Birch reduction mechanism begins with the formation of the radical anion by the addition of solvated electrons to the aromatic ring. The alcohol now supplies a proton to the radical anion and also to the next to last carbanion. With the alcohol present, cyclohexadiene and an alkoxide ion are formed as products.
The Birch Reduction was first reported by the Australian chemist Arthur Birch in 1944. Arthur Birch was building on the earlier work of Wooster and Godfrey, published in 1937 while he was working in the Dyson Perrins Laboratory at the University of Oxford.
What is Birch Reduction?
The Birch reduction is an organic chemical reaction where aromatic compounds which have a benzenoid ring are converted into 1,4-cyclohexadiene which have two hydrogen atoms attached at opposite ends of the molecule. It is a very useful reaction in synthetic organic chemistry.
The Birch reduction is an organic redox reaction. Here, an organic reduction of aromatic rings in liquid ammonia with sodium, lithium or potassium and alcohol occurs. An example of a Birch reduction reaction is the reduction of naphthalene which is given below.
Sodium and ethanal were used in the Birch reduction when it was first reported by Arthur Birch. It was discovered that the yield is improved with the usage of lithium.
Conjugated enamines can also be formed from the Birch reduction of aniline. Alkynes can also undergo Birch Reduction to form Alkenes as shown below.
Birch Reduction Mechanism
The solvated electrons (the free electrons in the solution of sodium in liquid ammonia which are responsible for the intense blue colour) add to the aromatic ring, giving a radical anion. This radical anion is supplied with a proton by the alcohol. The alcohol also supplies another proton to the penultimate carbanion. Now, with the alcohol present, the products – cyclohexadiene and an alkoxide ion are formed. The Birch reduction mechanism is illustrated below.
Thus, the required 1,4 cyclohexadiene where two hydrogen atoms are attached on opposite ends of the molecule is formed. Alternative organic solvents such as tetrahydrofuran can be employed instead of ammonia since liquid ammonia must be condensed into a flask and left to evaporate overnight post the completion of the reaction which is a time-consuming task.
Features of Birch Reduction
1. Alkali metals dissolve in liquid ammonia to give a blue solution. The aromatic rings take up the electrons one by one. A radical anion is formed on absorbing the first electron. Later, a C-H bond is formed when the alcohol molecule gives away its hydroxylic hydrogen. The second electron is picked up to offer a carbanion of the cyclohexadienyl type and is protonated by the presence of alcohol.
2. In 1961 it was revealed that the most negative and likely to protonate was the ortho site. Also, the second protonation occurred in the centre of the cyclohexadienyl anion and produced an unconjugated product.
3. In the year 1990 and 1993, an esoteric test was planned to show that ortho protonation of the radical anion was selected over meta.
4. Huckel computations disclosed that the centre carbon instead of the preferred one.
5. As the reaction proceeds, the reaction mechanism gives details regarding the molecular change.
6. The beginning protonation of radical-anion takes place para to the withdrawing substituent.
Frequently asked questions
What is meant by Birch Method?
It is a chemical reduction reaction to convert the base form pseudoephedrine into methamphetamine.
Birch method is also known as ________.
Who developed the Birch method and in which year?
It was developed by Authur John Birch in the year 1944.