Acetaldehyde dissociation mechanism

Lai SCS, Kleyn SEF, Rosea V, Koper MTM. 2008. Mechanism of the dissociation and electro-oxidation of ethanol and acetaldehyde on platinum as studied by SERS. J Phys Chem C 112 19080-19087. 

The steady state and stopped-flow kinetic studies on the horse liver enzyme are now considered classic experiments. They have shown that the oxidation of alcohols is an ordered mechanism, with the coenzyme binding first and the dissociation of the enzyme-NADH complex being rate-determining.15,26,27 Both the transient state and steady state methods have detected that the initially formed enzyme-NAD+ complex isomerizes to a second complex 27,28 In the reverse reaction, the reduction of aromatic aldehydes involves rate-determining dissociation of the enzyme-alcohol complex,27,29 whereas the reduction of acetaldehyde is... 

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. 

The EOR mechanism on Pt-based anodes has been studied by different methods. The proposed path in acid media involves the first steps of ethanol adsorption via an O-adsorption or a C-adsorption. From these dissociative reactions, acetaldehyde (AAL) is formed [17, 24—26] ... 

From these results it appears that the addition of tin to platinum greatly favors the formation of acetic acid comparatively to acetaldehyde. This can be explained by the bifunctional mechanism [17] where ethanol is adsorbed dissociatively at platinum sites, either via an 0-adsorption or a C-adsorption process [9], followed by the oxidation of these adsorbed residues by... 

The hydrolysis of carbohydrates in dodecylbenzene sulphonic acid in dioxane-water mixtures has been the subject of one study in which it Was found that the hydrolysis was accelerated by about 21 times in dioxane mixtures above 60% by volume [126], but no coherent mechanism was put forward for the catalysis. Non-ionic surfactants may form inverse micelles in non-aqueous solvents in the presence of small amounts of water. Triton X-1(X), for example, micellizes in carbon tetrachloride on addition of water. This system, which obviously does not suffer the problems which result from the dissociation of the head groups of ionic surfactants in the water pool, has been used to investigate the hydration reaction of acetaldehyde [127]. This acid-catalysed reaction is increased by a factor of 10000 over that in water (Table 11.9). In spite of the nonionic nature of the peripheral head groups surrounding and penetrating the aqueous core, the nature of the water is such that ionization of solubilized species is changed. 

Abstract In this chapter, we present new insights in direct alcohol fuel cell-related anode electrocatalysis based on quantitative differential electrochemical mass spectrometry (DEMS) studies. First, we review the history and development of the DEMS technique, as well as the calibration method for quantification. We then discuss some contributions of quantitative DEMS to the study of the mechanism of methanol electrooxidation on Pt and PtRu model catalysts. We also discuss quantitative DEMS studies of the mechanism of dissociative adsorption and electrooxidation of ethanol and acetaldehyde at Pt, Pt3Sn, PtRu, and PtRh nanoparticle catalysts. Finally, the mechanism of dissociative adsorption and electrooxidation of ethylene glycol and its oxidative derivatives on carbon-supported Pt, Pt3Sn, and PtRu nanoparticle catalysts are discussed, based on quantitative DEMS results. 

Photodecarboxylation of the dissociated form of a-hydroxy-substituted arylacetic acids and related substrates occurs from the singlet excited state (SJ and leads to the corresponding benzyl alcohols with high quantum yields (O = 0.2 to 0.7) via a heterolytic mechanism (Scheme 2). Photodecarboxylation of some diarylacetic acids (Scheme 2) also proceeds from Sj via heterolytic mechanism. It is remarkable that these compounds show dramatic differences in their relative photodecarboxylation efhciency (O = 0.04 to 0.6). The reaction is enhanced when a cyclic delocalized carbanion with 4n electrons is formed. By contrast, photodecarboxylation of m- and p-nitrophenylacetic acids in aqueous solutions occurs via a heterolytic mechanism from Tj." The photodecarboxylation of pyruvic and benzoylformic acids takes place with high quantum yields ( 5 > 0.6) in aqueous solutions, to give acetaldehyde and benzaldehyde, respectively as the primary photoproducts. On the other hand, 2-, 3-, and 4-pyridinylacetic acids undergo photodecarboxylation in aqueous solutions via a heterolytic mechanism from their zwitterionic forms. 

In 1961 Jimi proposed the two-site theory of the mechanism of acyloin formation by pyruvate decarboxylase [15]. This theory was later confirmed by others [18,28]. According to the model, at the first site pyruvate is decarboxylated to an aldehyde-diphosphatamine complex (HETPP) called active acetaldehyde. The active acetaldehyde moiety is then irreversibly transferred to the second site, where reversible dissociation to free aldehyde takes place. The model is based on the observation that pyruvate decarboxylase not only forms free acetaldehyde as the major end-product of decarboxylation of an a-keto acid but also catalyzes formation of C-C bonds via an acyloin reaction in which free aldehyde competes with a proton for bond formation with the a carbanion of EDETPP. Thus the addition of a C2 unit equivalent to acetaldehyde by means of HETPP to a carbonyl group results in an (i )-hydroxy ketone [29]. For instance, the production of acetoin (methylacetyl carbinol) results when acetaldehyde is allowed to accumulate or is added to the reaction mixture [28]. This phenomenon was confirmed using pyruvate decarboxylase from different sources (wheat germ, yeast, and bacteria) [15,28,30]. 

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