MHTCET 20 Apr 2026 Shift 2 Paper

Concept:In a free radical chain reaction, a propagation step is one in which a free radical reacts and a new free radical is formed, keeping the chain alive.Explanation:Free radical chlorination of methane occurs through initiation, propagation, and termination steps.Option A shows the homolytic cleavage of a chlorine molecule:$$\mathrm{Cl}-\mathrm{Cl} \rightarrow \mathrm{Cl}^{\bullet}+\mathrm{Cl}^{\bullet}$$This is the initiation step, as it produces the free radicals that start the reaction.Option B shows the methyl free radical reacting with chlorine gas:$$\mathrm{CH}_{3}^{\bullet}+\mathrm{Cl}_{2} \rightarrow \mathrm{CH}_{3}-\mathrm{Cl}+\mathrm{Cl}^{\bullet}$$Here, one free radical ($\mathrm{CH}_{3}^{\bullet}$) is consumed, and another free radical ($\mathrm{Cl}^{\bullet}$) is generated.This is the characteristic feature of a propagation step, as it sustains the chain reaction.Options C and D involve the combination of two free radicals to form a stable molecule without producing any new radical.Hence, both C and D are termination steps.Therefore, the propagation step is correctly represented by option B.Answer:Option B: $\mathrm{CH}_{3}^{\bullet}+\mathrm{Cl}_{2} \rightarrow \mathrm{CH}_{3}-\mathrm{Cl}+\mathrm{Cl}^{\bullet}$ is the propagation step.