Syllabus B.Sc. III , Affiliated to Sant Gadage Baba Amravti University ,Amravati , Topic -Organometallic Compound

Organometallic Compound
B.Sc. III
Affiliated to Sant Gadage Baba Amravti University ,Amravati

1. G.S.Tompe Arts Commerce and Science College,Chandur Bazar ORGANOMETALLIC COMPOUNDS Dr.Shrikrushan S. Ubarhande M.Sc. B.Ed.Ph.D . Assistant Professor Department of Chemistry

2. CONTENTS Introduction Reactivity Organomagnesium Compound Preparation of organomagnesium compound Applications of organomagnesium compound Orgnolithium compound Preparation of organolithium compound Application of organolithium compound References

3. Introduction Organic cmpounds that contain C-M bonds are called Organometallic compounds. Represented by R-M. The nature of C-M bond. Ionic character of C-M bond. More will be the electropositive character of metal, higher will be the ionic character between C-M bonds.

4. Organometallic compounds are defined as the compounds which contains direct carbon-metal bonds. Or Organometallic compounds are those compounds which contain at least one carbon-metal bond .     Carbon - Metal Bond or C-M bond M= Mg & Li Where, M = K, Na, Li, Ca, Mg, Al, Zn, Cd , Sn , Pb , Hg, etc. Nomenclature:

5. Nomenclature: Organometallic compounds are named by simply adding the name of the metal (M) to that of organic group, which may be alkyl, alkenyl or aryl. For examples: H 3 C Li CH3-MgBr Methyl Lithium Methyl magnesium bromide (G.R.) The carbon - metal bonds in an organometallic compounds are ionic and the extent of the ionic character of C-M bond depends on the electropositive nature of the metal (loss of electron easily).

6. Percent ionic character of some C-M bonds are in given table. C- M Bonds % Ionic Character 1. C-Li 60% 2. C-Mg 52% 3. C-Al 40% 4. C-Zn 38% 5. C-Pb 38% 6. C-Cd 32% 7. C-Sn 28% 8. C-Cu 28% 9. C-Hg 24%

7. Reactivity Reactivity of organometallic compound as nucleophile and as base depends mainly on electropositive character of metal. More will be the electropositve character higher will be the reactivity of compound. Organolithiun and organomagnesium compound have highest ionic character and behave as strong nucleophile.

8. Grignard Reagents: (R- MgX ) Victor Grignard discovered organomagnesium halides, i.e.,alkyl magnesium halides R- MgX called as organomagnesium halides or Grignard Reagents . Diethyl ether is an especially good solvent for the formation of Grignard reagents because ethers are non-acidic ( aprotic ). Water or alcohols would protonate and thus destroy the Grignard reagent , because the Grignard carbon is highly nucleophilic . This would form a hydrocarbon. The Grignard Reagents are highly reactive and used in the synthesis of alkanes , alkenes, alcohols, aldehydes , ketones and carboxylic acids. For his remarkable contribution to synthetic organic chemistry, Victor Grignard won Noble Prize in 1912 .

9. ORGANOMAGNESIUM COMPOUND OR GRIGNARD REAGENT French Chemist Victor Grignard

10. preparation of grignard reagent Dry ether

11. methyl bromide methyl magnesium bromide Order of Reactivity of Alkyl halides: R-I > R-Br > R- Cl Alkyl iodide is more reactive than the bromide, which are in turn more reactive than chloride. Alkyl bromide (intermediate property) is most suitable for the preparation of Grignard reagents

12. RMgX and RLi are also strong bases. Reaction with proton acids : Note- Grignard reagents are very reactive. In the presence of moisture, they react to give alkanes . Therefore, Grignard reagents should be prepared under anhydrous conditions .

13. Schlenk Equilibrium

14. THREE NECKED FLASK

15. Application of grignard reagent Electrophilic Substitution Reactions: Or Reactions with Active Hydrogens base acid acid Dry ether Preparation or Synthesis of Alkanes : When Grignard reagent (alkyl magnesium bromide) is treated with compounds containing active hydrogen , like, water (H-OH), alcohols (R-OH), ammonia (H-NH 2 ), carboxylic acids (R-COOH) & amines (RNH-H), etc; to form alkanes (hydrocarbon) corresponding to alkyl group ( R- ) of Grignard reagent ( R - MgX ).

16. Nucleophilic Substitution Reactions: Grignard reagent undergoes substitution reaction by the following general mechanism A-B = R-X, CH 2 =CH-Br, CH 2 =CH-CH 2 -Br, CH 3 CO-Cl e.g.

17. 2)Nucleophilic Addition Reaction:

18. 1. Reaction with Formaldehyde: Or Preparation of Primary (1o) alcohols: 2. Reaction with Acetaldehyde: Or Preparation of Secondary (2o) alcohols 3. Reactin with Acetone Or Preparation of tertiary (3 o ) alcohols: Or Reaction with Acetone: 4. Reaction with solid CO2 Or Preparation of Acetic acid: 5. Reaction with Cyanides: i . Reaction with Hydrogen cyanide: Or Preparation of Acetaldehyde: ii. Reaction with Methyl cyanide: Or Preparation of Acetone

19. 6. Reaction with epoxide . Reaction with Ethylene oxide: Or Preparation of n- propyl alcohol (Higher alcohol): 7. Other Reactions: Synthesis of Higher Alkynes: Or Preparation of 2-Butyne from Propyne :

20. 2) Organolithium Compounds (R-Li): Organolithium compounds are defined as the compounds which contains direct carbon-lithium bonds. CH 3 -Li CH3-CH2-Li CH3-CH2-CH2-Li Methyl lithium Ethyl lithium n- propyl lithium CH 3 -CH 2 -CH 2 -CH 2 -Li n-Butyl lithium C6H5-Li Phenyl lithium

21. Preparation or Synthesis: When alkyl halide (methyl bromide) is reacted with lithium metal in dry ether & intert atmosphere of nitrogen (Role of atmosphere Nitrogen is to displaces the air and keeps the inside of the container free of oxygen) in cold condition at -10oC (263K); to form alkyl lithium (methyl lithium).

22. Physical Properties: Organolithium compounds are colourless liquids or low melting solids, soluble in organic solvents. They are particularly sensitive towards air and moisture Organolithium compounds like Grignard reagents have a strongly polar covalent C-Li bond. Reactivity: Organolithium compounds behave in the same way as G.R., but with enhanced reactivity due to more % of ionic character . Since lithium is more electropositive than magnesium, the carbon-lithium bond is ionic to a greater extent. As a result, organolithium compounds are more reactive than Grignard reagents. The carbon atom is more electronegative than lithium. The bonding electron pair of the C-Li bond is more shifted towards the C-atom. As a result, the C-atom has a partial negative charge (δ-) and the lithium (Li) atom has a partial positive charge (δ+).  

23. A. Electrophilic Substitution Reactions: Or Reactions with Active Hydrogens 1) Preparation or Synthesis of Alkanes B. Nucleophilic Addition Reactions:

24. 1. Reaction with Formaldehyde: Or Preparation of Primary (1o) alcohols: 2. Reaction with Acetaldehyde: Or Preparation of Secondary (2o) alcohols 3. Reactin with Acetone Or Preparation of tertiary (3 o ) alcohols: Or Reaction with Acetone: 4. Reaction with solid CO2 Or Preparation of Acetic acid: 5. Reaction with Epoxide Reaction with Ethylene oxide: Or Preparation of n- propyl alcohol (Higher alcohol):

25. 3)Other application of grignard reagent

26. Organolithium compound n-butyllithium

27. Preparation of organolithium reagent Dry ether Method 1:

28. Method 2: Halogen - Metal Exchange CH 3 CH 2 CH 2 CH 2 -Li + CH 3 I CH 3 Li + CH 3 CH 2 CH 2 CH 2 I n-butyllithium methyllithium Dry ether

29. Organolithium Compounds (R-Li): Defination : Organolithium compounds are defined as the compounds which contains direct carbon-lithium bonds. C-Li bond   Or Organolithium compounds are those compounds which contain at least one carbon-lithium bond i.e. C-Li bond

30. Organolithium compounds are named by simply adding the name of the lithium (Li) to that of organic group, which may be alkyl, alkenyl or aryl. For example,   CH 3 -Li CH3-CH2-Li CH3-CH2-CH2-Li Methyl lithium Ethyl lithium n- propyl lithium CH 3 -CH 2 -CH 2 -CH 2 -Li n-Butyl lithium C6H5-Li Phenyl lithium

31. Preparation or Synthesis: When alkyl halide (methyl bromide) is reacted with lithium metal in dry ether & intert atmosphere of nitrogen (Role of atmosphere Nitrogen is to displaces the air and keeps the inside of the container free of oxygen) in cold condition at -10oC (263K); to form alkyl lithium (methyl lithium). R-X +2 Li dry Ether & N 2 R Li + Li-X cold condition -10o C alkyl lithium R = CH3, C2H5, C3H7, C4H9, C6H5, etc

32.   CH3Br + 2 Li dry Ether & N 2 cold condition CH 3 - Li+Li -Br methyl bromide   Physical Properties: Organolithium compounds are colourless liquids or low melting solids, soluble in organic solvents. They are particularly sensitive towards air and moisture.

33. Chemical Reaction of Methyl lithium (CH 3 -Li): Organolithium compounds like Grignard reagents have a strongly polar covalent C-Li bond. Reactivity: Organolithium compounds behave in the same way as G.R., but with enhanced reactivity due to more % of ionic character (C-Li = 43 %) than C-Mg bond (C-Mg = 35 %) Since lithium is more electropositive than magnesium, the carbon-lithium bond is ionic to a greater extent. As a result, organolithium compounds are more reactive than Grignard reagents. The carbon atom is more electronegative than lithium. The bonding electron pair of the C-Li bond is more shifted towards the C-atom. As a result, the C-atom has a partial negative charge (δ-) and the lithium (Li) atom has a partial positive charge (δ+). R Li Carbon - Lithium Bond  

34. Properties Organolithium compounds are colourless liquids or low melting solids, soluble in organic solvents. They are particularly sensitive towards air and moisture. Organolithium compounds like Grignard reagents have a strongly polar covalent carbon metal bond. They, therefore, resemble Grignard reagents in many of their reactions. Since lithium is more electropositive than magnesium, the carbon-lithium bond is ionic to a greater extent. As a result, organolithium compounds are more reactive than Grignard reagents.

35. Electrophilic Substitution Reactions: When alkyl-lithium is treated with compounds containing active hydrogen , like, water (H-OH), alcohols (R-OH), ammonia (H-NH2), carboxylic acids(R-COOH) & amines (RNH-H),etc; to form alkanes (hydrocarbon) corresponding to alkyl group ( R- ) of organolithium compound ( R -Li).

36. R-Li + H- A dry ether R – H + Li - A alkyl lithium a compound having Alkane acidic (active) hydrogen Where, A = OH, OR, RCOO, NH2, RNH, etc.

37. Reaction with Water: When methyl-lithium is reacted or treated with compounds containing active hydrogen , such as, water (H-OH); to form methane. CH3 – Li + H – OH dry ether CH 4 + Li – OH methyl-lithium Methane

38. Nucleophilic Addition Reactions Carbonyl compounds or other unsaturated compounds are attacked by G.R.; to form addition product, which on acid-hydrolysis yield alcohols (1o, 2o, 3o), acids, aldehyde , ketone , etc.

39. 1. Formation of Alcohols: a. Preparation of Primary (1o) alcohols b. Preparation of Secondary (2o) alcohols c. Preparation of tertiary (3o) alcohols 2. Reaction with solid CO2 (dry Ice): 3. Reaction with Epoxides

40. Synthetic Applications 1. Alkanes Organolithium compounds react with water to give alkane . For example 2. Alcohols Primary alcohols may be synthesized from alkyl lithium by its reaction with formaldehyde, secondary by reaction with aldehydes other than formaldehyde and tertiary by rection with ketones

41. Primary Alcohols

42. Preparation of Primary (1o) alcohols: When methyl lithium is reacted with formaldehyde (H-CHO); to form addition product, which on acid hydrolysis; to form ethyl alcohol (primary alcohol).

43. APPLICATIONS OF ORGRNOLITHIUM COMPOUND 1)Preparation of Alcohol from organolithium compound: R-CO-R + R-Li (R) 2 -C-OH R 1 ketone alcohol Dry ether

44. 2)In the synthesis of Gilman reagent Lithium diorganocopper reagents, known more commonly as Gilman reagents.

45. 3)Industrial use of butyllithium: Butyllithium is use in polymerisation of isoprene which leads to synthetic rubber. Synthetic rubber

46. A. Heterocyclics Heterocyclic compound, or heterocyclic, is cyclic compound in which one or more of the atoms of the ring are heteroatoms. A heteroatom is an atom other than carbon. The name comes from the Greek word heterose , meaning “different” variety of atoms, such as N, O, S, Se, P, Si, B and As can be incorporated into ring structures. Heterocycles make up an exceedingly important class of compound, more than half of known organic compounds are heterocycles . Almost all the compounds we know as drugs, most vitamins and many other natural products are heterocycles . (A natural product is a compound synthesized by a plant or an animal.) Alkaloids are natural products containing one or more nitrogen heteroatoms that are found in the leaves, bark, roots, or seeds of plants .In this chapter, however, we will study only nitrogen containing five membered pyrrole and six membered pyridine heterocycles .

47. Defination : Heterocyclic Compounds or Heterocycles : The cyclic compounds obtained by replacing one ormore carbon atom of a ring by heteroatom , like, N, O or S , are called as heterocyclic compounds. OR Heterocyclic compound, or heterocycles , is cyclic compound in which one or more of the atoms of the ring are heteroatoms . A heteroatom (such as N, O, S, etc.) is an atom other than carbon.

48. Nomenclature Pyrrole Furan Thiophene Pyridine The nitrogen containing five membered heterocycle’s names usually ends with –ole and that of six membered with – ine . The numbering start from hetero atom except isoquinolene .

49. PYRROLE ( Azacyclopenta -2, 4 – diene C 4 H 5 N) Pyrrole is an important heterocyclic compound having five- member ring containing nitrogen as the heteroatom. Many naturally occurring compounds like chlorophyll, alkaloids, hemoglobin etc. contain pyrrole nucleus. It occurs in coal tar (by product of coke and coal gas fromcoal ) and bone oil. Methods of Synthesis Pyrrole may be synthesized by passing acetylene mixed with ammonia through red hot tube

50. 2. From Succinimide Pyrrole can be synthesized by distilling succinimide with zinc dust 3. From Furan Pyrrole can be manufactured by passing a mixture of furan, ammonia and steam over heated alumina (catalyst).

51. Physical Properties It is a colorless liquid ( b.p 404K) sparingly soluble in water but readily soluble in alcohol and ether. It rapidly darkens on exposure to air and finally forms a resinous mass. Orbital Picture In pyrrole , each ring atom is in a state of sp 2 hybridization and joined to the adjacent ring atom by means of sigma bond formed by the sp 2 -sp 2 overlap. Also each ring atom is attached to the hydrogen atom by means of sigma bond formed by the sp 2 -s overlap. Thus there is formation of three carbon – carbon, two carbon – nitrogen, four carbon - hydrogen and one nitrogen – hydrogen sigma bonds. All these sigma bonds lie in one plane and are at 120 from each other.

52. Structure: According to molecular orbital theory (MOT); In pyrrole , all carbon and nitrogen undergoes sp2-hybridisation . All ring atoms in pyrrole ( Four carbon & one nitrogen) contains three sp2 H.O’s sp2 hybrid orbital of each atom is half-filled (singly filled). The unhybridised p-orbital (i.e., 2-pz) of each carbon atom is half-filled & that of nitrogen atom ( 2pz2) is full-filled having a lone pair of electrons (i.e., p-orbital on nitrogen contains two electrons). Pyrrole ring is planar due to sp2-hybridisation (All C, N & H atoms are in one plane). Formation of sigma (σ ) bonds M.O. diagram: The sp2 H.O’s overlap with each other and with s-atomic orbital of the five hydrogen forming three carbon – carbon, two carbon – nitrogen (by sp2-sp2 overlap) , four carbon - hydrogen and one nitrogen – hydrogen (by sp2-s overlap ) sigma bonds. So, total ten sigma (σ) bonds are formed. All these sigma bonds lie in one plane and are at 120 c from each other.

53. Formation of delocalized pi (π) bonds M.O. diagram: Each ring atom (C & N) in pyrrole also has an unhybridised p-orbital and these are perpendicular to the plane of sigma bonds and are parallel to each other. The p- orbitals on carbons contain one electron ( 2pz1) each and p-orbital on nitrogen contains two electrons , i.e , lone pair ( 2pz2 ). Lateral overlap of these five p- orbitals gives rise to the formation of delocalized π-molecular orbital (or π-electron clouds) above and below the plane of ring. This π-molecular orbital contains a total of six electrons , due to this pyrrole shows aromatic properties or aromaticity by satisfying Huckel’s (4n+2)π e-s, rule. Thus, pyrrole shows aromatic character (nature) as it contains aromatic sextet . This aromatic sextet opposes addition reaction and favours electrophilic substitution reaction

54. A common shorthand representation of pyrrole is as shown bellow-

55. Pyrrole as a Resonance Hybrid Pyrrole is the resonance hybrid of the following five resonating structures. However, structures I, II and III are the main contributing. Resonance hybrid structure is represented as shown below-

56. Basic Nature of Pyrrole Acidic Nature of Pyrrole

57. Orientation of Electrophonic Substitution

58. Electrophilic Substitution Reactions 1. Nitration 2. Sulphonation

59. 3. Halogenation (a) Chlorination ( i ) With sulphuryl chloride in ether at 273K pyrrole gives 2, 3, 4, 5 - tetrachloro pyrrole (ii) With chlorine, pentachoro pyrrole is the product

60. ( b ) Bromination With bromine in ethanol at 273K, pyrrole gives 2,3,4,5 –tetra bromo pyrrole (c) Iodination With iodine in aqueous potassium iodide, pyrrole gives 2,3,4,5 – tetraiodo pyrrole or iodole.

61. 4.Friedel –Craft Acylation With acetic anhydride at 523K, pyrrole undergoes acylation to give 2-acetyl pyrrole .No catalyst is required in this reaction.

62. PYRIDINE It occurs along with pyrrole in bone oil and in the light oil fraction (boiling point up to443K) of coal tar. Natural products like vitaminB 6 , nucleotides, coramine , isoniazid , etc. and alkaloids like nicotine, piperine , etc. contain pyridine ring in their molecules.

63. Preparation 1. From Acetylene By passing mixture of acetylene and hydrogen cyanide through red hot tube, pyridine is obtained.

64. 2. From Pentamethylene diamine hydrochloride By heating pentamethylene diamine hydrochloride, piperidine is obtained which on heating with conc. H 2 SO 4 at 573K or with nitrobenzene at 533K undergoes dehydrogenation to give pyridine

65. Pictorial Chemistry of Pyridine

66. Resonance in Pyridine

67. Basicity

68. Orientation of Electrophilic Substitution The nitrogen atom in pyridine,because of its electronegativity lowers the electron density around the ring carbon; and Electrophiles can coordinate with the lone pair of electrons on nitrogen to form resonance stabilizedpyridinium salts

69. Attack at C-3 position Attack at C-2 position Attack at C-4 position

70. Electrophilic Substitution Reactions Nitration Sulphonation

71. Orientation of Nucleophilic Substitution Attack at C-2 Attack at C-3

72. Attack at C - 4

73. Nucleophilic Substitution Reactions Reaction with Sodamide ( Chichbabin reaction) Reaction with Sodium hydroxide

74. Reaction with n-butyl lithium and Phenyl lithium

75. References 1) Advanced Organic Chemistry- By Jerry, March 2) Organic Chemistry- By Solomon 3) Organic Synthesis- By Jaya Singh & Jagdamba Singh 4) Modern Methods of Organic synthesis-By W. Carruthers . 5)Textbook of Organic synthesis-By R. O. C. Norman N. J. M. Coxon . 6) Organic chemistry- ByJ . Clayden .

76. Thank You….

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