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Question #44439

Difficulty: 8/10 | 🟢 Free

The Question

Consider the diketone $X$ , 2,2,6,6-tetramethylcyclohexane-1,4-dione. $X$ is subjected to two separate reduction protocols:

Protocol 1: $X$ is treated with excess $\ce{NaBH4}$ in $\ce{EtOH}$ at 298 K to give major product $P$ . Protocol 2: $X$ is treated with excess $\ce{LiAlH4}$ in dry $\ce{Et2O}$ at 298 K, followed by acidic workup ( $\ce{H3O+}$ ), to give major product $Q$ .

Identify the major products $P$ and $Q$ .

Options

A.
$P$ = 4-hydroxy-2,2,6,6-tetramethylcyclohexan-1-one ; $Q$ = 2,2,6,6-tetramethylcyclohexane-1,4-diol
B.
$P$ = 2,2,6,6-tetramethylcyclohexane-1,4-diol ; $Q$ = 2,2,6,6-tetramethylcyclohexane-1,4-diol
C.
$P$ = 2,2,6,6-tetramethylcyclohexane-1,4-diol ; $Q$ = 4-hydroxy-2,2,6,6-tetramethylcyclohexan-1-one
D.
$P$ = 4-hydroxy-2,2,6,6-tetramethylcyclohexan-1-one ; $Q$ = 4-hydroxy-2,2,6,6-tetramethylcyclohexan-1-one

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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

Nucleophilic Addition Sterics|Reduction Selectivity (Hydrides)

Why This Question Is Tricky

Common Mistakes

This question tests the 'Effective Steric Bulk' exception in nucleophilic addition.

Students commonly memorize the rote rule that 'both NaBH4 and LiAlH4 reduce ketones to secondary alcohols' and will incorrectly assume excess NaBH4 reduces both carbonyls (Option B).

The hidden variable is the steric demand of the solvated [BH4]- complex versus the accessibility of the B"urgi-Dunitz trajectory.

Highly hindered ketones (like those flanked by gem-dimethyl groups) are completely inert to NaBH4 but can be forced to reduce by the more reactive, smaller effective radius of the AlH4- nucleophile.

Quick Facts About This Question

Question Information

Subject	Chemistry
Chapter	Aldehydes, Ketones, and Carboxylic Acids - Class 12
Concept	Nucleophilic Addition Sterics
Difficulty	8
Cognitive Load	7
Target Solve Time	160 sec
Access	Free