Acetaldehyde dissociation products

After reviewing ethanol and acetaldehyde dissociation on Pt-based nanoparticle catalysts, in this section we will discuss the effects of various factors, such as ethanol concentration, potential and catalyst composition on the current efficiencies, and product yields of the bulk electrooxidation of ethanol. 

Carbinolamines are chemically unstable and, in the case of tertiary amines, dissociate to generate the secondary amine and aldehydes as products or eliminate water to generate the iminium ion. The iminium ion, if formed, can reversibly add water to reform the carbinolamine or add other nucleophiles if present. If the nucleophile happens to be within the same molecule and five or six atoms removed from the electrophilic carbon of the iminium ion, cyclization can occur and form a stable 5- or 6-membered ring system. For example, the 4-imidazolidinone is a major metabolite of lidocaine, which is formed in vivo or can be formed upon isolation of the A -deethyl metabolite of lidocaine if a trace of acetaldehyde happens to be present in the solvent used for extraction (116,118) (Fig. 4.52). 

The primary products are methyl and formyl radicals [36, 37] because potential energy surface crossing leads to a H shift at combustion temperatures [35], It is rather interesting that the decomposition of cyclic ethylene oxide proceeds through a route in which it isomerizes to acetaldehyde and readily dissociates into CH3 and HCO. Thus two primary addition reactions that can be written are... 

An alternate view regarding selectivity has been proposed for Si02-supported V2Os (20). In this proposal, ethane could be activated in two ways. One was by dissociative adsorption across a V=0 bond to form H—V—OC2H5. a- or -elimination of the ethoxide would result in acetaldehyde or ethene, respectively. The other way was by dissociative adsorption across a V—O— bond to form —OH and V—C2H5. The latter species would lead to combustion products. 

Cool flames were somewhat more difficult to establish with propion-aldehyde than with acetaldehyde and it was necessary to use a higher initial tube temperature (270 C). In its general features, this system resembled that of acetaldehyde, although the second stage flame was less sharply defined. The analytical data were more complex and considerable production of ethylene occurred, presumably via (42), the ethyl radicals being the result of C2H5 CO dissociation. 

This ester resembles its methyl homologue in possessing three modes of decomposition [131]. It also supports a self-decomposition flame, the multiple reaction zones of which are clearly separated at low pressures [122, 123, 125]. Temperature and composition profiles in the low-pressure decomposition flame have been measured [133]. The products include formaldehyde, acetaldehyde and ethanol with smaller amounts of methane and nitromethane. The activation energy derived from the variation of flame speed with final flame temperature was 38 kcal. mole", close to the dissociation energy of the RO—NO2 bond. The controlling reaction is believed to be unimolecular in its low pressure regime, and the rate coefficient calculated from the heat-release profile is... 

For Rh/Ti02 films the hydrogenation of CO produced acetone and acetaldehyde as oxygenated products the bridged carbonyl species was the likely precursor of these products. For the CO2 hydrogenation reaction the presence of potassium caused the dissociation or desorption of all CO species, and oxygenated products were not produced. Potassium significantly poisoned both reactions toward the production of methane. 

If product formation proceeded via an intermediate long-lived ion whose dissociation were governed by competitive channeling of energy into the possible dissociation coordinates, one would expect the complex to fragment at the weakest bond—i.e., in the most thermodynamically favorable direction. This is clearly not the case. The most exothermic reaction (17) does not dominate the fate of the complex formed by collision of methanol ion with acetaldehyde. Similarly, one would expect Reaction 4 to be favored and Reactions 5, 6, and 7 to occur with about equal probability. 

The dissociation rate of the acetate ligand of 10 was very fast (instantaneous) to form 11 and acetaldehyde, while the dissociation rate of the acetate ligand of 9 occurred in two or more hours at room temperature, forming 8 and oxidized product. 

Methane, acetic acid, acetaldehyde, and ethanol constitute approximately 90 carbon atom percent of the primary products from the hydrogenation of CO over Rh/SiO and Rhr-Mn/SiOi catalysts at 250 -300°C and 30-200 atm pressure in a back-mixed reactor with H /CO = 1. The rate of reaction and the ratio, CHj /C chemicals, vary with (Pjy / The addition of 1% Mn raises the synthesis rate of a 2.5% Rh/SiOfi catalyst about tenfold. The kinetics and the product distribution are consistent with a mechanism in which CO is adsorbed both associatively and dissodatively. The surface carbon produced by the dissociative CO chemisorption is hydrogenated through a Rh-CHs intermediate, and CO insertion in that intermediate results in formation of surface acetyl groups. 

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