Homogeneous catalysis 2021

Advanced organic chemistry

1. Homogeneous Catalysis Presented by Madhura Datar 1 st Mpharm Pharmaceutical chemistry Government college of Pharmacy,Bengaluru.

2. contents Homogeneous catalysis Hydrogenation Hydroformylation Hydrocyanation Wilkinson catalysts

3. Homogeneous catalysis H omogeneous catalysis is catalysis in a solution by a soluble catalyst. The catalyst and reactant are in the same phase. A great variety of dissolved homogeneous catalysts are known: Bronsted and Lewis acids and bases, metal complexes, metal ions, organometallic complexes, organic molecules, enzymes, artificial enzymes. H eterogeneous catalysis describes processes where the catalysts and substrate are in distinct phases, typically solid-gas, respectively . The term is used almost exclusively to describe solutions and implies catalysis by organometallic compounds.

4. General features Industrially less relevant; but complex organic or asymmetric transformations possible! 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. different ligands/additives easy possible.

5. 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 un d ersto o d . Theref o re, i t is easie r t o co n t 5 ro l and manipulate the process parameters.

6. Disadvantages The separation of homogeneous catalysts from products can be challenging. In some cases involving high activity catalysts, the catalyst is not removed from the product. In other cases, organic products are sufficiently volatile than they can be separated by distillation. Homogeneous catalyst have limited thermal stability compared to heterogeneous catalysts.

7. Acid catalysis: The proton is a pervasive homogeneous catalyst because water is the most common solvent. Water forms protons by the process of self-ionization of water. In an illustrative case, acids accelerate (catalyze) the hydrolysis of esters . CH 3 CO 2 CH 3 + H 2 O ⇌ CH 3 CO 2 H + CH 3 OH At neutral pH, aqueous solutions of most esters do not hydrolyze at practical rates. the lead chamber process during the manufacture of sulphuric acid, the presence of nitric oxide gas helps in catalyzing the oxidation of Sulphur dioxide.

8. During the decomposition of acetaldehyde, the catalysis is carried out by iodine. The presence of nitric oxide as catalyst during the combination of carbon monoxide and oxygen also clarifies the homogeneous catalysis.

9. Hydrogenation Hydrogenation is a chemical reaction between molecular hydrogen (H2 ) 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 typically constitutes the addition of pairs of hydrogen atoms to a molecule, often an alkene. Catalysts are required for the reaction to be usable, non catalytic hydrogenation takes place only at higher temperature. Hydrogenation reduces double and triple bonds in hydrocarbons.

10. Hydrogenation of alkenes Hydrogenation of alkenes is an exothermic reaction Hydrogenation reaction are having high free energy of activation. 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 .

11. MECHANISM Steps in the hydrogenation of a C=C double bond at a catalyst surface, for example Ni or Pt The reactants are absorbed on the catalyst surface and hydrogen disassociates. An h atom to on C atom. The other C atom is still attached to the surface. A second c atom bonds to an h atom. The molecule leaves the surface.

12. HYDROGEN SOURCE For hydrogenation, the source of hydrogen is H2 gas itself, which is typically available commercially within the storage medium of a pressurized cylinder. The hydrogenation process of H2 , usually conveyed from the cylinders and sometimes augmented by "booster pumps". TRANSFER HYDROGENATION : hydrogen is transferred from donor molecules formic acid, isopropanol, and dihydroanthracene. These hydrogen donors undergo dehydrogenation to, respectively, carbon dioxide, acetone, and anthracene. These processes are called transfer hydrogenations.

13. Substrates An important characteristic of alkene and alkyne hydrogenations, both the homogeneously and heterogeneously catalyzed versions, is that hydrogen addition occurs with "syn addition", with hydrogen entering from the least hindered side.

14. Inorganic substrate The hydrogenation of nitrogen to give ammonia is conducted on a vast scale by the Haber-Bosch process, consuming an estimated 1% of the world’s energy supply.

15. Catalysts HOMOGENEOUS CATALYSTS USED FOR HYDROGENATION Ruthenium Iridium (crab tree’s catalyst) Wilkinson’s catalyst HETEROGENEOUS CATALYST USED FOR HYDROGENATION Lindlar’s catalyst Raney Nickel Transfer hydrogenation

16. Palladium : active form of palladium is palladium chloride. More commonly the palladium chloride reduce in presence of charcoal or any other solid support on which metal is deposited in a very finely divided state.

17. Adam’s catalyst : Chloroplatinic acid is fused with sodium nitrate to give brown platinum oxide which can be stored. When required ,it is treated with hydrogen to give a very finely divided black suspension of the metal.

18. Applications Food industry : The food industry hydrogenates vegetable oils to convert them into solid or semi solid fats that can be used in spreads, butter etc. Vegetable oils are made from poly unsaturated hydrocarbons upon hydrogenation eliminates some of these double bonds.

19. Petrochemical industry : In petrochemical processes, hydrogenation is used to convert alkenes and aromatics into saturated alkanes and cycloalkanes , which are less toxic and less reactive. Organic chemistry : hydrogenation is a useful method of converting non saturated compound to saturated one. alkenes and alkynes, but also aldehydes, imines, and nitriles, which are converted into the corresponding saturated compounds, i.e. alcohols and amines.

20. 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. It is important because aldehydes are easily converted into many secondary products. the resulting aldehydes are hydrogenated to alcohols that are converted to detergents. Hydroformylation is also used in speciality chemicals, relevant to the organic synthesis of fragrances and drugs.

21. This chemical reaction entails the net addition of formyl group and a hydrogen atom to a carbon-carbon double bond(alkenes).

22. Mechanism Step1 : 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.

23. HYDROCYANATION Hydrocyanation is a process for conversion of alkenes to nitriles. The reaction involves the addition of hydrogen cyanide and requires a catalyst. 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. This conversion is conducted on an industrial scale for the production of precursors to nylon.

24. Hydrocyanation of unactivated alkenes. Industrially, hydrocyanation is commonly performed on alkenes catalyzed by nickel complexes of phosphite (P(OR)3 ) ligands. A general reaction is shown . RCH=CH2 + HCN → RCH2 -CH2 -CN 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.

25. 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.

26. Wilkinson’s Catalyst Wilkinson's catalyst is the common name for chloridotris (triphenylphosphine)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.

27. Synthesis Wilkinson's catalyst is usually obtained by treating rhodium(III) chloride hydrate with an excess of triphenylphosphine in refluxing ethanol.Triphenylphosphine serves as a two-electron reducing agent that oxidizes itself from oxidation state (III) to (V). In the synthesis, three equivalents of triphenylphosphine become ligands in the product, while the fourth reduces rhodium(III) to rhodium(I). RhCl3 (H2O)3 + 4 PPh3 → RhCl (PPh3 )3 + OPPh3 + 2 HCl + 2 H2O

28. The most effective homogeneous hydrogenation catalyst is Wilkinson’s catalyst having composition RhCl (PPh3)3 . Where PPh3 is tridentate phosphine ligand. Both monomeric and dimer [Rh2Cl2(PPh3)4 ] forms are active catalyst. Since Rh can exist in two oxidation states, it readily catalyzes oxidative addition and reductive elimination reactions. In hydrogenation, the first step is dissociation of one ligand L, which is replaced by a solvent molecule. After ligand dissociation, oxidative addition reaction of H2 takes place. This is followed by migration of hydride from metal to ethane, forming the ethyl group. Finally, reductive elimination of ethane completes the cycle.

29. RhCl(L)3 + S  RhCl(L)2S + L RhCl(L)2S + H2  RhH2Cl(L)2S RhH2Cl(L)2S + H2C=CH2  RhH2Cl(L)2(H2C=CH2) + S RhH2Cl(L)2(C2H4) + S  Rh(C2H5)Cl(L)2S RhH(C2H5)Cl(L)2S → H3CCH3 + RhCl(L)2S Here L=tri- arylphosphines ; S= solvent ( ethanol , toluene ) Wilkinson's catalyst is best known for catalyzing the hydrogenation of olefins with molecular hydrogen.

30. REFERENCES CATALYSIS , An integrated approach to homogeneous, heterogeneous and industrial catalysis; J A Mouljin ; Elsevier publication;199-219 https://en.m.wikipedia.org/wiki/Homogeneouscatalysis https://en.m.wikipedia.org/wiki/Hydrogenation https://en.m.wikipedia.org/wiki/Hydroformylation https://en.m.wikipedia.org/wiki/Hydrocyanation

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