BR Chem Ch. 4 #50

[dougkaye]

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Question asks how many acidic sites there are on acetylsalicylic acid. I put in two but there is only one, according to key. It seems like we spent a lot of time in orgo talking about how the alpha carbon (to a carbonyl) is also acidic. Why is that not a valid argument in this situation?

 

[attixx]

There is no acidic alpha hydrogen. Look at the picture; the carboxylic acid group is attached to a benzene ring. On the alpha carbon, there is no hydrogen.

Edit: If you're referring to the other carbonyl, I'm pretty sure you are right that the alpha hydrogens are somewhat acidic (my notes tell me pKa=19). Someone correct me if I'm wrong, but I'm pretty sure that when they are asking for "acidic sites", they mean the sites that give up a hydrogen when the molecule dissociates into its conjugate base.

This is the conjugate base (salt) of Aspirin, which only gave up 1 hydrogen.

 

[dougkaye]

There is no acidic alpha hydrogen. Look at the picture; the carboxylic acid group is attached to a benzene ring. On the alpha carbon, there is no hydrogen.

Edit: If you're referring to the other carbonyl, I'm pretty sure you are right that the alpha hydrogens are somewhat acidic (my notes tell me pKa=19). Someone correct me if I'm wrong, but I'm pretty sure that when they are asking for "acidic sites", they mean the sites that give up a hydrogen when the molecule dissociates into its conjugate base.

This is the conjugate base (salt) of Aspirin, which only gave up 1 hydrogen.

I meant at the other site, the carbonyl with the methyl group. Why is that considered acidic in some problems, but not acidic in this case? In other words, where do we draw the line? I certainly understand that the -0H hydrogen is MUCH more acidic, but the problem doesn't ask "which is fairly acidic" it just asks if it's acidic. Of course, the issue here is that all protons are acidic to a certain extent but it's a matter of where you draw the line and say, "under reasonable conditions that proton is not going to be abstracted". Perhaps using physiological pH is a good guideline?

 

[soby10]

Carboxylic acids are very strong organic acids of the two the -cooh is the most acidic. But I agree w/ you overall I see two acidic hydrogens the -cooh and the alpha hydrogens. But could the question be asking for the most acidic site?

 

[attixx]

I understand where the confusion arises.

I just think of acidic sites as those that play a role in dissociation of the acid. Think of the amino acid glutamine. There is an alpha hydrogen, but we don't consider that when discussing its pKas and different protonated forms. In aspirin, there is only one pKa, one dissociation constant, one hydrogen that is lost in the conversion from acid to conjugate base.

Maybe it will help to think of it in terms of its equilibrium reaction with water. Remember, no matter how weak the acid is, there is some dissociation due to the water accepting a hydrogen from the acid. Never does water abstract an alpha hydrogen. That only happens in a reaction with a strong base.
So the acidic sites would be those that donate it's hydrogen to water, whether is be 1 in a protic acid, 2 in a diprotic acid, or 3 in a triprotic acid.

Instead of trying to "draw the line" when considering hydrogen acidity, if you are asked to count the acidic sites, ask yourself how many hydrogens will donate to water to some extent.

 

[dougkaye]

I understand where the confusion arises.

I just think of acidic sites as those that play a role in dissociation of the acid. Think of the amino acid glutamine. There is an alpha hydrogen, but we don't consider that when discussing its pKas and different protonated forms. In aspirin, there is only one pKa, one dissociation constant, one hydrogen that is lost in the conversion from acid to conjugate base.

Maybe it will help to think of it in terms of its equilibrium reaction with water. Remember, no matter how weak the acid is, there is some dissociation due to the water accepting a hydrogen from the acid. Never does water abstract an alpha hydrogen. That only happens in a reaction with a strong base.
So the acidic sites would be those that donate it's hydrogen to water, whether is be 1 in a protic acid, 2 in a diprotic acid, or 3 in a triprotic acid.

Instead of trying to "draw the line" when considering hydrogen acidity, if you are asked to count the acidic sites, ask yourself how many hydrogens will donate to water to some extent.

Yeah I agree. Although I think MCAT writers need to understand that we read about "acidic alpha hydrogens" in all of the review materials, and therefore it's not really a good question in the way that they asked it.

 

[attixx]

I definitely agree. It seems most of my mistakes are due to little interpretive errors like those where I understood the concept perfectly fine but misread their convoluted/confusing wording. Makes me want to slap someone.

 

[collegestud2013]

Compared to the ester alpha-carbon(pka ~25) the carboxylic acid is WAY more acidic (pKa~4). Therefore it's safe to say that it's monoprotic.

As another example, look at acetic acid: it also has an alpha-hydrogen but as you all know from gen chem it is considered to be monoprotic.

 

[Rabolisk]

Water and alcohol aren't considered to be acidic, even though they are ionizable. Alpha hydrogens are much less acidic than a hydroxyl group of an alcohol, so there's no reason to consider them acidic. They are more acidic than a typical hydrogen bonded to a carbon, but not enough to warrant them an "acidic site".

 

[epsilonprodigy]

In particular, you'd think that if the proton on the methyl group were abstracted, the anion would be resonance-stabilized, i.e. the electrons from the previous bond with the proton could be "spat" in between the two carbons giving it partial pi-bond character, and the pi electrons from the C==O bond could jump up onto the oxygen, allowing it to assume a negative charge.


WTF? Guess it's one of those "best answer" scenarios? were 1 and 2 both answer choices?

 

[BerkReviewTeach]

collegestud2013
Great pull of pKa numbers. Because of the resonance donantion of the lone pair on the ester oxygen, the alpha hydorgen is even less acidic than normal.

So the structure in this question has a carboxylic acid proton on a benzene substituent, so it's pKa is around 5, significantly less than water (pKa = 15.7). This is an acidic proton.

As mentioned by collegestud2013, the H on the methyl group of the ester has a pKa = 25, which is considerably more than water (the generally accepted definition of a neutral species). Less acidic than neutral cannot be classified as acidic.

There is only one proton that is acidic (i.e., will dissociate more than H2O when added to water).