THERMAL DECOMPOSITION OF ACETALDEHYDE

Acetaldehyde decomposes at a measurable rate in the temperature range 450-600 °C. At the pressures generally employed (30-300 torr), the main products are CO and CH4, which are formed in nearly equimolar amounts. Ethane, ethylene, hydrogen and propionaldehyde have also been detected. Under certain conditions small amounts of ethanol and acetylene were observed. At very low pressures the reaction products and the kinetics of the reaction seem to differ completely from normal  

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A possible free-radical chain mechanism for the thermal decomposition of acetaldehyde (to CH4 and CO) is the Rice-Herzfeld mechanism (Laidler and Liu, 1967) ... 

Iodine accelerates the decomposition of acetaldehyde In the steady-state range, the order is approximately 1.0 and 0.5 in aldehyde and iodine, respectively. The experimental results of Rollefson and Faull have been reinterpreted and added to by O Neal and Benson . The iodine-catalysed reaction is a free radical chain process initiated by the attack of an iodine atom on the acetaldehyde molecule. The proposed mechanism fits the experimental data very well. The thermal decomposition of acetaldehyde is catalysed also by other halogens and halogen compounds . 

Quantitative investigations on the reactions sensitized by azomethane , bi-acetyF and di-t-butyl peroxide , as well as those on the iodide-catalyzed decomposition , all gave an order of with respect to the sensitizer and that of unity with respect to the acetaldehyde. The agreement between the overall reaction orders of the thermal decomposition of acetaldehyde and of the sensitized decompositions seems to support the suggestion that both involve chain mechanisms. 

The problem of the chain processes in the thermal decomposition of acetaldehyde could be settled, in theory, by carrying out experiments with inhibitors. However, the data, and especially the explanations suggested, are rather contradictory. 

The minor products of the thermal decomposition of acetaldehyde, around 500 °C, are Hj, CjHg, C2H4, CH3COCH3, C2H5CHO and CO2. A number ot these cannot be explained by the Rice-Herzfeld mechanism. Figs. 1 and 2 illustrate the amounts of these products as a function of time. 

Fig. 1. Formation of minor products in the thermal decomposition of acetaldehyde at 523 °C...

According to our present knowledge, the recombination of methyl radicals is the dominant chain termination step in the thermal decomposition of acetaldehyde, though ethane is only a minor product of the reaction ... 

The Rice-Herzfeld mechanism for the thermal decomposition of acetaldehyde is ... 

It not infrequently happens that a chain reaction and a molecular reaction take place concurrently and make contributions of comparable magnitude to the total observed chemical change. In the thermal decomposition of acetaldehyde vapour, for example, there are probably two major mechanisms, a direct molecular rearrangement CH3CHO = C0-[-CH4, and a chain process similar to (3) above. The activation energy, Ui, for the formation of radicals is very much higher than that for the rearrangement, Ii, and in consequence the number of molecules which initiate chains is smaller in about the ratio than the number which suffer simple... 

A following mechanism has been proposed for a thermal decomposition of acetaldehyde ... 

This next example is not quite perfect because it gives a solution with a leftover radical unaccounted for. However, it is shown here as an example of what to expect in research. Suppose we want to understand the thermal decomposition of acetaldehyde. Rice and Herzfeld [10] studied the thermal cracking of hydrocarbons as part of a very important study related to petroleum processing. Here, we present the thermal cracking of acetaldehyde. Consider the following scheme for the thermal decomposition reaction [11] ... 

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