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Question #43981

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The Question

Consider the reaction of 1,3,6-trimethylpyrimidine-2,4(1H,3H)-dione with 1.05 equivalents of Lithium diisopropylamide (LDA) in THF at $-78^\circ\text{C}$ , followed by the addition of 1.1 equivalents of benzaldehyde. The reaction mixture is subsequently subjected to acidic workup and heated ( $H_3O^+, \Delta$ ) to yield the major product $\textbf{P}$ . Identify the structure of $\textbf{P}$ .

Options

A.
1,3-dimethyl-6-((E)-2-phenylethenyl)pyrimidine-2,4(1H,3H)-dione
B.
5-(hydroxy(phenyl)methyl)-1,3,6-trimethylpyrimidine-2,4(1H,3H)-dione
C.
1,3-dimethyl-6-(2-hydroxy-2-phenylethyl)pyrimidine-2,4(1H,3H)-dione
D.
3,6-dimethyl-1-((E)-2-phenylethenyl)pyrimidine-2,4(1H,3H)-dione

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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

Alpha-Hydrogen Acidity|Nitrogenous Base Keto-Enol Tautomerism

Why This Question Is Tricky

Common Mistakes

The question tests the concept of vinylogous acidity in heterocyclic systems.

Students often memorize that alpha-protons to carbonyls are acidic but fail to recognize that protons on a methyl group attached to the beta-position of an alpha,beta-unsaturated carbonyl (vinylogous alpha-protons) are highly acidic.

In 1,3,6-trimethyluracil, the C-6 methyl group can be deprotonated to form an extended enolate.

Distractors include deprotonation at the vinylic C-5 position or N-methyl groups, and failure to dehydrate the aldol adduct.

Quick Facts About This Question

Question Information

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