Two protons have different pKas: 3.50 and 4.25. The molecule is 2-bromo-3-nitrophthalic acid (if I'm naming it correctly). Assign the correct pKa to each proton.
A lower pKa means a stronger acid, which means a more stable conjugate acid.
Carboxylates are stabilized by resonance and by induction.
The Attempt at a Solution
I say the carboxylate ortho to bromine and meta to nitro is the strongest acid, pKa = 3.50. The inductive effect of an adjacent bromo is a strong stabilizing effect on the conjugate base.
A resonance form may be drawn such that a positive charge is situated adjacent to the other carboxylate: the carboxylate meta to bromine and para to nitro. The resonance stabilization is compromised by the adjacent substituted positions to the carboxylate, which force the carboxylate out of planarity with the ring.
My professor disagrees. His solution:
Please weigh in with your answer. Also can anyone help me find a literature value for these? I think the professor made them up. I haven't been able to find pKa's for this compound myself. Where is a good place to look, that would have data for an obscure compound like this?