Before You Begin

Question #44237

Difficulty: 8/10 | 🟢 Free

The Question

Consider the reactions of benzoic acid ( $\ce{PhCOOH}$ ) with an excess of two different organometallic reagents:

\textbf{Reaction I:} $\ce{PhCOOH}$ is treated with excess $\ce{CH3MgBr}$ in anhydrous THF at 298 K, followed by acidic aqueous workup. The major organic product isolated is $\mathbf{A}$ .

\textbf{Reaction II:} $\ce{PhCOOH}$ is treated with excess $\ce{CH3Li}$ in anhydrous THF at 298 K, followed by acidic aqueous workup. The major organic product isolated is $\mathbf{B}$ .

Given that the Pauling electronegativities are $\text{Li} = 0.98$ and $\text{Mg} = 1.31$ , which of the following correctly identifies $\mathbf{A}$ and $\mathbf{B}$ ?

Options

A.
$\mathbf{A} = \ce{Ph-CO-CH3}$ , $\mathbf{B} = \ce{Ph-CO-CH3}$
B.
$\mathbf{A} = \ce{PhCOOH}$ , $\mathbf{B} = \ce{Ph-CO-CH3}$
C.
$\mathbf{A} = \ce{Ph-C(OH)(CH3)2}$ , $\mathbf{B} = \ce{Ph-CO-CH3}$
D.
$\mathbf{A} = \ce{PhCOOH}$ , $\mathbf{B} = \ce{Ph-C(OH)(CH3)2}$

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

Bond Enthalpy Pitfall|Grignard Reagent Sensitivity

Why This Question Is Tricky

Common Mistakes

The question tests the 'Bond_Enthalpy_Pitfall' and 'Grignard_Reaction_Pathways' by contrasting the reactivity of Grignard and organolithium reagents with carboxylic acids.

Students often assume Grignard reagents will attack the carboxylate, failing to account for the initial protonolysis that forms a highly stable, chelated magnesium carboxylate (an enthalpic sink).

The key hidden variable is the relative electronegativity of Li and Mg; students often mistakenly assume Mg is less electronegative, incorrectly predicting Grignard as the stronger nucleophile.

Furthermore, recognizing the stability of the tetrahedral dilithium intermediate is crucial, as it prevents over-addition to the tertiary alcohol.

Quick Facts About This Question

Question Information

Subject	Chemistry
Chapter	Thermodynamics - Class 11 Part 1
Concept	Bond Enthalpy Pitfall
Difficulty	8
Cognitive Load	7
Target Solve Time	160 sec
Access	Free