The Birch reduction is known as the chemical reaction where benzene is converted to 1,4-cyclohexadiene. In simple words, the Birch reduction reaction is an organic redox reaction used to convert aromatic compounds into dienes. In synthetic organic chemistry, the Birch reduction reaction is beneficial as the product formed, i.e., 1,4-cyclohexadiene contains two hydrogen atoms on either side of the molecule. We can also illustrate the chemical reaction with the help of the following diagram:-
The above illustration shows that benzene, when treated with sodium or lithium along with ammonia and proton sources such as ethanol, methanol, etc., is reduced to 1,4-cyclohexadiene. In the product formed, it can be seen that the hydrogen atoms are attached on opposite sides, i.e., different planes.
It has to be noted that phenols cannot undergo the birch reduction reaction. This is because the phenolic function of the compound becomes a phenolate ion under the reaction conditions and thus does not react further.
Birch Reduction Mechanism
Now, we know that ammonia is a gas, and at room temperature, it maintains its gaseous state. Only at --33 °C ammonia starts boiling. However, it can be condensed to the liquid state using dry acetone. This helps ammonia to be served as a solvent for alkaline metals even though the metals are partially soluble in the ammonia.
Hence, when sodium or lithium are mixed with ammonia, the entire mixture turns blue, representing the presence of the solvated electrons. When this mixture is now introduced to benzene, an electron is added to the existing system of electrons. This results in the formation of a radical anion. It is represented in the picture as follows:
Further on, as we can see from the above image, one pi bond is formed, whereas two pi bonds are broken. Now, this radical anion is further introduced to proton sources such as ethanol, etc. Being basic, the radical anion reacts to the proton source immediately. We can represent the following as:-
It has to be noted that the nature of the functional group attached to the aromatic benzene decides the position of protonation. Therefore, they can be named two types of functional groups attached to the aromatic compound, i.e., electron-withdrawing or electron-donating groups. For the electron-withdrawing group, the ipso and para positions of the aromatic compounds are preferred to be the most stable. Therefore, the Birch reduction reaction is promoted at these sites.
However, in the electron-donating functional group, aromatic compounds' ortho and meta position is preferred to be the most stable one. Suppose the electron-withdrawing group is said to activate the entire aromatic ring of the compound. In that case, the electron-donating group is said to deactivate the entire aromatic ring of the compound.
Later on, this product is transformed into an anion by an electron. At this stage of the reaction, the presence of alcohol, e.g., ethanol, becomes necessary. This is because NH3 is not a strong enough acid to protonate this anion, requiring a vital proton source. Protonation of this species of the compound at the central carbon will result in the 1,4-cyclohexadiene. We can represent the final step is:-
Birch reduction has found several applications in the total synthesis of several naturally occurring compounds. It is advantageous in aromatic compounds due to its selectivity of reducing certain double bonds, which is present in one of the starting materials out of the total steps involved in multi-step total synthesis. It is also applicable when further reduction of nonaromatic molecules as a part of natural product structures is required.
Following are some of the examples of Birch reduction reaction:-
1. The aromatic compound Naphthalene can be converted to 1,4,5,8-tetrahydronaphthalene
2. When undergoing birch reduction reaction, Benzoic acid results in the para, ipso form of the acid.
3. Functional groups of naphthalene produce the following products when it undergoes birch reduction reaction.
4. Electrons lacking heterocyclic aromatic compounds such as pyridine, etc., can also be reduced to products such as 1,4- dihydropyridine using the Birch reduction mechanism.
5. Alkali metals such as sodium, lithium, etc., are now found to be encapsulated in nanostructured oxides like silica gel instead of liquid ammonia-metal solutions. For example, the aromatic compound phenanthrene is reduced to 9,10- dihydrophenanthrene.