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

Table 12.6. Comparison of processes for hydroformylation.
Chemistry based on synthesis gas
Tributylphosphine
Cobalt modified cobalt Rhodium carbonyl
carbonyl carbonyl phosphine
Catalyst HCo(C0) 4 HCo(CO),PBu, HRh(C0) 2 (PPh,) 2
Temperature ( 0 C) 140-150 180-200 80-120
Pressure (atm) 260-350 50-100 12-25
n:iso ratio 3 or 4:1 9:1 > 10:1
%n product 67 67 83
Main product Aldehyde Alcohol Aldehyde
%Propane 2 10 2
By-products Aldehyde Alkane Aldehyde trimer
trimer
Formate ester
Ethers
Acetals
throughout the cycle given here, whereas only o ne is present at some stages in the
alternative mechanism. The mechanism in Fig. 12.1 7 is therefore the more
sterically demanding and is therefore consistent with the high proportion of
unbranched (n) product formed. Addition ofL increases the selectivity, although
it reduces the rate. It does have the advantage, however, ofreducing the activity
of the catalyst towards isomerization of the alkenes. There is thus an optimum
L:Rh ratio. It has been shown by n.m.r. spectroscopy that the monocarbonyl
HRh(CO)L 2 ~HRh(CO)L 3 does not react with alkene at 25°C, 1 atm. whereas the
dicarbonyl does. This excludes a mechanism in which dissociation of CO from
HRh(C0) 2 L 2 is the initial step.
12.4.3 Carbonylation of alkynes and of alkenes
During the period 1920-1960 ethyne was an important building block for a wide
range of substances. It can be produced from coal via calcium carbide
CaO+ 3C ~ CaC 1 +CO
CaC 1 + 2H 2 0 -------> C 2 H 2 + Ca(OH) 2
It can also be obtained by steam cracking of ethane at temperatures above
1200°C. Nowadays acetylene has generally been replaced by alkenes as a
chemical feedstock. India and East European countries however, continue to use
some acetylene technology as they have adequate coal reserves but are oil
importers.
A pioneer in the development of homogeneous transition metal catalysts was
W. Reppe of the German firm I.G. Farben. Working during the late 1930s and
early 1940s he discovered a series of carbonylation reactions of alkynes, alkenes
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