Which proton is most acidic?

[FROGGBUSTER]

My workbook says that protons A & B are both more acidic than C, with the answer being A (I do get that A should be more acidic than B).

I chose C though because de-protonation of C would access the more conjugated system. I would think there is more electron delocalization if you de-protonate C compared to A or B.

Why are A & B more acidic than C?

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[salim271]

Electron withdrawing carbonyl oxygen trumps electron delocalization of a double bond I guess. I'm a bit surprised by that too.

 

[whiteshadodw]

Electron withdrawing carbonyl oxygen trumps electron delocalization of a double bond I guess. I'm a bit surprised by that too.

not quite the exact reason.

they are alpha hydrogens, meaning hydrogens on a carbon next to a carbonyl carbon. when those protons get deprotonated, the oxygen can 'absorb' some of the charge. but really, its just resonance, draw out the resonance structures. also, this is like the bread and butter for aldol condensation and other important carbonyl reactions when the alpha hydrogen gets deprotonated.

 

[FROGGBUSTER]

not quite the exact reason.

they are alpha hydrogens, meaning hydrogens on a carbon next to a carbonyl carbon. when those protons get deprotonated, the oxygen can 'absorb' some of the charge. but really, its just resonance, draw out the resonance structures. also, this is like the bread and butter for aldol condensation and other important carbonyl reactions when the alpha hydrogen gets deprotonated.

I think resonance actually favors de-protonation of C.

I posted this question in the DAT forum and got the response Salim gave, that the reason is because the alpha hydrogens are closer to the Oxygen (electron-withdrawing), but I am not entirely convinced. In my undergrad Orgo course, I was told to (when determining acidity) always go with the proton that yields that more stable conjugate base, and I would think the base that results from de-protonation at C is clearly much more stable than the one that results from either A or B.

Is this a matter of kinetic control vs. thermo control or something? That's the only thing I can think of, where at low temperatures the proximity to the Oxygen makes A & B more acidic while at higher temperatures the yield would increasingly favor de-protonation of C. Just something I'm putting out there, but it's really just a shot in the dark.

 

[whiteshadodw]

I think resonance actually favors de-protonation of C.

I posted this question in the DAT forum and got the response Salim gave, that the reason is because the alpha hydrogens are closer to the Oxygen (electron-withdrawing), but I am not entirely convinced. In my undergrad Orgo course, I was told to (when determining acidity) always go with the proton that yields that more stable conjugate base, and I would think the base that results from de-protonation at C is clearly much more stable than the one that results from either A or B.

Is this a matter of kinetic control vs. thermo control or something? That's the only thing I can think of, where at low temperatures the proximity to the Oxygen makes A & B more acidic while at higher temperatures the yield would increasingly favor de-protonation of C. Just something I'm putting out there, but it's really just a shot in the dark.

a nitrogen containing a negative charge is not favored.

deprotonate the alpha hydrogens, and you'll see the oxygen holds a negative charge. this just a rule of thumb sort of thing and again:

http://en.wikipedia.org/wiki/Alpha_and_beta_carbon


scroll down to mechanism in this page and look at the base catalyzed mechanism, these are the structures you should get:
http://en.wikipedia.org/wiki/Aldol_condensation

also, the two "Bs" in the question you posted aren't exactly identical. one is an alkene alpha carbon, the other isn't. basically, at the end of the day, you need to memorize this, otherwise aldol condensation and other alpha carbon reactions will be difficult.

 

[FROGGBUSTER]

a nitrogen containing a negative charge is not favored.

deprotonate the alpha hydrogens, and you'll see the oxygen holds a negative charge. this just a rule of thumb sort of thing and again:

http://en.wikipedia.org/wiki/Alpha_and_beta_carbon


scroll down to mechanism in this page and look at the base catalyzed mechanism, these are the structures you should get:
http://en.wikipedia.org/wiki/Aldol_condensation

also, the two "Bs" in the question you posted aren't exactly identical. one is an alkene alpha carbon, the other isn't. basically, at the end of the day, you need to memorize this, otherwise aldol condensation and other alpha carbon reactions will be difficult.

That's a Hydrogen, not a Nitrogen. I drew it sloppily. If you look at my resonance structures that I drew, you will see that the oxygen on the right has the negative formal charge upon de-protonation of C when you push electrons.

As for the B on the right, that should be a D. It's probably a misprint.

I do understand that alpha hydrogens of ketones and aldehydes are relatively quite acidic (pka of 20-25 iirc), but I am not understanding why they would be more acidic in this case.

 

[loveoforganic]

C was my guess. Same rationale

 

[whiteshadodw]

That's a Hydrogen, not a Nitrogen. I drew it sloppily. If you look at my resonance structures that I drew, you will see that the oxygen on the right has the negative formal charge upon de-protonation of C when you push electrons.

As for the B on the right, that should be a D. It's probably a misprint.

I do understand that alpha hydrogens of ketones and aldehydes are relatively quite acidic (pka of 20-25 iirc), but I am not understanding why they would be more acidic in this case.

actually alpha hydrogen from ketone is more acidic than aldehyde proton.

furthermore, if you sandwich a carbon between two carbonyls, it becomes even MORE acidic, since the resonance structures allow for either oxygen to hold the negative charge allowing for more resonance.

look at the picture at the very top, and look at product 2 after the acetoacetic ester is deprotonated, those are the resonance forms:

http://en.wikipedia.org/wiki/Acetoacetic_ester_synthesis

 

[salim271]

actually alpha hydrogen from ketone is more acidic than aldehyde proton.

Nah, aldehyde is slightly more acidic. I checked pKas to make sure: http://www.chem.ucalgary.ca/courses/351/Carey5th/Ch21/ch21-2.html

 

[deleted329605]

Comparing protons A and B, couldn't you make an argument for each saying they were acidic? Like for A, you could say it's more acidic 1)because of less steric hindrance and 2) in for a carbanion intermediate that is flanked by less electron donating alkyl groups. Also you could say that for B although it is more sterically hindered and would have a less stable carbanion, when you delocalize electrons to form the enolate it will have the more substituted alkene(more stable). Is this correct? And if so which effects win out?

 

[whiteshadodw]

Nah, aldehyde is slightly more acidic. I checked pKas to make sure: http://www.chem.ucalgary.ca/courses/351/Carey5th/Ch21/ch21-2.html

what i obviously meant was that the proton attached to the the alpha carbon of a carbonyl is more acidic than the aldehyde proton, pKa 17 of the aldehyde and pKa 20 of the ketone refer to the same proton, the alpha proton. i'm not talking about the alpha proton, i'm talking about the proton directly attached to the carbonyl, its not acidic.

also, due to the electron donating effect of the carbon next to B, i would go further and say A is more acidic than B.

 

[FROGGBUSTER]

This was bothering me, so I ended up emailing my orgo TA from last semester. The answer is C.