Chemistry

Problem

Addition of HBr to propene can proceed by several pathways. Three proposed mechanisms are: I. Radical chain mechanism involving Br• addition to the double bond. II. Carbocation mechanism beginning with protonation of the double bond. III. Concerted one-step addition of H-Br across the double bond without discrete intermediates. Electron spin resonance (ESR) studies of the reaction carried out in the gas phase detect a strong Br• signal, while time-resolved infrared spectroscopy shows no evidence of carbocation formation. Rank the three mechanisms from MOST likely to LEAST likely in light of these observations. a)

\text{III} > \text{I} > \text{II}

\text{I} > \text{III} > \text{II}

\text{I} > \text{II} > \text{III}

\text{II} > \text{I} > \text{III}

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