Question

The reaction least likely to oecur is (1) \(\mathrm{C}_{6} \mathrm{H}_{6}+\mathrm{HNO}_{3} \stackrel{\mathrm{II}, \mathrm{so}}{\longrightarrow} \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NO}_{2}\) (2) \(\mathrm{C}_{6} \mathrm{H}_{6}+\mathrm{H}_{2} \mathrm{SO}_{4} \stackrel{\text { Heat }}{\longrightarrow} \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{SO}_{3} \mathrm{H}\) (3) \(\mathrm{C}_{6} \mathrm{H}_{6}+\mathrm{Cl}_{2} \stackrel{\text { UY }}{\longrightarrow} \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{Cl}\) (4) \(\mathrm{C}_{6} \mathrm{H}_{6}+\mathrm{Br}_{2} \longrightarrow \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{Br}\)

Short answer

4. \text{C}_6\text{H}_6 + \text{Br}_2 \rightarrow \text{C}_6\text{H}_5\text{Br} is least likely to occur without a catalyst.

Step-by-step solution

Analyze Reaction 1

Check the reaction \(\text{C}_6\text{H}_6 + \text{HNO}_3 \rightarrow \text{C}_6\text{H}_5\text{NO}_2\) using \(\text{H}_2\text{SO}_4\). This is the nitration of benzene, a common reaction for forming nitrobenzene. This reaction is quite feasible.

Analyze Reaction 2

Check the reaction \(\text{C}_6\text{H}_6 + \text{H}_2\text{SO}_4 \rightarrow \text{C}_6\text{H}_5\text{SO}_3\text{H}\) with heat. This is the sulfonation of benzene, a typical reaction used to form benzene sulfonic acid. It is also feasible.

Analyze Reaction 3

Check the reaction \(\text{C}_6\text{H}_6 + \text{Cl}_2 \rightarrow \text{C}_6\text{H}_5\text{Cl}\) using UV light. This is a chlorination reaction, which is feasibly carried out under UV light.

Analyze Reaction 4

Check the reaction \(\text{C}_6\text{H}_6 + \text{Br}_2 \rightarrow \text{C}_6\text{H}_5\text{Br}\). This looks like a bromination reaction, but benzene typically requires a Lewis acid catalyst, such as \(\text{FeBr}_3\), for bromination. Without the catalyst, this reaction is unlikely.

Key concepts

Nitration of Benzene

The nitration of benzene is a well-known and important chemical reaction.
Benzene reacts with concentrated nitric acid (HNO₃) in the presence of sulfuric acid (H₂SO₄) as a catalyst to form nitrobenzene (C₆H₅NO₂).
This process involves an electrophilic aromatic substitution.

Here’s a detailed breakdown of what happens:

• Sulfuric acid protonates nitric acid, turning it into a nitronium ion (NO₂⁺), a very strong electrophile.
• The nitronium ion attacks the benzene ring, creating a resonance-stabilized intermediate.
• Finally, a proton (H⁺) is abstracted, restoring the aromaticity of the benzene ring and forming nitrobenzene.

This reaction is highly feasible and widely used in the production of nitrobenzene, which is a precursor for many dyes, drugs, and explosives.

Sulfonation of Benzene

Sulfonation of benzene is another electrophilic aromatic substitution reaction.
This reaction occurs when benzene reacts with sulfuric acid (H₂SO₄) under heat to form benzene sulfonic acid (C₆H₅SO₃H).
Sulfonation involves the introduction of a sulfonyl group (SO₃H) into the benzene ring.

Here’s how it happens:

• Under heat, sulfuric acid generates sulfur trioxide (SO₃), a strong electrophile.
• Sulfur trioxide then attacks the benzene ring, forming a resonance-stabilized intermediate.
• This intermediate then loses a proton (H⁺), resulting in the formation of benzene sulfonic acid.

Sulfonation is a practical reaction used to manufacture detergents, dyes, and pharmaceuticals.

Chlorination of Benzene

Chlorination of benzene is a common reaction where benzene reacts with chlorine (Cl₂) in the presence of ultraviolet (UV) light to produce chlorobenzene (C₆H₅Cl).
This is also an electrophilic aromatic substitution reaction.

The steps include:

• UV light breaks down chlorine molecules (Cl₂) into chlorine radicals (Cl•).
• One of these chlorine radicals attacks the benzene ring, forming a resonance-stabilized intermediate.
• The intermediate then loses a proton (H⁺), leading to the formation of chlorobenzene.

This reaction requires UV light to generate the chlorine radicals and is widely used in organic synthesis and the production of pesticides, dyes, and pharmaceuticals.

Bromination of Benzene

Bromination of benzene is a reaction where benzene reacts with bromine (Br₂) to form bromobenzene (C₆H₅Br).
However, unlike chlorination, bromination requires the presence of a Lewis acid catalyst such as iron(III) bromide (FeBr₃).
This reaction is also an electrophilic aromatic substitution.

Here’s the reaction mechanism:

• FeBr₃ polarizes the Br₂ molecule, forming a more electrophilic bromine species (Br⁺).
• This more electrophilic bromine attacks the benzene ring, creating a resonance-stabilized intermediate.
• The intermediate then loses a proton (H⁺), resulting in the formation of bromobenzene.

Without the catalyst, the bromination of benzene is highly unlikely to proceed, making the presence of FeBr₃ crucial for the reaction's success. Brominated compounds are frequently used in fire retardants and pharmaceuticals.