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

Mechanisms of industrial processes
OH
OH- 1 -H 2 0 H 2
H3CCH2 CH 2 CHO - H3CCH 2 CH 2CHCHCHO- H3CCH 2CH 2CH=CCHO-
co
rAf~o
~co
I 1
C2H5 C2H5
The world capacity for its production from butyraldehydes is at least 3 m tonnes
per annum.
Hydroformylation was discovered in 1938 by Otto Roehlen (Ruhrchemie)
during investigations on the Fischer-Tropsch process (p. 393). An alkene
(propene to produce butyraldehydes ). carbon monoxide and hydrogen are heated
under pressure in the presence of a cobalt catalyst which operates homogeneously.
Typical conditions used today are 140-180°C/190-210atm. The
active catalyst is HCo(C0) 4 which is formed in situ via Co 1 (C0) 8 from a cobalt salt
such as the oleate or the acetate, present in 0.1-1% concentration. The active
species is stable only under at least 100 atm of carbon monoxide. Catalyst life
depends on maintaining a high pressure of CO. The products consist mainly of
aldehydes (Table 12.6). From propene, butanal and 2-methylpropanal (n- and
iso-butyraldehydes) are formed in a ratio of about 3:1. As only the former is
useful for making 2-ethylhexanol, the low selectivity towards the straight chain
product is a disadvantage.
Hydrogenation of an alkene is thermodynamically favoured over hydroformylation
and it is thus the function ofthe catalyst to direct the reaction into the latter
course by providing an energetically favourable path.
C 1 H 4 (g) + H,(g) + CO(g):;:::':C,H,CHO(g) ~G 6 =- 61 kJmol-'
C,H 4 (g)+H,(g):;:::':C,H 6 (g) ~G 9 = -9SkJmol-'
The rate expression shows inhibition by carbon monoxide. As the catalyst is
stable only above 100 atm CO, this sets a limit on the rates which can be achieved.
d[aldehyde] = k[alkene][Co]pH,
dt pCO
o+ o-
A conventional cycle is shown in Fig. 12.16. As the H-Co bond is polarized as
shown (HCo(C0) 4 is a strong acid), formal Markovnikov addition would be
predicted. On steric grounds, however, the bulky metal centre prefers to add to the
unsubstituted carbon atom of the alkene. Addition of CO and alkyl migration lead
to RCH 2 CH 2 C0Co(C0) 3 • Acyl tetracarbonyls, however, have been detected by
infrared spectroscopy under the conditions of hydroformylation and are therefore
in equilibrium with the tricarbonyls. The rate determining step seems to be the
hydrogenolysis. This can be thought of as proceeding either by addition of H 2 to
cobalt followed by elimination of aldehyde, or as a concerted process.
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