Acetaldehyde pyrolysis mechanism

Since the conversion of C2H5 into C2H4 + H is indeed the slow step in the ethane pyrolysis, the occurrence of this reaction does explain the non-zero rates at maximal inhibition and the increase in rate at high NO concentrations. On the other hand, for reactions like the acetaldehyde pyrolysis the g - P transition is not rate limiting, and the Norrish-Pratt mechanism then gives no explanation for the behavior. Also, the Norrish-Pratt mechanisms as originally written down do not explain the large amounts of products such as H2O, N2 and N2O that are found in the ethane pyrolysis. 

The decompositions of the formyl and acetyl radicals are certain to be in the fall-off region at the pressures used in the investigations of the acetaldehyde pyrolysis. However, the complexity of the mechanism impedes any conclusion to be drawn from this system. 

Whereas acetone shows little tendency to undergo chain decomposition in photolysis or pyrolysis, acetaldehyde has been found to decompose by a chain mechanism which tends to quite sizable chain lengths as the temperature is raised. As a consequence of this behavior, the dec(3mposition has been found to be remarkably sensitive to the presence of small amounts of substances that can form free radicals more readily than pure acetaldehyde does. A further result of this sensitivity is that the data on the pyrolysis obtained under different conditions or in different laboratories show quite important discrepancies. In compensation for these difficulties the stoichiometry of the pyrolysis seems to be quite simple, the products being CO + CH4, together with very small amounts of C2H6 and also some II2 at temperatures near 500°C. These can be represented by ... 

The fact that there is a significant increase in the rate of methane formation shows that the NO is providing some entirely different mechanism for CH4 production. In this connection, it is interesting that in the ethane pyrolysis these is no HCN, which is formed in significant amounts in the acetaldehyde decomposition. A likely source of HCN is... 

Morris analysed the methanes formed in the pyrolysis of acetaldehyde and acetaldehyde-d by infrared spectrophotometry. If the decomposition were an intramolecular reaction then only CH4 and CD4 could be expected, while CH3D, CH2D2 and CHD3 could not. However, in a chain reaction partially deuterated methanes should be formed. With equimolar mixtures of acetaldehyde and acet-aldehyde-d4 at temperatures 480 and 535 °C, respectively, only CH4 and CD4 were formed. On the basis of these findings it was concluded that in the thermal decomposition of pure acetaldehyde mainly a unimolecular mechanism is operative the contribution of the chain decomposition was regarded to be only 10-20 % at the most. 

It was proved by a separate experiment that isotope mixing in a mixture of methane and methane-i proceeds very slowly even above 600 °C. Thus, it must be concluded that, in the pyrolysis, the formation of the partially deuterated methanes is a result of free radical reactions and not of the secondary exchange of the methanes. Consequently, these results support the free radical mechanism of the acetaldehyde decomposition. 

As shown in the pyrogram of poly(L-lactide), the main pyrolysis products are CO and acetaldehyde. The pyrolysis probably takes place by a free radical mechanism including the following reactions ... 

Amorphous and semi-crystalline polypropylene samples were pyrolyzed in He from 388°-438°C and in air from 240°-289°C. A novel interfaced pyrolysis gas chromatographic peak identification system was used to analyze the products on-the-fly the chemical structures of the products were determined also by mass spectrometry. Pyrolysis of polypropylene in He has activation energies of 5-1-56 kcal mol 1 and a first-order rate constant of JO 3 sec 1 at 414°C. The olefinic products observed can be rationalized by a mechanism involving intramolecular chain transfer processes of primary and secondary alkyl radicals, the latter being of greater importance. Oxidative pyrolysis of polypropylene has an activation energy of about 16 kcal mol 1 the first-order rate constant is about 5 X JO 3 sec 1 at 264°C. The main products aside from C02, H20, acetaldehyde, and hydrocarbons are ketones. A simple mechanistic scheme has been proposed involving C-C scissions of tertiary alkoxy radical accompanied by H transfer, which can account for most of the observed products. Similar processes for secondary alkoxy radicals seem to lead mainly to formaldehyde. Differences in pyrolysis product distributions reported here and by other workers may be attributed to the rapid removal of the products by the carrier gas in our experiments. 

Poly(ethylene terephthalate) decomposes upon heating through a series of different reactions. These run either concurrently or consecutively. The result is a complex mixture of volatile and nonvolatile products. It was found that when poly(ethylene terephthalate) is maintained in molten condition under an inert atmosphere at 282-323°C, it slowly converts to a mixture of gaseous low molecular weight fragments [581]. The major products from pyrolysis of poly(ethylene terephthalate) are carbon dioxide, acetaldehyde and terephthalic acid. In addition, there can be detected trace amounts of anhydrides, benzoic acid, p-acetylbenzoic acid, acetophenone, vinyl benzoate, water, methane, ethylene, acetylene, and some ketones [505]. The following mechanism of degradation was postulated [505] ... 

Another pyrolysis whose kinetics is described by the Rice-Herzfeld mechanism is the decomposition of acetaldehyde ... 

EXAMPLE 3 Pyrolysis of acetaldehyde in the presence of iodine. The eight steps of the chain mechanism are replaced by the following... 

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