[xiaomeimei]

In my organic chemistry class, we're looking at the hypothetical reaction of salicyclic acid with water. Seeing that there is two places to deprotonate;
can someone example why a certain one is a pka of 10 and the other a pka of 5.

Please help explain why is there a pka difference depending on which hydrogen is deprotonated.
Here is my work:
http://oi44.tinypic.com/28a1hna.jpg

I'm thinking even though there is more resonance structures in the first one, the inductive effect of the other two oxygen is making the conjugate bases more unstable.

Thank you!

[Babcock_Hall]

The resonance available through the aromatic ring makes this proton more acidic than it otherwise would be.  Think about the pK_a of ethanol for a comparison where there is no resonance.  However, we also need to explain why it is less acidic than the proton on the carboxylic acid.  Some resonance forms make a greater contribution to the overall structure than others do.  See if you can put this idea to work.

[xiaomeimei]

Hmm, do you think proximity and inductive effect could explain why the carboxylic acid proton is more acidic?

In addition,
is it common to compare acidity in this order:
1. Charge: Neutral cb = more acidic
2. Proximity: Proximity of electronegative atoms = more acidic
3. # Electronegative atoms: More - more acidic
4. Size of atom: Bigger = more acidic
5. Electronegativity: More electronegative = more acidic

[Babcock_Hall]

We had a long thread about the subject of acidity a few months ago.  OrgoPete may recall which thread, and it might be worth looking over.  My own thoughts are One, We should ask which atom can more easily bear a negative charge.  Two, we should compare the pK_a of the carboxylic acid in salicylic acid with the pK_a values of acetic acid and benzoic acid.  The pK_a of acetic acid is close to 5, and the pK_a of benzoic acid is 4.20.
ETA
I don't think one should be too concerned about the difference in pK_a values between the carboxylic acid groups in benzoic acid vs. salicylic acid, until we have adequately addressed the difference in values between the carboxylic and phenolic groups within salicylic acid.   

[Altered State]

We had a long thread about the subject of acidity a few months ago.  OrgoPete may recall which thread, and it might be worth looking over.  My own thoughts are One, We should ask which atom can more easily bear a negative charge.  Two, we should compare the pK_a of the carboxylic acid in salicylic acid with the pK_a values of acetic acid and benzoic acid.  The pK_a of acetic acid is close to 5, and the pKa of benzoic acid is 4.20.

Im interested in that thread, if someone finds it, would appreciate if you post it here.

[xiaomeimei]

We had a long thread about the subject of acidity a few months ago.  OrgoPete may recall which thread, and it might be worth looking over.  My own thoughts are One, We should ask which atom can more easily bear a negative charge.  Two, we should compare the pK_a of the carboxylic acid in salicylic acid with the pK_a values of acetic acid and benzoic acid.  The pK_a of acetic acid is close to 5, and the pK_a of benzoic acid is 4.20.
ETA
I don't think one should be too concerned about the difference in pK_a values between the carboxylic acid groups in benzoic acid vs. salicylic acid, until we have adequately addressed the difference in values between the carboxylic and phenolic groups within salicylic acid.   

Thank you for the suggestion.
I appreciate the *delete me*

[orgopete]

We had a long thread about the subject of acidity a few months ago.  OrgoPete may recall which thread, and it might be worth looking over.

Ugh, I don't recall specifically, but this sounds like the thread that I got a lot of flags for thinking differently than everyone else.

At the risk of getting more negative comments, I will agree with the basic reasons noted by xiaomeimei and Babcock Hall. I'll refrain from comments regarding electronegativity, especially as larger atom, i.e., greater nuclear charge has been noted.

The acidity should become rational. O, increases acidity. HO attached to sp2 carbon increases acidity. HO attached to C=O, increases acidity. HOOC attached to sp2 carbons increase acidity. I think those are the major effects seen.

[xiaomeimei]

Thanks so much orgopete and Babcock Hall! Really appreciate the *delete me*

I understand this problem a lot better now, and in addition to comparing acidity in general.

[Babcock_Hall]

My point about the pK_a of the carboxylic acid in salicylic acid is that it is close to other carboxylic acids.  Likewise, the pK_a of the phenolic hydrogen is about what one would expect.