Which reaction coordinate plot represents the kinetics of the reaction shown below?

Question

BlackTom AI · Accepted Answer

Departure of the leaving group to give a discrete, planar carbocationic (or resonance-stabilized) intermediate requires two transition states, so the multistep (two‑peak) energy profile is correct.

1) Relevant knowledge points and concepts
- SN1 or stepwise substitution reactions proceed via a distinct intermediate (carbocation or stabilized cationic/enol/enamine species) and therefore show two transition states on a reaction coordinate (TS1 for ionization, intermediate valley, TS2 for nucleophilic capture).
- Formation of a planar intermediate leads to loss of stereochemical information and often gives mixtures of stereoisomers.
- The highest energy TS determines the rate (rate‑determining step).

2) Step‑by‑step reasoning
- The substrate is an α‑chloro ketone and the reaction conditions (heat, tertiary/weakly basic/nucleophilic amine) favor ionization to a stabilized cation (adjacent carbonyl stabilizes positive charge by resonance).
- Once the cation forms, Et2NH (nucleophile) attacks, producing two stereoisomeric products consistent with attack on a planar intermediate.
- Thus the mechanism is stepwise: (1) C–Cl bond heterolysis (TS1) → intermediate cation; (2) nucleophilic attack (TS2) → product.

3) Calculational/intermediate steps
- No numerical energy values are given, but conceptually: G(Reactants) → TS1 (highest) → Intermediate (local minimum) → TS2 (lower than TS1) → Products. The activation energy for step 1 (ΔG‡1) > ΔG‡2, making ionization rate‑determining.

4) Why the correct answer is correct
- The first energy diagram shows two distinct maxima with an intermediate valley, matching a multistep mechanism with an intermediate and two transition states. The redrawn profile corresponds to ionization then nucleophilic attack, consistent with observed stereochemical outcome.

5) Why the other option is incorrect
- The single‑peak diagram corresponds to a concerted (one‑step) mechanism with no intermediate; that would preserve stereochemistry (or give a different stereochemical outcome) and does not fit the evidence for a planar cationic intermediate and two distinct kinetic events.

6) Conclusion
- Therefore the correct reaction coordinate is the multistep plot with two transition states (the first answer option), which matches the provided answer.

Which reaction coordinate plot represents the kinetics of the reaction shown below? 单项选择题

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