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Question #44567

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The Question

Compound $\mathbf{X}$ (2,2,4,4-tetramethyl-3-oxopentanal) is treated with $0.25$ equivalents of $\ce{NaBH4}$ in dry methanol at $298\text{ K}$ to yield $\mathbf{P}$ as the major organic product. Let $\Delta G^\ddagger_{\text{ald}}$ and $\Delta G^\ddagger_{\text{ket}}$ represent the activation free energies for the nucleophilic attack of hydride on the aldehyde and ketone carbonyl carbons of $\mathbf{X}$ , respectively. Which of the following correctly identifies $\mathbf{P}$ and the primary kinetic relationship on the reaction progress coordinate?

Options

A.
$\mathbf{P}$ is 1-hydroxy-2,2,4,4-tetramethylpentan-3-one; $\Delta G^\ddagger_{\text{ket}} \gg \Delta G^\ddagger_{\text{ald}}$ because the B"urgi-Dunitz trajectory for hydride attack on the ketone is sterically blocked by the adjacent quaternary centers.
B.
$\mathbf{P}$ is 3-hydroxy-2,2,4,4-tetramethylpentanal; $\Delta G^\ddagger_{\text{ald}} > \Delta G^\ddagger_{\text{ket}}$ because ground-state steric repulsion between the quaternary centers selectively lowers the activation barrier for the ketone.
C.
$\mathbf{P}$ is 2,2,4,4-tetramethylpentane-1,3-diol; $\Delta G^\ddagger_{\text{ket}} \approx \Delta G^\ddagger_{\text{ald}}$ due to the formation of a cyclic borate intermediate that delivers hydride intramolecularly to both centers.
D.
$\mathbf{P}$ is 1-hydroxy-2,2,4,4-tetramethylpentan-3-one; $\Delta G^\ddagger_{\text{ket}} \gg \Delta G^\ddagger_{\text{ald}}$ exclusively because the $+I$ effect of the tert-butyl groups eliminates the partial positive charge on the ketone carbon.

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Explanation

How the Examiner Thinks

Mechanism_Trap

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Concepts Behind This Question

Reaction Progress Coordinate|Reduction Selectivity (Hydrides)|Nucleophilic Addition Sterics

Why This Question Is Tricky

Common Mistakes

This question tests the 'Hydride Shuttle Sterics' exception.

Students often calculate stoichiometric yields assuming uniform reactivity for all carbonyls, focusing solely on 2D electronic effects (like $+I$ inductive effects).

The hidden variable is the 3D steric reality: highly hindered ketones (like di-tert-butyl ketone analogs) have an inaccessible B"urgi-Dunitz trajectory.

This steric blockade drastically raises the activation free energy ( $\Delta G^\ddagger$ ) on the reaction progress coordinate, making the ketone kinetically inert to $\ce{NaBH4}$ reduction compared to the less hindered aldehyde.

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