Light initiates free radical chlorination

Steps:

• Identify the reaction as free radical substitution between Cl₂ and CH₄.
• Recall that light provides energy for homolytic bond fission to start the chain.
• Note Cl-Cl bond energy (243 kJ/mol) is lower than C-H (413 kJ/mol), so light breaks Cl₂ first.
• Confirm this generates Cl• radicals, propagating the reaction.

Why B is correct:

• In free radical halogenation, UV light causes homolytic cleavage of the Cl-Cl bond (Cl₂ → 2Cl•), initiating the chain reaction as defined by the mechanism.

Why the others are wrong:

• A: Light lacks sufficient energy to directly break strong C-H bonds; radicals attack them instead.
• C: Not enough information.
• D: Light acts photochemically, not thermally; heat alone would not initiate radicals efficiently.

Final answer: B