Chemistry – Organic chemistry - Alkanes | e-Consult
Organic chemistry - Alkanes (1 questions)
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The substitution of hydrogen in alkanes by chlorine requires ultraviolet (UV) light because the reaction proceeds via a free radical mechanism. This mechanism involves the homolytic cleavage of a chlorine molecule (Cl2) into two chlorine radicals (Cl•). UV light provides the energy needed to break the Cl-Cl bond.
The mechanism can be described in the following steps:
- Initiation: UV light causes Cl2 to split into 2Cl• radicals. Cl2 → 2Cl•
- Propagation: A chlorine radical abstracts a hydrogen atom from an alkane molecule, forming a hydrogen radical and a chlorinated alkane. Cl• + R-H → HCl + R• The hydrogen radical then reacts with another Cl2 molecule to form a chlorinated alkane and regenerate a chlorine radical. R• + Cl2 → R-Cl + Cl• This chain reaction continues.
- Termination: The chain reaction is terminated when two radicals combine. For example, two chlorine radicals combine to form Cl2, or a chlorine radical combines with a hydrogen radical to form HCl. Cl• + Cl• → Cl2 Cl• + R• → R-Cl
The UV light initiates the reaction by generating the chlorine radicals, and the reaction proceeds through a chain of radical reactions until termination occurs.