In a chain-branching radical mechanism, what is the typical effect on the overall rate when a branching step yields two radicals from one radical?

• A. The rate decreases
• B. The rate remains unchanged
• C. The rate increases significantly
• D. The rate becomes unpredictable

Answer: (C)

Chain-branching in radical mechanisms means one radical step can produce two radicals, doubling the number of reactive centers. This creates an autocatalytic boost: as soon as the branching step occurs, more radicals are available to propagate further steps, so the rate of the overall reaction climbs quickly. Because radical-chain rates depend strongly on radical concentrations, this doubling of radicals leads to a substantial, often exponential, increase in rate until termination steps or reactant depletion slow things down. In short, producing two radicals from one greatly accelerates the overall reaction.