Two positional isomers of fluorobenzoic acid are compared at 298 K
CHEMISTRY · Multi-part · AP · Chemistry · Acids and Bases · Acids and Bases
Problem
Two positional isomers of fluorobenzoic acid are compared at 298 K.
• Compound C: 2-fluorobenzoic acid (F at the ortho position).
• Compound D: 4-fluorobenzoic acid (F at the para position).
The pKₐ of unsubstituted benzoic acid under the same conditions is 4.20.
(a) Predict which isomer, Compound C or Compound D, is the stronger acid. Support your prediction with structural arguments.
(b) Explain how the balance between the resonance effect and the inductive effect of the fluorine substituent differs in the ortho versus para position, and how this balance governs the overall acid strength.
(c) State whether the expected pKₐ difference between the two isomers is likely to be large (≫1 pKₐ unit), moderate (≈1 pKₐ unit), or small (≪1 pKₐ unit). Justify your choice qualitatively.