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

Mechanisms of industrial processes
Table 12.5. Conditions for single stage and two stage Wacker oxidations.
% % %C,H 4
Temp ( 0 C) Press (atm) Oxidant Yield Selectivity Conversion Recycle
Single
stage 130 3 o, 95 98 40 C,H4
Two
Liquid
stage 130 8-10 Air 95 100 phase
Composition ofsolution; CuCI,.2H,O, 200 gdm- 3 ; PdCI,, 4 gdm- 3 ; CH,CO,H, 50 gdm -,
(solvent for C,H4); rest water.
The process can be operated either in one or two stages (Table 12.5). The first
method requires pure oxygen which has tobe separated from air by liquefaction.
Although this is expensive, it is exclusively adopted in the West because very pure
ethene is available, permitting recycling ofthe gas phase. The palladium/copper
catalyst is continuously regenerated by oxygen while acetaldehyde is removed.
One reason why this homogeneous process has been so viable commercially is
that the product acetaldehyde is very volatile and is therefore easily separated
from the solution of catalyst.
A mechanism has to account for the following observations.
(i) The rate is given by the expression
-d[alkene]
dt
k[PdCI;- ][alkene]
[CI )2[H,O+]
(ii) 1 moi of ethene is absorbed for each moi of palladium(II) which is reduced to
palladium(O).
(iii) Ethanol is not an intermediate, as it is oxidized more slowly than ethene
under the conditions of the Wacker reaction.
(iv) Vinyl alcohol is not an intermediate. Ifthe reaction is carried out in D 2 0, the
CH,CHO produced contains no deuterium.
(v) The rates of oxidation of C 2 H 4 and C 2 D 4 are very similar. This means that
C-H bond breaking is unim portant in the rate determining step.
The following mechanism has been proposed:
PdCI!- + C,H.~Pd(C,H 4 )Cl; + Cl-
Pd(C,H4)Cl; + H,O~transPd(C,H.)(H,O)Cl, + Cl-
The anion Pd(C 2 H 4 )Cl; is the palladium analogue of Zeise's anion (p.196).
Substitution of CI- by water occurs trans to ethene rather than to chlorine,
consistent with the greater trans effect ofthe former. The next step is thought tobe
nucleophilic attack of water on the ethene complex. Hydroxide cannot be the
nucleophile as its concentration is too low under the prevailing acid conditions to
account for the observed rate. There is still some controversy whether the
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