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Question #44534

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The Question

An alkene $X$ (\ce{3-methylbut-1-ene}) is subjected to hydroboration-oxidation using \ce{B2H6}/THF followed by alkaline \ce{H2O2} to yield a major organic product $Y$ . Exactly $7.0\text{ g}$ of $X$ is completely converted to $Y$ , and the entire yield of $Y$ is dissolved in $250\text{ g}$ of benzene. The freezing point of the resulting solution is depressed by $1.6384\text{ K}$ . Assuming $Y$ undergoes partial dimerization in benzene due to intermolecular hydrogen bonding, identify the IUPAC name of $Y$ and its percentage of dimerization. (Given: $K_f$ for benzene = $5.12\text{ K kg mol}^{-1}$ , atomic masses: C=12, H=1, O=16)

Options

A.
\text{3-Methylbutan-1-ol, } 40%
B.
\text{2-Methylbutan-2-ol, } 40%
C.
\text{3-Methylbutan-1-ol, } 20%
D.
\text{2-Methylbutan-2-ol, } 20%

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Correct Answer

Step-by-Step Solution

Explanation

How the Examiner Thinks

Numerical_Precision

Improve calculation stamina and avoid rounding errors in high-stakes competitive examinations.

Concepts Behind This Question

Freezing Point Depression (Pure Solvent)|Hydroboration-Oxidation Regiochemistry|Degree of Dissociation/Association

Why This Question Is Tricky

Common Mistakes

This problem integrates organic synthesis with physical chemistry, testing two specific traps.

First, it tests the regiochemistry of hydroboration-oxidation.

Students who incorrectly assume a carbocation intermediate will predict a 1,2-hydride shift, leading to the tertiary alcohol \ce{2-methylbutan-2-ol}.

Second, it tests the colligative properties of associating solutes.

Alcohols in non-polar solvents like benzene undergo intermolecular hydrogen bonding, leading to dimerization.

Students who misapply the van't Hoff factor formula for association (e.g., using $i = 1 - \alpha$ instead of $i = 1 - \alpha/2$ ) will incorrectly calculate a 20% dimerization rate.

Quick Facts About This Question

Question Information

Subject	Chemistry
Chapter	Solutions - Class 12
Concept	Freezing Point Depression (Pure Solvent)
Difficulty	8
Cognitive Load	7
Target Solve Time	160 sec
Access	Free