Q. 14 The total number of tautomers for the following molecule (including the structure provided below)
is ____ .

Below is a complete SEO-friendly chemistry article written for JEE/NEET–level organic chemistry, with a clear correct answer, conceptual explanation, and option-wise reasoning.
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Question Overview

Q. The total number of tautomers for the following molecule (including the structure provided) is ___.

This question tests understanding of keto–enol tautomerism, conjugation, aromatic stabilization, and hydrogen migration—topics frequently asked in JEE Advanced, NEET, and Olympiad exams.

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Understanding the Given Structure

The given compound is a hydroxy-substituted naphthoquinone derivative, structurally similar to 2-hydroxy-1,4-naphthoquinone (lawsone).

Key Structural Features:

• Two carbonyl (C=O) groups

• One enol –OH group

• Conjugated ring system

• Presence of α-hydrogen relative to carbonyl groups

These features make the molecule capable of keto–enol tautomerism.

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Key Concept: Keto–Enol Tautomerism

Tautomerism involves:

• Migration of a proton (H⁺)

• Shifting of a π-bond

• Interconversion between keto (C=O) and enol (C=C–OH) forms

Important Rule:

👉 Each carbonyl group with an α-hydrogen can form a distinct enol tautomer.

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Counting the Tautomers

Step-by-Step Logic

1. Given structure (enol form) → 1 tautomer

2. First keto form (enol → keto at one position) → 2nd tautomer

3. Second keto form (alternative enolization at another carbonyl) → 3rd tautomer

⚠️ No additional stable tautomers form because:

• Further proton shifts break aromatic/conjugation stability

• No extra α-hydrogens are available

• Symmetry does not create new distinct forms

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✅ Correct Answer

Total Number of Tautomers = 3

(including the structure given in the question)

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Explanation of All Possible Options

Option (A): 2

❌ Incorrect

• Counts only one keto and one enol form

• Ignores the second possible keto–enol shift

• Underestimates tautomeric possibilities

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Option (B): 3

✅ Correct Answer

• Includes:

  • The given enol form

  • Two distinct keto tautomers

• All are chemically valid and distinct

• Matches theoretical and exam-standard counting

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Option (C): 4

❌ Incorrect

• Assumes an extra tautomer

• No additional α-hydrogen or resonance-stable form exists

• Overcounting due to misunderstanding of conjugation

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Option (D): 5

❌ Incorrect

• Gross overestimation

• Ignores structural and stability constraints

• Such forms are not chemically feasible

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Final Answer

✅ Correct Answer: 3

The molecule exhibits three tautomeric forms, including the structure provided.