Chapter 9 : Alkynes

Summary

• Alkynes can be reduced to trans-alkenes using Na in NH₃ (l)
• This reaction is stereospecific giving only the trans-alkene via an anti addition.
• Note that the stereochemistry of this reaction complements that of catalytic hydrogenation (syn)
• The reaction proceeds via single electron transfer from the Na with "H" coming from the NH₃
• These reaction conditions do not reduce alkenes, so the reaction stops at the alkene.

• Catalytic hydrogenation of alkynes

MECHANISM FOR THE REDUCTION OF ALKYNES WITH Na / NH3
Step 1: Sodium transfers an electron to the alkyne giving a radical anion.
Step 2: The radical anion removes a proton from the ammonia in an acid / base reaction.
Step 3: A second atom of sodium transfers another electron to the alkyne giving a vinly carbanion. The stereochemistry is established at this step - the alkyl groups being trans.
Step 4: The carbanion removes a proton from the ammonia in an acid / base reaction to give the trans-alkene.

© Dr. Ian Hunt, Department of Chemistry, University of Calgary