Birch Reduction Mechanism

Birch reduction mechanism begins with the formation of the radical anion by the addition of solvated electrons to the aromatic ring. The alcohol now supplies a proton to the radical anion and also to the next to last carbanion. With the alcohol present, cyclohexadiene and an alkoxide ion are formed as products.

Birch Reduction

The Birch reduction is an organic chemical reaction where aromatic compounds which have a benzenoid ring are converted into 1,4-cyclohexadiene which have two hydrogen atoms attached at opposite ends of the molecule. It is a very useful reaction in synthetic organic chemistry. The Birch Reduction was first reported by the Australian chemist Arthur Birch in 1944. Arthur Birch was building on the earlier work of Wooster and Godfrey, published in 1937 while he was working in the Dyson Perrins Laboratory at the University of Oxford.

The Birch reduction is an organic redox reaction. Here, an organic reduction of aromatic rings in liquid ammonia with sodium, lithium or potassium and an alcohol occurs. An example for a Birch reduction reaction is the reduction of naphthalene which is given below.

Birch Reduction Example

Sodium and ethanal were used in the Birch reduction when it was first reported by Arthur Birch. It was discovered that the yield is improved with the usage of lithium.

Conjugated enamines can also be formed from the Birch reduction of aniline. Alkynes can also undergo Birch Reduction to form Alkenes as shown below.

Birch Reduction Example for Alkyne

Birch Reduction Mechanism

The solvated electrons (the free electrons in the solution of sodium in liquid ammonia which are responsible for the intense blue colour) add to the aromatic ring, giving a radical anion. This radical anion is supplied with a proton by the alcohol. The alcohol also supplies another proton to the penultimate carbanion. Now, with the alcohol present, the products – cyclohexadiene and an alkoxide ion are formed. The Birch reduction mechanism is illustrated below.

Birch Reduction Mechanism

Thus, the reuired 1,4 cyclohexadiene where two hydrogen atoms are attached on opposite ends of the molecule is formed. Alternative solvents such as tetrahydrofuran can be employed instead of ammonia, since liquid ammonia must be condensed into a flask and left to evaporate overnight post the completion of the reaction which is a time consuming task.

Practise This Question

The decomposition of dimethyl ether leads to the formation of CH4,H2 and CO and the reaction rate is given
By rate  = K[CH3OCH3]32
The rate of reaction is followed by increase in pressure in a closed vessel and the rate can be expressed in terms of the partial pressure of dimethyl ether i.e, Rate = K(pCH3OCH3)32
If pressure is measured in bar and time in minutes, the units of rate constant are