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Question #44637

Difficulty: 8/10 | 🟢 Free

The Question

Consider the following reaction sequence starting from 3,4-dihydro-2H-pyran (Compound \textbf{X}):

\textbf{X} $\xrightarrow[2. \ H_2O_2, \ NaOH]{1. \ BH_3 \cdot THF}$ \textbf{P} $\xrightarrow{PBr_3, \ \text{pyridine}}$ \textbf{Q} $\xrightarrow{\text{Mg turnings, dry ether, } \Delta}$ \textbf{R} $\xrightarrow[2. \ \text{dil. } H_2SO_4]{1. \ \text{Excess } CO_2(s)}$ \textbf{S} (Major Organic Product)

Identify Compound \textbf{S}.

Options

A.
Tetrahydropyran-3-carboxylic acid
B.
Pent-4-en-1-ol
C.
Tetrahydropyran-2-carboxylic acid
D.
5-Hydroxypentanoic acid

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

Grignard Reagent Sensitivity|Williamson Ether Synthesis Constraints|Ether Cleavage Patterns

Why This Question Is Tricky

Common Mistakes

This problem tests the student's awareness of the severe instability of Grignard reagents containing $\beta$ -leaving groups (even poor ones like ether alkoxides).

The 'textbook' assumption that the Grignard will simply form and wait to react with the electrophile ( $CO_2$ ) fails due to the rapid intramolecular elimination (Boord fragmentation).

The $CO_2$ addition acts as a logical distractor for students who miss the ring-opening step and assume standard intermolecular Grignard carboxylation.

Quick Facts About This Question

Question Information

Subject	Chemistry
Chapter	Haloalkanes and Haloarenes - Class 12
Concept	Grignard Reagent Sensitivity
Difficulty	8
Cognitive Load	7
Target Solve Time	160 sec
Access	Free