Hydrogenation of acetates

Hydrogenation of acetic anhydride to acetaldehyde (equation 23) has been demonstrated utilizing cobalt carbonyl under one atmosphere of hydrogen. However, the cobalt complex is short lived. A more efficient cobalt catalyzed reaction with substantial catalyst longevity was realized at a temperature of 190 and 3000 psi pressure CO and hydrogen. The main products were equal amounts of EDA and acetic acid. Upon investigation, this reaction was found exceptionally efficient at a more reasonable 1500 psi pressure provided that the temperature was maintained... 

The hydrogenation of acetic anhydride was also performed in the vapor phase over a supported palladium catalyst resulting in acetaldehyde and acetic acid in high yields (36). To avoid recycling, the reactor was designed for complete reaction of acetic anhydride. Minor selectivity loss was found in formation of ethyl acetate (0.5-1.5%) and methane (0.5%). Typical reaction conditions were 160-200 C, 30-100 psi, with a hydrogen-anhydride ratio of 3 1 to 10 1. A similar catalyst was used in the liquid phase (37). The simplicity and high selectivity of this process is certainly attractive. 

Vapor Phase Hydrogenation of Acetic Anhydride Acetic anhydride was pumped into an evaporator where it was mixed with hydrogen. The temperature of anhydride-hydrogen mixture was raised to the reaction temperature in a preheater zone, made of a 2 feet bed packed with 2 mm glass beads. The reaction took place in a 2 feet catalyst bed packed with 1 m.m. alpha-alumina coated with 0.5% Pd. The effluent was condensed and analyzed by G.C. 

The acid formed has a methyl and an ethyl group in place of two hydrogens of acetic acid and is therefore often referred to as a disubstituted acetic acid. 

The O-H hydrogen of acetic acid is more acidic than the C-H hydrogens. The -OH oxygen is electronegative, and, consequently, the -O-H bond is more strongly polarized than the -C-H bonds. In addition, the acetate anion is stabilized by resonance. 

For synthesis of ethylidene diacetate, other startmg materials have also been proposed. Hydrogenation of acetic anhydride by a palladium catalyst [3gj or caibonylafion of CH3CH(OCIl3)2 with RhCl3/Ph3P/MeJ (39] are two alternative routes. 

Tite acidaldehyde condensation products have to be anticipated, and ethers such as methyl ethyl ether can also be formed via catalytic hydrogenation of acetals, as is shown in Equation (34) [11. 

G=G bond is low. The CHgOH value of the bond ener of the carbonyl group in cJhIoH aldehydes has been ob- CjHJiOH tained from heat of CeHijOH hydrogenation of acet-aldehyde, 554 4 kcals. 

Another reaction catalyzed by the same anionic [Rh(CO)2l2] is the hydrogenation of acetic anhydride to ethylidene diacetate (8). Figure I is a plot of... 

Figure 1 Hydrogenation of acetic anhydride using Reillex with 1.1 wt% Rh as tRh(CO)2l2]-.

And there was more. The two hydrogen atoms that are removed and replaced by a single oxygen atom when alcohol is oxidized to acetic acid could only have come from the ethyl radical, where all the hydrogen of acetic acid that is not involved in acid formation was located. From this evidence, the rational formula for acetic acid had to be written ... 

The cluster [Ru4(/t-OMe)(/t4-N)(CO)i2] with a bridging methoxide unit has been previously discussed (compound 13). The complex [Ru4(/t-MeCOO)4(CO)8(PBu3)2] is catalytically active in the hydrogenation of acetic acid and its role in this capacity has been further investigated. The tetranuclear complex [Ru4(/t3-OH)- fi-rf-(0)CEt (CF3S03)2(CO)8] 48 was isolated (Equation (37)) in a study of the catalytic properties of the water-soluble Ti7r -[Ru(OCOCF3)2(CO)3(H20)] and its derivatives. There are no RuRu bonds in 48 consistent with its 72 c.v.e. count. 

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