Oxidation of olefins by palladium(II). Part XIV. Product distribution and kinetics of the oxidation of ethene by PdCl3 (pyridine)− in aqueous solution in the presence and absence of CuCl2: a modified Wacker catalyst with altered reactivity

✍ Scribed by John W. Francis; Patrick M. Henry

• Publisher: Elsevier Science
• Year: 1995
• Tongue: English
• Weight: 899 KB
• Volume: 99
• Category: Article
• ISSN: 1381-1169
• DOI: 10.1016/1381-1169(95)00041-0

✦ Synopsis

In the absence of CuC12, ethene was oxidized to ethanal by PdCl,(pyridine) -in aqueous solution by the rate expression:

* where K' is the equilibrium constant for n-complex formation between ethene and PdCl,(Py) -(Py =pyridine). This rate expression is of the same form as that previously found for the oxidation of ethene by PdCl:-in aqueous solution ( Wacker reaction). The value of K' for PdCl, (Py ) -was found to be 20.3 which is close to the value of 17.4 previously measured for n-complex formation between ethene and PdCli-. However, the value of k for PdCl:-was 750 times the value of k' for PdC&( Py) -. This result suggests that the hydroxypalladation adduct from PdCl,(Py) -is much more stable towards decomposition to ethanal than the corresponding one from PdClz-. A direct result of this higher stability is the expectation that the adduct from PdCl,( Py) _ should be more readily intercepted by CuCl, to produce 2-chloroethanol. At chloride concentrations as low as 0.2 M and [CuCl,] =4 M, the product was almost 50% 2chloroethanol. At [CuCl,] =g M, the product was 98% 2-chloroethanol. With PdCli-, a chloride concentration of 3 M is required before an appreciable amounts of 2-chloroethanol are produced at any cupric chloride concentration. For the reaction of ethene with PdCl$-, these results are consistent with a mechanism involving cis addition at low [Cl-] and trans addition at high [Cl-]. The pathway for ethanal formation may be different with PdC13( Py) -than it is with PdClz ~.