Chapter 9 : Alkynes

Summary

• Internal alkynes can be reduced to trans-alkenes using Na in NH₃ (l)
• Under these conditions, terminal alkynes are not reduced as they deprotonate to give acetylide ions
  
• A more modern alternate set of reagents uses (1) Na / THF then (2) CH₃OH
• 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.
• Overall, the "H₂" that is added is added as 2 e- and 2 H+

• 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 - electron replusion of the single electron and the lone pair forces them to be trans to each other and this defines the stereochemistry of the product.
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 vinyl carbanion. The stereochemistry is generated at this step - the alkyl groups are 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