Question

why p-fluoro aniline is less basic than p-chloro aniline?

Answer 1

Although resonance is a powerful effect then inductive effect, however in halogens inductive effect dominate over resonance effect. The order of inductive effect in halogens is: F>Cl>Br>I. Basicity increases with donor group attached to the ring(+I,+R) and decreases with withdrawing groups (-I,-R) attached to the ring. Since fluorine has stronger inductive effect than chlorine, therefore basicity of para chloro aniline is more than para fluoro aniline