Birch Reduction Mechanism

Reaction of Birch Reduction

When aromatic rings are reduced with sodium, potassium or lithium in liquid ammonia or amine in the presence of alcohol, addition of hydrogen takes place at posilions-1 and -4 to give an unconjugated diene. This is known as Birch reduction. Thus, benzene gives 1. 4-dihydro cyclohexadiene and naphthalene gives 1, 4-dihydronaphthalene.

Reaction mechanism of Birch Reduction

Liquid ammonia serves as solvent. Primary amines may also be used as solvent with advantage, since it permits higher temperature of reaction. (b.p. of ethyl amine is 19°C and b.p. of liquid ammonia is -33°C.)

The accepted mechanism of reduction involves the following sequential steps: The metal transfers one electron to the benzene ring to produce a resonance-stabilized anion radical (la-le) which accepts a proton from the alcohol to form a radical (II). The addition of an electron from the metal to the radical produces an anion (Ill) which subsequently takes up a proton from the alcohol to give the dihydro product. The repulsion between the anionic and radical centres Is minimum in (lb) which, therefore, adds a proton to give (II) and subsequently a 1, 4-dlhydro and not 1,2-dlhydro product Is formed.

At higher temperatures (50-120°C), ammonia becomes the proton source and alcohol need not be used. The amide Ion thus formed Is a strong base and isomerizes the 1, 4-dihydro product to 1,2-dihydro product.

Cyclohexene has a single olefinic bond which is unaffected by the reagent. The presence of electron-withdrawing gro1Jps in the aromatic rings makes the rings more electron-accepting and hence the reaction is facilitated. The presence of electron-releasing groups have, the reverse effect. With substituted benzene the electron-donating group remains on the unsaturated carbon and the electron-withdrawing group remain on the saturated carbon in the products.

Questions based on Birch Reduction