Unit: Name Reactions B.Sc-II Paper-IV
By Dr. Anindra Sharma Department of Chemistry A.P.S.M. College, Barauni
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 can be classified as 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 (illustrated below).
Example of Birch Reduction
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.
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 illustrated 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
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 upon the absorption of the first electron.
Now, a carbon-hydrogen bond is formed when the alcohol molecule gives away its hydroxylic hydrogen. A cyclohexadienyl-type carbanion is formed when the second electron is picked up. This carbanion is now protonated by the alcohol.
It is important to note that the protonation of the cyclohexadienyl carbanion takes place at its centre.
The structure of the product formed in the Birch reduction reaction is determined by the location at which the anionic radical is protonated.
When electron-donating groups are used, the protonation tends to occur at the ortho or meta positions (with respect to the substituent).
When electron-withdrawing groups are used, the protonation generally occurs at the para position.
Benzoin Condensation
The Benzoin Condensation is a coupling reaction between two aldehydes that allows the preparation of α-hydroxyketones. The first methods were only suitable for the conversion of aromatic aldehydes.
Mechanism of Benzoin Condensation
Addition of the cyanide ion to create a cyanohydrin effects an umpolung of the normal carbonyl charge affinity, and the electrophilic aldehyde carbon becomes nucleophilic after deprotonation: A thiazolium salt may also be used as the catalyst in this reaction
A strong base is now able to deprotonate at the former carbonyl C-atom:
A second equivalent of aldehyde reacts with this carbanion; elimination of the catalyst regenerates the carbonyl compound at the end of the reaction: