question_answer

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

Answer:

                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 +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 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.