Question

Although chlorine shows strong −I effect, it is ortho–para directing in electrophilic aromatic substitution. Why?

Hint:

Halogens are deactivating but ortho–para directing due to resonance.

Answer 1

Concept:
Chlorine (\(Cl\)) is a halogen that has a strong -I (inductive electron-withdrawing) effect due to its electronegativity. However, despite this electron-withdrawing nature, chlorine is an ortho–para directing group in electrophilic aromatic substitution reactions. This behavior can be explained by considering the resonance effect alongside the inductive effect.
Step 1: Understand the electron-withdrawing inductive effect of chlorine.
Chlorine, being highly electronegative, pulls electron density away from the aromatic ring through the sigma bond. This is known as the -I effect. The -I effect would, in principle, decrease the electron density on the ring, making it less reactive towards electrophilic substitution.
Step 2: Understand the resonance donating effect.
Chlorine also has lone pairs of electrons on its atom, which can participate in resonance with the benzene ring. This gives chlorine a +M (resonance donating) effect. Through resonance, the lone pairs on chlorine can donate electron density to the ring, particularly at the ortho and para positions, making these positions more reactive to electrophiles.
Step 3: Result of the opposing effects.
While chlorine’s -I effect decreases the electron density on the ring, its +M resonance donating effect increases the electron density at the ortho and para positions. This is stronger than the -I effect, especially in the context of electrophilic aromatic substitution, where the electrophile attacks the electron-rich positions.
Step 4: Conclusion.
Because the +M effect dominates the -I effect in this case, chlorine acts as an ortho–para directing group. The electron donation through resonance makes the ortho and para positions more reactive to electrophiles, despite the overall electron-withdrawing -I effect.