Free radical chain reactions acetaldehyde decomposition

Photodecomposition. The photochemistry of acetaldehyde has been studied by several investigators in the past. Both the primary processes and the free-radical chain processes have been studied, but it has not been until recently that a reasonable understanding of the primary processes has been achieved. If free-radical chain reactions are assumed to be secondary processes, three decomposition pathways can be envisaged ... 

The decomposition of acetaldehyde (CH3CHO) to methane and carbon monoxide is an example of a free radical chain reaction. The overall reaction is believed to occur through the following sequence of steps ... 

The possible extent to which free radical chains may account for the thermal decomposition of organic molecules in the gas phase was first emphasized by Rice and Herzfeld.26 They gave three examples showing how all the known facts in the decomposition of acetone, acetaldehyde and ethane could be explained by chain reactions involving free radicals. Their calculations showed that the first order character of the reaction could be maintained under proper conditions and they estimated reaction rates and temperature coefficients in agreement with the facts. 

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 . 

For the thermal decomposition of acetaldehyde, a chain reaction propagated by free radicals is postulated ... 

Experimental evidence of the part played by free radicals in a chemical reaction was soon forthcoming. In 1934 Frey24 found that butane decomposed very slowly at 525° but that if one per cent of dimethyl mercury was introduced the decomposition proceeded rapidly. In the same year Sickman and Allen25 found that acetaldehyde was stable at 300° but that it was decomposed completely when a few per cent of azomethane was added. The introductions of dimethyl mercury or azomethane at these temperatures apparently liberated free radicals which initiated chains. Moreover when mixed gases decomposed simultaneously they did not do so independently. The products contained groups from each in a way that could be easily explained on the assumption of the liberation and recombination of free radicals. Again the appearance of butane from the decomposition of propane is difficult to explain on any hypothesis except on the assumption that some free radicals of CH3 are split out and that they become attached to propane molecules. More direct examples will be given later in the discussion of photochemistry. 

Chemical Sensitization. Equally valuable for the demonstration of the existence of free radicals and for their study are methods of chemical sensitization. Free radicals may be demonstrated to exist in a reaction by their ability to produce a sensitized decomposition of a material normally inert at the temperature employed. Thus, it has been demonstrated that, whereas acetaldehyde does not decompose at an appreciable rate at 300 0, a fast decomposition can be induced at that temperature by adding azomethane (CH3)2N2 in small amounts. The role of the azomethane is to produce methyl radicals which can then start a chain decomposition. Oxygen is similarly a chemical sensitizer for the decomposition of many hydrocarbons and aldehydes. 

Later these experiments were repeated -with the conclusion that Morris findings were dubious. Zemany and Burton used equimolar mixtures of acetaldehyde and acetaldehyde- /4, at temperatures 510 and 465 °C, and found that partially deuterated methanes were formed in appreciable amounts. The ratio CHD3/ CD4 was found to be 1.2 and 1.0 at 510 and 465 °C, respectively (compared to the value of 1.6 obtained in the photolysis at 140 °C). These results clearly indicate the free radical origin of the methane. However, the fact that the CHD3/CD4 ratio is lower than the one found in the photolysis made the authors conclude that there IS some contribution from the molecular mechanism. An upper limit for the latter was estimated to be approximately 15 and 25 % of the total reaction at temperatures 510and465 °C, respectively. Zemany and Burton estimated the values for the ratios methane-rf3/ethane-d6 and methane-t /ethane-rfe, from which a chain length of 1000 can be derived, at 465 °C, for the Rice-Herzfeld type decomposition. 

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