Organic Acids

The p-orbital overlap is much more effective when the overlapping orbitals are of comparable size and so the +R effect of fluorine (which uses 2p orbital) is greater than the effect of bromine (which uses 4p orbital). Infact, the +R and +I effects of F are nearly counterbalanced, whereas the +R effect of Br is weaker than its +I effect. Therefore, although both halogen atoms tend to lower the electron density of the ring, the lowering is much effective in Br-acid than F-acid. Hence, p-fluorobenzoic acid is LESS acidic than p-bromobenzoic acid.