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pfaction

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I'm re-reading NMR and stuff before Saturday. We figured out Ketones and Ethers don't H-Bond, what about aldehydes? I was thinking about how pentanal and pentanol would be on TLC. Obviously Pentanol will have a low Rf, but what about compared to pentanal? One of the AAMC's asked about ketones vs alcohols (which I got right, thanks to the no h-bond thing), AAMC11 asked about acids and alcohols (which I got right but for wrong reasons).

EDIT: I'll throw this in here since it's also in TBR. Which would have the strongest H-Bond:
O-H----O
O-H----F
O-H----N
N-H----F
N-H----O
N-H----N
F-H----F
F-H----O
F-H----N

I think it's N-H-F, but maybe it's F-H-F....
 
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hellocubed

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Ketones and Ethers don't H-Bond, what about aldehydes
Ketones and ethers do actually H-bond. But they are only receivers (donate electrons) and require a proton-donor.

but yes you're correct.

edit: eek I was wrong about H-bonding strength.

F-H F
should be the strongest
 
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hellocubed

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Obviously Pentanol will have a low Rf, but what about compared to pentanal?


Oh, I suppose that Pentanol Rf <<< pentanal in a non-protic solvent?
Considering Pentanol can donate AND receive H-bonds. While pentanal can only receive.

But it is dependent on the polarity of the solvent (the TLC is always polar).
If the solvent is polar, it gets more complicated...
I have no idea because: more polar are slowed by TLC, AND more polar are carried further by solvent...
 

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Ketones and ethers do actually H-bond. But they are only receivers (donate electrons) and require a proton-donor.

but yes you're correct. The strongest H-bond would be N-H - - F

(weak EN)-H - - - :(Strong EN)

Cool I remember being confused by this ages ago when I first started, but never really encountered it later on.

Can you explain this? I vaguely remember the answer to the question being

NH----OH forms a very strong hydrogen bond compared to OH-----OH. Why is this so?
 
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pfaction

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but yes you're correct. The strongest H-bond would be N-H - - F

(weak EN)-H - - - :(Strong EN)

I looked it up on Wikipedia and it said F-H-F was the strongest?

H-bonds are due to the electronegativity between the atoms, so that was my thinking of N-H-F. But F-H-F would be stronger IMO since both fluorines want that hydrogen, and none are willing to give it up, so it would form shorter H-bonds between the two atoms than say, O-H-O?
 

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For the purposes of the MCAT ONLY molecules with hydrogens attached to F, O, or N hydrogen bond. Hydrogen bond meaning they can reciprocally hydrogen bond with other molecules of itself. Endo story.
 

hellocubed

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ohhh...................................

I can see where this is going. Considering greater EN---> more acidic.


Which begs the question:
What makes a better Hydrogen Bond Donor? Greater EN (more acidic), Less EN (loose H+).
 

pfaction

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For the purposes of the MCAT ONLY molecules with hydrogens attached to F, O, or N hydrogen bond. Hydrogen bond meaning they can reciprocally hydrogen bond with other molecules of itself. Endo story.

So I guess that's a no for pentanal, meaning it's RF value will be higher since it won't H-bond as much.

Any idea about my other question, stronger H-Bonds?
 

hellocubed

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Quote:
For the purposes of the MCAT ONLY molecules with hydrogens attached to F, O, or N hydrogen bond. Hydrogen bond meaning they can reciprocally hydrogen bond with other molecules of itself. Endo story.
So I guess that's a no for pentanal, meaning it's RF value will be higher since it won't H-bond as much.

Any idea about my other question, stronger H-Bonds?

Wait, there was a long discussion here about this issue.

For the MCAT, I believe the end resolve was that any F O N with a lone pair could participate in H-bond.


The final argument was a problem in the AAMC tests that cited this exact issue, saying ethers (or something) could be involved in H-bonding...
 

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I'm re-reading NMR and stuff before Saturday. We figured out Ketones and Ethers don't H-Bond, what about aldehydes? I was thinking about how pentanal and pentanol would be on TLC. Obviously Pentanol will have a low Rf, but what about compared to pentanal? One of the AAMC's asked about ketones vs alcohols (which I got right, thanks to the no h-bond thing), AAMC11 asked about acids and alcohols (which I got right but for wrong reasons).

EDIT: I'll throw this in here since it's also in TBR. Which would have the strongest H-Bond:
O-H----O
O-H----F
O-H----N
N-H----F
N-H----O
N-H----N
F-H----F
F-H----O
F-H----N

I think it's N-H-F, but maybe it's F-H-F....

Strongest covalent bond = weakest H-bond and vice versa, so NH---F should have the strongest H-bond since NH will have the weakest covalent bond since N is the least EN.

Also in regard to the continuing confusion about whether or not ketones and ethers can H-bond, the key is to remember that FON lone pairs can hydrogen bond no matter what. However, the FON must have a hydrogen on it in order to donate a hydrogen bond. So ketones and ethers can hydrogen bond with protic solvent, just not with themselves or any other non-H holding FON.

So for a question like "Which is the best explanation for aqueous alcohols having a higher boiling points than aqueous ketones?" The answer "Ketones cannot hydrogen bond" is completely valid because the answer choices implies ketones hydrogen bonding with other ketones. Obviously both the ketone and the alcohol will h-bond with the solvent, but the alcohol will h-bond with itself thereby increasing the number and strength of intermolecular interactions.
 

pfaction

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So then pentanal can't because it's not attached to O-H, unlike acids?

Strongest covalent bond = weakest H-bond and vice versa, so NH---F should have the strongest H-bond since NH will have the weakest covalent bond since N is the least EN.

Okay, thanks! I'll have to reconcile this with Wikipedia though.
 

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"Strongest covalent bond = weakest H-bond and vice versa, so NH---F should have the strongest H-bond since NH will have the weakest covalent bond since N is the least EN."

I want to look more closely at this.

please ignore "arbitrary rating" on the nitrogen I drew. turned out to be too complicated. here i just drew what i don't understand.
 

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This seems to be a problem though.

Covalent bonds with hydrogen get even weaker as the bonded molecule is more electronegative. MORE electronegative-H is more acidic....

Which should make F-H :F. The strongest??
 

gettheleadout

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This seems to be a problem though.

Covalent bonds with hydrogen get even weaker as the bonded molecule is more electronegative. MORE electronegative-H is more acidic....

Which should make F-H :F. The strongest??
This is exactly what I was going to say.

Reasoning that nitrogen's lower electronegativity makes it form a weaker covalent bond with hydrogen doesn't make any sense. Inductive effects alone can increase acidity of protons bonded to O, N, whatever. An example would be trichloroacetic acid. No resonance change adding those Cl's, but the O-H bond is weaker because electron density is drawn away, and the O holds on to it better than the H. It's the same reason why sp-hybridized carbons are more acidic than sp2 or sp3; greater s character means electrons more closely localized on the carbon and not contributing to C-H bond strength. An increase in the electronegativity of an atom Q bonded to H causes a decrease in the strength of the covalent Q-H bond.

I think the answer is F-H---F.
 
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chiddler

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This is exactly what I was going to say.

Reasoning that nitrogen's lower electronegativity makes it form a weaker covalent bond with hydrogen doesn't make any sense. Inductive effects alone can increase acidity of protons bonded to O, N, whatever. An example would be trichloroacetic acid. No resonance change adding those Cl's, but the O-H bond is weaker because electron density is drawn away, and the O holds on to it better than the H. It's the same reason why sp-hybridized carbons are more acidic than sp2 or sp3; greater s character means electrons more closely localized on the carbon and not contributing to C-H bond strength. An increase in the electronegativity of an atom Q bonded to H causes a decrease in the strength of the covalent Q-H bond.

I think the answer is F-H---F.

TBR gave a question that contradicts what you wrote. I wrote it above but I don't think I was very clear.

In comparing OH-OH, OH-NH, and NH-NH, it said OH-NH formed the strongest bonds.

i'll dig it up tomorrow.
 

hellocubed

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TBR gave a question that contradicts what you wrote. I wrote it above but I don't think I was very clear.

In comparing OH-OH, OH-NH, and NH-NH, it said OH-NH formed the strongest bonds.

i'll dig it up tomorrow.
That is not a contradiction at all. In fact it completely supports their claim.

What we are saying is that (I am nearly 100% sure) H-bonding is proportional to ACID STRENGTH of the donor, and NUCLEOPHILICITY (or Basicity?) of the acceptor.


In the example you gave, O is MORE electronegative than N.
If we were following along the lines that NH-F was stronger than FH-F because N is less electronegative, then the answer to the question you provided would have been NH-NH (a less electronegative atom bonded to the H). But this is not the case. The answer is a MORE electronegative atom bonded to the H, making the molecule more acidic.


So you want to attain a
(Strong EN)-H - - : (Strong EN)

strong acid.................Strong nucleophile (?)

(I was originally wrong)

But then we have to answer the question:
Is the H-bond acceptor a Nucleophile? or a Base?

If it is a Nucleophile
F-H :F

If it is a Base
F-H :N
 

hellocubed

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Chiddler's TBR problem is saying that it should be
(strong acid)-H (Strong Base)-H


BUT wiki is saying that it should be
(strong acid)-H (Strong nucleophile)-H


  • F&#8722;H…:F (161.5 kJ/mol or 38.6 kcal/mol)
  • O&#8722;H…:N (29 kJ/mol or 6.9 kcal/mol)
  • O&#8722;H…:O (21 kJ/mol or 5.0 kcal/mol)
  • N&#8722;H…:N (13 kJ/mol or 3.1 kcal/mol)
  • N&#8722;H…:O (8 kJ/mol or 1.9 kcal/mol)

hmmm strange
 

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"Strongest covalent bond = weakest H-bond and vice versa, so NH---F should have the strongest H-bond since NH will have the weakest covalent bond since N is the least EN."

I want to look more closely at this.

please ignore "arbitrary rating" on the nitrogen I drew. turned out to be too complicated. here i just drew what i don't understand.


Well TBR said strong covalent=weak hbond. It made sense when I was studying so I just accepted it.
 

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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 a hydrogen bonded to O
C. the lone pair of O and a hydrogen bonded to N
D. the lone pair of N and a hydrogen bonded to N

Answer is B.

"Strongest Hbond comes from the more basic lone-pair donor (found on the nitrogen atom) being donated to the most protic hydrogen (found covalently on the oxygen)."
 

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"Strongest Hbond comes from the more basic lone-pair donor (found on the nitrogen atom) being donated to the most protic hydrogen (found covalently on the oxygen)."

So, the answer according to BR is

(strong acid)-H - - - (strong base)


But how come wiki says something different?
F-H - - F

doesn't follow this pattern.
 

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it has to do with partial neg and pos charges. H-F has the greatest diff in electronegativity therefor the H has the greatest partial pos charge. F is the most electronegative, so the hydrogen of H-F will H-bond the strongest with other F atoms.
 

gettheleadout

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If it is a Base
F-H :N

"Strongest Hbond comes from the more basic lone-pair donor (found on the nitrogen atom) being donated to the most protic hydrogen (found covalently on the oxygen)."
So the answer should be F-H---N then right?
So, the answer according to BR is

(strong acid)-H - - - (strong base)


But how come wiki says something different?
F-H - - F

doesn't follow this pattern.

it has to do with partial neg and pos charges. H-F has the greatest diff in electronegativity therefor the H has the greatest partial pos charge. F is the most electronegative, so the hydrogen of H-F will H-bond the strongest with other F atoms.
But according to TBR, the strong electronegativity of the F atom means that, while it makes for a very positive covalently-bound H, it is a poor donor of the hydrogen bond that H will accept.

So what's the truth? Anyone want to ask a chem prof? I'll make an effort to this week if we can't resolve it.
 

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This 99% won't appear on any MCAT. And if it does, it most likely won't be F-H-F, it'll be OH-N because of bio applications.
 

gettheleadout

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Bumping this to resolve the issue: http://www.annualreviews.org/doi/full/10.1146/annurev.physchem.48.1.511

The strongest hydrogen bond is F-H---:N and hydrogen bond strength of A-H---:B increases with acidity of A-H and basicity of :B.

I don't recall what distinction was being made in this thread between nucleophilicity and basicity of the bond acceptor :)B), but as far as I can tell basicity and nucleophilicity parallel each other along the 3rd period for nonmetals C, N, O, and F. Carbanions are extremely strong bases and powerful nucleophiles (think Grignards), alkoxides are weaker but still strong bases and nucleophiles, and fluorides are weak bases and weak nucleophiles.
 

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is this the answer: F-H ---- N ?

"strong_acid ---- strong_base rule" makes sense here.

we are interested in the strength of the dotted bond. if the proton is loose (weak bond on the left, i.e. strong acid on the left) then it can form ----bond easier.

on the other hand, what do we want on the right to grab that proton? we want a base (the stronger the better).

HF is the most acidic from the choices on the left.
N is the most basic from the choices on the right (think ammonia).
 
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