Two different mechanisms have been proposed for converting compound P to compound R at

CHEMISTRY · Multi-part · AP · Chemistry · Kinetics · Kinetics

Problem

Two different mechanisms have been proposed for converting compound P to compound R at 298 K298\,\text{K}. Each mechanism proceeds through two elementary steps and has been experimentally characterized, yielding the energy maxima (transition states) shown below. All energies are expressed in kJ mol−1\text{kJ}\,\text{mol}^{-1} relative to P. Mechanism I: • TS1=+105 kJ mol−1TS_1 = +105\,\text{kJ}\,\text{mol}^{-1}, TS2=+95 kJ mol−1TS_2 = +95\,\text{kJ}\,\text{mol}^{-1}, Products R=−50 kJ mol−1R = -50\,\text{kJ}\,\text{mol}^{-1} Mechanism II: • TS1=+85 kJ mol−1TS_1 = +85\,\text{kJ}\,\text{mol}^{-1}, TS2=+120 kJ mol−1TS_2 = +120\,\text{kJ}\,\text{mol}^{-1}, Products R=−50 kJ mol−1R = -50\,\text{kJ}\,\text{mol}^{-1} (Intermediates are not shown.) (a) For each mechanism, identify the rate-determining step and state its activation energy. (b) Using only the information provided, determine which mechanism is kinetically favored at 298 K298\,\text{K}. Support your answer with appropriate reasoning. (c) Both mechanisms yield the same value of ΔHrxn\Delta H_{\text{rxn}}. Explain why a difference in transition-state energies, rather than in ΔHrxn\Delta H_{\text{rxn}}, controls the relative rates of the two mechanisms.

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