JEE Main · 2012 · 29 Apr 2012 · Offline · Hydrocarbons

2-Hexyne gives trans-2-Hexene on treatment with :

(A) $Pt / H_{2}$
(B) $Li / NH_{3}$ ✓ Correct
(C) $Pd / BaSO_{4}$
(D) $LiAlH_{4}$

Answer: (B)

Solution

Turning an internal alkyne into the trans alkene calls for a dissolving-metal reduction. Lithium (or sodium) in liquid ammonia adds through a radical-anion pathway in which the two bulky groups end up on opposite sides, delivering the trans (E) alkene from 2-hexyne. A platinum catalyst would reduce all the way to hexane, Lindlar's

Pd/BaS O_{4}

would give the cis alkene, and

LiAl H_{4}

does not reduce an isolated alkyne here. Hence

Li/N H_{3}

gives trans-2-hexene.

JEE Main · 2012 · 29 Apr 2012 · Offline · Single correct option

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