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Birch Reduction: Mechanism, Procedure & Examples

Instructor: Laura Foist

Laura has a Masters of Science in Food Science and Human Nutrition and has taught college Science.

Strong reducers typical fully reduce a compound. The Birch reduction is unique because it only selectively reduces compounds. In this lesson we will learn about the mechanism by which this occurs.

A Reaction Stopper

Think about a reaction - you're likely to think of a typical situation where certain reactants form certain products. But think of the compounds you could produce if you could stop a reaction mid-way through! Well, in a way, we can, and this has helped us make antibacterials and other medicines, which are derived from natural compounds, including modern methods for making Penicillin.

Often these compounds start out as an aromatic, benzene ring. These benzene rings are very stable, and so in order to reduce the double bonds in the benzene rings we typically need to use a very strong reducing agent.

The problem with using these agents is that it reduces all of the double bonds - it can't stop at reducing only one double bond. But there is a reduction method that is able to selectively reduce only one of the double bonds: the Birch reduction.

Mechanism of the Birch Reduction Reaction

In general, the birch reduction takes a benzene ring and uses sodium in liquid nitrogen and an alcohol to make a six-membered ring with two double bonds opposite each other.


The Birch reduction turns a benzene into a six-membered ring with two double bonds opposite each other
General reaction


The sodium in the liquid nitrogen forms a free electron on the sodium, and this electron can easily react with one of the double bonds, putting a negative charge on the opposite carbon:


The electron, from the sodium, attacks a carbon, moving the double bond, and forming a negative charge on the opposite carbon
Mechanism step 1


The alcohol acts as a hydrogen donator, and the negative charge on the carbon can easily take this hydrogen, removing the negative charge:


The hydrogen comes from the alcohol, which can be removed with the negative charge on the carbon
Mechanism step 2


Another electron (from the sodium) is added, forming a negative charge on this carbon:


An electron is again added, forming a negative charge on the carbon
Mechanism step 3


Hydrogen is again removed from the alcohol, completing the reaction:


The negative charge on the carbon can remove the hydrogen from the alcohol
Mechanism step 4


Examples of the Birch Reduction Reaction

This Birch reduction reaction is selective for putting a double bond on the most highly substituted carbon. So if one of the carbon atoms is attached to a CH3 group instead of a hydrogen, one of the double bonds needs to end up attached to that carbon, and the other double bond will be opposite it.


The Birch reduction selectively puts the double bond on the most highly substituted carbon
Select highly substituted


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