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Question #44568

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The Question

Consider the solvolysis of (2S)-3-methyl-2-butyl tosylate (labeled with $\ce{^{18}O}$ at the sulfonyl oxygens) in anhydrous trifluoroacetic acid ( $\ce{CF3COOH}$ ) at $298 \text{ K}$ . At $30\%$ conversion, the unreacted starting material is recovered and found to exhibit extensive $\ce{^{18}O}$ scrambling into the alkyl oxygen position. The product mixture consists of a major rearranged product (A) and a minor unrearranged product (B). Based on the Reaction Progress Coordinate and ion-pair dynamics, which of the following statements correctly identifies the kinetic bottleneck and the stereochemistry of product B?

Options

A.
The formation of the intimate ion pair (IIP) is the rate-determining step (RDS); product B is a racemic mixture because the secondary carbocation local minimum is long-lived.
B.
The formation of the IIP is a rapid pre-equilibrium, making the 1,2-hydride shift the RDS for the major pathway; product B exhibits predominantly inversion of configuration (2R).
C.
The RDS is the solvent collapse from the solvent-separated ion pair (SSIP); product B exhibits predominantly retention of configuration (2S) due to front-side attack.
D.
The highest energy transition state is the initial ionization; the extensive scrambling indicates thermodynamic control, yielding product B with net retention (2S).

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Correct Answer

Step-by-Step Solution

Explanation

How the Examiner Thinks

Mechanism_Trap

Don't get tricked by the process! Learn the mechanical traps used in competitive exam questions.

Concepts Behind This Question

Reaction Progress Coordinate|E1/E2 Solvent and Base Effects|Steric Hindrance in SN2

Why This Question Is Tricky

Common Mistakes

This question tests the Winstein ion-pair mechanism and targets the common misconception that the initial ionization in an $S_N1$ reaction is always the Rate-Determining Step (RDS).

Extensive isotopic scrambling proves that internal return from the Intimate Ion Pair (IIP) is faster than product formation, making ionization a rapid pre-equilibrium and shifting the RDS to the subsequent 1,2-hydride shift.

Additionally, it tests the stereochemical trap that unrearranged products derived from an IIP do not racemize but undergo inversion due to front-face shielding by the leaving group.

Quick Facts About This Question

Question Information

Subject	Chemistry
Chapter	Chemical Kinetics - Class 12
Concept	Reaction Progress Coordinate
Difficulty	8
Cognitive Load	7
Target Solve Time	160 sec
Access	Free