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GATE 2021 CY – Question 48

Organic Chemistry · Types of Reactions: Nucleophilic and electrophilic substitution reactions (both aromatic and aliphatic). Addition reactions to carbon-carbon and carbon-heteroatom (N and O) multiple bonds. Elimination reactions. Reactive intermediates – carbocation, carbanion, carbenes, nitrenes, arynes and free radicals. Molecular rearrangements. Barton decarboxylation and Barton-McCombie reaction, Hunsdiecker reaction. · 2 marks · Multiple choice

The major product formed in the reaction of (2R,3R)-2-bromo-3-methylpentane with NaOMe is:

  1. (Z)-3-methylpent-2-ene
  2. (E)-3-methylpent-2-ene
  3. (2R,3R)-2-methoxy-3-methylpentane
  4. (2S,3R)-2-methoxy-3-methylpentane

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Correct answer: (B) (E)-3-methylpent-2-ene

Explanation

Methoxide is a strong base and the substrate is secondary, so E2 elimination dominates. Rotate the C2–C3 bond to place C–Br antiperiplanar to the β-C–H bond; removal gives the more substituted C2=C3 alkene. The specified R,R stereochemistry then places the higher-priority carbon groups on opposite sides. The major product is **(E)-3-methylpent-2-ene, B**.