Birch Reduction - Mechanism, Features, Example

What is 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 can be classified as an organic redox reaction. Here, an organic reduction of aromatic rings in liquid ammonia with sodium, lithium or potassium and alcohol occurs. An example of a Birch reduction reaction is the reduction of naphthalene (illustrated below).

Birch Reduction Example

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.

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.

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 illustrated below.

Birch Reduction - 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 required 1,4 cyclohexadiene where two hydrogen atoms are attached on opposite ends of the molecule is formed. Alternative organic 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).

Features of Birch Reduction

  • Alkali metals dissolve in liquid ammonia to give a blue solution. The aromatic rings take up the electrons one by one. A radical anion is formed upon the absorption of the first electron.
  • Now, a carbon-hydrogen bond is formed when the alcohol molecule gives away its hydroxylic hydrogen.
  • A cyclohexadienyl-type carbanion is formed when the second electron is picked up. This carbanion is now protonated by the alcohol.
  • It is important to note that the protonation of the cyclohexadienyl carbanion takes place at its centre.
  • The structure of the product formed in the Birch reduction reaction is determined by the location at which the anionic radical is protonated.
  • When electron-donating groups are used, the protonation tends to occur at the ortho or meta positions (with respect to the substituent).
  • When electron-withdrawing groups are used, the protonation generally occurs at the para position.

Frequently Asked Questions

What are the Reactants and Products in Birch Reduction Reactions?

The reactant in a Birch reduction is an aromatic compound consisting of a benzenoid ring. The final product formed in these reactions is 1,4-cyclohexadiene. Alkynes are also known to undergo Birch reduction to form alkenes.

What Type of Reaction can Birch Reduction be classified as?

The Birch reduction reaction can be classified as an organic redox reaction. Here, aromatic rings undergo reduction in the presence of liquid ammonia (along with alcohol and sodium, lithium, or potassium).

Which compound is used as a Solvent in Birch Reduction?

Generally, liquid ammonia is used as a solvent in these reactions. However, tetrahydrofuran can be used as an alternative.

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