I want to compare the relative acidity of benzoic acid with para substituted chlorine vs para substituted fluorine.
Now, the carboxylic acid group is too far away from the halogen for inductive effect to have any significance. The +M effect of fluorine is greater than chlorine (due to more effective orbital overlap).
+M effect it not distance dependant and hence benzoic acid with para substituted fluorine should be less acidic since the negative charge on its conjugate base is destabilized.
Also, there is resonance of the negative charge of the carboxylate ion with the d orbitals of chlorine which would stabilize it.
Both of the above facts point to benzoic acid with para substituted chlorine being more acidic.
However, this is not the case. Why?