Formation and decomposition of intermediates

2 Formation and decomposition of intermediates Depending on the hydroperoxide concentration, the unimolecular decomposi- 

However, radicals originating from the solvent or from fragmentation of alkoxy radicals , viz. 

The rate of decomposition of the tetroxide, as determined by oxygen evolution at low temperatures, is given in Table 116. Equilibriiun between peroxy radicals and the tetroxide has been studied and the pertinent data are reported in Table 117. The y4-factors reported in Table 116 are lower than expected from a unimolecular decomposition of the tetroxide. This is probably due to the equilibrium between the peroxy radicals and the tetroxide . Secondary tetroxides appear to be considerably less stable than tertiary tetroxides in agreement with the suggested non-radical decomposition of secondary tetroxides to ketone, alcohol and oxygen (ref. 488). 

Additional kinetic data for the termination reaction of various peroxy radicals according to 

THE EQUILIBRIUM CONSTANT AND ENTHALPY FOR THE I-BUTYL FBROXY RADICAL TBTROXIDE EQUILIBRIUM IN METHYLENE CHLORIDE  

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Cyclopropene (545) and thiophene (546) were isolated from the reaction of betaine (542) with dimethyl acetylenedicarboxylate (543) (equation 143). This reaction is understood quite well by the formation and decomposition of intermediate 544 ... 

The essential aspects of mechanism are the dissociative adsorption of water on ultrafine gold particles, followed by the spillover of active OH groups onto adjacent sites of the ferric oxide. The formation and decomposition of intermediate species are accompanied by redox transfer of Fe Fe in Fe304 and the reverse step during the dissociation of water molecule. 

Figure 2 Observed absorbance-time traces showing formation and decomposition of intermediate Co(CN)5 at = 280 nm for the reaction of Co(CN)5X with V + at 25-0 "C and fx = 1-00 mol 1 . Upper curve 5 x 10 M-Co(CN)5Cl , 0-0105M-V + lower curve 7 x 10- M-Co(CN)5OHj -, 0 0053M-V +...

Figure 2 The formation and decomposition of intermediate V -QH2 complexes at fjt, = 5-00 mol 1 , T = 25 °C. The initial vanadium(v) concentration is 1-0 X 10 moll and that of hydroquinone 0-02 mol ITrace A in 5-OOM HCIO4, trace B in 0-15M HCIO4...

A theory of kinetics is first of all a theory of the intermediate substances [1,2]. From this point of view any more or less complicated chemical reaction should proceed via the formation and decomposition of intermediate substances state-by-state, that is, via the total combination of elementary reactions. That is why a theory of an intermediate substances... 

Scheme 14 Hypothetical pathways for the formation and decomposition of intermediate formed during Cu ll)-promoted methanolysis of 44d in d4-methanol. Adopted from J. Org. Chem. 2075 80 1357-1364.

Salts with the [Au(CO)2]+ cation have been proposed to be important intermediates in the transport of gold in hydrothermal fluids. Gold deposits may accumulate through formation and decomposition of (carbonyl)gold salts under geochemically relevant conditions, similar to the mechanism advanced with salts based on the [Au(SH)2]- anion.303... 

The displacement of both alkoxy groups at sulfur in 239 with predominant inversion of configuration can be explained by the simultaneous formation of two different intermediates (or transition states) 242 and 243, which undergo decomposition before pseudorotation. In this context, it is interesting to note that the formation and decomposition of only one such sulfurane intermediate would give two sulfinate products with inverted and retained configuration at sulfur. 

Fig. 2. The Bonnichsen, Chance, and Theorell 34) mechanism for the dismutation of hydrogen peroxide by catalase. (A) The simple ping-pong mechanism (ferric-peroxide compound (ycle) involves only the successive formation and decomposition of the compound 1 intermediate by two successive molecules of H2O2. (B) Reversible ES(Fe -H202) and ternary (compound I-H2O2]) complexes are added to the mechanism in A.

A detailed study of the CO insertion, or methyl migration, observing formation and decomposition of the transients, was performed so far only for one Cu(I) model system (93). It was reported that methyl radicals form transient complexes containing metal carbon -bonds with carbonmonoxide (n = 1, 3, 4) complexes of Cu(I). These complexes decompose yielding Cu(II) and acetaldehyde as final products via an copper acetyl intermediate formed by insertion of /migration of CH3 as described in Scheme 4. 

The decomposition of liquid water and the following reactions are the results of a typical chemical effect. In this case, however, overall water splitting does not occur because oxygen is not obtained but hydrogen and hydrogen peroxide are. On the other hand, it is impossible to decompose water by photochemical reaction under illumination with a xenon lamp. Although it is possible to decompose water by photocatalytic reaction using a desirable photocatalyst and photoirradiation, it is difficult to decompose in practice because of rapid backward reaction, the formation and accumulation of intermediates onto the surface of photocatalyst,10) and other reasons. 

A radical chain oxidation mechanism, involving the formation and decomposition of an intermediate hydroperoxide, is consistent with the observed kinetics in the oxidation of cumene and acenaphthene by oxygen in the presence of alkylammonium perchlorates.128... 

The basic mechanism of autoxidation at elevated temperatures is similar to that of room-temperature oxidation, i.e., a free radical chain reaction involving the formation and decomposition of hydroperoxide intermediates. Although relative proportions of the isomeric hydroperoxides, specific for oleate, linoleate and linolenate, vary with oxidation temperatures in the range 25°C -80°C, their qualitative pattern is the same (. Likewise, the major decomposition products isolated from fats oxidized over wide temperature ranges are those reflecting autoxidation of their constituent fatty acids (2 -6). The mechanisms and products of lipid oxidation have been extensively studied. The reader is referred to the numerous monographs, reviews and research articles available in the literature (1,A,7,8,9,10,11). 

Often oxidation reactions are possible via intermediate formation and decomposition of hydride derivatives for example (96,117),... 

K. Eichner and M. Ciner-Doruk, Formation and decomposition of browning intermediates and visible sugar-amine browning reactions, in Water Activity Influences on Food Quality, L. B. Rockland and G. F. Stewart (eds), Academic Press, New York, 1981, 567-603. 

Kinetic evidence for synergic adsorption of carbon monoxide and water on the low-temperature shift catalyst Cu/ZnO/Fe203 was obtained by van Herwijnen and deJong (113), and IR spectra of surface formate were detected on several oxide catalysts, including CuO/MgO, at temperatures as low as 20 JC and pressures of 20 Torr, as reported by Davydov et al. (104). Decomposition of the surface formate to C02 and H2 occurred at 100-150°C over the Cu/MgO catalyst and at 250 300°C over the MgO catalyst, and the promotion effect of copper was attributed to the formation and decomposition of a labile surface formate (HCOO)2Cu. Ueno et al. (117) have shown earlier that surface formates are formed on zinc oxide, from CO and H20 as well as from C02 and H2, and hence an associative mechanism of the shift and reverse-shift reaction, involving formate intermediate, is believed to operate on many oxide catalysts. 

Many compounds, especially various metallic oxides, also induce very rapid decomposition of hydrogen peroxide without themselves being permanently changed.4 In addition to the solutions of the alkali hydroxides already,mentioned, manganese dioxide, cobalt oxide, and lead oxide (massicot) are remarkably active, and as might be expected a colloidal solution of manganese dioxide 5 is also able to exert powerful catalytic influence.6 The effect in such cases may be partly a surface effect, but is also probably due in part to the intermediate formation and decomposition of unstable highly oxidised derivatives. 

The formation and decomposition of the enol sulphite (195) derived from the ketol (194) is now reported in full.174 A cyclopropanone (196), or an equivalent dipolar intermediate resulting from loss of S02, has been trapped as the diene-addition product (197) with furan. 

The interest in the mechanisms of SchifF base hydrolysis stems largely from the fact that the formation and decomposition of SchifF base linkages play an important role in a variety of enzymatic reactions, for example, carbonyl transfers involving pyridoxal phosphate, aldol condensations, /3-decarboxylations and transaminations. The mechanisms for the formation and hydrolysis of biologically important SchifF bases, and imine intermediates, have been discussed by Bruice and Benkovic (1966) and by Jencks (1969). As the consequence of a number of studies (Jencks, 1959 Cordes and Jencks, 1962, 1963 Reeves, 1962 Koehler et al., 1964), the mechanisms for the hydrolysis of comparatively simple SchifF bases are reasonably well understood. From the results of a comprehensive kinetic investigation, the mechanisms for the hydrolysis of m- and p-substituted benzylidine-l,l-dimethylethylamines in the entire pH range (see, for example, the open circles in Fig. 13) have been discussed in terms of equations (23-26) (Cordes and Jencks, 1963) ... 

The second-order rate constant of the cleavage of PNPA with this polymer, D( 10%)-PEI-Im(lS%) (10% dodecyl group, 15% methyleneimidazole group) 7 was 45 see" at pH 7.3,25° C on the baas of methyleneimidazole group a value more fium 100 times as effective as imidazole itself. The catalysis includes formation and decomposition of acetylimidazole intermediate, and the latter rate was estimated to be (1—6) X 10 sec 10 times greater than that of simple acetylimidazole under the same condition. The formation of acetylimidazole unit was detected spectroscof caHy 121). 

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