Homogeneous Catalytic Processes
Hydroformylation RCH2CH2 O RCH CH2 + CO + H2 Co(I), Rh(I) or Pt(II) H
Oxidation H3C O H2CCH2 + O2 Pd(II) or Cu(II) H Carbonylation H3C O - CH3OH + CO [RhI2(CO)2] OH
Hydrocyanation [Ni{P(OR) } ] H CCC CH + 2HCN 3 4 NCCH CH CH CH CN 2 H H 2 2 2 2 2
Cyclotrimerization Ni(acac) 3 HC CH 2 Hydrogenation of Alkenes
The most commonly used catalyst is the Wilkinson’s Catalyst
Many alkenes are hydrogenated with hydrogen at 1atm pressure or less .
Wilkinson’s catalyst is highly sensitive to the nature of the phosphine ligand and the alkene substrate. Analogous catalysts with alkyl phosphine ligands are inactive Highly hindered alkenes and ethylene are not hydrogenated by the catalyst Wilkinson’s Catalyst
Ph3P PPh3 R CH3 Rh H2 Ph3P Cl Reductive Elimination Oxidative Addition
R H H PPh3 H PPh3 Rh Rh Ph3P Cl Ph3P Cl
PPh3 PPh3
Ligand Dissociation Ligand Association PPh3 PPh3
H R H PPh3 Rh H PPh3 Cl Rh Ph3P Cl PPh3 H Migratory Insertion H PPh3 Rh R Ph3P Cl Alkene Coordination
Catalyst + CO + H2 RCH2CH2CHO R A less common,,pppyy but more appropriate name is hydrocarbonylation
Both cobalt and rhodium complexes are used as catalysts. Alkene iitiisomerization, alkene hdhydrogena tion and ftiformation of bhdbranched aldeh yd es are the possible side reactions.
Cobalt catalysts operate at 150 ºC and 250 atm, whereas Rhodium catalysts operate at moderate temperatures and 1 atm. Rhodium catalysts promotes the formation of linear aldehydes. Cobalt catalysts do so if modified with alkylphosphine ligands. Hydroformylation
Co2(CO)8 + H2
H CH CH CH CH CHO 3 2 2 2 OC Co CO CO Reductive Elimination OC CO Ligand Dissociation CO H H OC H Co Co OC CO COCH2CH2CH2CH3 OC CO OC Oxidative Addition CO H 2 Alkene Coordination H O CH2CH2CH2CH3
OC Co CO Co OC OC CO CO OC Ligand Association CH CH CH CH CO Migratory Insertion 2 2 2 3 Ligand Association CO OC Co CO Migratory Insertion OC CO Monsanto Acetic Acid Synthesis
All three members of the ggproup 9 ((,Co, Rh and Ir) can catalyze this reaction. A cobalt complex was initially used, which was replaced with the rhodium complex later on.
Methanol initially reacts with hydroiodic acid to give methyliodide and H2O. Methyliodide reacts with the 16e- catalyst, which forms the rate-determining step.
The final product formed after the catalytic cycle is acetyl iodide, which is hydrolyzed bywater to acetic acid and HI. Monsanto Acetic Acid Synthesis
I CO Rh CH3COI I CO CH3I Reductive Elimination Oxidative Addition
I CH3 CH3 I CO I CO Rh Rh OC CO I CO
I I
Sol CH3 Migratory Insertion CO I CO Rh I CO
I Alkene Polymerization
If only a few monomers couple together, the short chains formed are called oligomers.
Initiation
Propagation
Termination Alkene Polymerization
The commonly used catalyst has a tetrahydrofuran molecule occupying the fourth coordination site of zirconium. Ziegler-Natta Catalyst
Karl Ziegler in 1953, discovered a catalyst based on TiCl3 and AlCl3 for the polymerization of ethylene. Giulio Natta extended the method for the polymerization of other olefins like propylene and made a number of different varitiiations. The Ziegler-Natta catalyst family includes halides of titanium, chromium, vanadium and zirconium, typically activated by aluminum halide compounds.
Ziegler and Natta received the Nobel Prize in Chemistry in the year 1963. Alkene Metathesis Alkene Metathesis Alkene Metathesis Nobel Prize in Chemistry 2005. Ring Closing Metathesis Ring Opening Metathesis Polymerization