Mechanisms of decomposition

In the present review of the kinetics and mechanisms of decomposition and chemical interactions of inorganic solids, emphasis is placed on reports appearing in the approximate period 1955—1976, though many references to earlier work have been included to maintain a balance in the treatment. Some of the citations have been particularly selected to... 

Mechanisms of decomposition reactions at interfaces are conveniently considered with reference to the diagrammatic representations in Fig. 8 (R = reactant, 1,1 = intermediates and P = product) and classified under the following headings. 

Fig. 8. Mechanisms of decomposition reactions occurring at interfaces. R = reactant I, I = intermediates and P = product. A more detailed account of these phenomena is given in the text.

It is apparent from DTA studies [1021] of the decompositions of Group IA formates in inert or oxidizing atmospheres that reaction is either preceded by or accompanied by melting. Anion breakdown leading to carbonate production may involve formation of the oxalate, through dimerization [1022] of the postulated intermediate, C02, especially during reaction of the Na and K salts in an inert atmosphere and under isothermal conditions. Oxalate production is negligible in reactions of the Li and Cs formates. Reference to oxalate formation is included here since this possibility has seldom been considered [1014] in discussions of the mechanisms of decompositions of solid formates. 

Fig. 18. Schematic representation of the mechanism of decomposition of copper(II) formate, proposed by Galwey et al. [97], (Reproduced, with permission, from Journal of Physical Chemistry.)...

An unusual variation in kinetics and mechanisms of decomposition with temperature of the compound dioxygencarbonyl chloro-bis(triphenyl-phosphine) iridium(I) has been reported by Ball [1287]. In the lowest temperature range, 379—397 K, a nucleation and growth process was described by the Avrami—Erofe ev equation [eqn. (6), n = 2]. Between 405 and 425 K, data fitted the contracting area expression [eqn. (7), n = 2], indicative of phase boundary control. At higher temperatures, 426— 443 K, diffusion control was indicated by obedience to eqn. (13). The... 

Formulation of the detailed mechanisms of decomposition of coordination compounds are likely to remain difficult. Reliable kinetic and supporting observations are not easily obtained where several initiating reactions are possible and subsequent chemical changes may occur, before the first-formed product has left the crystallite of reactant. 

Current views on the main features of the mechanisms of decomposition of benzoyl peroxide are summarized in Scheme 2. 

The CVD of graphite is theoretically simple and is based on the thermal decomposition (pyrolysis) of a hydrocarbon gas. The actual mechanism of decomposition, however, is complex and still not completely understood. This may be due, in part, to the fact that most of the studies on the subject of hydrocarbon decomposition are focused on the improvement of fuel efficiency and the prevention of carbon formation (e.g., soot), rather than the deposition of a coating. 

KINETICS AND MECHANISM OF DECOMPOSITION OF N-Br-AMINO ACIDS IN ALKALINE MEDIUM... 

We present here a kinetic study of the decomposition of the N-bromo derivatives of alanine, aminoisobutyric acid and proline in alkaline medium, where the mechanism of decomposition is not fiilly understood. A discussion of the different microscopic mechanisms that can be proposed is done in the light of the obtained experimental results. 

Predictive equations for the rates of decomposition of four families of free radical initiators are established in this research. The four initiator families, each treated separately, are irons-symmetric bisalkyl diazenes (reaction 1), trans-phenyl, alkyl diazenes (reaction 2), tert-butyl peresters (reaction 3) and hydrocarbons (reaction 4). The probable rate determining steps of these reactions are given below. For the decomposition of peresters, R is chosen so that the concerted mechanism of decomposition operates for all the members of the family (see below)... 

Since the quantum chemical calculations used to parameterize equations 6 and 7 are relatively crude semiempirical methods, these equations should not be used to prove or disprove differences in mechanisms of decomposition within a family of initiators. The assumption made in the present study has been that the mechanism of decomposition of initiators does not change within a particular family of initiators (reactions 1-4). It is generally accepted that trow5-symmetric bisalkyl diazenes (1) decompose entirely by a concerted, synchronous mechanism and that trans-phenyl, alkyl diazenes (2) decompose by a stepwise mechanism, with an intermediate phenyldiazenyl radical (37). For R groups with equal or larger pi-... 

Epoxide is formed side by side with oligomeric peroxide during monomer oxidation. Miller and Mayo [54] assumed the following mechanism of decomposition of formed radicals ... 

Such decay is known as concerted fragmentation. Peroxides have the weak O—O bond and usually decompose with dissociation of this bond. The rate constants of such decomposition of ROOR into RO radicals demonstrate a low sensitivity of the BDE of the O—O bond to the structure of the R fragment [4], Bartlett and Hiat [8] studied the decay of many peresters and found that the rate constants of their decomposition covered a range over 105 s-1. The following mechanism of decomposition was proposed in parallel with a simple dissociation of one O—O bond [3,4] ... 

Three different mechanisms of perester homolytic decay are known [3,4] splitting of the weakest O—O bond with the formation of alkoxyl and acyloxyl radicals, concerted fragmentation with simultaneous splitting of O—O and C—C(O) bonds [3,4], and some ortho-substituted benzoyl peresters are decomposed by the mechanism of decomposition with anchimeric assistance [3,4]. The rate constants of perester decomposition and values of e = k l2kd are collected in the Handbook of Radical Initiators [4]. The yield of cage reaction products increases with increasing viscosity of the solvent. 

The nonconcerted mechanism of decomposition was observed by Szwarc et al. [67], They studied the photolytic decomposition of cyclopropyltrifluoromethyl diazene in the gas phase... 

Other very convincing evidences for free radical-mediated mechanism of decomposition and reactions of peroxynitrite and nitrosoperoxocarboxylate were demonstrated by Lehnig [140] with the use of CIDNP technique. This technique is based on the effects observed exclusively for the products of free radical reactions their NMR spectra exhibit emission characterizing a radical pathway of their formation. Lehnig has found the enhanced emission in the 15N NMR spectra of N03- formed during the decomposition of both peroxynitrite and nitrosoperoxocarboxylate. This fact indicates that N03- was formed from radical pairs [ N02, H0 ] and [ N02, C03 ]. Emission was also observed in the reaction of both nitrogen compounds with tyrosine supposedly due to the formation of radical pair [ N02, tyrosyl ]. 

Lewis et al.106 calculated four possible decomposition pathways of the ot-HMX polymorph N-N02 bond dissociation, HONO elimination, C-N bond scission, and concerted ring fission. Based on energetics, it was determined that N-N02 dissociation was the initial mechanism of decomposition in the gas phase, whereas they proposed HONO elimination and C-N bond scission to be favorable in the condensed phase. The more recent study of Chakraborty et al.42 using density functional theory (DFT), reported detailed decomposition pathways of p-HMX, which is the stable polymorph at room temperature. It was concluded that consecutive HONO elimination (4 HONO) and subsequent decomposition into HCN, OH, and NO are the most energetically favorable pathways in the gas phase. The results also showed that the formation of CH20 and N20 could occur preferably from secondary decomposition of methylenenitramine. 

The kinetics and mechanism of decomposition of organic hydroperoxides in presence of transition metal complexes has been reviewed. 

Y. Yamazaki, J. McEntagart, K. Shinozaki, H. Yazawa, Kinetics and Mechanism of Decomposition of Cephazolin Ester in Phosphate Buffer Solution , Chem. Pharm. Bull. 

Further theoretical study of the mechanism of decomposition of /3-propiolactone and jS-butyrolactone, to form CO2 and ethene or propene, respectively, has concluded that the process can best be described as asynchronous and concerted. Calculations also suggest that concerted processes are preferred for both decarbonylation and decarboxylation of jj-thiobutyrolactone. ... 

The mechanism of decomposition of the Criegee intermediates is believed to occur via several reaction channels shown for the [(R,CH2)(R2)CHOO] Criegee intermediate in Fig. 6.4. The oxygen-atom elimination channel for simple alkenes is not believed to be important. However, the ester and hydroperoxide channels are important and explain the production of free radicals such as OH. Theoretical calculations have shed some light on this (e.g., Gutbrod et al., 1996, 1997a ... 

The structural aspects and thermal decomposition mechanisms of some alkaline earth salts of NTO have also been reported and their mechanism of decomposition is similar [163, 164]. 

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