Azomethane, chain sensitization

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. 

The extreme sensitivity of CH3CHO to chain decomposition makes it very susceptible to free radical sensitization. Thus OH3 radicals from the pyrolysis of azomethane can induce the chain decomposition in CIIsCHO at 300°C, the chain length being as great as 500. The photolysis of azomethane at room temperature can also sensitize the decomposition. Letort showed that CH3 radicals from dtBP will decompose as many as 50 molecules of CH3CHO per molecule of dtBP at 160 0. 

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. 

Steel and Trotman-Dickenson studied the reaction in a static system and showed that it is inhibited by propene at high pressures of azomethane, but accelerated at low pressures. The accelerating effect was evident with inert gases as well and was attributed to the increased rate of energization. The inhibiting effect along with the observed surface sensitivity of the reaction was taken as proof of the chain nature of the decomposition. The reaction was further shown to be homogeneous and first order with a rate coefficient of 10 exp (—51,200/RT) sec when fully inhibited with propene. 

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See also in sourсe #XX -- [ ]

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