Answer to Question #85848 in Organic Chemistry for anuj

Question

Reduction of an alkyne in the presence of sodium metal gives a trans alkene

Answer 1

Answer on Question #85848 – Chemistry – Organic Chemistry

Task:

Reduction of an alkyne in the presence of sodium metal gives a trans-alkene.

Solution:

The reduction of alkynes by sodium in liquid ammonia at –33°С proceeds completely stereospecifically with the formation of the trans-isomer only. For this process, the mechanism of two-electron reduction of the triple bond with the formation of anion radical, vinyl radical and vinyl anion as successive intermediates was proposed. The first electron is directed to the loosening π-orbital with the formation of a radical anion. Further, the radical anion is protonated by ammonia and gives the vinyl radical, which adds another electron to form the vinyl anion. In the end, the vinyl anion cleaves the proton from ammonia, which acts as the N – H acid, resulting in a trans-alkene and an imide anion:

Example: