Initiation reaction, mechanism thermal

The energy available in various forms of irradiation (ultraviolet, X-rays, 7-rays) may be sufficient to produce in the reactant effects comparable with those which result from mechanical treatment. A continuous exposure of the crystal to radiation of appropriate intensity will result in radiolysis [394] (or photolysis [29]). Shorter exposures can influence the kinetics of subsequent thermal decomposition since the products of the initial reaction can act as nuclei in the pyrolysis process. Irradiation during heating (co-irradiation [395,396]) may exert an appreciable effect on rate behaviour. The consequences of pre-irradiation can often be reduced or eliminated by annealing [397], If it is demonstrated that irradiation can produce or can destroy a particular defect structure (from EPR measurements [398], for example), and if decomposition of pre-irradiated material differs from the behaviour of untreated solid, then it is a reasonable supposition that the defect concerned participates in the normal decomposition mechanism. 

The first and rate-determining step involves carbon monoxide dissociation from the initial pentacarbonyl carbene complex A to yield the coordinatively unsaturated tetracarbonyl carbene complex B (Scheme 3). The decarbonyla-tion and consequently the benzannulation reaction may be induced thermally, photochemically [2], sonochemically [3], or even under microwave-assisted conditions [4]. A detailed kinetic study by Dotz et al. proved that the initial reaction step proceeds via a reversible dissociative mechanism [5]. More recently, density functional studies on the preactivation scenario by Sola et al. tried to propose alkyne addition as the first step [6],but it was shown that this... 

Considerable attention has been directed to the formation of nitroarenes that may be formed by several mechanisms (a) initial reaction with hydroxyl radicals followed by reactions with nitrate radicals or NO2 and (b) direct reaction with nitrate radicals. The first is important for arenes in the troposphere, whereas the second is a thermal reaction that occurs during combustion of arenes. The kinetics of formation of nitroarenes by gas-phase reaction with N2O5 has been examined for naphthalene (Pitts et al. 1985a) and methylnaphthalenes (Zielinska et al. 1989) biphenyl (Atkinson et al. 1987b,c) acephenanthrylene (Zielinska et al. 1988) and for adsorbed pyrene (Pitts et al. 1985b). Both... 

Mercuric carboxylates, which decarboxylate by a chain mechanism when initiated by peroxides, also decarboxylate under UV irradiation (123,128,129,131-140,142,144-146,153-155). In addition, decarboxylation was observed for mercuric benzoate and mercuric a-naphthoate (123). Side reactions [Eqs. (24), (25), (109)] observed in peroxide initiated reactions also occurred on UV irradiation, and mercurous salt formation [Eq.(24)] was more extensive under the latter conditions. Decarboxylation giving methylmercuric acetate occurred on irradiation of mercuric acetate in aqueous solution and is considered to be of environmental significance (156,157). Stepwise decarboxylation giving (CF3)2Hg occurred on irradiation of solid mercuric trifluoroacetate at -196° C (158), but, at 20° C, trifluoromethyl radicals diffused from the solid and dimerized (158). No other diorganomercurial has been formed by radical decarboxylation, and the reaction is not preparatively competitive with the thermal decarboxylation synthesis of (CF3)2Hg (26,27) (Section III,A). 

The chlorination of hydrocarbons proceeds via the chain mechanism [195]. Chlorine atoms are generated photochemically or by the introduction of the initiator. However, liquid-phase chlorination occurs slowly in the dark in the absence of an initiator. The most probable reaction of thermal initiation in RH chlorination is the bimolecular reaction... 

The decomposition of some materials into smaller, more stable molecules can be initiated by mechanical shock alone, and they are known as shock-sensitive. Many commercially important chemicals are thermally sensitive and decompose with the addition of heat. For storage situations, the critical temperature at which the thermal energy is sufficient to start an uncontrolled reaction in a particular storage configuration for a specified time is known as the self-accelerating decomposition temperature (SADT), as described in NFPA 49. 

The deoxygenation of nitroxides by (TMS)3SiH is shown in Reaction (4.43) [79]. Indeed, the reaction of this silane with TEMPO, in the presence of thermal or photochemical radical initiators, afforded the corresponding amine in quantitative yield, together with the siloxane (TMS)2Si(H)OSiMc3. The apparently unexpected detection of the siloxane can be accounted for by the reaction mechanism shown in Scheme 4.4. 

Mechanically initiated reactions can be used to create thermally stable polymer films. Such films form on various surface factors. They are very dense, although amorphous (Simonescu et al. 1983). The films are thermally and frictionally more stable than the thermally stable polymers obtained by conventional methods (Krasnov et al. 2002). The discussed case of polymerization can be of interest if amorphous polymers with moderate molecular weights are needed. 

The preparation of N-alkyl imides by exchange reaction of an imide with an alkyl amine was documented [104] well before the application of this chemistry to the preparation of polyimides [105], see Scheme 30. Although no experimental details are provided, the initial reaction of pyromellitimide with p,p-methylene dianiline in NMP takes place at reflux temperatures to apparently yield a poly(amic amide). Subsequent heating of this intermediate at elevated temperatures ( 300 °C) provides the desired polyimide with evolution of ammonia. The final polyimide is quoted to be thermally and chemically stable, however, no mechanical properties are given. 

The initial products may be considered to be excited cyclopropane derivatives formed by addition to the double bond and excited olefins formed by insertion into CH bonds. (The detailed reaction mechanisms are discussed in Sec. V.) The isomerization products are expected to be similar to those found in the thermal isomerizations of the corresponding cyclopropane derivative or olefins, the excitation energy being at least 80-85 kcal. in the former case and 85-90 kcal. in the latter (taking AHf° (CH2) 80-85 kcal.). The excitation energy is increased by any excess energy of methylene. 

The reaction mechanism for this process is fairly well established and may serve as a more complex example of a chain reaction. The initiation step in ethane pyrolysis is thermal dissociation of the ethane molecule,... 

The products of the thermolysis of 3-phenyl-5-(arylamino)-l,2,4-oxadiazoles and thiazoles have been accounted for by a radical mechanism.266 Flash vacuum pyrolysis of 1,3-dithiolane-1-oxides has led to thiocarbonyl compounds, but the transformation is not general.267 hi an ongoing study of silacyclobutane pyrolysis, CASSF(4,4), MR-CI and CASSCF(4,4)+MP2 calculations using the 3-21G and 6-31G basis sets have modelled the reaction between silenes and ethylene, suggesting a cyclic transition state from which silacyclobutane or a trcins-biradical are formed.268 An AMI study of the thermolysis of 1,3,3-trinitroazacyclobutane and its derivatives has identified gem-dinitro C—N bond homolysis as the initial reaction.269 Similar AMI analysis has determined the activation energy of die formation of NCh from methyl nitrate.270 Thermal decomposition of nitromethane in a shock tube (1050-1400 K, 0.2-40 atm) was studied spectrophotometrically, allowing determination of rate constants.271... 

As already mentioned, once the reaction was initiated by mechanical activation, it continued without additional activation. This phenomenon suggests that the reaction is au-tocatalytic. As known, activators such as iodine and alkylaluminum halide initiate the conventional thermal reaction of aluminum with alkyl halides. In the case discussed, the reaction was initiated by mechanical working without any activator. 

The mechanism by which emulsifiers could influence the rate of the thermal initiation reaction is obscure. Most probably the emulsifiers increase the efficiency with which one of the radicals produced in the thermal initiation process escapes into the aqueous phase so that emulsion polymerization may begin. If so those emulsifiers for which exchange between the micelle or the adsorbed layer on a latex particle and true solution in the aqueous phase is most rapid should be most effective in promoting the thermal polymerization. Recently the kinetics of micellization has attracted much attention (29) but the data which is available is inadequate to show whether such a trend exists. 

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