Conversion to acetals

In 1968 a new methanol carbonylation process using rhodium promoted with iodide as catalyst was introduced by a modest letter (35). This catalyst possessed remarkable activity and selectivity for conversion to acetic acid. Nearly quantitative yields based on methanol were obtained at atmospheric pressure and a plant was built and operated in 1970 at Texas City, Tex. The effect on the world market has been exceptional (36). 

Conversion to acetates, trifluoroacetates (178), butyl boronates (179) trimethylsilyl derivatives, or cycHc acetals offers a means both for identifying individual compounds and for separating mixtures of polyols, chiefly by gas—Hquid chromatography (glc). Thus, sorbitol in bakery products is converted to the hexaacetate, separated, and determined by glc using a flame ionisation detector (180) aqueous solutions of sorbitol and mannitol are similarly separated and determined (181). Sorbitol may be identified by formation of its monobensylidene derivative (182) and mannitol by conversion to its hexaacetate (183). 

An example of a reversible reaction in the liquid phase is afforded by the esterification reaction between ethanol and acetic (ethanoic) acid forming ethyl acetate and water. Since, however, ethyl acetate undergoes conversion to acetic acid and ethanol when heated with water, the esterification reaction never proceeds to completion. 

The use of the enolsilyl ether of 1-menthone [16, 19, 21-23] and of some free triflic acid favors the formation of the thermodynamically controlled products as with free 2,2 -dihydroxydiphenyl [22] and only subsequently added HMDS 2 [22]. On reacting silylated alcohols and carbonyl compounds with pure trimethylsilyl triflate 20 under strictly anhydrous conditions no conversion to acetals is observed [24]. Apparently, only addition of minor amounts of humidity to hydrolyze TMSOTf 20 to the much stronger free triflic acid and hexamethyldisiloxane 7 or addition of traces of free triflic acid [18-21, 24, 26] or HCIO4 [25] leads to formation of acetals. 

Conversion to acetals is a very general method for protecting aldehydes and ketones against nucleophilic addition or reduction.245 Ethylene glycol, which gives a cyclic dioxolane derivative, is frequently employed for this purpose. The dioxolanes are usually prepared by heating a carbonyl compound with ethylene glycol in the presence of an acid catalyst, with provision for azeotropic removal of water. 

It is important that in the two organic equilibria involving iodide reaction (Equation (2)) shows complete conversion of methanol to methyl iodide, whereas the reaction with acetyl iodide shows complete conversion to acetic acid and hydrogen iodide (Equation (3)) ... 

Fermentation of the wastes - conversion to acetates Acetogenesis - conversion to acids, formaldehyde and Hydrogen, and Methanogenesis - conversion of formaldehyde, acetates, and acids to CO2 and Methane... 

As mentioned in the previous section, the carbonylation of methanol to acetic acid is an important industrial process. Whereas the [Co2(CO)s]-catalyzed, iodide-promoted reaction developed by BASF requires pressures of the order of 50 MPa, the Monsanto rhodium-catalyzed synthesis, which is also iodide promoted and which was discovered by Roth and co-workers, can be operated even at normal pressure, though somewhat higher pressures are used in the production units.4,1-413 The rhodium-catalyzed process gives a methanol conversion to acetic acid of 99%, against 90% for the cobalt reaction. The mechanism of the Monsanto process has been studied by Forster.414 The anionic complex m-[RhI2(CO)2]- (95) initiates the catalytic cycle, which is shown in Scheme 26. 

These results could be explained if, as was suggested by the data of Dillwith et al. (14). there is a pathway In which propionate is directly converted to acetate with the loss of carbon 1 and the oxidation of carbon 3. With a pathway of this type, propionate labeled in carbon 1 would lose its label if it were converted to acetate prior to incorporation into other compounds. However, if propionate were labeled in carbon 2 or carbon 3 it would retain its label upon conversion to acetate. Therefore, any label incorporated into JH or hydrocarbon from propionate labeled in carbon 1 would have to be the result of propionate being utilized as an intact unit. Incorporated label from propionate labeled in carbon 2 or carbon 3 could result from direct Incorporation of propionate or from conversion of propionate to acetate prior to being utilized for JH or hydrocarbon biosynthesis. Thus, the labeling patterns seen by Schooley, ejt al. (O and Dillwith, et a l. (14) could be the result of the retention of label from carbon 2 or carbon 3 of propionate, and the loss of label from carbon 1 of propionate during the conversion of propionate to acetate. 

Pankhania I. P., Spormann A. M., and Thauer R. K. (1988) Lactate conversion to acetate, CO2 and H2 in cell suspensions of Desulfovibrio vulgaris (Marburg) indications for the involvement of an energy driven reaction. Arch. Microbiol. 150, 26—31. 

The rhodium-catalyzed process gives a methanol conversion to acetic acid of 99%, against 90% for the cobalt reaction. The mechanism of the Monsanto process has been studied by Forster. The anionic complex c -[Rhl2(CO)2] (95) initiates the catalytic cycle, which is shown in Scheme 26. 

Henderson MA (2005) Photooxidation of acetone on TiO ) ) conversion to acetate via methyl radical ejection. J Phys Chem B 109 12062... 

The production of methyl formate by carbonylation of methanol with basic catalysts [134] can be used to separate carbon monoxide from by-product synthesis gas streams, e.g., steel-mill off-gases [135], to generate clean sources of CO for production of acetic acid by methyl formate isomerization. Therefore methyl formate could be produced near cheap CO sources and then transported to an appropriate site for conversion to acetic acid. This route to acetic acid is potentially competitive with a classic grass-roots methanol carbonylation process. Though the process has not been commercialized, numerous companies have patented the isomerization of methyl formate [136]. 

H-Mordenite catalyzes the smooth conversion of simple aldehydes and alcohols to form acetals at 30° in the liquid phase. From the examples in Table XXVII, it is apparent that in these heterogeneous catalytic systems, acetal formation is dependent on the structures of both the aldehyde and the alcohol involved. Thus, for a given aldehyde, yields of acetal decreased in the order primary > secondary > tertiary that is, branching at the a-carbon of the alcohol reduced the equilibrium conversion to acetal. In the isobutyraldehyde reactions, an extremely sharp drop in conversion was observed upon changing from isopropanol to fert-butanol as reactant. This observation suggests that, in addition to the increased steric interactions between organic reactants encountered in the tert alcohol system, molecular sieving-type interactions within the narrow mordenite pore system are operative. 

Aldehyde Alcohol Alcohol/aldehyde molar ratio Conversion to acetal (%) ... 

The synthesis of acetaldehyde, essentially intended for conversion to acetic acid, was the main outlet for ethanol until a few years ago. The industrial development of methanol carbonylation to acetic arid has relegated this application to the background. Ethanol is chiefly used today as a solvent and for the preparation of esters (ethyl acetate, ethyl chloride). It is also employed in cosmetics and pharmaceuticals. 

Acetic acid and ethanol are also good carbon sources for glutamate production. Ethanol seems to be used after conversion to acetic acid in the cells of the... 

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