Q20. The CORRECT combination(s) of Y and T for the following elimination
reaction is(are)
E2 Elimination Reaction: Correct Y and T Combination for Major Product
The question involves identifying the correct leaving group (Y) and base (T) combination for an E2 elimination reaction yielding the specified major alkene product from a substrate with OMe and Me substituents.
Reaction Analysis
E2 eliminations require anti-periplanar alignment of the β-hydrogen and leaving group (Y) in the transition state, favoring the most stable alkene (Zaitsev product). The substrate likely features a cyclohexane ring or acyclic chain where OMe (methoxy) and Me (methyl) are positioned such that only specific Y and T enable the required geometry for the trans-disubstituted alkene shown as major.
Option Breakdown
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Option A (○Me and T): OMe as Y is a poor leaving group (requires protonation or conversion to mesylate/tosylate first); neutral T unspecified fails E2’s strong base need. Incorrect.
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Option B (NMe₂ and T): NMe₂ (dimethylamino) is a worse leaving group than halides, resisting E2 without quaternization. Incorrect.
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Option C (Br and Me): Br is excellent Y (good leaving group), but Me (methyl) as T is ineffective base (pKa ~50 for conjugate acid). Cannot deprotonate β-H. Incorrect.
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Option D (Br and tBu): Br enables anti-periplanar departure; bulky tBuO⁻ (tert-butoxide) as T favors Hofmann (less substituted) alkenes but here drives correct stereochemistry by accessing anti geometry, yielding the major trans product. Correct.
The E2 elimination reaction determines the correct Y and T combination for the major alkene product through stereospecific anti-periplanar requirements. In CSIR NET exams, identifying Br as optimal leaving group (Y) and bulky tert-butoxide (tBuO⁻ as T) is key, as this setup overcomes steric barriers to form the stable trans alkene.
E2 Mechanism Essentials
E2 proceeds concertedly: base T abstracts β-H as Y departs, demanding 180° dihedral angle. Cyclohexyl substrates need trans-diaxial Y-H; acyclic cases favor anti staggered conformations minimizing gauche interactions.
Why Option D Succeeds
Br (Y) departs readily; tBuO⁻ (T) bulk selectively deprotonates accessible β-H, yielding the depicted major product over alternatives. Other options fail: poor Y (A,B) or weak T (C).
CSIR NET Tips
Practice Newman projections: anti-periplanar governs stereoselectivity (trans > cis). Zaitsev holds unless bulky T enforces Hofmann.
