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Question #44419

Difficulty: 8/10 | 🟢 Free

The Question

Consider the unsymmetrical ketone $\textbf{X}$ , 3-methylbutan-2-one. \ \textbf{Reaction 1:} $\textbf{X}$ is treated with 1 equivalent of $\ce{Br2}$ in glacial acetic acid at 298 K to yield the major monobrominated product $\textbf{Y}$ . \ \textbf{Reaction 2:} $\textbf{Y}$ is then reacted with sodium methanethiolate ( $\ce{NaSMe}$ ) in DMF to give the major organic product $\textbf{Z}$ . \ \textbf{Reaction 3:} In a separate experiment, $\textbf{X}$ is treated with excess $\ce{I2}$ in aqueous $\ce{NaOH}$ , followed by careful acidification, to yield a yellow precipitate and a carboxylic acid $\textbf{W}$ . \ Identify the correct IUPAC names for structures $\textbf{Z}$ and $\textbf{W}$ .

Options

A.
$\textbf{Z}$ = 3-methylbut-3-en-2-one; $\textbf{W}$ = 2-methylpropanoic acid
B.
$\textbf{Z}$ = 1-(methylthio)-3-methylbutan-2-one; $\textbf{W}$ = 2-methylpropanoic acid
C.
$\textbf{Z}$ = 3-methylbut-3-en-2-one; $\textbf{W}$ = 2,2-dimethylpropanoic acid
D.
$\textbf{Z}$ = 1-(methylthio)-3-methylbutan-2-one; $\textbf{W}$ = 3-methylbutanoic acid

Check Your Answer

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

Oxidation State Traps|Haloform Reaction Logic|Enolate Regioselectivity

Why This Question Is Tricky

Common Mistakes

This problem tests the critical distinction between kinetic and thermodynamic control in enolate chemistry, coupled with steric constraints in substitution reactions.

The primary trap lies in Reaction 1: students often mistakenly apply the basic Haloform regioselectivity (kinetic, less substituted) to acidic halogenation (thermodynamic, more substituted).

If they incorrectly identify Y as 1-bromo-3-methylbutan-2-one, they will predict an unhindered S_N2 reaction with NaSMe to form 1-(methylthio)-3-methylbutan-2-one (Option B).

By correctly identifying Y as the tertiary bromide, the student must recognize that S_N2 is sterically forbidden, forcing an E2 elimination to yield the conjugated enone Z.

The parallel Haloform reaction confirms the substrate's dual regiochemical reactivity profile.

Quick Facts About This Question

Question Information

Subject	Chemistry
Chapter	Alcohols, Phenols, and Ethers - Class 12
Concept	Oxidation State Traps
Difficulty	8
Cognitive Load	7
Target Solve Time	160 sec
Access	Free