Advanced organic chemistry

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

2. contents
1.Homogeneous catalysis
2.Hydrogenation
3.Hydroformylation
4.Hydrocyanation
5.Wilkinson catalysts

3. Homogeneous catalysis
Homogeneous 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.
Heterogeneous 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.

5. 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.

6. 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 cont5
rol and manipulate the process parameters.

7. 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.

8. 1.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.
CH3CO2CH3 + H2O ⇌ CH3CO2H + CH3OH
At neutral pH, aqueous solutions of most esters do not hydrolyze at practical rates.
2. the lead chamber process during the manufacture of sulphuric acid, the presence of nitric
oxide gas helps in catalyzing the oxidation of Sulphur dioxide.

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

10. 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.

11. 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.

12. MECHANISM
Steps in the hydrogenation of a C=C double
bond at a catalyst surface, for example Ni
or Pt
1. The reactants are absorbed on the
catalyst surface and hydrogen
disassociates.
2. An h atom to on 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.

13. 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.

14. 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.

15. 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.

16. Catalysts
HOMOGENEOUSCATALYSTS USED FOR HYDROGENATION
1. Ruthenium
2. Iridium (crab tree’s catalyst)
3. Wilkinson’s catalyst
HETEROGENEOUSCATALYST USED FOR HYDROGENATION
1. Lindlar’s catalyst
2. Raney Nickel
3. Transfer hydrogenation

19. 1. 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.

20. 2. 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.

21. Applications
1. 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.

22. 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.

23. 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.

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

25. Mechanism
Step1 : The process begins with dissociation of
CO from cobalt tetracarbonyl hydride to give
the 16-electronspecies.
Step 2-Subsequent binding of alkene gives an
18e species.
Step 3- The olefin inserts to give the
16ealkyl tricarbonyl.
Step 4-Coordination of another equivalent of
COgive alkyl tetracarbonyl .
Step 5-Migratory insertion of CO gives the 16e
acyl.
Step 6- oxidative addition of hydrogen
givesa dihydrido complex,
Step 7-this dihydrido complex releases
aldehydeby reductive elimination.
Step 8-is unproductive and reversible.

26. 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.

27. 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.

28. 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.

29. 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.

30. 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

31. 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.

32. 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.

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