Stepwise decomposition reaction

While reaction with the vessel is more severe in closed systems under pressure, avoidance of even traces of water is more difficult to achieve in flow systems. Only when NH3 is rigorously dried with sodium metal, is Bi3NF6 obtained free of oxygen from a stepwise decomposition reaction in an NH3 atmosphere [71] ... 

In the stepwise decomposition of azo-compounds such as 4, products can arise from reactions within the primary diazenyl-alkyl radical pair or from the secondary radical pair produced by loss of nitrogen from the... 

The thermolysis products of silaazetidines support the formulation of stepwise decomposition to alkene and Cl2Si=NR derivatives [3,4], The silene is not liberated from the silaazetidines when performing thermolysis reactions, but the Si-dichloro substituted silanimine (Cl2Si = N/Bu) is formed. This can be trapped by M SiOMe or Ph2C=NfBu, to give the addition products. 

The mechanism of thermal decomposition (pyrolysis) of methane has been extensively studied [90,91]. Because C-H bonds in the methane molecule are significantly stronger than C-H and C-C bonds of the products, the secondary and tertiary reactions contribute at the very early stages of the reaction, which obscure the initial processes. According to Holmen et al. [92], the overall methane thermal decomposition reaction at high temperatures can be described as a stepwise dehydrogenation as follows ... 

This is followed by a series of surface stepwise dissociation reactions leading to elemental carbon and hydrogen (the stepwise mechanism was suggested for Ni-catalyzed methane decomposition in Refs 100 and 122) ... 

More detailed consideration of light absorption and consequent chemical changes is left to Chapter 13, but it is appropriate here to summarize briefly the types of compounds that are convenient photochemical radical sources. Many of the substances we have been discussing as thermal radical sources absorb light in the visible or ultraviolet and can be decomposed photochemically. The azoalkanes are particularly versatile they absorb around 350 nm and decompose cleanly to nitrogen and two radicals just as in the thermal reaction. As we have already noted, a preliminary photochemical isomerization to the cis isomer precedes the homolysis, which is actually a thermal decomposition of this unstable form.78 CIDNP observations confirm a stepwise decomposition pathway, and clarify the various reactions of the radicals produced.79... 

Thiol Decomposition. As explained previously, the elimination of sulfur from benzothiophene occurs stepwise after the aromatic thiol (o-thiocresol) has formed and not in a concerted fashion from the thiophenic ring system. Extrapolation of this implies that thio-phenic sulfur in coal is eliminated by conversion to an aromatic thiol that subsequently undergoes desulfurization. Since the aromatic thiol is the apparent organosulfur species that undergoes desulfurization, it is of interest to understand the chemistry involving the elimination of sulfur from aromatic thiols. Thio-phenol was used as a model compound to examine reactions, primarily the thermal decomposition reactions that might lead to sulfur elimination. In the experiments with caustic and benzothiophene, the intermediate (o-thiocresol) most likely exists in the salt... 

Path 5 has not been confirmed in the ylid, but products which can be attributed to this path are noted in the decomposition of the fluor-enylid and in alkoxy-substituted quaternary ammonium salts. Although ethylene and polymethylene are observed in the decomposition of the ylid, it has been suggested that these products are formed via a stepwise alkylation reaction. However, the ylid may be regarded as a carbenoid, with the N+(CH3)3 group behaving as a pseudohalogen, which reacts either as the free ylid or as the lithium halide complex. 

In this section we wiii consider the energies associated with various types of bonds and see how the bonding concept is usefui in deaiing with the energies of chemical reactions. One important consideration is to estabiish the sensitivity of a particuiar type of bond to its moiecuiar environment. For exampie, consider the stepwise decomposition of methane ... 

It was discussed above that all chlorosiloxanes must decompose for entropy reasons, preferably according to Eq. 3. Thus, the stepwise built-up chlorosiloxane is decomposed, but the stoichiometry differs from that of the formation step. Whereas Si and O atoms are always introduced in equal numbers in the growth reaction (Eq. 9 and 10), only Si and Cl atoms are cleaved in the decomposition reaction (Eq. 3). All oxygen atoms remain in the rest of the decomposing molecule. In this way, the molecules become smaller in each decomposition step, but at the same time oxygen-richer. 

Weak emission of light has been observed in the thermal decomposition of cw-1,2-cyclobutylene dinitrite. Since this reaction possibly proceeds through the same type of biradical as would be involved in stepwise decomposition of dioxetans, the difference between this case and the decomposition of the analogous dioxetan has been tentatively suggested as support for a concerted mechanism for the latter. For simple unsymmetrical dioxetans, undergoing... 

Homogeneous chemical nucleation is similar both to stepwise polymerization reactions in fluid media and to spinodal decomposition in mixtures. The characteristic differences with a binodal-type nucleation reaction are a strong sensitivity to the presence of impurities, the absence of an activation barrier and a concentration threshold, and no incubation time. 

In Utis section we wfll consider the eneigies associated with various types ctf bonds and see how the bonding concept is useful in dealing with the enemies ot chemical reactions. One inf rtant conaderation is to establish the sensitivity ot a particular type ot bond its molecular environmeot. For example, consider the stepwise decomposition of lhane ... 

The decomposition of the peroxyketals (53) follows a stepwise, rather than a concerted mechanism. Initial homolysis of one of the 0-0 bonds gives an aikoxy radical and an a-peroxyalkoxy radical (Scheme 3.36).306"08"210 This latter species decomposes by P-scission with loss of either a peroxy radical to form a ketone as byproduct or an alkyl radical to form a peroxyester intermediate. The peroxyester formed may also decompose to radicals under the reaction conditions. Thus, four radicals may be derived from the one initiator molecule. 

Co(en)3]X3 complexes [1113,1130] decompose in nitrogen to yield CoX2 and give Co304 in air, the sequence of stability with variation in X is SCN < Cl < I" = Br" < S04 < NOJ. Mass spectrometric analysis of the products of reaction in vacuum indicated the formation of large quantities of NH3 and no ethylenediamine. Decomposition does not, therefore, involve stepwise release of the ligand. 

In the case of reaction 3, entries 1 and 2, that is, iert-butyl peracetate and (ert-butyl perpropionate, almost certainly decompose by a stepwise mechanism, rather than the concerted mechanism assumed for reaction 3. Entry 3, tert-butyl perisobutyrate, probably forms the least stable R radical by the perester decomposition mechanism which is still mostly concerted in nature (36). 

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-... 

Although the concerted mechanism described in the preceding paragraph is available only to those azo compounds with appropriate orbital arrangements, the nonconcerted mechanism occurs at low enough temperatures to be synthetically useful. The elimination can also be carried out photochemically. These reactions presumably occur by stepwise elimination of nitrogen, and the ease of decomposition depends on the stability of the radical R ... 

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