Birch Reduction
Reduction of aromatic rings by means of alkali metals (sodium or potassium ) in liquid ammonia with ethanol as proton donor, to give mainly unconjugated dihydro derivatives is known as Birch Reduction.
Aromatic rings and conjugated dienes are reduced by this method while isolated double bonds are normally not affected. sometimes ethers are used as a co-solvent to dissolve the aromatic compounds. Use of tert-butanol fulfills the dual role of proton donor and a co-solvent.
Alkynes are also reduced stereo-specifically to trans alkenes with sodium or lithium in liquid ammonia and no added proton donor is needed.
Birch reduction mechanism
The alkali metal donates an electron to the aromatic compound , forming the alkali metal cation and a radical anion (1) . radical anion is very basic and abstracts a rpoton from protic solvent to give a radical (2) , which picks up another electron to give an anion (3). It is quenched again by the proton source to give unconjugated dihydro compound (4).
The mechanism of the reduction of alkynes follows the same course as the reduction of aromatic rings, but the vinyl anion is basic enough to abstract proton from ammonia, so there is no need to add proton source.
Some other important reactions
Aldol condensation
Some other important reactions
Aldol condensation