Chloroarenes (aryl chlorides) and bromoarenes (aryl bromides) can be easily prepared by direct chlorination or bromination of benzene or other aromatic hydrocarbons. These reactions are carried out in dark, at ordinary temperatures (310–320 K) in the presence of Lewis acid catalyst such as anhydrous ferric or aluminium halides (FeCl₃, FeBr₃, AlCl₃). These reactions are called electrophilic substitution of arenes. The Lewis acid acts as a catalyst and its function is to carry the halogen to the aromatic hydrocarbon and is also called halogen carrier. In actual practice, iron filings in the presence of chlorine or bromine are commonly used. The halogens react with iron filings to form corresponding Lewis acids.

The mechanism of direct halogenation is a three-step process. Which is the rate-determining step of the reaction?

Step 2

The correct answer is option 2. Step 2.

Arenes are halogenated by electrophilic substitution reaction. The halogen carrier's function is to generate an electrophile, which then attacks the benzene ring to produce the product. For instance, benzene is chlorinated as follows:

Step 1. Generation of electrophile: The electrophile is generated as:

Step 2. Formation of carbocation (s-complex) intermediate: The electrophile, then attacks the benzene ring to form a carbocation (arenium ion or s-complex) which gets resonance stabilized. This is a slow and rate-determining step.

Step 3. Loss of proton from the carbocation intermediate: The resonance stabilized carbocation loses a proton (H⁺) to FeCl₄^– to give chlorobenzene. This is a fast step and therefore, does not affect the rate of reaction.

This step is fast and hence does not affect the rate of the reaction.

Because halogens are o-, p-directing groups, if excessive quantity is utilized, the second halogen also enters the ring, mostly at the o- and p-positions with regard to the first halogen.