Reactivity controlled by the stronger inductive effect and orientation is controlled by resonance effect. How?
Answers
The -I effect of chlorine withdraws electrons from the benzene ring. Hence tends to destabilize the intermediate carbocation formed during the electrophilic substitution.
Conversely Cl donates its lone pair of electrons to the aromatic ring and hence increase the electron density at ortho and para positions. Thus, the lone pair of electrons on the chlorine atom stabilize the intermediate carbocation due to resonance.
Since -I effect of Cl is stronger than its +R effect, hence Cl cause net deactivation. Also further since resonance effect opposes the inductive effect, it makes deactivation less at o- and p- positions.
Therefore, in chlorobenzene, reactivity is controlled by stronger -I effect while orientation of incoming substituent is controlled by the weaker resonance effect.