What happens when Benzene is treated with chlorine in the dark in the presence of anhydrous FeC{l_3} ?

When benzene is treated with chlorine in the dark in the presence of anhydrous FeCl₃, an electrophilic substitution reaction takes place, specifically chlorination of benzene. Here’s the detailed explanation:

### Step 1: Role of FeCl₃
Anhydrous FeCl₃ acts as a catalyst and helps generate the electrophile. The catalyst reacts with chlorine (Cl₂) to form a complex, which makes chlorine more reactive.

The reaction is:
\[ \text{Cl}_2 + \text{FeCl}_3 \rightarrow [\text{FeCl}_4]^- + \text{Cl}^+ \]

Here, \(\text{Cl}^+\) is the electrophile, which will attack the benzene ring.

### Step 2: Electrophilic attack on benzene
The generated \(\text{Cl}^+\) acts as an electrophile and attacks the π-electron system of the benzene ring. Since benzene is electron-rich due to its delocalized π-electrons, the electrophile (Cl⁺) replaces one hydrogen atom in the ring, forming chlorobenzene (C₆H₅Cl).

### Step 3: Regeneration of catalyst
The intermediate complex formed after the substitution releases a proton (H⁺), which combines with the \([\text{FeCl}_4]^{-}\) ion to regenerate FeCl₃ and HCl.

The overall reaction can be written as:
\[
\text{C}_6\text{H}_6 + \text{Cl}_2 \xrightarrow{\text{FeCl}_3} \text{C}_6\text{H}_5\text{Cl} + \text{HCl}
\]

### Conclusion:
When benzene reacts with chlorine in the dark in the presence of anhydrous FeCl₃, chlorobenzene (C₆H₅Cl) is formed as the major product along with the byproduct hydrogen chloride (HCl). The reaction is an example of electrophilic aromatic substitution (EAS).