Two alternative mechanisms have been proposed for the transformation of substrate S…
CHEMISTRY · Multi-part · A Level · Chemistry · Introduction to organic chemistry · Introduction to organic chemistry
Problem
Two alternative mechanisms have been proposed for the transformation of substrate S to product P.
Mechanism A (one-step): S→PEa=80kJ mol−1ΔH=−30kJ mol−1
Mechanism B (three-step):
Step 1 S→JEa1=45kJ mol−1ΔH1=+5kJ mol−1
Step 2 J→KEa2=40kJ mol−1ΔH2=−10kJ mol−1
Step 3 K→PEa3=55kJ mol−1ΔH3=−25kJ mol−1
A research group records the following observations at 298K:
1. The measured rate law is first-order in S.
2. Addition of a catalyst that selectively stabilizes transition states but not intermediates lowers all activation energies by 20kJ mol−1, yet the rate enhancement is modest.
Use this information to evaluate which mechanism is more consistent with the data.
(1) Part 1: (a) Construct separate qualitative energy profiles for Mechanism A and Mechanism B on the same set of axes. Clearly label all relevant species and energy barriers.
(2) Part 2: (b) Using the magnitudes of the individual activation energies, predict which mechanism would be expected to proceed more rapidly in the absence of a catalyst. Explain your reasoning.
(3) Part 3: (c) Relate the experimentally observed first-order rate law to the proposed mechanisms and identify which mechanism aligns better with the kinetic evidence. Provide a justification based on your profiles.
(4) Part 4: (d) Explain why the catalyst described above produces only a modest rate enhancement and discuss how this observation further supports your chosen mechanism.