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

Explanation

Halogens are more electronegative than carbon and also possess lone pairs of electrons. Therefore, halogens can exert - I and + R effects. In F atom, the lone pairs of electrons are present in 2p-orbitals but in Clatom, they are present in 3p-orbitals. Since 2p-orbitals of F and Care of almost same size, therefore, + R effect is more pronounced in p-fluorobenzoic acid than in p-chlorobenzoic acid.

Thus, in p-fluorobenzoic acid, + R effect outweighs the - I effect and therefore, it is weaker acid than p-chlorobenzoic acid (having more - I effect than + R effect).