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Which condition is required for methane to undergo a substitution reaction with chlorine?

Acidic medium

Heat

Electric current

Sunlight

Correct Answer

🔬 SC • Science Concept & PrincipleGeneral ScienceChemistry

Option D

Sunlight

Quick Summary:

Methane undergoes a free-radical substitution reaction with chlorine only in the presence of sunlight or ultraviolet (UV) radiation. This energy is essential to initiate the cleavage of the chlorine molecule into highly reactive free radicals.

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💡 Explanation

💡 Everyday Analogy (Real-World Intuition)

Think of sunlight as the 'spark' needed to light a campfire; the fuel (methane and chlorine) is ready, but it needs an energy catalyst to get the chemical reaction moving.

🧠 Memory Mnemonic / Shortcut Aid

SUN: Substitution Under Neutron/Light (Use 'UV light starts the reaction').

🔢 Key Formulas

$CH_{4} + Cl_{2} \xrightarrow{h\nu} CH_{3}Cl + HCl$ — The overall chemical equation for chlorination of methane.

$Cl_{2} \xrightarrow{h\nu} 2Cl^{\cdot}$ — Homolytic cleavage step initiated by photons ( $h\nu$ ).

⚙️ Working Principle

The reaction proceeds via a free-radical mechanism involving three stages: Initiation, Propagation, and Termination. Sunlight provides the energy for the homolytic fission of $Cl_{2}$ into two $Cl^{\cdot}$ free radicals. These radicals then attack $CH_{4}$ to form a methyl radical ( $CH_{3}^{\cdot}$ ), which subsequently reacts with $Cl_{2}$ to yield $CH_{3}Cl$ and regenerate a chlorine radical.

📌 Key Points

▸
The reaction is a photochemical process.
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It is a free-radical substitution reaction.
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If excess chlorine is used, the reaction proceeds further to form $CH_{2}Cl_{2}$ , $CHCl_{3}$ , and $CCl_{4}$ .
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It does not occur in the dark as there is insufficient energy to break the $Cl-Cl$ bond.

✅ Advantages

▸
Used in industrial processes for producing chloromethanes.
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Facilitates the conversion of unreactive alkanes into functionalized organic compounds.

❌ Disadvantages / Limitations

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Lack of selectivity (often results in a mixture of products).
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Difficult to control the degree of substitution.

🛠️ Applications / Uses

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Production of dichloromethane ( $CH_{2}Cl_{2}$ ) used as a solvent.
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Synthesis of chloroform ( $CHCl_{3}$ ) and carbon tetrachloride ( $CCl_{4}$ ).

📄 Additional Information

▸
The term $h\nu$ represents the energy of a photon, where $h$ is Planck's constant and $\nu$ is the frequency of light.
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Option B (Heat) can lead to different reaction pathways (like cracking) and is not the specific condition for free-radical halogenation initiation.

📊 Diagram / Illustration

✅

D is correct — Sunlight (ultraviolet radiation) provides the energy required for the homolytic fission of chlorine molecules into reactive free radicals.

Core Concepts Used

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Free-radical mechanism Photochemical reaction Substitution reaction

💡 EXAM TIP

In organic chemistry, always look for photochemical symbols ( $h\nu$ or UV light) when dealing with alkane halogenation; it distinguishes radical mechanisms from ionic ones.

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