Birch Reduction

Birch Reduction

It occurs when aromatic molecules with benzene rings are transformed into 1,4-cyclohexadiene molecules with two hydrogen atoms attached at opposite ends, and this chemical reaction is known as the Birch reduction. This reaction plays a vital role in synthetic organic chemistry. Redox reactions classified as organic can be classified as Birch reductions. To produce an organic reduction of aromatic rings, sodium, lithium or potassium react with alcohol. A Birch reaction can be illustrated below with the reduction of naphthalene.



By adding solvated electrons onto the aromatic ring, the Brich reduction mechanism forms the radical anion. Alcohol supplies a proton along with the radical anion and the next-to-last carbanion. An alkoxide ion is created when an alcohol is present along with cyclohexadiene. During World War II, Australian chemist Arthur Birch reported the Birch Reduction for the first time. The work of Wooster and Godfrey was expanded upon by Arthur Birch in 1937, when he worked at the Dyson Perrins Laboratory at Oxford University.

Arthur Birch first reported the Birch reduction using sodium and ethanal. There is evidence that lithium improves yield. Conjugated enamines can also be produced from the Birch reduction of aniline. It is also possible to convert Alkynes into Alkenes by undergoing Birch Reduction, as shown below.



Birch Reduction Mechanism
In the presence of sodium in liquid ammonia, the solvated electrons (free electrons in the solution) give a radical anion due to adding them to the aromatic ring. Alcohol provides a proton to this radical anion. Alcohol provides the penultimate carbanion with another proton. Once the alcohol is present, the alkoxide ion and cyclohexadiene are formed. Below is an image explaining Birch's process.



This produces the 1,4 cyclohexadiene molecule containing two hydrogen atoms at opposite ends. In place of liquid ammonia, other organic solvents can be used instead, such as tetrahydrofuran, since liquid ammonia must be condensed into a flask and allowed to evaporate overnight after completing the reaction.

Features of Birch Reduction

• Blue liquid results from the dissolution of alkali metals in liquid ammonia. Electrons are taken up one by one by aromatic rings. An ion is formed when the first electron is removed.
• Alcohol molecules form carbon-hydrogen bonds when their hydroxylic hydrogen is given away.
• Cyclohexadienyl carbanion is formed when the second electron is removed. After the alcohol protonates the carbanion, it is rendered inactive.
• This carbanion protonates at its center since it is cyclohexadienyl.
• By protonating the anionic radical at a specific location, Birch reduction products are formed that have different structures.
• When electron-donating groups are used, protonation occurs either at the ortho or meta position (regardless of the substituent).
• When electron-withdrawing groups are used, protonation usually occurs at the para position.

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