Exothermic thermal decomposition

Some unsaturated ketones derived from acetone can undergo base- or acid-catalyzed exothermic thermal decomposition at temperatures under 200°C. Experiments conducted under adiabatic conditions (2) indicate that mesityl oxide decomposes at 96°C in the presence of 5 wt % of aqueous sodium hydroxide (20%), and that phorone undergoes decomposition at 180°C in the presence of 1000 ppm iron. The decomposition products from these reactions are endothermic hydrolysis and cleavage back to acetone, and exothermic aldol reactions to heavy residues. 

From TG/DTA analysis of the dried resin with Fe, the resin was found to show a major weight loss with an exothermic thermal decomposition at 473 873 K followed by a slight endothermic weight decrease at above 873K. About 20wt% of the carbonized resin remained at 1273K. 

Ozone can be destroyed thermally, by electron impact, by reaction with oxygen atoms, and by reaction with electronically and vibrationaHy excited oxygen molecules (90). Rate constants for these reactions are given ia References 11 and 93. Processes involving ions such as 0/, 0/, 0 , 0 , and 0/ are of minor importance. The reaction O3 + 0( P) — 2 O2, is exothermic and can contribute significantly to heat evolution. Efftcientiy cooled ozone generators with typical short residence times (seconds) can operate near ambient temperature where thermal decomposition is small. 

Thermal Decomposition. Differential thermal analysis shows phase transitions at 295° and 370° (see above) and a sharp decompn exotherm at 485—500° (Ref 20). G.F. Smith claims that it is unchanged after heating to 400° for long periods of time (Ref 5). Workers in the USSR further investigated the thermal decompn and found that decompn took place betw 320 and 465° with formn of Ba oxide, Ba chloride, and evolution of O. No Cl was evolved below 520— 50°(Ref 14)... 

A considerable amount of research has been conducted on the decomposition and deflagration of ammonium perchlorate with and without additives. The normal thermal decomposition of pure ammonium perchlorate involves, simultaneously, an endothermic dissociative sublimation of the mosaic crystals to gaseous perchloric acid and ammonia and an exothermic solid-phase decomposition of the intermosaic material. Although not much is presently known about the nature of the solid-phase reactions, investigations at subatmospheric and atmospheric pressures have provided some information on possible mechanisms. When ammonium perchlorate is heated, there are three competing reactions which can be defined (1) the low-temperature reaction, (2) the high-temperature reaction, and (3) sublimation (B9). 

In summary thermal decomposition of chlorinated phenols does not generally lead to dioxins. There are, however, several conditions which by themselves or combined would favor dioxin formation. First, of all chlorinated phenols either in bulk or in solution, only pentachlorophenol produced measurable amounts of dioxin. Secondly (Table II), only sodium salts in salid state reactions produced dioxins in reasonable yields. In contrast, the silver salt of pentachlorophenol (Figure 8) undergoes an exothermic decomposition at considerably lower temperatures and produced only higher condensed materials. No dioxin was detected. 

Contact of a drop of glycerol with a flake of sodium tetrahydroborate leads to ignition, owing to thermal decomposition of the latter at above 200°. Other glycols and methanol also react exothermally, but do not ignite. 

Thermal decomposition under hydrogen of a series of pentacyanocobaltate complexes (CN-, N02-, NO- or N3-ligands) revealed that the latter complex is the most exothermic by far. Presence of iron powder suppresses hydrogen cyanide formation. 

The acid is prepared by sulfonation of nitrobenzene with oleum, and the reaction product consists essentially of a hot solution of the acid in sulfuric acid. A completed 270 1 batch exploded violently after hot storage at 150C for several hours. An exotherm develops at 145°C, and the acid is known to decompose at 200 C [ 1], A similar incident arose from water leaking from a cooling coil into the fuming sulfuric acid reaction medium, which caused an exotherm to over 150°C and subsequent violent decomposition [2], Detailed examination of the thermal decomposition of the acid shows that it is much slower for the isolated acid than for the reaction mass, and that the concentration of sulfur trioxide in the oleum used for sulfonation bears... 

In our detailed theoretical study of reaction pathways of the model A-azido-A-methoxyformamide 82b we showed that decomposition by loss of nitrogen was the energetically most favourable process with an EA of only 5.3 kcal mol-1 at B3LYP/6-31G. 36 In addition this step is exothermic by 42-44kcalmol-1. Thermal decomposition of 68b to methyl formate 67b and nitrogen has an EA of only 2.9 kcal mol-1 and is exothermic by 95 kcal mol-1. Overall, the conversion of 82b to methyl formate 67b and two molecules of nitrogen is thus predicted to be exothermic by 137-139 kcalmol-1. 

Cronin, J. L., and P. F. Nolan, "The Comparative Sensitivity of Test Methods for Determining Initial Exotherm Temperatures in Thermal Decompositions of Single Substances," J. Haz. Mat., 14 (1987). 

Neither o-NCB nor 2-EHA, individually, demonstrates exothermic activity at the normal Yellow 96 process temperatures. Morton s initial research and development for the Yellow 96 process identified the existence and described the two exothermic chemical reactions that can occur when the chemicals used to produce Yellow 96 are mixed and heated. The desired exothermic reaction to form Yellow 96 is initiated at an onset temperature of 38°C (100°F) and begins to proceed rapidly at a temperature of approximately 75°C (167°F). The undesired, exothermic reaction that results from the thermal decomposition of the Yellow 96 product is initiated at an onset temperature 195°C (383°F). 

A sketch of the thermogram obtained for the thermal decomposition of Fe(CO)(l,3-C4H6)2 at 418 K is shown in figure 9.5 [163]. The endothermic part reflects the heating (from 298 K to 418 K) and the melting of the sample and probably also some thermal decomposition. The exothermic peak of the thermogram was attributed to the polymerization of butadiene. Because area B is larger than A, the overall process (equation 9.10) is exothermic. 

The result obtained for Af//°[Cr(CO)6, cr)] is some 50 kJ mol-1 more positive than the recommended value, -980.0 2.0 kJ mol-1 [149], a weighted mean of experimental results determined with several types of calorimeter. The large discrepancy is not due to an ill-assigned thermal decomposition reaction but to a slow adsorption of carbon monoxide by the chromium mirror that covered the vessel wall. This is an exothermic process and lowered the measured Ar//°(9.13). 

Dining preparation of tris(ketoximino)silanes, two violent explosions attributed to acid-catalysed exothermic rearrangement/decomposition reactions occurred. Although these silane derivatives can be distilled under reduced pressure, the presence of acidic impurities (e.g. 2-butanone oxime hydrochloride, produced during silane preparation) drastically reduces thermal stability. Iron(III) chloride at 500 ppm caused degradation to occur at 150°, and at 2% concentration violent decomposition set in at 50°C. 

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