Light initiates free radical chlorination of methane

Steps:

• Identify the reaction as free radical substitution between Cl₂ and CH₄.
• Recognize that light (UV) provides energy for the initiation step.
• Understand initiation involves homolytic cleavage of the weak Cl-Cl bond.
• Confirm chlorine atoms then propagate the chain by abstracting H from methane.

Why B is correct:

• UV light breaks the Cl-Cl bond (bond energy 242 kJ/mol) homolytically into Cl• radicals, starting the radical chain per free radical mechanism.

Why the others are wrong:

• A: C-H bonds (413 kJ/mol) are broken by Cl• radicals in propagation, not directly by light.
• C: C-Cl bonds form in the product; light doesn't break them.
• D: Light specifically excites Cl₂ for bond fission, not general heating (which would cause side reactions).

Final answer: B