A research team investigates the addition of HBr to propene under two different sets of conditions.
• Trial 1 is performed in the dark at room temperature.
• Trial 2 is performed under ultraviolet irradiation at the same temperature.
Electron-spin resonance (ESR) spectroscopy performed in real time on Trial 2 reveals a spectrum whose hyperfine pattern is assigned to a 1-propyl radical, whereas no radical signal is detected in Trial 1. No spectroscopic features attributable to a carbocation are observed in either trial.
Two elementary-step mechanisms have been proposed for the formation of the organic product C₃H₇Br.
Mechanism A (ionic sequence)
1. HBr + CH₃–CH=CH₂ → CH₃–CH⁺–CH₃ + Br⁻
2. CH₃–CH⁺–CH₃ + Br⁻ → CH₃–CH(Br)–CH₃
Mechanism B (radical chain sequence)
Initiation: HBr → H· + Br·
Propagation step 1: Br· + CH₃–CH=CH₂ → CH₃–CH·–CH₂Br
Propagation step 2: CH₃–CH·–CH₂Br + HBr → CH₃–CH(Br)–CH₃ + Br·
Termination: radicals combine in various bimolecular steps.
The overall stoichiometric equation for both trials is identical:
CH₃–CH=CH₂ + HBr → CH₃–CH₂–CH₂Br.
Use the experimental observations and the two mechanistic proposals to answer the following.
(a) Write the overall balanced chemical equation for the reaction that produces C₃H₇Br from propene and identify the structural isomer expected when the reaction proceeds through Mechanism A.
(b) For each proposed mechanism, list every species that qualifies as a reaction intermediate and justify why it meets the definition of an intermediate.
(c) Using the ESR evidence, determine which mechanism is more consistent with the data. Provide a reasoned argument that references specific spectral observations and mechanistic steps.
(d) Predict the regiochemical orientation (Markovnikov or anti-Markovnikov) of the product obtained in Trial 2. Explain how the preferred mechanism leads to that orientation.
(e) Design one additional, feasible experiment that could further distinguish between Mechanism A and Mechanism B. Clearly state what new observation would support your conclusion.