For the isomerization of compound P to compound R, two alternative two-step mechanisms have been suggested:
Mechanism I:
Step 1 P → Q (fast)
Step 2 Q → R (slow)
Mechanism II:
Step 1 P → Q₁ (slow)
Step 2 Q₁ → R (fast)
A calorimetric study provides the following potential-energy profile (all energies in kJ mol⁻¹ relative to P):
• TS₁: +65
• Intermediate: –5
• TS₂: +50
• Products (R): –70
Use these data to evaluate which mechanism is consistent with the observed profile.
(a) For each mechanism, state the expected number of transition states and intermediates, and sketch the qualitative ordering of energies for those species.
(b) Compare the predicted ordering from part (a) to the calorimetric profile. Identify which mechanism is consistent with the experimental data, and justify your answer by referencing both the number and relative energies of transition states and intermediates.
(c) The experimental profile labels the first transition state at a higher energy than the second. Explain how this information supports your selection of the rate-determining step.
(d) Discuss one kinetic experiment (other than calorimetry) that could further corroborate the mechanism you selected.