Question

Discuss the mechanism of nuclear halogenation of benzene and explain the role of halogen carrier.

Answer 1

The mechanism of nuclear (electrophilic) halogenation of benzene involves the substitution of a hydrogen atom on the benzene ring with a halogen atom (Cl, Br).

Mechanism steps:

1. Formation of the electrophile: The halogen carrier reacts with the halogen molecule to form a complex, generating the electrophilic halogen species.
   For example: \[\mathrm{Cl}_2 + \mathrm{FeCl}_3 \rightarrow \mathrm{Cl}^+ - \mathrm{FeCl}_4^- \]
2. Electrophilic attack: The electrophile (Cl⁺) attacks the π-electrons of benzene, forming a sigma complex (arenium ion), disrupting aromaticity temporarily.
3. Deprotonation: A base (often \[\ce{FeCl^-_4}\]) abstracts a proton from the sigma complex, restoring aromaticity and yielding halobenzene.
   Equation example: \[\mathrm{C_6H_6} + \mathrm{Cl}_2 \xrightarrow{\mathrm{FeCl}_3} \mathrm{C_6H_5Cl} + \mathrm{HCl}\]

Role of halogen carrier:

• It acts as a Lewis acid.
• It polarizes the X₂ molecule, generating a strong electrophile (X⁺) capable of attacking the stable benzene ring.
• Stabilizes the leaving group as the complex ion (e.g., \[\ce{FeCl^-_4}\]).
• Facilitates the reaction under milder conditions by lowering activation energy.
• Without it, chlorine alone is not reactive enough to substitute hydrogen for benzene.

Thus, a halogen carrier is essential for the electrophilic substitution halogenation of benzene to proceed efficiently, enabling aromatic substitution rather than addition reactions.