The GATE Grind

GATE 2021 CY – Question 16

Organic Chemistry · Reduction: Catalytic homogeneous and heterogeneous hydrogenation. Metal based reductions using Li/Na in liquid ammonia, magnesium, zinc, titanium, and samarium. Hydride transfer reagents: NaBH4, L-selectride, K-selectride, Luche reduction, LiAlH4 and DIBAL-H. · 1 mark · Multiple choice

The reagent(s) required for the conversion of hex-3-yne to (E)-hex-3-ene is/are:

  1. $H_2$, $Pd/BaSO_4$
  2. $Bu_3SnH$
  3. Li / liquid $NH_3$
  4. $LiAlH_4$

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Correct answer: (C) Li / liquid $NH_3$

Explanation

Dissolving-metal reduction with Li/liquid NH3 proceeds through electron transfer, protonation, a second electron transfer and protonation. The stereochemical outcome is anti addition of hydrogen, giving the trans or E alkene. A poisoned palladium catalyst instead favours syn reduction to the Z alkene.

Therefore **C**.