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Question #44382

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The Question

Consider 2,6-di-tert-butylphenol ( $\textbf{X}$ ) and phenol ( $\textbf{Y}$ ) at $298\text{ K}$ . Which of the following statements correctly identifies the thermodynamic and kinetic behavior of $\textbf{X}$ compared to $\textbf{Y}$ , along with the correct underlying mechanistic rationale?

Options

A.
$\textbf{X}$ is insoluble in $10\%\text{ aq. } NaOH$ and fails to undergo azo coupling with $PhN_2^+Cl^-$ . The bulky tert-butyl groups force the $-OH$ group out of the aromatic plane, causing Steric Inhibition of Resonance (SIR) which destabilizes the phenoxide ion and suppresses the $+M$ activating effect.
B.
$\textbf{X}$ is completely soluble in $10\%\text{ aq. } NaOH$ due to steric relief upon deprotonation, but fails to undergo azo coupling because the bulky tert-butyl groups sterically block the approach of the electrophile to the para position.
C.
$\textbf{X}$ is less acidic than $\textbf{Y}$ solely due to the $+I$ effect of the tert-butyl groups, but undergoes azo coupling at a faster rate than $\textbf{Y}$ because the para position is sterically unhindered and electron-rich.
D.
$\textbf{X}$ is insoluble in $10\%\text{ aq. } NaOH$ due to strong intermolecular hydrogen bonding, and undergoes electrophilic aromatic substitution exclusively at the meta position due to the deactivating nature of the twisted $-OH$ group.

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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

Phenol Acidity vs. Substituents|EAS in Phenols|Electrophilic Substitution Control

Why This Question Is Tricky

Common Mistakes

The question tests the student's ability to recognize the 'Steric Inhibition of Resonance' (SIR) effect as a dominant hidden variable over standard electronic effects.

Students typically memorize that $-OH$ is a strongly activating, ortho/para-directing group ( $+M > -I$ ) and that phenols dissolve in aqueous $NaOH$ .

The trap is assuming that 2,6-di-tert-butylphenol will behave similarly, perhaps just reacting at the para position due to ortho blocking.

However, the extreme steric bulk forces the $-OH$ group out of coplanarity, effectively 'turning off' its $+M$ effect.

This simultaneously kills its ability to undergo EAS with weak electrophiles (like diazonium salts) and drastically lowers its acidity by preventing resonance stabilization of the phenoxide conjugate base.

Quick Facts About This Question

Question Information

Subject	Chemistry
Chapter	Alcohols, Phenols, and Ethers - Class 12
Concept	Phenol Acidity vs. Substituents
Difficulty	8
Cognitive Load	7
Target Solve Time	160 sec
Access	Free