Partial reduction of alkynes to trans alkenes using sodium and ammonia

by James

Description: Alkynes treated with sodium in ammonia are reduced to trans alkenes. Alternatively, potassium in ammonia can also be used.
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Notes: The reaction is complementary to the Lindlar reduction of alkynes to give cis-alkenes. Note that the alkenes here are not further reduced to the alkane.

Examples

Notes: The byproduct in each of these reactions is NaNH2 (2 equiv)

Mechanism: Sodium metal has an extremely low ionization energy and will readily give up its electron to the alkynes (Step 1, arrow A). This will form the trans alkene “radical anion”. Protonation of carbon by NH3 (Step 2, arrows B and C) gives the radical, which is reduced by a second equivalent of sodium (Step 3, arrow D) to give a new anion, which is then protonated (Step 4, arrows E and F) by NH3 to give the trans alkene.

Notes: It’s also reasonable to show direct formation of the trans isomer from the first reduction step (step 1).

Video Walkthrough:

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{ 2 comments… read them below or add one }

Seah

Why is the radical anion able to undergo flipping when there is a pi bond? Does that require a great amount of energy to first break the pi bond, flip and then reform the pi bond again?

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Ryan McCauslin

I was wondering the same thing….? Prehaps the alkyne pi bond cleavage actually takes electrons from each of the separate pi bonds which allows temporary alkane rotation to form the more stable trans intermediate?

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