Determining Relative Acidity

[ieatshrimp24]

I have a question as to why benzoic acid (C6H5COOH) is more acidic than bicarbonate ion (HOCO2-). When you remove the H from the bicarbonate ion, isn't the charge spread around more than the charge on benzoic acid based on resonance?

Another comparison I don't understand is why acetic acid (CH3COOH) is more acidic than carbonic acid (HOCOOH). Doesn't the methyl group on acetic acid become electron donating so it reverses the inductive effect?

I'm very confused about determining relative acidity in general as you can tell. Just for future reference, between the factors 1) electronegativity of the atom with the negative charge, 2) size of the atom with the negative charge, 3) and delocalization due to resonance, which ones take priority over the other? As in order of 1, 2, and 3 with 1 being the most important. Thanks for the help.

---

Members don't see this ad.

 

[N7chell]

Benzoic acid is more acidic because it's conjugate base can form 4 resonance structures total(negative charge can be delocalized in ring). The conjugate base of bicarbonate has less resonance structures. Also, it as a -2 charge, so it's less stable.

I think the second comparison is a special case because carbonic acid exist in solution partly as H2O + CO2. You are correct in that theoretically carbonic acid should be stronger. However, since not all of it is in acid form in solution, its actual pKa is higher than that of acetic acid.

For your last question, I'd say resonance would be the most important factor in determining the stability of the conjugate base. But don't quote me, I'm sure there are exceptions.

 

[ieatshrimp24]

Oh wow. I didn't realize that benzoic acid is a ring. "Benz-" should have tipped me off. Exceptions... LOL Thanks for your help.

 

[yoyohomieg5432]

Benzoic acid is more acidic because it's conjugate base can form 4 resonance structures total(negative charge can be delocalized in ring). The conjugate base of bicarbonate has less resonance structures. Also, it as a -2 charge, so it's less stable.

I think the second comparison is a special case because carbonic acid exist in solution partly as H2O + CO2. You are correct in that theoretically carbonic acid should be stronger. However, since not all of it is in acid form in solution, its actual pKa is higher than that of acetic acid.

For your last question, I'd say resonance would be the most important factor in determining the stability of the conjugate base. But don't quote me, I'm sure there are exceptions.

how are you spreading a negative charge in the ring? i can only see to spread the negative charge between 2 oxygens. i can pull a double bond from the ring out and push the carbonyl up to move a positive charge around. not seeing how you got - in the ring though