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 positions–1 and –4 to give an unconjugated diene. This is known as Birch reduction. Thus, benzene gives 1, 4-dihydrocyclohexadiene and naphthalene gives 1, 4-dihydronaphthalene.
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.)
Mechanism of Birch Reduction
The metal transfers one electron to the benzene ring to produce a resonance-stabilized anion radical (Ia–Ic) 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 (III) 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 (Ib) which, therefore, adds a proton to give (II) and subsequently a 1, 4-dihydro and not 1,2-dihydro 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.
The 1, 2-dihydro product has a conjugated double bond and hence undergoes further reduction to form a tetrahydro derivative.
Cyclohexene has a single olefinic bond which is unaffected by the reagent. The presence of electron-withdrawing groups 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 remains on the saturated carbon in the products.
This is because, calculations by molecular orbital method indicate that in the anion-radical
(a) the electron density is the greatest at the ortho or meta positions with respect to the electron-releasing substituent in benzene and
(b) the electron density is the greatest at the para position with respect to the electron-withdrawing substituent in benzene.
Phenols and isolated double bonds are not reduced by this method.