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Question #44222

Difficulty: 8/10 | 🟢 Free

The Question

Consider the SN1S_N1 solvolysis of the optically active alkyl halide (2S,3S)-2-chloro-3-methylpentane in \ce{H2O} at 298 K. The reaction proceeds via an initial carbocation C1+C_1^+ which undergoes a rapid 1,2-shift to form a more thermodynamically stable carbocation C2+C_2^+, ultimately yielding the major product PP. The global enthalpy path shifts due to the superior hyperconjugative stabilization of C2+C_2^+.

Let n1n_1 and n2n_2 be the number of hyperconjugable α\alpha-hydrogens in C1+C_1^+ and C2+C_2^+, respectively. Identify the major product PP, its optical activity, and the difference in hyperconjugable hydrogens (Δn=n2−n1\Delta n = n_2 - n_1) that provides the thermodynamic driving force (ΔHrearr<0\Delta H_{\text{rearr}} < 0) for the shift.

Options

  • A.

    P=P = 3-methylpentan-3-ol; Optically Inactive; Δn=3\Delta n = 3

  • B.

    P=P = 3-methylpentan-2-ol; Optically Active; Δn=0\Delta n = 0

  • C.

    P=P = 3-methylpentan-3-ol; Optically Active; Δn=4\Delta n = 4

  • D.

    P=P = 2-methylpentan-2-ol; Optically Inactive; Δn=3\Delta n = 3

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Correct Answer

Step-by-Step Solution

Explanation

How the Examiner Thinks

Mechanism_Trap

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Concepts Behind This Question

Enthalpy Definition|SN1 Carbocation Rearrangement|Hyperconjugation Logic

Why This Question Is Tricky

Common Mistakes

This question tests the intersection of thermodynamic control in intermediates (Hammond Postulate) and stereochemistry.

Students often rote-learn that SN1S_N1 reactions of chiral substrates lead to 'racemization' (and thus optical inactivity via a racemic mixture), but fail to recognize when a rearrangement completely destroys the chiral center, yielding a fundamentally achiral molecule.

Furthermore, it forces the quantitative evaluation of hyperconjugation (α\alpha-hydrogen counting) as the primary thermodynamic driver for the enthalpy shift (ΔHrearr<0\Delta H_{\text{rearr}} < 0), overriding simple inductive effect assumptions.

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Question Information

SubjectChemistry
ChapterThermodynamics - Class 11 Part 1
ConceptEnthalpy Definition
Difficulty8
Cognitive Load7
Target Solve Time160 sec
AccessFree