Question 34 of 34
Q.Why chlorination of methane is not possible in dark ?
Tamil Nadu DgeTamil Nadu HSC First Year (DGE) Board 2026Subjective· 2mImportance★★★★★est
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Start your 14-day free trial to unlock the full solution →We need to explain why methane does not get chlorinated in the absence of light.
Chlorination of methane is a free-radical substitution whose initiation step requires UV light (or heat) to generate chlorine radicals; in the dark this step cannot occur, so there is no reaction.
The chlorination of methane (CH4 + Cl2 → CH3Cl + HCl) proceeds by a free-radical chain mechanism, which has three stages:
- Initiation: Cl2 --(UV light or heat)--> 2Cl•. The Cl-Cl bond is broken homolytically to give two chlorine free radicals. This step needs an energy source — ultraviolet light or high temperature.
- Propagation: Cl• + CH4 → •CH3 + HCl, then •CH3 + Cl2 → CH3Cl + Cl•, which continues the chain.
- Termination: radicals combine (e.g. Cl• + Cl• → Cl2). …
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