Ethylene oxide operation/reaction mechanism

Wacker (1) A general process for oxidizing aliphatic hydrocarbons to aldehydes or ketones by the use of oxygen, catalyzed by an aqueous solution of mixed palladium and copper chlorides. Ethylene is thus oxidized to acetaldehyde. If the reaction is conducted in acetic acid, the product is vinyl acetate. The process can be operated with the catalyst in solution, or with the catalyst deposited on a support such as activated caibon. There has been a considerable amount of fundamental research on the reaction mechanism, which is believed to proceed by alternate oxidation and reduction of the palladium ... 

Mechanisms There is a derth of knowledge about the mechanisms operative in selective oxidation reactions. The only exceptions are the reactions of ethylene to ethylene oxide on supported silver catalysts and of propylene to acrolein on bismuth molybdate type catalysts. For the latter, it is well established through isotopic labeling experiments that a symmetric allyl radical is an intermediate in the reaction and that its formation is rate-determining. Many studies simply extrapolate the results substantiated for this case to other reactions. New ideas on mechanisms are presented by Oyama, et oL, Parmaliana, et aL, and Laszlo. 

All these reactions and especially the latter two. which correspond to the complete combustion of ethylene and of its oxide, are highly exothermic and complete. in the operating conditions of ethylene oxide synthesis. To guide the transformation in the direction of the first reaction, the operations require the presence of a metallic catalyst. The catalyst is generally considered to act according to the foliowring reaction mechanism ... 

An alternative mechanism by which additives may protect polymers from photo-oxidation is radical trapping. Additives which operate by this mechanism are strictly light stabilizers rather than antioxidants. The most common materials in this class are the hindered amines, which are the usual additives for the protection of poly (ethylene) and poly (propylene). The action of these stabilisers is outlined in Reactions 8.3-8.5. 

We note also that the schemes discussed until now only show the oxidation of the ethylene glycol moiety. In the PECT copolymer, the 1,4-cyclohexylenedi-methylene moiety is also available for oxidation. Indeed, given that the oxidizable hydrogens are tertiary, one reasonably expects a greater ease of production of a radical from that center. Grossetete et al. [11] reported such to be the case with the observation that photo-oxidation reactions occurred much faster with the PECT copolymer than with PET itself. The aliphatic acids that they reported, as identified by the SF4 treatment, could also account for previous aliphatic acid reports [25], This is also additional support that the photo-oxidation mechanism is operating as proposed (Scheme 18.4). 

However, more significant was the introduction of a new one step process for the oxidation of ethylene to acetaldehyde using a PdCl2-CuCl2 catalyst (equation [16]) sometime between 1957-1959. The process, now known as the Wacker Process, is operated at ca. 10 atm and 100-110°C, requires a large excess of chloride, and produces AcH in about 95% yield. The mechanism for this reaction is shown in figure 2. 

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