Acetaldehyde dimethyl acetal, carbonylation

The carbonylation of acetaldehyde dimethyl acetal or the hydrocarbonyla-tion of methyl acetate by use of rhodium catalysts yields ethylidene diacetate, which can be cracked thermally to give vinyl acetate ... 

The equilibrium constants for addition of alcohols to carbonyl compounds to give hemiacetals or hemiketals show the same response to structural features as the hydration reaction. Equilibrium constants for addition of metiianoHb acetaldehyde in both water and chloroform solution are near 0.8 A/ . The comparable value for addition of water is about 0.02 The overall equilibrium constant for formation of the dimethyl acetal of... 

In addition to the successful reductive carbonylation systems utilizing the rhodium or palladium catalysts described above, a nonnoble metal system has been developed (27). When methyl acetate or dimethyl ether was treated with carbon monoxide and hydrogen in the presence of an iodide compound, a trivalent phosphorous or nitrogen promoter, and a nickel-molybdenum or nickel-tungsten catalyst, EDA was formed. The catalytst is generated in the reaction mixture by addition of appropriate metallic complexes, such as 5 1 combination of bis(triphenylphosphine)-nickel dicarbonyl to molybdenum carbonyl. These same catalyst systems have proven effective as a rhodium replacement in methyl acetate carbonylations (28). Though the rates of EDA formation are slower than with the noble metals, the major advantage is the relative inexpense of catalytic materials. Chemistry virtually identical to noble-metal catalysis probably occurs since reaction profiles are very similar by products include acetic anhydride, acetaldehyde, and methane, with ethanol in trace quantities. 

For the preparation of an acetal, the carbonyl compound is generally used directly. However, where the aldehyde is rather volatile, a polymeric form is preferred. For example, the trimer, paraldehyde, is often used instead of acetaldehyde. In other cases, the carbonyl compound has been used as its diethyl or dimethyl acetal. It has been found that, whereas the paraldehyde-sulfuric acid reagent often yields the seven-membered, oxido-diethylidene ring (2), 1,1-dimethoxyethane or 1, l-diethoxyethane, and an acid catalyst, afford only the six-membered, ethylidene ring. 

The overall equilibrium constant for formation of the dimethyl acetal of acetaldehyde is 1.58 The comparable value for the addition of water is about Because the position of the equilibrium does not strongly favor product, the synthesis of acetals is carried out in such a way as to drive the reaction to completion. One approach is to use a dehydrating reagent or azeotropic distillation so that the water that is formed is irreversibly removed from the system. Because of the unfavorable equilibrium constant and the relative facility of the hydrolysis, acetals are rapidly converted back to aldehydes and ketones in acidic aqueous solution. The facile hydrolysis makes acetals useful carbonyl protecting groups (see Part B, Section 3.5.3). 

Low boiling substances are removed from the chilled reactor product by distilling up to a cut point of 80 °C. These low boilers are gaseous dimethyl ether, methyl acetate, acetaldehyde, butyraldehyde, and ethyl acetate. The bottoms are flash-distilled to recover the rhodium catalyst. Flash distilled acid is azeotropically dehydrated. In the final distillation, glacial acid is obtained. Traces of iodine that may remain in the finished acid may be removed by fractional crystallization or by addition of a trace of methanol followed by distillation of the methyl iodide that forms. Somewhere in the carbonylation reaction, a minute amount of propionic acid seems to be made. It typically is found in the residues of the acetic acid finishing system and can be removed by purging the finishing column bottoms. 

Subsequent developments of Eastman carbonylation technology are yet to be commercialized production routes to acetaldehyde, propionic acid, propionic anhydride, methacrylates, and acrylates (29). It is also relevant to note that dimethyl ether, readily available from methanol by dehydration, has recently achieved some prominence as a suitable raw material in carbonylation technology, via the intermediacy of methyl acetate and acetic anhydride in the case of acetic anhydride manufacture. It also offers the advantage of processing under totally anhydrous conditions. 

In subsequent developments, Eastman has reported two new alternative manufacturing routes to vinyl acetate (38). The first uses the carbonylation of dimethyl ether to acetic anhydride, followed by the reaction between acetic anhydride and acetaldehyde in a reactive distillation column to 3ueld vinyl acetate, whereas the second involves the intermediacy of ketene. Here ketene is hydrogenated to acetaldehyde, and the acetaldehyde is reacted with a second equivalent of ketene to produce vinyl acetate. Both of these routes are claimed to avoid the problematic and expensive acetic acid recycle. 

In the BASF carbonylation process, methanol and CO are converted in the liquid phase (solvent dimethyl ether, water) at 250 °C and 700bar. The reaction rate depends strongly on the concentration of methanol and the partial pressure of CO. The proposed mechanism for the Co-catalyzed carbonylation of methanol is presented in detail in Example 16.6.2. Acetic acid yields are typically 90% (based on methanol) and 70% (based on carbon monoxide). Selectivities are high, with the production of 100 kg of acetic acid affording 4 kg by-products (mainly CO2, CH4, ethanol, acetaldehyde, and propionic acid). 

Carbonylation acetic acid, acetic anhydride, methyl acetate, methyl formate Reductive carbonylation acetaldehyde, ethanol, ethyl acetate, ethylidene diacetate Oxidative carbonylation dimethyl carbonate, dimethyl oxalate... 

In contrast, in the Eastman process, dimethyl ether is first carbonylated to acetic anhydride by reaction 4.7.8 (see Section 4.4). The acetic anhydride is then reacted with acetaldehyde to give acetic acid and vinyl acetate according to reaction 4.7.9. 

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