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, hence the product is 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 an anion.
Step 4: The anion removes a proton from the ammonia in an acid / base reaction.