See image — Hydrocarbons Chemistry Question

💡 Solution & Explanation

In free-radical chain reactions, the mechanism consists of three types of steps: initiation, propagation, and termination. Concept: Chain propagation steps are steps in which a radical reacts with a stable molecule to produce a new radical and a new stable molecule. The radical is consumed but another radical is regenerated, allowing the chain to continue. Step-by-step analysis: Step (1): Cl2 --> 2Cl• — This is an INITIATION step. A stable molecule (Cl2) is split into two radicals by UV light/heat. No radical is consumed to produce another radical. This is NOT propagation. Step (2): Cl• + CH4 --> CH3Cl + H• — This is incorrect chemistry for direct free-radical chlorination. Cl• does not abstract H from CH4 to give CH3Cl directly in one step; this is not a recognized propagation step in the standard mechanism. Step (3): Cl• + CH4 --> CH3• + HCl — This IS a propagation step. A chlorine radical abstracts a hydrogen atom from methane, producing a methyl radical (new radical) and HCl (stable molecule). A radical is consumed and a new radical is generated. Step (4): H• + Cl2 --> HCl + Cl• — Although this could regenerate a radical, step (2) producing H• is not part of the standard mechanism, so step (4) is not part of the standard propagation cycle. Step (5): CH3• + Cl2 --> CH3Cl + Cl• — This IS a propagation step. The methyl radical reacts with Cl2 to produce chloromethane (stable molecule) and a chlorine radical (new radical). A radical is consumed and a new radical is generated. The standard propagation cycle in free-radical chlorination of methane consists of: - Step 3: Cl• + CH4 --> CH3• + HCl - Step 5: CH3• + Cl2 --> CH3Cl + Cl• These two steps together form a cycle: Cl• generates CH3•, and CH3• regenerates Cl•, allowing the chain to propagate. Steps (2) and (4) together form an alternative non-standard cycle and are not part of the accepted mechanism. Step (1) is initiation only. Why other options fail: - (a) 2, 3, 5: includes step 2, which is not a valid propagation step in the standard mechanism. - (b) 1, 3, 6: includes step 1 (initiation) and step 6 (not listed/does not exist). - (d) 2, 3, 4: includes steps 2 and 4, which are not standard propagation steps. Only option (c) 3, 5 correctly identifies the two chain propagation steps. Therefore, the correct answer is C.