Birch Reduction Mechanism

What is Birch Reduction

Birch Reduction Reaction is an organic reaction which is used to produce cyclohexadienes from aromatic compounds. It is named after an Australian Chemist, Mr. Arthur John Birch, who reported it in 1944. 

In this reaction, we use 3 reagents, namely – 1) Liquid Ammonia (NH3), 2) Organic Alcohol (ROH) and 3) Alkali metal (Li, Na or K). Arthur Birch had originally used Sodium (Na), but later studies revealed that Lithium (Li) gives much better results.

Examples of Birch Reduction Reaction

Example 1 : Reduction of Benzene

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Example 2 : Reduction of Naphthalene

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Birch Reduction Mechanism

As we all know, ammonia is a gas at room temperature. In Birch Reduction Reaction, we need to use liquid ammonia. Hence, it is condensed to a liquid state by lowering its temperature to -78oC. This is usually done by using either dry ice or acetone cold finger. A cold finger is an equipment in which a coolant fluid (such as dry ice or acetone) flows and it can be used to create a localized extremely cold area. 

Once in liquid form, ammonia becomes a solvent for alkali metals. Please note the solubility of metals in ammonia is not very good, and only a small amount of metal can be dissolved. But once the solution is formed, it gives an intense blue colour due to presence of solvated electrons. Metal and ammonia actually form an electride salt, along with solvated electrons.

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The cation thus formed, i.e. [Na(NH3)6]+, is an octahedral coordination complex, which is out of purview of this topic.

The solvated electron released by Sodium is very important because it initiates the reaction. An organic alcohol (ROH) is also added to the solution. We will look at the various steps in the mechanism of Birch Reduction Reaction below.

We will take the first example, i.e. reduction of Benzene, which we gave earlier to explain the reaction mechanism in more detail.

  1. In the first step, the Benzene ring is reduced by the solvated electron to form Benzene radical anion.

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As you can see in the image above, 2 pi bonds are broken and 1 new pi bond is formed in this step.

  1. The anion formed in the first step is highly basic in nature. The alcohol (ROH) present in the solution readily reacts with this anion and protonates it.

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It is worth to be noted that the anion being highly basic can also be protonated by NH3 if ROH wasn’t available in the solution. But we need to use ROH because of step 4 explained below.

  1. Step 2 forms a pentadienyl radical, which reduces further and accepts one more solvated electron.

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This forms a pentadienyl radical anion in the solution.

  1. The anion formed in step 3 is not as basic as the one formed in step 2. So, a strong acid like ROH is required to protonate it. Just ammonia wouldn’t be able to drive this reaction further.

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The RO- group thus formed, reacts with Na+ to form NaOR.

Birch Reduction Reaction Mechanism in Presence of Ring Substituents

In the example which we saw in the previous section, we reduced a simple benzene ring using Birch Reduction Reaction. What if we have another group attached to a benzene ring? Let’s see.

Case 1 : When an electron withdrawing group is attached to the benzene ring.

In such a case, the protonation happens on the carbon atom which is bearing the substituent group. For example, in case of benzoic acid, which has an electron withdrawing group in form of COOH, the reaction works as follows :

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Case 2 : When an electron donating group is attached to the benzene ring.

In this case, the protonation happens to the carbon which is adjacent to the one bearing the substituent group. For example, in case of anisole, which has an electron donating group in form of RO, the reaction works as follows :

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Alternatives forms of Birch Reduction Reaction

From an exam point of view, you may be tested on detailed concepts of the reaction mechanism. One of the variations that the examiner can introduce is to have alcohol in the solution mixture which has an isotope of Hydrogen, say Deuterium. Hence the formula of such an alcohol will be R-OD instead of R-OH. You may be asked to detect or identify the end result if such a variation is introduced. For this, you need to keep in mind that the protonation of aromatic rings happens by the H+ supplied by alcohol which acts as a strong acid in the reaction. So, when R-OD is used instead of R-OH, it will introduce D+ instead of H+ in the solution. Hence, it will be the D which will be added to the ring where hitherto H was being added. 

Remember the positions of H additions in electron withdrawing and electron donating substituent groups from the previous section to quickly answer the question in case ROH is replaced by ROD. Usage of a heavier isotope may impact the rate of reaction, but that is out of purview of this topic.

This ends our coverage of Birch Reduction Reaction and its mechanism. We hope you enjoyed learning and were able to grasp the concepts behind this reaction. If you are looking for solutions to NCERT Textbook problems based on this topic, then log on to Vedantu website or download Vedantu Learning App. By doing so, you will be able to access free PDFs of NCERT Solutions as well as Revision notes, Mock Tests and much more.