Stepwise decomposition

Stepwise decomposition of thietane oxides should be influenced by the relative stabilities of the developing radical centers, whereas the subsequent selection between... 

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. 

However, (11) is approximately 180 kcal.mole-1 endothermic. If it occurs it must be a stepwise process. Partial stepwise decomposition may take place but complete dissociation to liberate Si atoms would be inconsistent with the observation that the carbon content of the solid product under all decomposition conditions is approximately half that of the parent alkyl (low decomposition — grey solid, 25.3 % C high decomposition — black solid, 28.3 % C Si(CH3)4 = 54 % C)130. 

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... 

Stereochemical information on the mode of cyclodehydration of unsymmetrical diols to cyclic ethers could obviously have important consequences regarding useful, preparative routes to chiral cyclic ethers of high enantiomeric purity. For example, dioxyphos-phorane promoted cyclodehydration of a chiral diol can, in principle, give the enantiomeric ethers by either of two stereochemi-cally distinct routes. Separate stepwise decomposition of oxy-phosphonium betaines, A and B, although proceeded by a number of equilibria could ultimately afford a nonracemic mixture of cyclic ethers. 

Participation of an HO(X)COf intermediate can also account for the observation that different OH rotational levels display different production rates, since this intermediate might yield rotationally cold OH via stepwise decomposition processes ... 

However the stepwise decomposition of the NH3 molecule at 298 K proceeds as follows ... 

Among monomeric metal carbonyls, Mo(CO)6, Cr(CO)6, Fe(CO)5, and Ni(CO)4 have been most studied. Their stepwise decomposition on the support may lead to the formation of grafted species. For example, Mo(CO)6 is first physically adsorbed onto hydroxylated alumina at room temperature [2, 62, 63, 68]. Thermal decomposition leads to the formation of adsorbed subcarbonyl species such as Mo(CO)s and then Mo(CO)3 at 373 K. Complete dccarbonylation at 573 K is observed upon oxidative addition of the metal on surface hydroxy groups ... 

Stepwise decomposition of (NH4)2MF6 type compounds crosses mark phases characterized... 

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] ... 

Therefore, melts-solvents of the first kind are of interest in the following scientific aspects determination of the acid-base product of the ionic solvent and estimation of the upper limit of acidity of these solvents (such as nitrates, sulfates). The decrease of stability of the solvent acid can be used for the stepwise decomposition of acidic solutions of cations and synthesis of complex oxide compounds and composites by coprecipitation [53-56], It is possible to obtain complex oxides containing alkali metals by precipitation of multivalent metal oxides with the alkali metal oxide as a strong Lux base, as was reported by Hong et al., who used 0.59LiNO3-0.41LiOH mixed melt to obtain electrochemically active lithium cobaltate, LiCo02 [57]. 

More detailed investigations [118] included a deeper look at the SO rovibrational spectrum and a resonance enhanced multiphoton ionization (REMPI) analysis of the methyl radicals. These resulted in total rovibrational energies for the fragments. The translational energy component was not measured. The quantum yield for formation of SO in its ground electronic state ( Z) was unity within experimental error. Evidence for two types of methyl radicals, as might be expected for stepwise decomposition, was not found, so the authors suggested that the three-body dissociation pathway was dominant [118]. 

Multiphoton infrared excitation of the sulfoxide stretching chromophore (ca. 1100 cm ) of DMSO also leads to production of CH3 and SO in their respective electronic ground states [120]. The direct formation of ethane was once again eliminated, this time by product analysis from double label experiments with DMSO and its Dg isotopomer. Time resolved detection of IR absorptions was used to analyze products. Though, once again, CHjSO- was not directly observed, these authors favor a stepwise decomposition because of a nonthermal rotational level distribution of the SO fragment. 

Photofragment mass spectroscopy of thionyl chloride molecular beams irradiated at 193 and 248 nm was studied as early as 1984 [121]. The same three mechanistic possibilities, stepwise decomposition to 2 Cl- -I- SO (A), concerted decomposition to 2 Cl- -I- SO (B), and concerted elimination to CI2 and SO (C), were all considered. 

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 ... 

Figure 2 Stepwise decomposition of particulate organic nitrogen (PON) in arbitrary units versus time during a dark incubation. Nitrogen is transformed, first to NH4+ by bacterial ammonification and finally to NOa and NOs by the two-step process of bacterial nitrification. These same processes are responsible for the global ocean formation of NOs in the deep sea. These data are the idealized results of pioneering nitrogen cycle investigators, T. von Brand and N. Rakestraw, who unraveled these processes more than 50 years ago.

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... 

Write two balanced net ionic equations to demonstrate how the addition of H+ leads to the stepwise decomposition of CO32 " to liberate CO2(g) ... 

Yet more papers have appeared on the hexafluoroisobutenylidene sulphate (32) derived from bis(trifiuoromethyl)keten (or its precursor, 2fl-hexafluoroisobutyric acid) by treatment with sulphur trioxide. - Extension of the earlier chemistry to diiluoroacetic acid generated not difluoroketen but analytically pure fluorosulphonic acid. Stepwise decomposition of the sulphate (32) has now been found to give various sulphur-oxygen heterocycles, the main product obtained depending rather critically on the procedure (see Scheme 5l).328,33o... 

Accordingly, the photolysis of the dihalomaleic anhydrides was examined. In the solid state the dichloro compound 211 gave the same polymer 212 as was obtained from thermolysis,a result which permits-but does not require-an aryne interpretation. In dilute cyclohexane solution, however, a monosubstitution product (220) (and no polymer) was isolated, indicating a stepwise decomposition. Irradiation of the dibromo compound 209... 

Scheme 7.17 Stepwise decomposition of methane on active metal sites...

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