Answer
The highlighted proton on the right is more acidic.
Work Step by Step
Draw the conjugate bases:
In the first one, we are going to remove the proton on the left:
Remove the highlighted hydrogen on the left, and give a lone pair to the oxygen that was connected to it. (It will get a negative charge).
In the second one, we are going to remove the proton on the right:
Remove the highlighted hydrogen on the right, and give a lone pair to the oxygen that was connected to it. (It will get a negative charge).
Now, check which one is more stable, use the ARIO rules:
1. Atom:
In both structures, the charge will be on a oxygen, so there is no difference.
2. Ressonance:
- The first conjugate base will have 4 resonance structures, and the charge will be spread between an oxygen and 3 carbons.
- The second conjugate base will have 2 resonance structures, and the charge will be spread between 2 oxygens.
- Therefore, the second base should be more stable.
** 2 Oxygens > 1 Oxygen and 3 Carbons (for stability)
** An oxygen stabilizes the charge better than many carbons.
3. Induction:
- The induction won't make much difference on the stability of the bases, in this case.
4. Orbitals:
The orbitals won't make much difference in this case.
- Since the second conjugate base is more stable, the proton on the right is more acidic.
