IIT JEE REvision - Reduction of Alkynes

Reaction Type: Addition

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.

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.

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.