Azomethane

The pyrolysis of azomethane which yields mainly nitrogen and ethane, has been extensively studied [138, 139]. At high temperatures the decomposition is explosive [140]. The evidence suggests that the explosion is of thermal origin above 636 °K, while below this temperature there are indications that chain-branching plays a significant part. 

This reaction has been studied over an unusually wide range of temperature [141] and the activation energy is known to be about 53 kcal. mole , from about 500 to 1300 °K. The results from the study of explosive reaction [140], however, lead to a value of 32 kcal. mole .  

The first studies on the thermal decomposition of this molecule had been carried 

more detailed analytical studies have indicated the intervention of free radicals in the mechanism and the operation of short chains. It is interesting to speculate though as Trotman-Dlckenson has remarked that if all these decompositions had been studed analytically, much confusion might have been avoided, but probably the development of the theory of energy transfer would have been delayed by ten or twenty years . 

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. 

The inhibited decomposition was studied again by Forst and Rice using ethylene, propene and nitric oxide. The addition of any of these scavengers was found to reduce the rate of decomposition as monitored by nitrogen evolution and also the ratio of CH4/N2 in the product, but each inhibitor affected both quantities to a different extent. Nitric oxide appears to be the most efficient inhibitor. As is the usual case, nitre oxide functions not only as an inhibitor but at higher pressures as an accelerator as well. The fully NO inhibited reaction was thought to correspond to the initial homogeneous unimolecular decomposition reaction 

The radical N2CH3, if formed at all, would probably decompose in the first vibration for thus far it has not been possible to scavenge this radical N2 (formed)/ azomethane (decomposed) is not affected by scavengers. 

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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.

Heterocyclics of all sizes, as long as they are unsaturated, can serve as dipolarophiles and add to external 1,3-dipoles. Examples involving small rings are not numerous. Thiirene oxides add 1,3-dipoles, such as di azomethane, with subsequent loss of the sulfur moiety (Section 5.06.3.8). As one would expect, unsaturated large heterocyclics readily provide the two-atom component for 1,3-dipolar cycloadditions. Examples are found in the monograph chapters, such as those on azepines and thiepines (Sections 5.16.3.8.1 and 5.17.2.4.4). 

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 ... 

Benzyl ethers of phenols can also be prepared by reaction with phenyldi-azomethane. 

Azomethane decomposes into nitrogen and ethane at high temperatures according to the following equation ... 

Azomethane. CA Registry No 503-28-6. The following supplements the article in Vol 1, A655-R under Azomethane Preparation. It has been prepd by the oxidn of N,N -dimethylhydrazine with K dichromate (Ref 2). The action of Cu(II) sulfate in aq Na acetate contg HC1, Na chloride, or Cu(II) chloride on the same hydrazine gives the Cu(l) chloride complex of azomethane (Refs 3 13)... 

Its addn at a level of 0.1—5.0% was found to improve the octane rating of diesel fuels (Ref 10) Cuprous Chloride Complex. For prepn see above. It is a red cryst solid, readily decompd into its components at 135—40° (Refs 4 13). X-ray diffraction showed that the azomethane mols lie betw the infinite folded sheets of the Cu(I) chloride (Ref 11). The complex is used in the prepn of highly pure samples of azomethane (Ref 7)... 

Self-Test 13.6B Azomethane, CH N2CH3, decomposes to ethane and nitrogen gas in the reaction CH N2CH (g) - CH CH3(g) + N2(g). The reaction was followed at 460. K by measuring the partial pressure of azomethane over time ... 

Confirm that the reaction is first order of the form Rate = kP, where P is the partial pressure of azomethane, and find the value of k. 

Azoxy-Verbindungen werden durch Pentacarbonyleisen3,4 bzw. Dinatrium-decacarbo-nyl-dichrom5 zu den entsprechenden Azo-Verbindungenreduziert. So erhalt manz. B. aus Hexafluor-azoxymethan 50% d.Th. Hexafluor-azomethan (Kp7,l0 -320)3 bzw. aus 2,2 -Dimethyl-azoxybenzol 65% d.Th. 2,2 -Dimethyl-azobenzol4. 

Phenyldiazonium-Salze bilden mit Methyl-magnesiumjodid ein Gemisch aus Biphenyl, Jodbenzol und Ben-zol-azomethan (s.ds. Handb., Bd. X/3, S. 167). 

Recently, Stair and coworkers [10, 11] developed a method to produce gas-phase methyl radicals, and used this to study reactions of methyl groups on Pt surfaces [12] and on molybdenum oxide thin films [13]. In this approach, methyl radicals are produced by pyrolysis of azomethane in a tubular reactor locat inside an ulttahigh vacuum chamber. This method avoids the complications of co-adsorbcd halide atoms, it allows higher covraages to be reached, and it allows tiie study of reactions on oxide and other surfaces that do not dissociate methyl halides effectively. 

Methyl radicals were produced by pyrolysis of azomethane (CH3N2CH3). Azomethane was synthesized as describe earlier [18]. It was purified periodically by fteeze-pump cycles at 77 K, and the gas purity verified by RGA. The methyl radical source was similar to that developed by Stair and coworkers. [10, 11] The source was made of a quartz tube with 3 mm OD and 1 mm ID, resistive heating was supplied by means of a 0.25 mm diameter tantalum wire wrapped outside the quartz tube. The len of the heating zone was 4 cm, recessed from the end of the tube by 1 cm. An alumina tube around the outside of the heating zone served as a radiation shield. Azomethane was admitted to the hot tube at a pressure of 1x10-8 to 1x10-7 Torr via a high-vacuum precision leak valve. The pyrolysis tube was maintained at about 1200 K, adequate to decrease the major peaks in the mass sp trum of the parent azomethane at 58 and 43 amu by at least a factor of 100. 

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. 

Shortly after, Doetschman and Hutchison reported the first example of a reactive carbene in the crystalline solid state, by preparing diphenylcarbene from diphenyldi-azomethane in mixed crystals with 1,1-diphenylethylene 84 (Scheme 7.23). When the mixed crystals were irradiated, carbene 85 was detected by electron paramagnetic resonance (EPR) and the disappearance of the signal was monitored to determine its kinetic behavior. Two reactions were shown to take place under topochemical... 

The photochemistry of the simplest member of the azo family, azomethane, has been extensively investigated in the vapor phase(1) and in solution(2) ... 

Photolysis in the gas phase leads to the quantitative production of nitrogen and methyl radicals. Photolysis in solution, however, results in a shift in the absorption spectrum to longer wavelengths due to the production of a new species, which is identified as the cw-azomethane (the trcms configuration is the normal isomer). Similarly, irradiation of tro/u-azoisopropane<3) results in trans-cis isomerization to the cis isomer ... 

Nitromethan 1942 Methylamin 1950, 1957 MW (47) Methylisocyanat 1940 Azomethan 1935 Athylamin 1950 1,2-Dimethylhydrazin 1948 N.N -Dichlorpiperazin 1949 Hexamethylentetramin 1938... 

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

Using an alternative route involving [3+2] cycloaddition of phosphoranediyldi-azomethane (43) with trimethylsilylphosphaethyne (44), 5-trimethylsilyl derivative (45) of the zwitterionic 3-phosphino-[l,2,4]diazaphospholide (42) was obtained (Scheme 13) [44],... 

Kobaek-Larsen M, Christensen L P, Vach W, Ritskes-Hoitinga J and Brandt K (2005), Inhibitory effects of feeding with carrots or (-) falcarinol on development of azomethane-induced preneoplastic lesions in the rat colon , J Agric Food Chem, 53, 1823-1827. 

The regioselective ozonation of alkylidene-sultams 282 followed by reaction with di azomethane leads to the formation of highly reactive bicyclic trioxo-isothiazolidine 284... 

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 ... 

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