Birch reduction

Mechanism

1. Birch Reduction Mr. Yashvant V. Khandare

2. M liq. NH 3 M [H 3 N-------e-------NH 3 ] M = Na / Li (Solvated Electron) The Birch reduction is an organic reaction where aromatic compounds undergo partial reduction to 1,4- unconjugated cyclohexadiene compounds in presence of alkali metals in liquid ammonia i.e. solvated electrons. H H ( A r o m a ti c ) H H ( Non - a r o m a tic ) M / Liq. NH 3 M = Na / Li The reduction is conducted by Sodium or Lithium metal in liquid ammonia at -33 o C 3

3. Radical In chemistry, a radical is an atom, molecule, or ion that has an unpaired valence electron. It may be generated in a number of ways, but typical methods involve redox reactions. Ionizing radiation, heat, electrical discharges, and electrolysis are known to produce radicals. A radical is the hydroxyl radical, a molecule that has one unpaired electron on the oxygen atom.

4. The exam p les are trip l et oxygen and trip l et carbene which have two unpaired electrons.

5. E xam p les of Birch Reduction

6. The alkali metal donate an electron to the aromatic comp , forming the alkali metal cation and radicle anion. The radicle anion is basic and abstract the proton from protic solvent to give ion. This pick up another electron to give ion. Its is quenched again by the proton to give dihydro comp.

7. Mechanism It consist of three steps Step-1 :- Sodium oxidises to Na+ and radical anion I. Step-2:- Radical anion accepts a proton from the alcohol to give radical II which is reduced to carbanion III by another sodium atom. Step-3:- Carbanion III then accept a proton from alcohol to give product 1,4-Cyclohexadiene.

8. Mechanism

9. e M / liq.NH 3 H H H H e H H H H H H H HO R R O H H (Radical Anion) 9 MECHAN I SM