Homogenous catalysis

Catalysis

1. HOMOGENOUS CATALYSIS Presented by, D.Keerthana - M.pharm 1st yr, Dept.of pharmaceuticalchemistry

2. INTRODUCTION Catalysis is the ability of some species to rapidly speed up the rate at which a chemical reaction proceeds. For historical reasons, the discipline is normally split into two sub-categories; homogeneous (homo = same, geneous = phase) and heterogeneous (hetero = different). Homogeneous catalysis is concerned with catalysts that are in the same phase as the chemical reactions they are speeding up. These reactions are normally in the liquid phase and include all of biology's enzymes.  While the majority of homogeneous catalysis is in the liquid phase there are gas phase and solid phase homogeneous catalytic reactions.

3. HOMOGENOUS CATALYSIS: It refers to catalytic reactions where the catalyst is in the same phase as the reactants. It applies to the reactions in the gas and liquids phase and even in solids. In homogeneous catalysis, all the reactants and catalysts are present in a single fluid phase and usually in the liquid phase. GENERAL FEATURE: Liquid phase reactions dominate the field. Industrially less relevant; but complex organic or asymmetric transformations possible.

4. Reaction conditions milder than required for heterogeneous reactions (-78 °C - ~200 °C). Investigation of reactions by spectroscopic methods (NMR, MS, IR, UV-Vis) directly in solution possible. Fine-tuning of catalyst properties using different ligands/additives easy possible. Major challenge: Separation of products and catalysts/additives ADVANTAGES : In many reactions, homogeneous catalysts are more active and/or selective compared to heterogeneous catalysts. In homogeneous catalysis, the catalysts are molecularly dispersed within the fluid. Hence, pore diffusion limitations are absent. However, bulk phase mass transfer limitation may occurs. Catalytic chemistry and mechanism for homogeneous catalysis are better studied and understood. Therefore, it is easier to control and manipulate the process parameters.

5. EXAMPLES OF HOMOGENOUS CATALYSIS Many of the homogeneous catalysed reactions have been studied in both gas and liquid phases and some of the common examples in gas phase are as follows. In the lead chamber process during the manufacture of sulphuric acid, the presence of nitric oxide gas helps in catalysing the oxidation of sulphur dioxide. During the decomposition of acetaldehyde, the catalysis is carried out by iodine vapours. The presence of nitric oxide as catalyst during the combination of carbon monoxide and oxygen also clarifies the homogeneous catalysis.

6. HYDROGENETION CATALYSIS Hydrogenation – meaning, to treat with hydrogen – is a chemical reaction between molecular hydrogen and another compound or element, usually in the presence of a catalyst such as nickel, palladium or platinum. The process is commonly employed to reduce or saturate organic compounds. Hydrogenation reduces double and triple bonds in hydrocarbons. The Hydrogenation of alkenes to alkanes at low pressure (1-4 atm) and moderate temperature (0-100 C) contain nobel metals such as platinum, palladium or rhodium.

7. Example , Hydrogenation of alkenes is an exothermic reaction. Mostly Hydrogenation reactions are having high free energies of activation.

8. MECHANISM: Steps in the hydrogenation of a C=C double bond at a catalyst surface, for example Ni or Pt : (1) The reactants are adsorbed on the catalyst surface and Hydrogen dissociates. (2) An H atom bonds to one C atom. The other C atom is still attached to the surface. (3) A second C atom bonds to an H atom. The molecule leaves the surface.

9. Catalyst used in catalytic hydrogenation reaction are following Palladium Adam's Catalyst Raney Nickel Copper Chromite Transfer Hydrogenation Rhodium Ruthenium Triethylamine

10. ADVANTAGES: 1. Relatively high specificity 2. Relatively low reaction temperatures 3. Far more easily studied from chemical & mechanistic aspects 4. Far more active 5. Generally Far more selective for single product. DISADVANTAGES: 1. More difficult for achieving product/ catalyst separations. APPLICATIONS: Applications of Hydrogenation , Requires metal catalyst (Pd, Pt, or Ni). Used for converting polyunsaturated oils into Margarine

11. HYDROFORMYLATION Hydroformylation, also known as oxo synthesis or oxo process, is an industrial process for the production of aldehydes from alkenes. This chemical reaction entails the net addition of a formyl group (CHO) and a hydrogen atom to a carbon-carbon double bond(alkenes)

12. MECHANISM step 1-Mechanism of cobalt- catalyzed hydroformylation. The process begins with dissociation of CO from cobalt tetracarbonyl hydride to give the 16-electron species. step 2-Subsequent binding of alkene gives an 18e species. step 3- The olefin inserts to give the 16e alkyl tricarbonyl. step 4-Coordination of another equivalent of CO give alkyl tetracarbonyl . step 5-Migratory insertion of CO gives the 16e acyl . step 6- oxidative addition of hydrogen gives a dihydrido complex, step 7-this dihydrido complex releases aldehyde by reductive elimination. step 8- is unproductive and reversible.

13. HYDROCYANATION Hydrocyanation is, the process whereby H+ and –CN ions are added to a molecular substrate. The substrate is an alkene and the product is a nitrile. Cyanide is both a good σ–donor and π– acceptor its presence accelerates the rate of substitution of ligands. A key step in hydrocyanation is the oxidative addition of hydrogen cyanide to low–valent metal complexes. 

14. MECHANISM  Hydrocyanation is commonly performed on alkenes catalyzed by nickel complexes of phosphite (P(OR)3) ligands. The reaction proceeds via the oxidative addition of HCN to Ni(0) to give a hydridonickel(II) cyanide complex, abbreviated Ni(H)(CN)L2. Subsequent binding of the alkene gives the intermediate Ni(H)(CN)L(alkene), which then undergoes migratory insertion to give an alkylnickel(II) cyanide Ni(R)(CN)L2. The cycle is completed by the reductive elimination of the nitrile. 

15. APPLICATIONS: Hydrocyanation is important due to the versatility of alkyl nitriles (RCN), which are important intermediates for the syntheses of amides, amines, carboxylic acids, and esters. The most important industrial application is the nickel- catalyzed synthesis of adiponitrile (NC–(CH2)4–CN) synthesis from 1,3–butadiene (CH2=CH–CH=CH2). Adiponitrile is a precursor to hexamethylenediamine (H2N–(CH2)6–NH2), which is used for the production of certain kinds of Nylon

16. WILKINSON’S CATALYST Wilkinson's catalyst, is the common name for chlorido-tris(triphenylphosphane)rhodium(I), a coordination complex of rhodium with the formula RhCl(PPh3)3 (Ph = phenyl). It is a red-brown colored solid that is soluble in hydrocarbon solvents such as benzene, and more so in tetrahydrofuran or chlorinated solvents such as dichloromethane. The compound is widely used as a catalyst for hydrogenation of alkenes. It is named after chemist and Nobel Laureate, Sir Geoffrey Wilkinson, who first popularized its use.

17. Wilkinson's catalyst is usually obtained by treating rhodium(III) chloride hydrate with an excess of triphenylphosphine in refluxing ethanol. RhCl3(H2O)3 + 4 PPh3 → RhCl(PPh3)3 + OPPh3 + 2 HCl + 2 H2O USES: It is used in the selective hydrogenation of alkenes and alkynes without affecting the functional groups like: C=O, CN, NO2, Aryl, CO2R etc. Wilkinson catalysts Rh+ PH PH PH Cl-

18. REFERENCE  https://en.m.wikipedia.org/wiki/Homogen eous_catalysis . https://chemistry.tutorvista.com/inorganic -chemistry/homogeneous- catalyst.htmlview=simple https://en.m.wikipedia.org/wiki/Hydrofor mylation https://en.m.wikipedia.org/wiki/Hydrocya nation Green Chemistry and Catalysis”, Roger A. Sheldon, Isabel Arends, Ulf Hanefold, WILEY-VCH Verlag GmbH & Co. KGaA, Weinhein, Germany; Pg. no. 106, 223-244, 250, 304, 310, 314. “Multistep Continuous-flow Synthesis of (R)- and (S)-Rolipram using Heterogeneous Catalyst”, Tetsu Tsubogo, Hidekazu Oyamada & Shu Kobayashin. (DOI: 10.1038/nature14343). 

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