Reactions intermediate

carbocation, carboanions and other reaction intermediate were mentioned

1. Generation and fate of reaction intermediates UNDER THE GUIDENCE OF: Prof.Dr. K.MADHAVI M.Pharm.,Ph.D Dept.of Pharmaceutical chemistry PRESENTED BY: S.SAISWATHIVARMA M PHARMACY (2015MPH40005) Dept.of pharmaceutical chemistry SPMVV - TIRUPATHI

2. CONTENTS Introduction of reactions intermediate Generations and fate of reactions and their stability of Carbocations Carboanions Carbenes Free Radicals Nitrenes Nitrenium ions References

3. A reaction intermediate or an intermediate molecular entity (atom, ion, molecule..) with a lifetime appreciably longer than a molecular vibration that is formed (directly or in directly) from the reactants and reacts further to give (either directly or indirectly) the products of a chemical reaction. Main carbon reactive intermediates: Carbocations and their stabilized equivalents such as oxonium ions. Carbanions and their stabilized equivalents such as enolates. Carbenes Free radicals Nitrenes and Nitrenium ions. REACTION INTERMEDIATES

4. CARBOCATIONS A carbocation is an ion with a positively-charged carbon atom. Carbenium ion (trivalent positive species) CH 3 , C 2 H 5 Carbonium ion ( pentavalent positive species,non classical carbocation) Carbocation

5. CARBOCATION STRUCTURES : PLANAR SP 2 HYBRID

6. By the addition of a cation to a neutral molecule.

7. V. By the heterolytic fission of a C-heteroatom bond.

8. Vi. By the heterolysis fission of a C- heteroatom bond to form onium salt Cl, Br and I Onium salt Carbocation

9. By the heterolysis of alkyl diazonium salt By the removal of an electron from a neutral molecule or a free radical

10. Reactions of carbocation R + cation act as an electrophile to react with nucleophiles. For Eg; R + + Nu - R Nu Some carbocations act as Bronsted acid to lose a proton . For Eg;

11. 1º or 2º-carbocation often undergo Wagner – Meerwein rearrangement by an anionotropic 1,2- shift of a hydride or an alkyl anion. For Eg;

12. Internal alkylation of a C=C bond sometimes may take place with a carbocation. Fragmentation of carbon chain of carbocation is also known. Carbocations can be reduced to a free radical or carboanions by cathodic reduction.

13. STABILITY In solution, the carbocation may be free (this is more likely in polar solvents, in which it is solvated) or it may exist as an ion pair. Ion pairs are more likely in nonpolar solvents. A. Simple alkyl carbocations Stability: tertiary > secondary > primary

14. The most stable of all alkyl cations is the tert-butyl cation. Methane, ethane , and propane , treated with superacid, also yield tert-butyl cation as the main product. The Field Effect : The electron-donating effect of alkyl groups increases the electron density at the charge-bearing carbon, reducing the net charge on the carbon, and in effect spreading the charge over the α carbons. < < < +I effect

15. Hyperconjugation : Tertiary carbocations are more stable (and form more readily) than secondary carbocations; primary carbocations are highly unstable because, while ionized higher order carbons are stabilized by hyperconjugation, unsubstituted (primary) carbons are not. NONE Three for 3C-H bonds Six for 6C-H bonds Nine for 9C-H bonds

16. +R groups stabilize the carbocations. For eg; Some carbocations are stabilized due to aromatization. For eg; Cyclopropenyl cation is stable due to aromatization

17. STABLE ALLYLIC-TYPE CATIONS Allyl cation and benzyl cation are more stable than most othercarbocations. Molecules which can form allyl or benzyl carbocations are especially reactive. Stable allylic cations have been obtained by the reaction between alkylhalides,alcohols, or alkenes (by hydride extraction) and SbF 5 in SO 2 or SO 2 ClF .

18. Detection of carbocation : Formation of carbocation can be detected by NMR spectroscopy as the cation formation shifts the proton signals appreciably downfield due to deshielding of protons. For example;

19. CARBANION A carbanion is an anion in which carbon has an unshared pair of electrons and bears a negative charge usually with three substituents for a total of eight valence electrons. Formally a carbanion is the conjugate base of a carbon acid. Stable carbanions do however exist although in most cases they are reactive. Olmstead(1984):

20. THE GENERATION AND FATE OF REACTIONS CARBANIONS A group attached to a carbon leaves without its electron pair. A negative ion adds to a carbon-carbon double or triple bond.

21. Strongly reduced metals like Na or Li or Mg can convert alkyl halide to alkyl sodium or alkyl lithium or Grignard reagent. Reactions of carboanions Carboanions often act as a nucleophiles to react with electrophones species.

22. In rare cases ,the carboanions may undergo cationotropic 1,2-shift to give rearranged products, for eg; The carboanions may be oxidized to free radicals. For eg;

23. STABILITY AND STRUCTURE The stability of the carbanion is directly related to the strength of the conjugate acid. The weaker is the acid, the greater is the base strength and the lower is the stability of the carbanion. Stabilization by sulfur or phosphorus.

24. Field effect: Ylides are more stable than the corresponding simple carbanions. Carbanions are stabilized by a field effect if there is any heteroatom (O, N or S) connected to the carbanionic carbon, provided that the hetero atom bears a positive charge in at least one important canonical form .

25. RELATIVELY STABLE CARBANIONS WITH CERTAIN STRUCTURAL FEATURES

26. Factors determining the stability and reactivity of a carbanion: The inductive effect : Electronegative atoms adjacent to the charge will stabilize the charge; Hybridization of the charge-bearing atom. The greater the sp3character of the charge-bearing atom, the more stable the anion; The extent of conjugation of the anion . Resonance effects can stabilize the anion. This is especially true when the anion is stabilized as a result of aromaticity

27. Detection Carboanions are type of CH 3 - , where the negative charge is not delocalized, due to conjugation can be distinguished from their 1 H NMR spectra. Formation of carbanions can be detected by the UV and Visible spectra of the different from the starting compounds. Trapping of carboanions with an electrophile may also show their formation of reaction.

28. CARBENES A carbene is a highly reactive species containing a carbon atom with six valence electrons and having the general formula RR′C :, practically all having lifetimes considerably under 1 sec. Structure and bonding the total spin=0 Bond angle=102º For single Methylene the total spin=1 Paramagnetic ,may be observed by electron spin resonance spectroscopy

29. Triplet carbenes are generally stable in the gaseous state, while singlet carbenes occur more often in aqueous media. The C-atom in singlet carbene is sp 2 - hybridized in which the spin –paired electrons occupy an sp 2 orbital. Shape of carbenes is planar.

30. GENERATION AND FATE OF REACTIONS OF CARBENES Disintegration of diazoalkanes and their analogs, via photolytic,thermal , or transition metal ( Rh , Cu)-catalyzed routes.

31. Base-induced elimination Carbenes are intermediates in the Wolff rearrangement.

32. Carbenes being the electrons-deficient species may take part in electrophilic aromatic substitution reactions. For eg; in Riemer –Tiemann reaction.

33. Detection Electron paramagnetic resonance spectroscopy(EPRS) can be used to detect the formation of the triplet carbenes. Rotational fine structure of the UV and Visible spectra can detect the formation of the singlet (bent form) or triplet (linear form) carbenes. π- Insertion reaction also can detect and distinguish the formation of singlet and triplet.

34. FREE RADICAL A free radical may be defined as a species that contains one or more unpaired electrons. Radicals play an important role in combustion, atmospheric chemistry, polymerization, plasma chemistry, biochemistry, and many other chemical processes, including human physiology. The first organic free radical identified was triphenylmethyl radical, by Moses Gomberg (the founder of radical chemistry) in 1900.

35. A carbon -based free radical is a trivalent C-species having single p-electron in the valence shell. It has tetrahedral geometry where the C-atom is sp 3 hybridized.

36. GENERATION AND FATE OF REACTIONS OF FREE RADICALS Thermolysis or photolysis of organic peroxides and azo compounds generates free radicals. Alkyl peroxide Alkoxy radical Azo Alkyl nitrile

37. Bimolecular redox reactions also generate free radicals. For eg; Stabilization of free radicals: Resonance effect due to conjugation stabilizes the free radicals.

38. For aromatic Benzyl radical

39. Reactions of free radical Free radical often take part in radical-propagating reactions.

40. In some cases, the free radical itself may be fragmented and trigger the propagation of a chain reaction. For eg; Suitably substituted free radical may isomerize.for eg;

41. Disproportion and radical coupling are the common reactions of the termination of free radicals. For eg; DISPROPORTIONATION RADICAL COUPLING

42. DETECTION OF FREE RADICALS By using electron spin resonance (ESR) Highly stabilized free radicals may be detected UV and visible spectroscopy. NMR spectroscopy can detect free radicals (by studying the overhauser effect ). The unpaired electron in a free radical is accommodated in a single occupied molecular orbital(SOMO). If the SOMO is a high-energy orbital, the free radical shows a tendency to loose an electron. If the SOMO is a low-energy orbital ,it shows a tendency to accept an electron.

43. NITRENES These are neutral reaction intermediates where the central nitrogen atom is electron-deficient and has a sextet of electrons. Sp 2 hybridized and planar in shape.

44. GENERATIONS OF NITRENES Like carbenes , these are also generated by protolytic ,thermal, or base –catalysed α -elimination reactions. For eg; Acyl azides Acyl nitrene Alkyl azides Alkyl nitrene

45. By reduction of nitro compounds with trialkyl phosphites. N- bromo amides Acyl nitrene

46. REACTIONS OF NITRENES Singlet nitrene undergoes a σ -insertion to give 2 - amines. For eg; 2 -Amine

47. π - Insertion of nitrenes into a C=C bond gives aziridines. Acyl nitrenes undergo skeletal rearrangement to give alkyl isocyanates.this rearrangements is involved in Crutius and Hoffmann rearrangements. Singlet Nitrene Acyl nitrene Alkyl isocyanate

48. Detection of nitrenes Triplet nitrenes can be detected and distinguished from singlet nitrenes, like carbenes ,by EPR.

49. Nitrenium ion A  nitrenium ion  (also called:  aminylium ion ) on nitrogen  with both an electron lone pair and a positive charge and with two substituents (R 2 N + ). Sp 3 hybridized and non linear or bent structure.

50. Stability Type of ligand attached; example; if aromatic ring is attached to the nitrogen they are called arylnitrenium ions and similarly halo and alkyl nitrenium ions if halo and alkyl groups are attached to the nitrogen. Arylnitrenium ions are characterised by delocalization of the positive charge on nitrogen into the aromatic ring making the ring susceptible to nucleophilic attack.

51. Nitrogen is doubly bonded to one atom hence they are called vinylidene nitrenium ions.

52. Generation and Fate of Reactions N-alkyl anthranilium on photolysis produced primary nitrenium ion by protonation of the nitrogen followed by heterolysis of N-O bond.

53. Photolysis of azides to form nitrene followed by protonation in presence of weakly acidic solution gave nitrenium ions. Electrochemical oxidation of amines yields nitrenium ions. But principle disadvantage of this method is formation of polymers.

54. N- aminopyridinium ions when subjected to heat in a nonpolar medium formed nitrenium ions in a nonpolar media.

55. REFERENCES Advanced Organic Chemistry By Bhal And Bhal. Textbook Of Organic Chemistry By O.P.Agrwal. Organic Chemistry By R.T.Morrison And R.N.Boyed

56. Thank you