FREE RADICALS , CARBENES AND NITRENES.pptx

ORGANIC CHEMISTRY

1. FREE RADICALS , CARBENES AND NITRENES TENZIN PALMO PHARMACEUTICAL CHEMISTRY (1 ST M.pharm ) AL-AMEEN COLLEGE OF PHARMACY

2. CONTENT INTRODUCTION TYPES FORMATION STABILTY SYNTHETIC APPLICATION

3. FREE RADICALS When a covalent bond undergoes homolytic fission , the two departing atoms takes one electron of the bonding pair of electrons . The two fragments produced carry an odd electron each and are called free radicals. Example:

4. CHARACTERISTIC OF FREE RADICALS Odd electron . Eg Methyl free radicals Incomplete octet / electron deficient / electrophile. Unstable, short lived and highly reactive , Shape / hybridisation : sp2 Paramagnetic is nature .

5. TYPES OF FREE RADICALS There are three types of free radicals based on the number of alkyl group attached to the carbon carrying odd electron: Primary Secondary Tertiary

6. FORMATION OF FREE RADICALS The homolytic cleavage of covalent bond generates a pair of free radicals. Following are the some of the method used for the generation of free radicals. 1. Photolysis : In this method , a suitable organic molecule is subjected to UV radiation . The organic molecule absorbs radiation in the ultra violet or visible range. The cleavage of molecule by two parts by using UV light is known as photolysis. Example :

7. 2. Thermolysis : In this method , the organic substances is heated at suitable temperature . Example: 3. From radicals : Radicals molecule may undergo decomposition to form another free radical. Example:

8. STABILITY OF FREE RADICALS 1. Resonance > hyperconjugation > inductive. 2. Tertiary > secondary > primary > methyl free radical.

9. 3. Allylic free radical < benzylic free radicals

10. SYNTHETIC APPLICATION 1. Sandmayer reaction : It is used to synthesize aryl halides from aryl diazonium salts using copper salts as reagents or catalysts.

11. 2. Gomberg reaction : It is an aryl-aryl coupling reaction via diazonium salt .

12. CARBENE The carbenes are highly reactive species having a life time less than one second. These are defined as neutral divalent carbon species in which the carbon has six electron in its valance orbital –four covalent bonded electron and two non-bonded electron. methylene

13. CHARACTERISTIC OF CARBENE It has six electrons . Neutral species . Incomplete octet Electron deficifient species . Electrophile Highly unstable , very short-lived . Example :

14. TYPES OF CARBENE : Singlet carbene : The non bonded electrons are paired and are in antiparallel spin. Shape –trigonal. Diamagnetic (no magnetic moment ) Violates hund’s rule. Hot methylene. Spin state = I 2 sI+ 1 =1 sp2 hybridised.

15. 2. triplet carbene (Diradical) : The non bonding electron in the triplet carbene occupy different orbitals and has opposite spin. Shape – linear. Paramagnetic (magnetic moment ). It follows hund’s rule . Cold methylene. Spin state : I 2 sI+ 1 = 3 sp hybridised.

16. FORMATION OF CARBENE 1. From aliphatic diazo compounds : Aliphatic diazo compounds can be decomposed either photolytically or thermally to generate carbene.

17. 2. From ketenes : Ketenes can be decomposed thermally or photolytically to generate carbene. 3. From epoxide : Photolytic decomposition of epoxide generates carbenes.

18. 4. From diazirines : Decomposition of diazirines generates carbene.

19. STABILITY OF CARBENE Triplet carbene are more stable than singlet carbene because with two electrons in different orbitals there is less electrostatic repulsion than both are in same orbital. In halogen substituted carbene , the singlet carbene are more stable than triplet carbene due to overlap of their lone pair of electron to the empty p orbital thereby stabilising the singlet carbene. The order of stability of halogen substituted carbene : CF2 is more stable than CCl2 which is more stable than CBr2. This is due to bond length which provides more extensive lateral overlap of the respective p orbital .

20. SYNTHETIC APPLICATION Carbylamine reaction: It is a synthesis of an isocyanide by the reaction of a primary amine , chloroform and base . The conversion involves the intermediary of dichlorocarbene.

21. 2. Reimer- Tiemann reaction : It is a chemical reaction used for the ortho formylation of phenol.

22. NITRENE Nitrene is the nitrogen analogue of carbene ,these are defined as neutral monovalent nitrogen species in which the nitrogen has 6 electrons in its valence orbital – 2 covalent bonded and four non bonded electrons.

23. CHARACTERISTIC OF NITRENE It has 6 electrons and are electron deficient. They are regarded as electrophile or lewis acid. They are short lived and highly reactive due to their strong tendency to accept two more electron s in order to complete its octet.

24. TYPES OF NITRENE 1. Singlet nitrene : In singlet nitrene , the four non bonding electrons on nitrogen are present as two pairs having antiparallel spin and has no magnetic moment (diamagnetic). They are sp2 hybridised .

25. 2. Triplet nitrene : In triplet nitrene , the four non bonding electrons are present on nitrogen in form of one lone pair and two unpair electron having parallel spin and has magnetic moment (paramagnetic). They are sp hybridised .

26. FORMATION OF NITRENE 1 . From elimination : Example :

27. 2. From azides : Thermal or photolysis decomposition of azides gives nitrene. 3. From sulfinylamine : Pyrolysis of sulfinylamines generates nitrene.

28. STABILITY OF NITRENE Nitrenes are very reactive species and cannot be isolated .however , a nitrene has been trapped by its reaction with carbon monoxide to isocyanate. Nitrenes can also be trapped in presence of ethylene.

29. SYNTHETIC APLLICATION 1. Hofmann rearrangement : When an amide is treated with bromine in alkali solution , it is converted to a primary amine that has one carbon atom less than the starting amide . This reaction is known as Hofmann degradation/ Hofmann rearrangement .

30. 2. Schmidt reaction : It is an organic reaction in which an azide react with a carboxyl derivatives , usually an aldehyde or ketone or carboxylic acid under acidic condition gives an amine or amide , with expulsion of nitrogen.

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