Acetaldehyde dimethyl acetal, carbonylation from methanol

The equilibrium constants for addition of alcohols to carbonyl compounds to give hemiacetals or hemiketals show the same response to structural features as for the hydration reaction. Equilibrium constants for addition of methanol to acetaldehyde in both water and chloroform solution are near 0.8 M" The comparable value for addition of water is about 0.02 M The overall equilibrium constant for formation of the dimethyl acetal of acetaldehyde is 1.58 M . Because the position of the equilibrium does not strongly favor product, the formation of acetals and ketals must be 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 released is irreversibly removed from the system. 

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. 

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. 

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