User:Benjah-bmm27/degree/3/PGP
Appearance
Organometallic Chemistry and Homogeneous Catalysis 2, PGP
[edit]Feedstocks
[edit]- → aldehydes from alkenes, by Rh catalysis (hydroformylation)
- → methanol by Cu catalysis
- → alkanes and alkenes by Fe catalysis (Fischer-Tropsch)
- Methanol
- → alkanes, alkenes, aromatics by zeolite catalysis
- → formaldehyde and ethers, e.g. MTBE
- → acetic acid by Ir catalysis (Cativa process)
Hydroformylation
[edit]Hydroformylation is formally the addition of a molecule of formaldehyde across the C=C double bond of an alkene.
- R2C=CR2 + H–CHO → R2CH–CR2CHO
Formaldehyde isn't actually used. Synthesis gas (a mixture of hydrogen and carbon monoxide, H2 + CO) is used instead.
Regioselectivity
[edit]Hydroformylation of monosubstituted alkenes (vinyl alkanes, if you like: RHC=CH2) can give two possible regioisomers: linear (n) and branched (iso)
- The regioselectivity of a particular hydroformylation reaction is expressed as the ratio of linear to branched, i.e. n : iso
- Example: hydroformylation of propene can yield both butyraldehyde (linear) and isobutyraldehyde (branched)
- Butyraldehyde is a precursor for two very important molecules
- hydrogenation of butyraldehyde gives n-butanol, an important industrial solvent (4 Mt/year, industry prefers it to EtOH, which is more of a fire risk and is heavily taxed)
- aldol reaction followed by hydrogenation gives 2-ethylhexanol (2Mt/year, used to make bis(2-ethylhexyl) phthalate, the most important phthalate plasticizer for polymers)
Cobalt carbonyl catalysis of hydroformylation
[edit]- Cobalt added as either dicobalt octacarbonyl, Co2(CO)8, or as a cobalt(II) salt
- Synthesis gas is a good reducing agent, and rapidly reduces Co(II) to Co2(CO)8
- Co2(CO)8 + H2 → 2HCo(CO)4
- HCo(CO)4 is cobalt tetracarbonyl hydride
- Co(I), d8, 18 e, trigonal bipyramidal molecular geometry
- volatile
- unstable above 20 °C unless an atmosphere of CO is present
- Co catalysis requires high pressures of CO
- Mechanism begins with the precatalyst HCo(CO)4
- Dissociation of a CO ligand generates HCo(CO)3: Co(I), d8, 16 e, vacant coordination site
- Coordination of alkene generates HCo(CO)3(alkene): Co(I), d8, 18 e, coordinatively saturated
- Migration of hydride from Co to alkene C generates Co(CO)3(alkyl): Co(I), d8, 16 e, coordinatively unsaturated
- Coordination of a new CO ligand to Co gives Co(CO)4(alkyl): Co(I), d8, 18 e, coordinatively saturated
- Migratory insertion of alkyl (formally R−) to one of the CO ligands gives Co(CO)3(acyl): Co(I), d8, 16 e, coordinatively unsaturated
- Oxidative addition of H2 generates H2Co(CO)3(acyl): Co(III), d6, 18 e, coordinatively saturated, octahedral molecular geometry
- Reductive elimination of acyl hydride (aldehyde!) regenerates HCo(CO)3
Acetic acid synthesis by methanol carbonylation
[edit]- MeOH + CO → MeCO2H
- Catalysed by {Co, Rh, Ir}-iodine species
- BASF, 1960: Co-I
- Monsanto process, 1970: Rh-I
- Cativa process, 2000: Ir-Ru-I
MMA synthesis
[edit]- Methyl methacrylate for poly(methyl methacrylate) production