Class NEET 2026 ALL Q #1977
COMPETENCY BASED
APPLY
4 Marks 2026 NTA-RE-NEET-2026 MCQ SINGLE
Among the following, the compound having conjugated double bonds is
(A) hepta-1,6-diene
(B) hepta-1,3-diene
(C) hepta-1,4-diene
(D) hepta-1,5-diene
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Correct Answer: B

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Detailed Solution

Step 1: Define Conjugated Double Bonds

Conjugated double bonds occur when two double bonds are separated by exactly one single bond (alternating pattern: double-single-double). This allows for the delocalization of pi-electrons across the system.

Step 2: Analyze the Options

We examine the structure of each heptadiene isomer to identify the position of the double bonds:

  • (A) hepta-1,6-diene: $CH_2=CH-CH_2-CH_2-CH_2-CH=CH_2$ (Double bonds separated by three $CH_2$ groups; isolated).
  • (B) hepta-1,3-diene: $CH_2=CH-CH=CH-CH_2-CH_2-CH_3$ (Double bonds at positions 1 and 3 are separated by one single bond; conjugated).
  • (C) hepta-1,4-diene: $CH_2=CH-CH_2-CH=CH-CH_2-CH_3$ (Double bonds separated by two $CH_2$ groups; isolated).
  • (D) hepta-1,5-diene: $CH_2=CH-CH_2-CH_2-CH=CH-CH_3$ (Double bonds separated by two $CH_2$ groups; isolated).

Step 3: Conclusion

Only in hepta-1,3-diene are the double bonds separated by a single bond, satisfying the condition for conjugation.

Final Answer: hepta-1,3-diene

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Pedagogical Audit
Bloom's Analysis: This is an APPLY question because the student must apply the structural definition of conjugation to specific IUPAC nomenclature provided in the options.
Knowledge Dimension: CONCEPTUAL
Justification: The question tests the understanding of the electronic structure and bonding patterns in organic molecules rather than rote memorization of facts.
Syllabus Audit: In the context of NEET, this is classified as COMPETENCY. This question assesses the student's ability to visualize molecular geometry and bonding types, which is a core skill for the Organic Chemistry section of the NEET syllabus.

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