Organic Chem Help Needed

[Dochopeful13]

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@aldol16

Can you please help me clarify some confusion with acid base chemistry? In TBR it asks why carboxylic acids are stronger than carbonic acids. The book says the answer is C, (the OH group is more electron donating than the "R" group). However, in my Kaplan Orgo book it says Aldehydes are more acidic than ketones, because Ketones have an extra alkyl group which is electron donating. Electron donating groups decrease anion stability and electron withdrawing groups increase anion stability. Basically Kaplan if contradicting saying electron withdrawing is what makes an acid stronger? If this is the case I would assume carbonic acids would be stronger because they have more electron withdrawing groups as an anion than carboxylic acids do. In addition the book also asks about carbonic acid and acetic acid. I would assume carbonic acid would be stronger because as an anion acetic acid has an akyl group which are electron donating and decrease anion stability. However, according to the book carbonic acid is weaker. Can you please explain what concept I am mixing up or confusing here?

Any help is greatly appreciated.

 

[aldol16]

Acidity is a thermodynamic concept and thus we're talking about the equilibrium between an acid and its conjugate base when talking about acidity. The more stable the conjugate base, the stronger the acid. So if you have a carboxylic acid, R-COOH, the more electron-withdrawing the R group, the stronger the acid because the anion in the conjugate base will be stabilized. Compare trifluoroacetic acid and acetic acid. But here, you're asking to compare carbonic acids and carboxylic acids and the comparison isn't entirely valid. Here's why. So carbonic acid will actually be in equilibrium with H2O and CO2 - you should know this because it's a crucial part of the blood pH buffer system. So it's actually incredibly hard to measure the pKa of carbonic acid because the equilibrium strongly favors CO2 (which is a gas that readily escapes) and the H2O and CO2 equilibrium doesn't yield any protons. So you actually overestimate the amount of carbonic acid you have in solution, which makes calculations really difficult. Plus you don't really know the position of that equilibrium for a given system. That's why in many cases you'll see that carbonic acid is actually a weaker acid because the person who measures it actually doesn't know exactly how much carbonic acid he or she has in the system. So I would disregard that particular question - it's a shoddy example to use.

 

[Dochopeful13]

Acidity is a thermodynamic concept and thus we're talking about the equilibrium between an acid and its conjugate base when talking about acidity. The more stable the conjugate base, the stronger the acid. So if you have a carboxylic acid, R-COOH, the more electron-withdrawing the R group, the stronger the acid because the anion in the conjugate base will be stabilized. Compare trifluoroacetic acid and acetic acid. But here, you're asking to compare carbonic acids and carboxylic acids and the comparison isn't entirely valid. Here's why. So carbonic acid will actually be in equilibrium with H2O and CO2 - you should know this because it's a crucial part of the blood pH buffer system. So it's actually incredibly hard to measure the pKa of carbonic acid because the equilibrium strongly favors CO2 (which is a gas that readily escapes) and the H2O and CO2 equilibrium doesn't yield any protons. So you actually overestimate the amount of carbonic acid you have in solution, which makes calculations really difficult. Plus you don't really know the position of that equilibrium for a given system. That's why in many cases you'll see that carbonic acid is actually a weaker acid because the person who measures it actually doesn't know exactly how much carbonic acid he or she has in the system. So I would disregard that particular question - it's a shoddy example to use.

Thank you so much Aldol. Much appreciated. Their answer choice is confusing. If the Oh group is more donating than the R group that would not have any affect on acidity correct?

 

[5words]

Thank you so much Aldol. Much appreciated. Their answer choice is confusing. If the Oh group is more donating than the R group that would not have any affect on acidity correct?

pretty much what @aldol16 said, It's a very bad example to test acidity.And to answer your question, you're kaplan book is right, and it depends on the R group. because a R group can be anything (Although, most of the time , it's an Alkyl group), in general though, OH is more electron withdrawing than an Alkyl group, so less electron donating in a sense since it attracts electron towards itself, unless they are referring to the resonance within H2CO3.

 

[aldol16]

Thank you so much Aldol. Much appreciated. Their answer choice is confusing. If the Oh group is more donating than the R group that would not have any affect on acidity correct?

If the OH group is more donating than the R group, that would de-stabilize the anion and therefore make it less acidic. But the effect of the OH groups is more net withdrawing and thus it does actually stabilize the anion. So based on that alone, one would expect carbonic acid to be quite acidic. It's the equilibrium with H2O and CO2 that makes it less apparently acidic.