Propagation steps regenerate radicals in free radical chlorination

Steps:

• Identify propagation as chain-maintaining steps where a radical reacts with a molecule to produce a product and another radical.
• In ethane chlorination, the first propagation forms the alkyl chloride from the ethyl radical and Cl₂.
• The second propagation abstracts H from ethane by Cl• to form HCl and ethyl radical.
• Verify each option against these definitions to find the match.

Why B is correct:

• It depicts the ethyl radical (CH₃CH₂•) reacting with Cl₂ to yield CH₃CH₂Cl and regenerate Cl•, sustaining the chain per free radical mechanism rules.

Why the others are wrong:

• A: Reverses H-abstraction, endothermic and not a propagation step.
• C: Identical to A, invalid for the same reason.
• D: Combines two radicals to form a molecule, a termination step that breaks the chain.

Final answer: B