Q. 19 The major product formed in the following reaction is
Major Product Formed in the Following Reaction
Question Overview
A bicyclic brominated compound is treated with sodium ethoxide (NaOEt), a strong base. The major product P formed in this reaction must be identified from the given options.
Reagent Analysis
Role of NaOEt
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Strong base
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Promotes E2 elimination
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Requires anti-periplanar arrangement of H and leaving group (Br)
Structural & Mechanistic Insight
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The molecule is a fused bicyclic (decalin-type) system
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Br is attached at the ring junction
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Elimination must follow:
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Anti-periplanar geometry
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Bredt’s Rule (no double bond at bridgehead in small bicyclic systems)
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Important Rule: Bredt’s Rule
A double bond cannot form at a bridgehead carbon of a small bicyclic system because it cannot achieve planar geometry.
Thus:
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Any option showing a bridgehead double bond is not possible
Correct Answer
✅ Option (A)
Why Option (A) is Correct
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E2 elimination occurs between Br and a β-hydrogen
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The resulting double bond is not at the bridgehead
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The alkene formed is:
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Structurally allowed
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Conjugationally stable
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Free from Bredt’s rule violation
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✔ Hence, Option (A) represents the major and correct product
Explanation of All Options
Option (A) – Correct ✅
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Double bond formed away from the bridgehead
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Fully satisfies:
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E2 elimination requirements
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Anti-periplanar geometry
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Bredt’s rule
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Most stable and feasible alkene
Option (B) – Incorrect ❌
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Double bond lies at the ring junction
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Violates Bredt’s Rule
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Such an alkene cannot exist
Option (C) – Incorrect ❌
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Requires ring rearrangement
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Not a direct E2 elimination product
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Mechanistically unfavorable
Option (D) – Incorrect ❌
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Forms a strained system
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Does not follow anti-periplanar elimination
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Less stable than option (A)
Final Conclusion
The reaction proceeds via E2 elimination using NaOEt. Due to Bredt’s rule and anti-periplanar requirements, the most stable and allowed alkene formed is:
