Problem 14 - Chemistry in 30 days

Question

Benzene reacts with \(\mathrm{Cl}_{2}\) to form BHC in the presence of (a) \(\mathrm{Ni}\) (b) Anh. \(\mathrm{AlCl}_{3}\) (c) Sunlight (d) \(\mathrm{Zn}\)

Step-by-Step Solution

Answer

BHC forms in the presence of sunlight (c).

Step 1: Understanding the Reaction

The reaction mentioned involves benzene reacting with chlorine ( extrm{Cl}_2). The process can yield different products based on the catalyst or condition present. The potential outcomes are highly dependent on the environment and reagent mixing.

Step 2: Exploring Condition (a): Nickel Catalyst

When benzene reacts with  extrm{Cl}_2 in the presence of a Nickel ( extrm{Ni}) catalyst, this usually results in catalytic hydrogenation rather than chlorination directly. Since our context is chlorination in specific conditions and Ni doesn't favor substitution, no direct reaction to form BHC occurs.

Step 3: Exploring Condition (b): Anhydrous Aluminum Chloride Catalyst

In the presence of anhydrous  extrm{AlCl}_3, benzene reacts with  extrm{Cl}_2 via Friedel-Crafts alkylation or acylation reactions. Here, AlCl3 acts as a catalyst for electrophilic aromatic substitution, leading to chlorobenzene formation but not BHC.

Step 4: Exploring Condition (c): Sunlight

Under sunlight, benzene can undergo a photochemical reaction with  extrm{Cl}_2. This light-induced condition favors the formation of a compound known as benzene hexachloride (BHC), or hexachlorocyclohexane. This happens as chlorine radicals add across the benzene double bonds in six steps.

Step 5: Exploring Condition (d): Zinc Catalyst

Zinc is often used as a reducing agent rather than a catalyst for chlorination reactions. It doesn't commonly participate in promoting the formation of chlorinated benzenes like BHC.

Step 6: Conclusion of Reaction Analysis

Given the conditions listed, the formation of BHC is primarily influenced by the presence of sunlight, as it provides the necessary energy to break the  extrm{Cl}_2 bond leading to radical formation and subsequent addition to the benzene ring.

Key Concepts

Electrophilic Aromatic Substitution

In the world of chemical reactions, Electrophilic Aromatic Substitution (EAS) stands out as a significant process for modifying aromatic compounds. Benzene, being a highly stable aromatic hydrocarbon, undergoes EAS to substitute a hydrogen atom with another atom or group. This reaction involves the aromatic ring reacting with an electrophile, which is a species that accepts an electron pair.

The process begins when an electrophile approaches the π electrons of benzene, temporarily disrupting the aromaticity. This step yields an arenium ion intermediate. The aromaticity is then restored by eliminating a proton from the carbon bearing the new substituent. Throughout this process, the benzene ring maintains its aromatic core while allowing the substitution.

• Key feature: Substitution occurs without destroying the aromatic nature of benzene.
• Typical products include chlorobenzene and nitrobenzene, depending on the reacting electrophile.
• Common catalysts: Lewis acids like \({\text{AlCl}}_3\) or \({\text{FeCl}}_3\).

Friedel-Crafts Reaction

The Friedel-Crafts Reaction is a renowned method used in organic chemistry to introduce alkyl or acyl groups onto an aromatic ring. There are two main types: Friedel-Crafts Alkylation and Friedel-Crafts Acylation. Both rely on the activation of an electrophile by a Lewis acid catalyst, typically \({\text{AlCl}}_3\).

In Friedel-Crafts Alkylation, an alkyl group is added by forming a carbocation as the electrophile, facilitated by the catalyst. For Friedel-Crafts Acylation, an acyl chloride in the presence of \({\text{AlCl}}_3\) generates an acylium ion, which then adds to the benzene.

• This reaction is widely used to synthesize complex organic compounds.
• Limitations: Polyalkylation and carbocation rearrangement can occur in Alkylation.
• Acylation is favored for its lack of rearrangement, producing ketones directly.

The Friedel-Crafts reactions are valuable in molecular synthesis, providing routes to high-value aromatic compounds by introducing functional groups directly onto the benzene ring.

Photochemical Reaction

Photochemical reactions involve chemical changes initiated by light energy, generally in the ultraviolet or visible spectrum. In the context of benzene, exposure to sunlight can lead to reactions that do not occur under thermal conditions. A prime example is the formation of benzene hexachloride (BHC) when benzene undergoes a photochemical reaction with chlorine.

When benzene is exposed to sunlight in the presence of chlorine gas, the energy from the light breaks the \({\text{Cl}}_2\) bond, generating chlorine radicals. These radicals react with benzene through a chain reaction mechanism, ultimately transforming the aromatic ring into BHC, a fully saturated compound containing six chlorine atoms.

• Photochemical reactions offer unique pathways that are inaccessible under normal conditions.
• They play a crucial role in synthetic chemistry, as light can drive reactions selectively and efficiently.
• Such reactions expand the versatility of chemical transformations, offering novel synthetic challenges and opportunities.