Q. 18 The reactants P and Q in the following reaction are
Identification of Reactants P and Q in the Given Reaction
Question Overview
The reactants P and Q in the following reaction are to be identified:
P + Q
Δ (heat)
→ Given bicyclic product with a phenyl (Ph) substituent
Several pairs of reactants are provided as options. We must determine which pair correctly produces the given product under thermal conditions.
Key Concept Involved
The reaction shown proceeds under heat (Δ) and results in a bicyclic compound, which is a strong indication of a:
Diels–Alder Reaction
A Diels–Alder reaction involves:
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A conjugated diene
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A dienophile (alkene or substituted alkene)
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Formation of a six-membered ring (often bicyclic when cyclic dienes are used)
Product Analysis
From the given product:
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A bicyclic framework is present
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A phenyl (Ph) group is attached
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The structure strongly resembles a norbornene-type skeleton
This clearly suggests that:
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One reactant must be cyclopentadiene (a common cyclic diene)
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The other reactant must be a phenyl-substituted alkene (dienophile)
Correct Answer
✅ Option (A)
P = II (cyclopentadiene)
Q = substituted phenyl alkene
Why Option (A) is Correct
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Cyclopentadiene readily undergoes Diels–Alder reactions
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The phenyl-substituted alkene acts as an excellent dienophile
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Thermal conditions favor the endo product, which matches the given structure
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The resulting bicyclic system perfectly matches the product shown
✔ Hence, Option (A) correctly identifies reactants P and Q
Explanation of All Options
Option (A) – Correct ✅
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Contains cyclopentadiene (diene)
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Contains a phenyl-substituted alkene (dienophile)
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Produces a bicyclic Diels–Alder adduct
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Matches the given product exactly
Option (B) – Incorrect ❌
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Uses an inappropriate diene arrangement
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Does not allow correct orbital overlap
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Cannot form the observed bicyclic framework
Option (C) – Incorrect ❌
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The alkene shown is not a strong dienophile
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Regioselectivity does not match the product
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Product structure cannot be formed
Option (D) – Incorrect ❌
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Linear diene system
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Does not form the rigid bicyclic system shown
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Fails to explain the observed substitution pattern
Final Conclusion
The given reaction proceeds via a Diels–Alder cycloaddition, and the only option that correctly supplies a conjugated cyclic diene and a phenyl-substituted dienophile is:
