMC must be specifically defining "hydrogen bonding" as both acceptance and donation of hydrogen bonds. That's fine, but it needs to be specified somewhere (and it's good you've pointed it out) because outside of their apparent definition the answer is incorrect as worded.

You already know all of this, I'm just saying.

 

[syoung]

Well, the question I was referring to went something like this:

Which of the following reasons best explains why an aqueous solution of ethanol has a higher boiling point than an aqueous solution of compound X? (compound X is an ester)

A. something stuipd
B. something stupid
C. something stupid
D. Esters cannot form hydrogen bonds

The answer was D, even though esters can for hydrogen bonds in aqueous solution. I guess the fact that ethanol can h-bond with the water and with other ethanols makes the BP go up even more though.

key word would be that EtOH can H-bond with both Water and EtOH, whereas esters could only H-bond to Water?
Or are they saying aqueous meaning EtOH by itself and Esters by itself?

 

[gettheleadout]

key word would be that EtOH can H-bond with both Water and EtOH, whereas esters could only H-bond to Water?
Or are they saying aqueous meaning EtOH by itself and Esters by itself?

An "aqueous solution" of esters/ethanol/whatever by definition contains water.

 

[SaintJude]

Like MedPr is saying, AAMC is correct b/c:

I guess the fact that ethanol can h-bond with the water and with other ethanols makes the BP go up even more though.

 

[gettheleadout]

Like MedPr is saying, AAMC is correct b/c:

No, AAMC's answer is still wrong because the ester and ethanol are in aqueous solutions where the ester still has a chance to be a bond acceptor. Is ethanol better? Yes. Is an ester incapable of hydrogen bonding in aqueous solution? No, and this is why by the typical definition they are wrong.

 

[pm1]

Can ethers hydrogen bond in an aqueous solution with water then? Would the same reasoning for esters apply?

Kaplan says: "Ethers have no hydrogen atoms bonded to the oxygen atoms, so they can't possibly participate in hydrogen bonding."

 

[Buttafuoco]

This is one of those dumb genchemmy things where I wish they'd just leave it out. Like for sure the AAMC treats beta sheets and alpha helices as H-bonds (their definition of secondary protein structure is local geometry due to H-bonding) even those the carbonyls are H-bond acceptors only. I realize that a Carbonyl O is more negative than an ether O, but still.

 

[gettheleadout]

Can ethers hydrogen bond in an aqueous solution with water then? Would the same reasoning for esters apply?

Kaplan says: "Ethers have no hydrogen atoms bonded to the oxygen atoms, so they can't possibly participate in hydrogen bonding."

Ethers can function as hydrogen bond acceptors: http://www.cliffsnotes.com/study_guide/Ethers.topicArticleId-23297,articleId-23273.html

Any F, O, or N can be an H-bond acceptor. See large polymers like nylon.

 

[pm1]

Ethers can function as hydrogen bond acceptors: http://www.cliffsnotes.com/study_guide/Ethers.topicArticleId-23297,articleId-23273.html

Any F, O, or N can be an H-bond acceptor. See large polymers like nylon.

okay so just to make sure I understand it.
Any compound that has F, O, or N can be an H-bon acceptor. But compounds that do not have a H is not a donor, so they can't form H bond amongst themselves.
Nevertheless they can H bond when with water or other H donor compounds.

 

[gettheleadout]

okay so just to make sure I understand it.
Any compound that has F, O, or N can be an H-bon acceptor. But compounds that do not have a H is not a donor, so they can't form H bond amongst themselves.
Nevertheless they can H bond when with water or other H donor compounds.

That's correct.