Birch Reduction Mechanism

ChemistryNamed Reactions

Introduction

The Birch reduction is a reaction where arenes are transformed into cyclohexadiene. Prof. Arthur J. Birch, an Australian chemist who determined the product's structure in the reduction reaction as a 1,4-cyclohexadiene derivative in 1944, gave the reaction the name "Birch reduction." For the reduction of benzene (inactivated arenes), birch reductions generally use alkali metals (e.g.,Li, Na, or K), which dissolve in Liq. ammonia to provide a solvated electron.

Hence, the resulting solution is an astounding hue of blue, even though given metals are only faintly soluble in Liq. ammonia. The object has a deep blue colour because of the electrons with solvation that is present in the solution of ammonia. The types of groups attached—either electron-withdrawing or electron-giving groups—determine the sites of protonation on substituted benzenes. The point at which the anionic radical is protonated determines the structure of the product produced in the Birch reduction reaction.

What is Birch Reduction?

The Birch reduction reaction falls within the category of an organic redox reaction. In this reduction, benzenoid-ringed aromatic molecules are transformed into 1,4-cyclohexadiene, which has two hydrogen atoms connected to its opposite ends. Here, sodium, potassium or lithium, and alcohol cause the reduction of aromatic rings in the solution of liquid ammonia.

Images Coming soon

Fig 1: Birch Reduction Reaction.

Examples of the Birch Reduction Reaction

  • Birch reduction of Naphthalene: Birch reduction conditions can convert naphthalene to 1,4-Dihydronaphthalene.

Images Coming soon

Fig 2: Birch Reduction of Naphthalene.

  • Birch reduction of aniline or aniline derivatives: Conjugated enamines are generated immediately as the main products without the presence of a catalyst, even though aniline or aniline derivatives are electron donors.

Images Coming soon

Fig 3: Birch Reduction of Aniline Derivatives.

  • Birch reduction of differently substituted benzene ring: The following reaction demonstrates the synergistic effects of the directional influences of the alkoxy and alkyl groups.

Images Coming soon

Fig. 4: Birch Reduction of Substituted Benzene.

Birch Reduction Mechanism

In the Birch reduction reaction, mainly three reagents are used. These are:

  • ${NH_3}$ (liquid ammonia)
  • ${ROH}$ (alcohol) and
  • ${Li,Na}$ or ${K}$ as alkali metal

Only a little quantity of metal may be dissolved in ammonia due to the poor solubility of metals. An electrode salt is created when metal and ammonia combine with solvated electrons. Because it starts the process, the solvated electron emitted by sodium is quite significant. All the steps involved in the Birch reduction mechanism are discussed below:

Step 1: Benzene is reduced to a benzene radical anion.

Images Coming soon

Fig. 5: Formation of Benzene Radical Anion

When benzene and metal ${Na}$ in liquid ammonia come into contact, an electron is added to the ${Π}$ system, creating a seven pi electrons radical anion. One C - C ${Π}$ -bond is created while two C - C ${Π}$-bonds break down during this procedure.

Step 2: Benzene radical anion is protonated.

Images Coming soon

Fig. 6: Protonation of Radical Anion

Being extremely basic, the alcohol rapidly protonates the radical anion at the carbon to create the pentadienyl radical.

Step 3: The radical formed is then reduced to an anion by an electron.

Images Coming soon

Fig. 7: Reduction of Radical.

The "pentadienyl anion" is a new anion that is created when the pentadienyl radical accepts a second electron.

Step 4: The anion is protonated by alcohol.

Images Coming soon

Fig. 8: Protonation of Anion.

At this point, the presence of alcohol is required since ${NH_3}$ is insufficiently acidic to protonate this anion. This compound produces 1,4-cyclohexadiene when the central carbon undergoes protonation.

Birch Reduction Reaction Mechanism in the Presence of Ring Substituents

The sites of protonation on substituted benzenes vary in the type of group present when substituents are present on the ring. Which product forms if a substituent is attached is determined by the creation of the C− H  bond formed first. The two cases where substituents are attached to the benzene ring that we will discuss here are:

  • When an electron-withdrawing group is attached

Ipso & para reduction are promoted by electron-withdrawing groups like ${−COOH,−CONH_2}$ aryl groups, etc. These teams turn on the ring's birch reduction mechanism. The protonation first takes place at a para-position to the EWG. Example:

Images Coming soon

Fig. 9: Birch Reduction of Benzoic Acid.

Protonation takes place at the para and ipso positions about the ${-COOH}$ group present in the benzene ring during the birch reduction of benzoic acid.

  • When an electron-donating group is attached

Ortho and meta reduction are encouraged by electron-donating groups like, ${−R,\:−OR,\:−CHO,\:−C(O)R}$ etc. Example:

Images Coming soon

Fig. 10: Birch Reduction of Anisole.

The creation of an anion next to a group that is a strong pi donor ${(−OCH_3)}$ is discouraged. In the case of anisole, protonation takes place at the ortho and meta locations, resulting in the formation of additional substituted double bonds. This is because at the ortho- and meta-positions the radical anion is stabilized by electron-donating groups.

Alternative forms of Birch Reduction Reaction

In alternative forms, we can study the change in the form of alcohol in the case of Birch Reduction Reaction. We can introduce an alcohol ${R − OD}$, where isotope deuterium is set and the hydrogen is replaced in the solution mixture, instead of ${R − OH}$.Now that such a variation has been introduced, we can attempt to detect or identify the outcome. For this, it is essential to remember that alcohol, acting as a strong acid in the process, provides the ${H^+}$ necessary for the protonation of aromatic rings. ${R − OD}$ will therefore inject ${D^+}$ rather than ${H^+}$ into the solution when it is employed in place of ${R − OH}$

Conclusion

Alkali metals in liquid ammonia convert aromatic rings to 1,4-dienes in the Birch reduction process. In contrast to catalytic hydrogenation, birch reduction does not entirely degrade the aromatic ring into cyclohexane. Instead of sodium, the nucleophile in this case is an electron. Different products are produced in this reaction depending on the sorts of groups linked to the aromatic ring.

Frequently asked questions

Q1. What makes the Birch reduction significant?

Answer: By reducing just one of the double bonds rather than all of them, the Birch reduction is used to selectively decrease benzene molecules.

Q2. Are alkynes reduced by birch reduction?

Answer: Alkynes can indeed undergo Birch reduction to generate alkenes.

Q3. What chemical is the solvent in the Birch reduction process?

Answer: In this process, liquid ammonia often serves as the solvent. Tetrahydrofuran, however, is an available substitute.

Q4. Why is ${NH_3}$ first distilled before being used in Birch reduction?

Answer: Iron is a common contaminant in commercial ammonia. Therefore, before utilizing ammonia in the Birch reduction, it is frequently required to evaporate it.

Q5. What substance results from the dissolution of sodium in liquid ammonia?

Answer: The blue electrode salt ${[Na({NH_3})_6]^+e^{-}}$ is formed in sodium in a liquid ammonia solution.

References

Images Coming soon

raja
Updated on 13-Oct-2022 11:19:47

Advertisements