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Question #44346

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The Question

The acidity of ortho-substituted phenols in aqueous solution at $298\text{ K}$ is strongly influenced by local orbital re-hybridization, an effect often missed when relying solely on Hammett $\sigma$ constants. According to Bent's Rule, the electronegativity of the ortho-substituent alters the fractional $s$ -character of the $C_1-O$ bond, which directly modulates the effective electronegativity of the ipso-carbon ( $C_1$ ) and the stability of the phenoxide ion.

Consider the following substituted phenols: (I) Phenol (II) $o$ -Fluorophenol (III) $o$ -Chlorophenol (IV) $o$ -Cresol

Based on Bent's Rule, what is the correct decreasing order of the fractional $s$ -character in the $C_1-O$ bond?

Options

A.
II > III > I > IV
B.
III > II > I > IV
C.
IV > I > III > II
D.
II > I > III > IV

Check Your Answer

Correct Answer

Step-by-Step Solution

Explanation

How the Examiner Thinks

Exception_Exposure

Learn to identify edge cases and boundary exceptions that JEE examiners hide in math problems.

Concepts Behind This Question

Bent's Rule|Phenol Acidity vs. Substituents|Electrophilic Substitution Activation/Deactivation

Why This Question Is Tricky

Common Mistakes

The question tests a 'Chemical Exception' where students rely solely on standard inductive/resonance effects or final $pK_a$ values.

The 'Hidden Variable' is the local orbital re-hybridization dictated by Bent's Rule.

Students often overlook that an electronegative ortho-substituent makes the adjacent ring carbon more electronegative, forcing the ipso-carbon to direct more p-character towards it and leaving more s-character for the C-O bond.

Furthermore, it traps students who confuse the final acidity order (where o-chlorophenol > o-fluorophenol due to H-bonding) with the fundamental orbital hybridization trend, which strictly follows electronegativity.

Quick Facts About This Question

Question Information

Subject	Chemistry
Chapter	Chemical Bonding and Molecular Structure - Class 11 Part 1
Concept	Bent's Rule
Difficulty	8
Cognitive Load	7
Target Solve Time	160 sec
Access	Free