A researcher wishes to design a benzoic-acid derivative with a target pKₐ of approximately 3.0. She can introduce exactly one trifluoromethyl group (CF₃, strong electron-withdrawing) and one methoxy group (OCH₃, electron-donating by resonance) onto the aromatic ring.
Three candidate molecules are proposed:
• Compound F: CF₃ at the para position and OCH₃ at the meta position relative to COOH.
• Compound G: OCH₃ at the para position and CF₃ at the meta position.
• Compound H: CF₃ at the para position and OCH₃ at the ortho position.
Assume all other positions are unsubstituted.
(a) Rank Compounds F, G, and H in order of expected acid strength (1 = strongest) and justify your ranking in terms of how each substituent influences the conjugate base.
(b) Identify which compound is most likely to meet the target pKₐ of 3.0. Provide a detailed argument that weighs the competing resonance and inductive effects present in that molecule.
(c) For the compound selected in part (b), draw or describe a resonance structure of its conjugate base that shows electron donation from the methoxy group into the ring. Explain how this resonance interaction competes with the inductive effect of the CF₃ group.
(d) Predict qualitatively how adding a second CF₃ group at the meta position of the chosen compound would alter the pKₐ. Would the acid become significantly stronger, slightly stronger, or weaker? Support your prediction with reasoned analysis.