Azomethane decomposition

In a separate set of experiments designed to follow the gas phase reactions of CHj-radicals with NO, CHj- radicals were generated by the thermal decomposition of azomethane, CHjN NCHj, at 980 °C. The CH3- radicals were subsequently allowed to react with themselves and with NO in a Knudsen cell that has been described previously [12]. Analysis of intermediates and products was again done by mass spectrometry, using the VIEMS. Calibration of the mass spectrometer with respect to CH,- radicals was carried out by introducing the products of azomethane decomposition directly into the high vacuum region of the instrument. 

Figure B2.5.7. Oscilloscope trace of the UV absorption of methyl radical at 216 mn produced by decomposition of azomethane after a shock wave (after [M]) at (a) 1280 K and (b) 1575 K.

Both symmetrical and unsymmetrical azo compounds can be made, so that a single radical or two different ones may be generated. The energy for the decomposition can be either thermal or photochemical. In the thermal decomposition, it has been established that the temperature at which decomposition occurs depends on the nature of the substituent groups. Azomethane does not decompose to methyl radicals and nitrogen until temperatures above 400°C are reached. Azo compounds that generate relatively stable radicals decompose at much lower temperatures. Azo compounds derived from allyl groups decompose somewhat above 100°C for example ... 

The slow thermal decomposition of gaseous azomethane becomes explosive above... 

Recently the two-step decomposition of azomethane was proved in the study of the femtosecond dynamics of this reaction [68]. The intermediate CH3N2 radical was detected and isolated in time. The reaction was found to occur via the occurrence of the first and the second C—N bond breakages. The lifetime of CH3N2 radical is very short, i.e., 70fsec. The quantum-chemical calculations of cis- and /nmv-azomcthanc dissociation was performed [69]. 

Free radicals formed from an initiator in the gas phase take part in other reactions and recombine with a very low probability (0.1-2%). The decomposition of the initiator in the liquid phase leads to the formation of radical pairs, and the probability of recombination of formed radicals in the liquid phase is high. For example, the photolysis of azomethane in the gas phase in the presence of propane (RH) gives the ratio [C2H6]/[N2] = 0.015 [76]. This ratio is low due to the fast reactions of the formed methyl radicals with propane ... 

Transition complex, nonchain mechanism. The spontaneous decomposition of azomethane... 

No spin correlation effect was seen in photolysis of azocumene,ls9b perfluoroazomethane,196 or in azomethane itself.160,193 These results are again explained by the lack of any real triplet sensitized decomposition. 

Fig. 10.7 QRRK analysis [207] of azomethane, CH3N2CH3, unimolecular decomposition at 603 K (solid curve), and comparison with experimental data (points) from Ramsperger [326].

As an example calculation using QRRK theory, we consider the unimolecular decomposition of azomethane, CH3N2CH3, from Kassel s original paper [207], Kassel tested... 

Use QRRK theory to calculate kan as a function of pressure for the decomposition of azomethane at T = 563 and 603 K. Parameters needed for the calculation are given in Section 10.4.4, in the discussion of Fig. 10.7. Plot calculated rate constant in units of 1/s versus pressure (in atm) include in the same plot a comparison with experimental data of Ramsperger [326], which can be found in the data file azomethanedata.csv. 

H.C. Ramsperger. Thermal Decomposition of Azomethane over a Large Range of Pressure. J. Am. Chem. Soc., 49 1495-1512,1927. 

The constant for the decomposition of gaseous propionic aldehyde falls away steadily below about 80 mm., that for the decomposition of diethyl ether below about 150 mm., that for the decomposition of diethyl ether below about 300 mm. Several other ethers, dipropyl ether, methyl propyl ether and methyl ethyl ether behave in a similar manner. The velocity constant for the decomposition of azomethane also diminishes but not until lower pressures are reached for example at 290° C. k at 0-259 mm. has one-fourth of its value at 707-9 mm. In several reactions, such as the racemization of pinene, and the decomposition of gaseous acetone the falling off of the velocity constant has not actually been looked for. The decomposition of azoisopropane is unimolecular down to pressures of 0-25 mm. 

Rice and Ramsperger find that the constants for the decomposition of azomethane fall off sooner at higher... 

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