I want to compare the relative acidity of p-chlorobenzoic acid and p-flurobenzoic acid. Both of the facts below point to p-chlorobenzoic acid being more acidic:
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 is not distance dependant and hence p-fluoroobenzoic acid should be less acidic since the negative charge on its conjugate base is destabilized.
There is resonance of the negative charge of the carboxylate ion with the empty d-orbitals of chlorine which would stabilize it.
However, in reality, p-chlorobenzoic acid is less acidic than p-fluorobenzoic acid . Why?