Question:

Product, P is: \[ \text{C}_6\text{H}_6 + \text{Cl}_2(\text{excess}) \xrightarrow{\text{Anhy.AlCl}_3}{\text{dark, cold}} \text{P} \]

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In electrophilic aromatic substitution, "excess" of the halogenating agent with a strong Lewis acid catalyst can lead to exhaustive substitution, especially when conditions (like "dark") rule out other reaction pathways.

Updated On: May 29, 2025

C$_6$H$_5$Cl
C$_6$H$_4$Cl$_2$
C$_6$H$_6$Cl$_6$
C$_6$Cl$_6$

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The Correct Option is D

Solution and Explanation

Step 1: Identify the reactants and reaction conditions.

Reactant: Benzene (C$_6$H$_6$), an aromatic compound.
Reagent: Chlorine (Cl$_2$) in excess.
Catalyst: Anhydrous Aluminum Chloride (Anhy.AlCl$_3$), a strong Lewis acid.
Conditions: Dark and cold. Step 2: Determine the type of reaction.
The reaction of an aromatic compound (benzene) with a halogen (Cl$_2$) in the presence of a Lewis acid catalyst (Anhy.AlCl$_3$) is a classic example of electrophilic aromatic substitution (EAS), specifically chlorination. The "dark, cold" conditions are crucial: "dark" prevents free radical reactions that would occur under UV light, and "cold" helps to control the reaction rate and selectivity, while still being sufficient for EAS. Step 3: Analyze the effect of "excess Cl$_2$" on the product.
The first substitution yields chlorobenzene (C$_6$H$_5$Cl).
Although a halogen substituent (-Cl) is a \textit{deactivating group} (makes the ring less reactive towards further EAS), it is an \textit{ortho-para director}.
When Cl$_2$ is present in "excess" along with a strong Lewis acid catalyst like Anhy.AlCl$_3$, the reaction can be driven to multiple substitutions. With sufficient excess of the halogen and appropriate reaction time (even if 'cold' implies controlling the rate to prevent side reactions or decomposition, it doesn't necessarily preclude exhaustive substitution over time), all hydrogen atoms on the benzene ring can eventually be replaced by chlorine atoms.
This process leads to the formation of hexachlorobenzene. \[ \text{C}_6\text{H}_6 + 6\text{Cl}_2 \xrightarrow{\text{Anhy.AlCl}_3, \text{excess}, \text{dark, cold}} \text{C}_6\text{Cl}_6 + 6\text{HCl} \] Step 4: Evaluate the given options.
(A) C$_6$H$_5$Cl (Chlorobenzene): This is the product of monosubstitution. The "excess" condition suggests that the reaction proceeds beyond this stage.
(B) C$_6$H$_4$Cl$_2$ (Dichlorobenzene): This is a disubstituted product. While plausible with excess Cl$_2$, the presence of C$_6$Cl$_6$ as an option suggests the reaction can be driven to completion of substitution.
(C) C$_6$H$_6$Cl$_6$ (Benzene Hexachloride - BHC): This compound is formed via an \textit{addition reaction} of chlorine to benzene under UV light (free radical mechanism), which results in a saturated cyclic compound. The given conditions "dark, cold" specifically rule out this type of reaction.
(D) C$_6$Cl$_6$ (Hexachlorobenzene): This is the fully substituted aromatic product where all hydrogens are replaced by chlorines. With excess reagent and a strong catalyst under electrophilic substitution conditions, this exhaustive substitution is a known possibility, particularly when conditions (like "dark" to avoid radical addition) are set. The "cold" condition ensures selectivity for substitution over other highly energetic reactions. Step 5: Conclusion.
Given the condition of "excess Cl$_2$" and the nature of electrophilic aromatic substitution catalyzed by a strong Lewis acid like Anhy.AlCl$_3$, the reaction can proceed to replace all hydrogen atoms with chlorine atoms, leading to the formation of hexachlorobenzene, C$_6$Cl$_6$.

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