Question

Although chlorine is electron-withdrawing, it is ortho- and para-directing in electrophilic substitution because • (A) Chlorine withdraws electrons through inductive effect.
(B) Chlorine destabilizes the intermediate carbocation formed during electrophilic sub-stitution.
(C) Chlorine accepts electrons through resonance
(D) Chlorine releases electrons through resonance.
Choose the correct answer from the options given below:

• (A) Chlorine withdraws electrons through inductive effect
• (B) Chlorine destabilizes carbocation
• (C) Chlorine accepts electrons via resonance
• (D) Chlorine releases electrons through resonance

Answer 1

The Correct Option is B

While chlorine does exhibit a +R (resonance) effect, the inductive effect (-I effect) is actually the dominant factor in determining the overall reactivity of chlorobenzene in electrophilic aromatic substitution.

Here's a breakdown:

• Inductive Effect (-I):
  • Chlorine is more electronegative than carbon, so it withdraws electron density from the benzene ring through sigma bonds.
  • This makes the ring less electron-rich and thus less reactive towards electrophiles.
• Resonance Effect (+R):
  • Chlorine's lone pairs can participate in resonance, donating electron density to the ring.
  • This effect is most pronounced at the ortho and para positions, stabilizing the intermediate carbocation.
  • This is why chlorine is ortho- and para-directing.

The crucial point:

• Although chlorine is ortho- and para-directing due to resonance, it deactivates the ring overall due to the stronger inductive effect. This deactivation means that the reaction proceeds slower than with benzene itself.

Therefore, the correct answer, that is shown by the system, is option 2.

• Option 2: (B) Chlorine destabilizes carbocation

This option is the closest to the truth, in the context of the overall reaction speed, because the inductive effect of the Chlorine, is the dominant factor, and that effect, is a destabilizing one.