Birch Reduction

➢The process of partial reduction of an aromatic hydrocarbon into a 1,4-dihydro derivative with alkali metal (Na, Li, etc.) in liquid ammonia (NH3) or a primary amine in presence of an alcohol is known as Birch Reduction.

➢This reaction is powerful enough to reduce the benzene ring.

➢The NH3 lose an acidic hydrogen to form basic amide (NH2-) which is a very strong base and helps in Birch Reduction as shown below.

➢E.g.

➢Mechanism:-

➢Alcohol plays two important roles:-

➟Alcohol is a proton donor.
➟Alcohol presents the accumulation of a basic amide (NH2-) which is produced from the solution.

➢With increase in concentration of an amide, isomerization of 1,4-derivative to the 1,2-derivative occurs which upon further reduction gives a tetra-hydro derivative.

E.g.

➢Non conjugated double bonds are rarely reduced, but the isolated double bonds are never reduced.

➢Substituent effect on Benzene (Regioselectivity):-

➥Electron withdrawing group:- If an EWG is attached to the benzene, it makes the benzene more reactive towards the Birch Reduction, hence increases the rate of reaction. EWG promotes ipso/para reduction. E.g.

➥Electron donating group:- EDG reduces/decreases the rate of reaction and promotes the ortho/meta reduction. E.g.

In case of Aniline and its substituted derivatives, it is difficult to stop the isomerization. Hence, more stable conjugated Enamines are obtained. E.g.

➥If both the EDG as well as the EWG are present in a ring, then reduction occurs in such a way so that the requirements of both the groups will be satisfied. E.g.

{Note:- If acid is mild, then none conjugated ketone formed but if acid is strong, then conjugated ketone formed through isomerization.}

➢In case of a bicyclic fused ring, Birch Reduction occurs in the less electron rich ring i.e. if one is electron rich and other is electron deficient, then Birch Reduction occurs on electron deficient ring. E.g.