Thermal Decomposition of the Azides

Nevertheless, recent studies of the thermal decomposition of single crystals and compacts of the inorganic azides (Chapter 6) have brought out certain general features of the processes which increase our understanding of the macroscopic mechanisms by which fast reactions may be initiated and propagated. The general picture is one in which reaction occurs at preferred faces of the crystals [those perpendicular to the planes of azide ions in the crystal structure 

Using single crystals, it has been shown that thermal decomposition rates depend on the available areas of preferred surface and that induction periods decrease with increase in the surface area. However, virtually no other information is available on the sensitivities of different crystallographic faces of the azides. It is a matter of interest to determine whether the different thermal sensitivities are reflected in different sensitivities to adiabatic compression of the gas in the vicinity of the surfaces, or to electrostatic discharge, impact, or friction. Current- or field-initiation experiments on different azide surfaces (see the previous section) are also suggested by these observations. 

While it is plausible that dislocations also act as internal surfaces which serve as centers for the initiation of decomposition, detailed studies of the role of dislocations require further examination. Recent advances in electron-microscope techniques may permit direct observation of the role of dislocations and impurities in the mechanisms of colloid or nucleus formation. Indeed, such studies on alkali halide crystals (at liquid-helium temperatures, to avoid radiation damage by the electron beam) confirmed the special role of dislocations in sensitizing the nucleation of colloids [52]. Such techniques should be possible for at least the (more stable) alkali azides. 

Most seriously lacking at the present time is information which relates to regimes of fast thermal decomposition. The degree of control now possible with 

That some of the crystal parameters which affect slow decomposition are 

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In general, acyl azides are too unstable to survive at the temperatures required for addition to acetylenes, although benzoyl azide adds readily to ynamines in toluene. Ethoxycarbonyl azide also gives triazoles in good yield with ynamines. The azide adds to propargylic alcohols in boiling ethanol, and to acetylene at 100° under pressure. Addition to phenylacetylene and to electron-deficient acetylenes has been carried out at 130°. Oxazoles are also formed at this temperature by competing thermal decomposition of the azide, and addition of ethoxycarbonylnitrene to the acetylenes. The triazole obtained from phenylacetylene is 2-ethoxycarbonyl-4-phenyltriazole the two 1-ethoxycarbonyltriazoles can be isolated if the addition is carried out at 50° over several weeks. Since the IH- to -triazole isomerization takes place readily in these systems, a IH-structure cannot be assumed for a triazole formed by addition of these azides. 

The obtention of nitrides by thermal decomposition of the azides was only successful for the synthesis of [MNBr4]2- from [MN3Br5] . Thermolysis of MX5 with ammonium salts afforded a more general access to nitride halides in oxidation state V, or IV if higher temperatures were used.316,317 Oxo nitrides were obtained if moisture was admitted during the process.318... 

Thieno[2,3-c][l,2,5]oxadiazole 1-oxide (12) was prepared in 45% yield by thermal decomposition of the azide (89a). This oxadiazole oxide (12) was also prepared, albeit in lower yield, from the amine (89b) <74JOC2956>. 

Cesium is an alkali metal that reacts explosively with water and melts just above room temperature. The word cesium is derived from caesium (Latin for sky blue ). The name was chosen because of the blue lines observed by Robert Bunsen and Gustav Kirchhoff during their analysis of springwater with a spectroscope in 1860. Currently, cesium metal is generated via thermal decomposition of the azide, electrolysis of molten CsCN, or reduction of molten CsCl with calcium vapor followed by fractional distillation. 

It can be isolated by elecytrolysis of the fused cyanide and by a number of other methods. Very pure, gas-free cesium can be prepared by thermal decomposition of cesium azide. 

The air bag industry has become one of the principal users of pyrotechnic compositions in the world. Most of the current air bag systems are based on the thermal decomposition of sodium azide, NaN, to rapidly generate a large volume of nitrogen gas, N2. Air bag systems must function immediately (within 50 ms) upon impact, and must quickly deploy a pulse of reasonably cool, nontoxic, unreactive gas to inflate the protective cushion for the driver or passenger. These formulations incorporate an oxidizer such as iron oxide to convert the atomic sodium that initially forms into sodium oxide, Na20. Equation 1 represents the reaction. 

Generation of phenylnitrcne by thermal decomposition of phenyl azide in the same solvent mixture, or by deoxygenation of nitrosobenzene with triethyl phosphite in the absence of the trifluoroethanol, fails to yield the 1//-azepine. The role of the alcohol in promoting l//-azepine formation is not understood. 

The thermal, and more importantly, the photolytic decomposition of aryl azides in the presence of nucleophiles, generally amines or alcohols, is the commonest method for preparing 3H-azepines. In fact, jV-phenyl-3//-azepin-2-amine (32, R = Ph), the first example of a 3//-azepine, was prepared by thermal decomposition of phenyl azide in aniline.32... 

Sulfonylnitrenes are formed by thermal decomposition of sulfonyl azides. Insertion reactions occur with saturated hydrocarbons.255 With aromatic compounds the main products are formally insertion products, but they are believed to be formed through addition intermediates. 

Ferrous chloride-hydrochloric acid mixtures catalyzed the thermal decomposition of sulphonyl azides in isopropyl alcohol to give occasionally almost quantitative yields of sulphonamide and acetone, and the molar ratio of azide consumed to ferric chloride formed was typically of the order of 20 to 1 21>. 

Copper catalyzes the decomposition of sulphonyl azides in benzene very slowly. When methanesulphonyl azide was boiled under reflux in benzene solution in the presence of an excess of freshly reduced copper powder, some decomposition occurred to give methanesulphonamide and azide was recovered 78>. Transition metal complexes have been found to exert a marked effect upon the yields of products and isomer ratios formed in the thermal decomposition of methanesulphonyl azide in methyl benzoate and in benzotrifluoride 36>. These results will be discussed in detail in the section on the properties of sulphonyl nitrenes and singlet and triplet behaviour. A sulphonyl nitrene-iron complex has recently been isolated 37> and more on this species will be reported soon. 

Starting materials other than sulphonyl azides have been used as possible sources of sulphonyl nitrenes. The decomposition of the triethyl-ammonium salt of iV- -nitrobenzenesulphonoxybenzenesulphonamide (26) in methanol, ethanol, and aniline gave products derived from a Lossen-type rearrangement 20> (Scheme 3). It was felt that the rearrangement did not involve a free sulphonyl nitrene since, when the decomposition was carried out in toluene-methylene chloride or in benzene, no products (benzenesulphonamides) of substitution of the aromatic solvent nucleus were found (as are usually found with sulphonyl nitrenes from the thermal decomposition of the corresponding azides). On the other... 

Dining the preparation of cellular rubber by thermal decomposition of calcium, strontium or barium azides, various additives were necessary to prevent explosive decomposition of the azide in the blended mixture. 

Formation of 2//-azirines by thermal decomposition of vinyl azides has been shown to exhibit small entropy of activation and insensitivity to solvent polarity acyclic vinyl azides decompose more readily than analogous cyclic ones and it is advantageous to have a hydrogen atom cis to the azido group ( -are more reactive than Z-isomers). These results and the linear correlation found for ring-substiment effects on decomposition of a-styryl azides are consistent with a nonconcerted mechanism in which elimination of nitrogen and cyclization into a three-membered ring proceeds synchronously. 

Example VI Prediction and Detection of 2H-Azirine as Intermediate in the Thermal Decomposition of Vinyl Azide. Predictions concerning the hard-to-prove nonexistence of molecules are less convincing to the lay-colleague than those, which have stimulated successful experiments. Therefore, this last example has not only been selected because of its higher complexity, but rather due to the PE spectroscopic verification of the results anticipated by a preliminary hypersurface study (4,36). 

Thermal decompositions of alkyl azides are advantageously studied in millimole quantities using a PE spectroscopically controlled flow system under low pressure ( ), thereby reducing the hazards involved in handling these explosive compounds in bulk. Our investigations started with methyl azide, which splits off nitrogen unexpectedly only at temperatures above 500° C (37) ... 

Cesium is obtained from its ore pollucite. The element in pure form may be prepared by several methods (i) electrolysis of fused cesium cyanide, (ii) thermal reduction of cesium chloride with calcium at elevated temperatures, and (iii) thermal decomposition of cesium azide. It is stored under mineral od. The element must be handled under argon atmosphere. 

A few years ago an old reaction, that of the thermal decomposition of arylsulfonyl azides in pyridines, was re-examined (Scheme 42) (72JOC2022). Among the products identified was the 3-phenylsulfonamido derivative (96), which was proposed to arise by an electrophilic attack by phenylsulfonylnitrene. 

Despite patient and exhaustive effort by many researchers, all attempts to isolate or trap a benzazirine intermediate (214) have so far failed, and unequivocal evidence for their participation in either the photolytic or thermal decomposition of aryl azides is still awaited. Evidence in favor of the proposed reaction pathway (Scheme 22) comes from the work of Sundberg and coworkers, who succeeded in identifying 3-alkyl-2-diethylamino-lff-azepines as oxygen-sensitive, metastable intermediates in the photolysis of o-alkylphenyl azides in diethylamine (72JA513). Later studies on the flash photolysis of aryl azides in dialkylamine solution provided kinetic data which not only confirmed the Iff- to 3/f-azepine tautomer-ism, but also strongly supported the involvement of a benzazirine intermediate (74JA7491). 

According to Garner and Gomm [37] the activation energy of the thermal decomposition of lead azide is 38.0 kcal/mole, assuming that the reaction can be expressed by an equation of the form p=kt. 

The researches of Wischin [113] and those of Garner and Maggs [84] have shown that metallic nuclei are formed during the slow thermal decomposition of silver azide. These researches were carried out by means of an optical microscope. 

Other authors quote the following values for the activation energy of the thermal decomposition of silver azide ... 

Other tetraazapentalenes have been produced by photochemical or thermal decomposition of azidophenyl-l - and -2//-benzotri-azoles.339-341 Hall, Stephanie, and Nordstrom341 found that decomposition of the azide 259 in decalin at 170° led to a mixture of 260 and 261 in the ratio 1 9. The preference for cyclization to the position para to the methyl group to give 261 is consistent with the known electrophilic character of the intermediate nitrene. 

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