Proton acidity

[Eye Cue]

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I figured it would be easier to draw than to explain:

So I know that the hydrogen at A is more acidic than the hydrogens at B because A is surrounded by 3 EWG's, as opposed to B being surrounded by 2 EWG's. I also know that the carboxylic acid hydrogen at C is quite acidic, although I do not know which is more acidic.

I don't know if this is beyond the MCAT's scope, but regardless, I'd still like to know, and be able to understand to improve my reasoning skills.

 

[JFK90787]

I would guess A since it will have 3 resonance forms with a negative charge on the electronegative oxygens, as opposed to only two negative oxygen for the carboxylic acid group

 

[loveoforganic]

I've never seen a pKa value for a proton such as proton A, but I would bet on the carboxylic acid being more acidic. In A, you have 4 resonance structures, 3 of them equal, one of them not (carbanion). In B, you only have 2 resonance structures, but they are equal with the charge on only oxygen.

To give some kind of comparison, the pKa of diethyl malonate is around 13, while the pKa of acetic acid is around 5.

 

[BennieBlanco]

I figured it would be easier to draw than to explain:

So I know that the hydrogen at A is more acidic than the hydrogens at B because A is surrounded by 3 EWG's, as opposed to B being surrounded by 2 EWG's. I also know that the carboxylic acid hydrogen at C is quite acidic, although I do not know which is more acidic.

I don't know if this is beyond the MCAT's scope, but regardless, I'd still like to know, and be able to understand to improve my reasoning skills.

I've never seen a pKa value for a proton such as proton A, but I would bet on the carboxylic acid being more acidic. In A, you have 4 resonance structures, 3 of them equal, one of them not (carbanion). In B, you only have 2 resonance structures, but they are equal with the charge on only oxygen.

To give some kind of comparison, the pKa of diethyl malonate is around 13, while the pKa of acetic acid is around 5.

👍

 

[Eye Cue]

Ah, you guys are great, thanks!

 

[boaz]

I've never seen a pKa value for a proton such as proton A, but I would bet on the carboxylic acid being more acidic. In A, you have 4 resonance structures, 3 of them equal, one of them not (carbanion). In B, you only have 2 resonance structures, but they are equal with the charge on only oxygen.

To give some kind of comparison, the pKa of diethyl malonate is around 13, while the pKa of acetic acid is around 5.

Interesting... but in the enol form a similar compound has a pKa of about 3: http://physchem.org.uk/symp04/symp04_ec.pdf (page 5)

 

[loveoforganic]

That's interesting, I wouldn't expect such a large drop in pKa. That compound is probably a fairly accurate analogous structure too. I could be wrong! Anyway, above the scope of MCAT 😛

 

[Ferocity]

I've never seen a pKa value for a proton such as proton A, but I would bet on the carboxylic acid being more acidic. In A, you have 4 resonance structures, 3 of them equal, one of them not (carbanion). In B, you only have 2 resonance structures, but they are equal with the charge on only oxygen.

To give some kind of comparison, the pKa of diethyl malonate is around 13, while the pKa of acetic acid is around 5.

So is the C hydrogen most acidic because of resonance and induction?

 

[loveoforganic]

That was my guess, but the linked article has an analogous structure that would lead one to believe that isn't the case.