Chemistry – Organic chemistry - Alkanes | e-Consult

UV light is necessary for the substitution reaction of alkanes with chlorine because it provides the activation energy (E_a) required to initiate the reaction. The Cl-Cl bond is strong and requires energy to break. UV light photons have sufficient energy to break this bond homolytically, generating highly reactive chlorine radicals (Cl•). Without UV light, the Cl-Cl bond would not break, and the reaction would not proceed.

The reaction proceeds via a free radical chain mechanism, which involves three main stages:

1. Initiation: UV light breaks the Cl-Cl bond, producing chlorine radicals. Cl₂ + UV light → 2Cl•
2. Propagation: Chlorine radicals abstract a hydrogen atom from the alkane, forming an alkyl radical and hydrogen chloride. The alkyl radical then reacts with another chlorine molecule to form a chloroalkane and regenerate a chlorine radical. This cycle repeats. For example:

   • Cl• + CH₄ → CH₃• + HCl
   • CH₃• + Cl₂ → CH₃Cl + Cl•

3. Termination: The chain reaction stops when two radicals combine to form a stable molecule. For example:

   • Cl• + Cl• → Cl₂
   • CH₃• + CH₃• → C₂H₆
   • CH₃• + Cl• → CH₃Cl

The overall reaction for the substitution of ethane with chlorine is:

Reaction:

CH₃CH₃ + Cl₂ → CH₃CH₂Cl + HCl

Ethane + Chlorine → Ethyl Chloride + Hydrogen Chloride

The main organic product formed is ethyl chloride (CH₃CH₂Cl).