1. BIRCH REDUCTION Prepared By Dr. Krishnaswamy. G Faculty DOS & R in Organic Chemistry Tumkur University Tumakuru 1

2. Arthur John Birch 1915–1995 2

3. The Birch reduction is an organic reaction where aromatic compounds undergo partial reduction to 1,4- unconjugated cyclohexadiene compounds in presence of alkali metals in liquid ammonia i.e. solvated electrons. The reduction is conducted by Sodium or Lithium metal in liquid ammonia at -33 o C 3

4. The solvated electrons give an intense blue color to the solution and have to be captured as the metal releases them, otherwise with time the blue color fades as the electrons reduce the ammonia to NH 2 - and H 2. 4

5. The mechanism begins with a single electron transfer (SET) from the metal to the aromatic ring, forming a radical anion. The anion then picks up a proton from the alcohol which results in a neutral radical intermediate. Formed neutral radical intermediate picks up one electron to form anion followed by abstraction of proton from alcohol results in the final unconjugated cyclohexadiene. MECHANISM 5

6. 6

7. Regioselectivity arises when there are substituents around the aromatic ring. REGIOSELECTIVITY Electron Donating Groups such as alkyl or alkoxy groups remains on the unreduced or Sp 2 hybridised carbon atom or ortho and meta carbon atoms are reduced. Ortho Carbon reduced from Sp 2 to Sp 3 Meta Carbon reduced from Sp 2 to Sp 3 7

8. The Aldehydes and ester groups are in the electron donation side because these are reduced to the corresponding alcohols in Birch reduction prior to the reduction of aromatic ring. 8

9. Electron Withdrawing Groups such as carboxylic acid or primary amide groups remains on the reduced or Sp 3 hybridised carbon atom or ipso and para carbon atoms are reduced. Ipso Carbon reduced from Sp 2 to Sp 3 Para Carbon reduced from Sp 2 to Sp 3 9

10. If both Electron Withdrawing Groups and Electron Donating Groups are present on the same ring. 10

11. Different products are obtained depending upon the positions of EDG. In case of both EWG & EDG present in the same ring. EWG placed on reduced carbon and EDG on the unreduced carbon 11

12. Selective reduction of less electron rich aromatic ring occurs in the case of bicyclic aromatic compounds. Electron rich due to EDG OH Electron poor due to electronegative atom Nitrogen 12

13. If one wants conjugated dienes, it is quite simple to isomerise them using an acid catalyst. 13

14. REFERENCES  Birch, A. J. J. Chem. Soc. 1944, 430–436.  https://www.name-reaction.com/birch-reduction https://www.name-reaction.com/birch-reduction  https://en.wikipedia.org/wiki/Birch_reduction https://en.wikipedia.org/wiki/Birch_reduction 14