CHEMISTRY · Multi-part · AP · Chemistry · Acids and Bases · Acids and Bases
Problem
Three monoprotic aromatic acids are examined at 298K.
• Benzoic acid, C6H5COOH, has a measured pKa=4.20.
• Compound A is 4-nitrobenzoic acid (NO2 at the para position).
• Compound B is 4-methoxybenzoic acid (OCH3 at the para position).
Assume all acids behave ideally in dilute aqueous solution.
(1) Part 1: (a) Rank the three acids (benzoic acid, Compound A, Compound B) from strongest to weakest. Provide your reasoning.
(2) Part 2: (b) For both Compound A and Compound B, draw or describe at least two distinct resonance structures of the conjugate base that illustrate how the para substituent interacts with the ring and the carboxylate group.
(3) Part 3: (c) For each compound, discuss how resonance and inductive effects combine (or oppose) to produce the net stabilization or destabilization of the conjugate base. Explicitly state which effect dominates in each molecule and why.
(4) Part 4: (d) Using benzoic acid as a calibration point (pKa=4.20), predict an approximate pKa (±0.3 pKa units is sufficient) for Compound A and for Compound B. Justify each estimate with reference to your analysis in parts (b) and (c).