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Question #44013

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The Question

Compound $\textbf{X}$ ( $\ce{C_{11}H_{14}O}$ ) reacts with 2,4-dinitrophenylhydrazine to form a distinct yellow-orange precipitate. However, when $\textbf{X}$ is treated with $\ce{I2}$ in the presence of aqueous $\ce{NaOH}$ at 298 K, no yellow precipitate of iodoform is observed. Clemmensen reduction of $\textbf{X}$ yields 2-ethyl-1,3,5-trimethylbenzene. Which of the following best explains the primary mechanistic reason for the failure of the iodoform test for compound $\textbf{X}$ ?

Options

A.
The +I and hyperconjugative effects of the aromatic ring destabilize the intermediate carbanion, raising the activation energy for enolate formation.
B.
Steric crowding from the \textit{ortho}-methyl groups completely inhibits the abstraction of the $\alpha$ -protons by $\ce{OH^-}$ , preventing the initial halogenation.
C.
The bulky \textit{ortho}-methyl groups and the $\ce{-CI3}$ group sterically block the nucleophilic attack of $\ce{OH^-}$ on the carbonyl carbon, preventing the formation of the tetrahedral intermediate required for cleavage.
D.
The intermediate $\alpha,\alpha,\alpha$ -triiodoketone undergoes a rapid intramolecular Cannizzaro-type hydride shift instead of the standard haloform cleavage.

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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

Reaction with DNP|Haloform Reaction Logic|Alpha-Hydrogen Acidity

Why This Question Is Tricky

Common Mistakes

This question tests a profound 'Chemical Exception' to the Haloform reaction.

Students are trained to blindly associate the 'methyl ketone' moiety with a positive iodoform test.

When confronted with a failure, the common trap (Option B) is to assume that steric hindrance blocks the enolization step (the usual rate-determining step).

The 'Hidden Variable' is the mechanistic divergence: the methyl protons are accessible enough for exhaustive halogenation, but the true kinetic barrier arises in the final cleavage step.

The extreme steric clash between the newly formed bulky -CI3 group and the ortho-methyls completely shields the carbonyl carbon from nucleophilic attack by hydroxide, arresting the reaction at the triiodo intermediate.

Quick Facts About This Question

Question Information

Subject	Chemistry
Chapter	Aldehydes, Ketones, and Carboxylic Acids - Class 12
Concept	Reaction with DNP
Difficulty	8
Cognitive Load	7
Target Solve Time	160 sec
Access	Free