Acetaldehyde ions, decomposition

No metastable peak has been observed for loss of a methyl group following El of acetaldehyde, CH3CHO however, the metastable ion decomposition to lose CD3 has been seen with CD3CDO [695], This intermolecular isotope effect has been interpreted in terms of the isomerisation (CH3CHO)f - (CH2=CHOH)t occurring in the unlabelled molecule and precluding methyl loss [695]. 

Two examples are (1) the thermal, gas-phase decomposition of acetaldehyde at high temperatures and (2) the reaction of the hydrated 2-propylchromium ion with molecular oxygen in aqueous solution. The reactions and their rate laws are as follows ... 

The mechanism of the acid-catalyzed decomposition of 1-alkyltriazolines has been studied <93JOC2097>. The hydrolytic decomposition of these triazolines in aqueous buffers leads predominantly to 1-alkylaziridines with lesser amounts of 2-(alkylamino)ethanol, alkylamines, and acetaldehyde. The rate of hydrolysis of 1-alkyltriazolines is about twice as fast as that of the analogous acyclic 1,3,3-trialkyltriazenes and varies in the order t-butyl > isopropyl > ethyl > butyl > methyl > propyl > benzyl <92JOC6448>. The proposed mechanism, involving rate-limiting formation of a 2-(alkylamino)ethyldiazonium ion, is shown in Scheme 65. A theoretical study ab initio calculation) of the acid-induced decomposition of 4,5-dihydro-l,2,3-triazolines has also been reported <91JA7893>. 

Alcohols. In a reaction reminiscent of diazonium ion chemistry, 26 is reduced by ethanol to 13 (R = H). The ethanol is oxidized to acetaldehyde (72UP1). Like water, decomposition of 26 in f-butanol gives nitrogen in quantitative yield, but the other product is intractable. 

To understand the significant effect of catalyst nature, a better understanding of the main reactions, peracetic acid decomposition, and its reaction with acetaldehyde was needed. A literature -survey showed that the kinetics were not well studied, most of the work being done at very low catalyst concentration 1 p.p.m.), and there is disagreement with respect to the kinetic expressions reported by different authors. The emphasis has always been on the kinetics but not on the products obtained, which are frequently assumed to be only acetic acid and oxygen. Consequently, the effectiveness of a catalyst was measured only by the rates and not by the significant amount of by-products that can be produced. We have studied the kinetics of these reactions, supplemented by by-product studies and experiments with 14C-tagged acetaldehyde and acetic acid to arrive at a reaction scheme which allows us to explain the difference in behavior of the different metal ions. 

With manganese and cobalt acetate the reaction of peracetic acid with acetaldehyde is very fast, and AMP is not detected. By comparing our rates with literature values of k17, k.17, and k18 we cannot propose a mechanism in which the only role of the metal ion is to catalyze the decomposition of AMP. The experimental rates in the presence of either manganese or cobalt acetates are much faster than the noncatalytic rate of formation of AMP. Thus, AMP per se is probably not an intermediate in the presence of these catalysts. 

We believe that catalysis occurs by formation of a complex between acetaldehyde, peracetic acid, and the metal ion in the 3+ oxidation state. The metal ion could be acting as a superacid as for peracetic acid decomposition, although oxidation-reduction reactions within the complex cannot be ruled out. Here again, we have found a disturbing lack of catalytic activity of other trivalent metals (aluminum, iron, and chromium). Simple acid catalysis is not as effective as proved when using p-toluenesulfonic acid and acetyl borate. This indicates that at least more than one coordination position is needed to obtain a complex of the proper configuration. 

Oxidation of Acetaldehyde. When using cobalt or manganese acetate the main role of the metal ion (beside the initiation) is to catalyze the reaction of peracetic acid with acetaldehyde so effectively that it becomes the main route to acetic acid and can also account for the majority of by-products. Small discrepancies between acetic acid efficiencies in this reaction and those obtained in acetaldehyde oxidation can be attributed to the degradation of peracetoxy radicals—a peracetic acid precursor— by Reactions 14 and 16. The catalytic decomposition of peracetic acid is too slow (relative to the reaction of acetaldehyde with peracetic acid) to be significant. The oxidation of acetyl radical by the metal ion in the 3+ oxidation state as in Reaction 24 is a possible side reaction. Its importance will depend on the competition between the metal ion and oxygen for the acetyl radical. 

The dependence of the rate upon the inverse of the hydrogen ion concentration (base-catalysis) is reasonably attributed to the necessity for the coordinated water molecule to lose a proton. The resulting ethyl-ene-hydroxypalladium species (the cis isomer), I, is then believed to undergo an internal addition reaction of the hydroxyl group to the coordinated ethylene to form the dichloro-2-hydroxyethylpalladium anion, II. The final step is a decomposition of the last compound into acetaldehyde, palladium metal, hydrogen ion and chloride anions. 

The existence of a free carbonium ion such as VII in a strongly solvating medium is highly improbable. Only if VII could exist in association with the palladium could decomposition to vinyl acetate be expected to occur with a reasonable degree of frequency, in competition with the reaction with acetate to form ethylidene diacetate. Similar results have been reported in the Wacker acetaldehyde synthesis when D2O is used as the solvent (25). Stern (54) has reported results in which 2-deuteropropylene was used as substrate in the reaction. Based on assumed /J-acetoxyalkylpalladium intermediates, on the absence of an appreciable isotope effect in the proton-loss step, and on the product distribution observed, excellent agreement between calculated (71%) and observed (75%) deuterium retention was obtained. Several problems inherent in this study (54) have been discussed in a recent review (I). Hence, considerable additional effort must be expended before a clear-cut decision can be made between a simple / -hydrogen elimination and a palladium-assisted hydride shift in this reaction. 

The effect of substituent groups was also studied. In a series of N-substituted hydrazones, PhNRN CMeEt, cyclization proceeded with increasing difficulty in the series R = H, Me, Et, Ph this was ascribed to the effect of increasing steric hindrance [12]. From acetaldehyde phenylhydrazones with methoxy substituents on the aromatic ring, yields were poor, which was attributed to decomposition of the substrate to non-volatile products on the alumina catalyst surface [13]. In contrast, with an acidic ion-exchange resin as catalyst, higher yields were obtained with the 4-methoxy derivative than with the 4-nitro this was interpreted as the effect of methoxy in facilitating adsorption on the resin [14]. 

Catalyst studies have promoted attention with description of the use of iron salts to prevent ether formation during ester exchange polymerization. Model compounds have been employed to elucidate the meehanisms of metal ion catalysis in both transesterification and polycondensation reactions. A differential microcalorimeter has been used to assess the relative reactivities of catalyst systems for the poly-transesterification of bis-(2-hydroxyethyl tere-phthalate) and the relationship between the viscosity of the polymerizate and the temperature of the maximum rate of heat production has been investigated. Studies on antimony(v) compounds have indicated that their activity increases during the course of 2GT synthesis. This observation has been ascribed to the reduction of the antimony(v) compounds by acetaldehyde produced by 2GT decomposition. 

Other procedures are based upon determination of the decomposition products of poly-oxyalkalene compounds. By reaction with phosphoric acid, ethylene oxide and propylene oxide moieties are converted, with 80-90% yield, to acetaldehyde and propionaldehyde, which may be measured spectrophotometrically (106). By more drastic decomposition with perchloric acid and periodate ion, the oxyalkalene compounds are converted to formaldehyde, with 55-65% yield, which may be distilled and determined colorimetrically (107). Neither of these methods has been much used. 

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