Similar to the chlorination of benzene with Cl2/FeCl3, the iodination of benzene with ICl can be catalyzed with FeCl3, which bonds to the Cl to make a Lewis acid-base complex. Briefly explain why FeCl3 accelerates the rate of the reaction. HINT: The rate of a reaction is determined by the slowest step, and accelerating the slow step will accelerate the overall reaction rate – think about what the slow step is in the halogenation of benzene.

Introduction

Chlorination and iodination are two common methods for halogenating benzene. In both reactions, the halogen atom substitutes for a hydrogen atom on the benzene ring, generating a halogenated aromatic compound. However, the mechanisms of the two reactions differ somewhat.

In chlorination, chlorine gas (Cl₂) is typically used as the halogenating agent, and a Lewis acid catalyst, such as aluminum or iron chloride, is used to activate the chlorine and increase its reactivity toward the benzene ring. The reaction proceeds through a cyclic intermediate called the chloronium ion, which is formed by the attack of the chlorine molecule on the benzene ring.

In iodination, iodine monochloride (ICl) is often used as the halogenating agent, and a Lewis acid catalyst, such as iron(III) chloride, is also used to activate the iodine and increase its reactivity towards the benzene ring. The reaction proceeds through an electrophilic iodine species, such as iodine cation (I⁺), which attacks the benzene ring.