Partial reduction of alkynes to trans alkenes using sodium and ammonia
Description: Alkynes treated with sodium in ammonia are reduced to trans alkenes. Alternatively, potassium in ammonia can also be used.
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.
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).