Fluorine is more electronegative than chlorine but p-fluorobenzoic acid is a weaker acid than p-chlorobenzoic acid. Explain.

Explanation

Since halogens are more electronegative than carbon and also possess lone pairs of electrons, therefore, they exert both -I- and +R- effects. Now in F, the lone pairs of electrons are present in 2p-orbitals but in Cl, they are present in 3p-orbitals. since 2p-orbitals of F and C are of almost equal size, therefore, the effect is more pronoiunced in p-fluorobenzoic acid than in p-chlorobenzoic acid.

Thus, in p-fluorobenzoic acid, +R-effect outweighs the -I-effect but is p-chlorobenzoic acid, it is the -I- effect which outweighs the +R-effect. consequently, p-fluorobenzoic acid is a weaker acid than p-chlorobenzoic acid.