Wacker oxidation of alkenes

Key points: The Wacker process is used to produce acetaldehyde from ethene and oxygen; the most successful system uses a palladium catalyst to oxidize the alkene, with the palladium being reoxidized via a secondary copper catalyst. The Wacker process is used primarily to produce ethanal (acetaldehyde) from ethene and oxygen:

C₂H₄ +1/2 O₂ → CH₃CHO ∆_r G^O -197 kJ mol^-1

Its invention at the Wacker Consortium für Elektrochemische Industrie in the late 1950s marked the beginning of an era of production of chemicals from petroleum feedstock. Al though the Wacker process is no longer of major industrial concern, it has some interesting mechanistic features that are worth noting. The actual oxidation of ethene is known to be caused by a palladium (II) salt:

C₂H₄+PdCl₂+H₂O → CH₃CHO+Pd (0) +2HCl

The exact nature of the Pd (0) species is unknown, but it probably is present as a mixture of compounds. The slow oxidation of Pd (0) back to Pd (II) by oxygen is catalysed by the addition of Cu (II), which shuttles back and forth to Cu(I):

Pd (0) +2[CuCl₄]^-2 → Pd⁺²+2[CuCl₂ ]^-+4Cl^-

2 [CuCl₂]^-+1/2O₂+2H⁺ + 4Cl^- → 2[CuCl₄]^-2+H₂O

The overall catalytic cycle is shown in Fig. 26.9. Detailed stereochemical studies on related systems indicate that the hydration of the alkene–Pd (II) complex (B) occurs by the attack of H2 O from the solution on the coordinated ethene rather than the insertion of coordinated OH. Hydration, to form (C), is followed by two steps that isomerize the coordinated alcohol. First, -hydrogen elimination occurs with the formation of (D), and then migration of a hydride result in the formation of (E). Elimination of the ethanal and an H ion then leaves Pd (0), which is converted back to Pd (II) by the auxiliary copper (II)-catalysed air oxidation cycle. One important observation that the mechanism must account for is that, when the reaction is carried out in the presence of D₂O, no deuterium is incorporated into the final product. This observation suggests that either intermediate (D) is very short lived and does not exchange the Pd–H for a Pd–D, or that intermediate (C) rearranges directly to (E). Alkene ligands coordinated to Pt (II) are also susceptible to nucleophilic attack, but only palladium leads to a successful catalytic system. The principal reason for palladium’s unique behaviour appears to be the greater lability of the 4d Pd (II) complexes in comparison with their 5d Pt (II) counterparts. Furthermore, the potential for the oxidation of Pd (0) to Pd (II) is more favourable than for the corresponding Pt couple.

Figure 26.9 The catalytic cycle for the palladium-catalysed oxidation of alkenes to aldehydes.

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مواضيع ذات صلة

Surface acidic and basic sites

Surface area and porosity

Methanol carbonylation: ethanoic acid synthesis

Palladium-catalysed C-C bond-forming reactions

Asymmetric oxidations

Hydroformylation

Hydrogenation of alkenes

Alkene metathesis

Homogeneous and heterogeneous catalysts

Selectivity

Catalytic efficiency and lifetime

Catalytic cycles