Birch reduction.pptx

Named reactions -Birch reduction

1. welcome

2. Subject – Pharmaceutical Organic Chemistry- Topic: Birch Reduction Group No:12 MAYURI KOLTE – 35 SHASHANK KUMBHAR - 36 AKANKSHA KUNJIR – 37 Guided By- Prof. Jayshree Jagtap Mam Prof. Pooja Petkar Mam Seth Govind Raghunath Sable College Of Pharmacy,Saswad

3. Introduction History Reagents General reactions Mechanism Reaction kinetics Effect of substituent Catalytic hydrogenation Vs Birch reduction Applications Acknowledgement References Content

4. DEFINATION :- Reduction of aromatic rings with sodium(Na) , Potassium (K) or Lithium (Li) in liquid ammonia(NH3) or amine(RNH2) in presence of alcohol , addition of Hydrogen (H) take place at 1 & 4 positions to give unconjugated diene . This is known as Birch reduction. Reduction:- Addition of Hydrogen(H) to an organic molecule or gain of electron e- is regarded as reduction BIRCH REDUCTION

5. HISTORY In 1944 Arthur Birch, building on earlier work by Wooster and godfrey, developed the reaction while working in the Dyson Perrins Laboratory at university of oxford. Birch`s original procedure used sodium and ethanol Alfered L-wilds later discovered that lithium gives better yield

6. Reaction : -

7. Reagents :- Alkali metals :- Na , K or Li in liquid NH3 (ELECTRON SOURCE) Alcohol (Proton donor) Solvents :- Liquid ammonia (NH3) or Primary Amines (RNH2) Choice of solvent :- Benkar & Coworkers suggested that the use of primary amines as solvents because Amine have higher boiling point (Ethyl amine – 17 ˚C) than NH3 (-33˚C ) which Permits higher temperature for reactions . This reduces the yield of 1˚ reduced products. A photochemical Birch type reduction using OH ̄ (as electron source) in isopropyl Alcohol has recently been reported.

8. Working of Reagent:- Group 1 metal like Li or Na readily give up their single valance electron as thy dissolve in liq.NH 3 or amine. NH 3 + Na (NH3)……e ̄ (NH3) (deep blue solution) + Na⁺ These free electrons impart blue colour to the solution.

9. Reduction of conjugated dienes :- Reduction of arenes :-

10. Mechanism:- Transfer of one electron from metal to the benzene ring to produce a resonance stabilized anion radical ( Ia – Ib) in low concentration. Anion radical accepts a proton from the alcohol to form a radical ( II ). The strongly basic radical anion abstract a proton from alcohol to form radical The addition of an electron from the metal to the radical produce an anion ( III ) Anion ( III ) which takes up a proton from the metal alcohol to give the dihydro product

11. WHY 1 , 4 – DIHYDRO & NOT 1 , 2 – DIHYDRO PRODUCT IS FORMED ? In ( Ib ) , the steric repulsion between the anionic & radical centers is minimum , therefore , adds a proton to give ( II ) & subsequently a 1 , 4 – dihydro product & not 1 , 2 – dihydro product is formed .

12. Rate of reaction :- Krapcho and Buthnerby on the basis of a kinetic study , found that the reaction was consistent with a THIRD ORDER rate law , being single order in the concentration of substrate , alkali metal and alcohol . The rate limiting step is :- Protonation of radical atom

13. When Ammonia(NH 3 ) becomes proton source … At higher temperatures (50°C - 120°C) , ammonia becomes the proton source and Alcohol need not be used . The amide ion thus formed is a strong base and isomerises the 1 , 2 – dihydro Product.

14. Reduction of 1 , 2 – dihydro product :- The 1 , 2 – dihydro product has a conjugated double bond & hence undergoes Further reduction to form a tetrahydro derivates . Cyclohexone has a single olefinic bond which is unaffected by the reagent .

15. Effect of substituents :- For monosubstituted benzenes , the reduction is highly regioselective and this Is controlled by the polar nature of the substituent . If electron withdrawing substituent (E W S) is attached : The E W S withdraws electron from the aromatic ring and decreases electron cloud on Benzene ring. This stabilize the negative (-) charge of anion radical. This makes the ring more electron-accepting and hence the reaction is facilitated .

16. If electron donating substituent (EDS) is attached : The EDS donates electron to the aromatic ring and increases electron cloud at Benzene ring. This EDS destabilize the negative (-) charge of anion radical.

17. With substituted benzene , the electron – donating group remains on unsaturated Carbon and the electron withdrawing group remains on the saturated carbon in the products.

18. Note :- I) The presence of bulky groups on the aromatic ring retards the reduction , probably because of steric interference to solvation of radical anion. II) Phenols are not reduced presumably they react with metal. More elaborate studies have revealed that , Phenol may be reduced by increasing the concentration of the metal four fold.

19. Catalytic hydrogenation Birch reduction Difficult process Easy process Requires reagent in large concentration Small concentration of reagent is sufficient for reaction to be carried out Aromatic ring is reduced totally. Aromatic ring is not reduced totally and reaction steps at diene stage

20. Catalytic hydrogenation :-

21. Applications :- I)Synthesis of cyclohexenones Reduction of Methoxybenzene is carried out via hydrolysis of ether

22. II) Birch reduction can partially reduce C≡C of internal alkyene to form alkene

23. III) Synthesis of 3-oxocyclohexene Reduction of m- anisic acid which earlier gave 3-oxocyclohexane carboxylic acid can be controlled to yield 3-oxocyclohexene carboxylic acid in good yield.

24. IV) Synthesis of Non-conjugated hydrocarbons From Naphthalene B) From Anthracene

25. C) From Phenanthrene Here rings are not completely reduced

26. Acknowledgement I thank Prof. Jayashree jagtap mam for explaining us the reaction very well. I thank Prof. Pooja petkar mam for providing us information.

27. References:- 1) Prof . J. R. Jagtap , Name reaction of synthetic importance , sgrs college of pharmacy, saswad 2) Stanley H Pine (2007) , Organic Chemistry , New Delhi , Tata McGraw-Hill publishing company Ltd , Pg no. 110 3) P.S.Kalsi (2010) , Organic Reaction and Their Mechanisms , New Delhi , New Age International Ltd , Pg no. 514-515 4) Morrison , Boyd and S.K.Bhattacharjee (2011) , Organic Chemistry , South Asia , Dorling Kindersley (India) Pvt Ltd , Pg no. 1003-1005

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