Answer
The highlighted proton on the left is more acidic.
Work Step by Step
Draw the conjugate bases:
In the first one, I am going to remove the proton on the left:
Remove the highlighted hydrogen on the left, and give a lone pair to the nitrogen that was connected to it. (It will get a negative charge).
In the second one, I am going to remove the proton on the right:
Remove the highlighted hydrogen on the right, and give a lone pair to the carbon that was connected to it (It will get a negative charge).
Now, check which one is more stable, use the ARIO rules:
1. Atom:
- The first conjugate base will have its charge on a nitrogen.
- The second one will have its charge on a carbon.
- Therefore, the first one is more stable, since nitrogen is more electronegative.
2. Ressonance:
- The first conjugate base will have 2 ressonance structures, and the charge will be spread between 2 nitrogens.
- The second conjugate base will have 2 ressonance structures too, but the charge will be spread between 2 carbons.
- Therefore, the first base should be more stable.
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 first conjugate base is more stable, the proton on the left is more acidic.