_P.-Powell-auth.-Principles-of-Organometallic-Chemistry-Springer-Netherlands-1988

Chemistry based on synthesis gas
CO these bands change to peaks at 2l41(w), 2084(vs) and 1708(s) which are
assigned to an acetyldicarbonyl species. A cycle which accommodates these
findings is given in Fig. 12.15. The rate determining step is the addition of methyl
iodide to [Rh(C0)) 2 ]-. The immediate product of this addition has never been
observed, on account of its rapid conversion into the acetyl. The final step is
elimination of acetyl iodide. This reacts with water to give acetic acid or with
methanol forming methyl acetate. Hydrolysis of the ester takes place as methanol
is consumed.
The process has been modified to yield acetic anhydride. Here the last step is
[MeCORhi,(C0) 1 r + MeCOOH-->{MeC0) 2 0 + [Rhi 1 (C0) 1 r + HI.
An important goal is a single stage process for making acetic acid directly from
syn. gas. There are indications that this is a real possibility in the near future. A
homogeneous process to make ethylene glycol (ethane 1, 2-diol) has been
developed by Union Carbide. Rhodium carbonyl clusters are probably implicated.
Unfortunately very high pressures, 1000 atm, are required. The construction of
C 2 and longer units directly from syn. gas tends to be unselective. The very first
step
H
MH(CO)~MCHO
1 o
M- II
c
1\
H H
is usually strongly disfavoured thermodynamically and formation of CH 2 0
(methanal) becomes rate limiting. Even at high temperatures and pressures
commercially useful rates are not normally achieved. Under such forcing
conditions the reaction becomes unselective, following many concurrent
pathways.
The Monsanto process affords acetic acid even in the presence of hydrogen.
Modification of ligands or introduction of a second catalytic centre could in
principle permit hydrogenation ofthe acyl intermediate rather than its solvolysis.
In this way ethanal or ethanol are possible products.
12.4.2 Hydroformylation
(a) COBALT CATALYSTS. Production of aldehydes by catalytic addition of
carbon monoxide and hydrogen to alkenes is known as hydroformylation.
Formally, hydrogen and a formyl group (Hand CHO) are added across the double
bond. Aldehydes are used to make acids by oxidation, amines by reductive
amination, polyols by reaction with methanal and alcohols by reduction. Butanal
(n-butyraldehyde) is a precursor to 2-ethylhexanol and hence to di-iso-octyl
phthalate, which is used as a plasticizer for PVC.
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