Decomposition monomolecular

The mechanistic investigations presented in this section have stimulated research directed to the development of advanced ruthenium precatalysts for olefin metathesis. It was pointed out by Grubbs et al. that the utility of a catalyst is determined by the ratio of catalysis to the rate of decomposition [31]. The decomposition of ruthenium methylidene complexes, which attribute to approximately 95% of the turnover, proceeds monomolecularly, which explains the commonly observed problem that slowly reacting substrates require high catalyst loadings [31]. This problem has been addressed by the development of a novel class of ruthenium precatalysts, the so-called second-generation catalysts. 

A question that intrigued several kineticists around 1920 was the following. For bi-molecular reactions of the type A -1- B = Products collision theory gave at least a plausible conceptual picture If the collision between A and B is sufficiently vigorous, the energy barrier separating reactants and products can be crossed. How, though can one explain the case of monomolecular elementary reactions, e.g. an isomerization, such as cyclopropane to propylene, or the decomposition of a mol-... 

The initiation of free radical reactions by ozone in the gas phase at elevated temperatures occurs due to ozone monomolecular decomposition [131,132] ... 

If radicals are produced in the reactions of unimolecular hydroperoxide decomposition and the reaction of ROOH with hydrocarbon whose concentration at the initial stages of oxidation is virtually constant, the production of radicals from ROOH can be regarded as a pseudo-monomolecular process occurring at the rate V = [ROOH] = + iRH[RH]). The... 

The decomposition of a single hydroperoxyl group in the absolutely pure polymer can proceed by two mechanisms by the reaction of POOH with the C—H bond of a macromolecule and by the monomolecular cleavage of the O—O bond (see Chapter 4). If the first reaction prevails, the single POOH group of PP should break down more rapidly than those of PE, because the latter polymer has stronger C—H bonds (the BDE difference between the secondary and tertiary C—H bonds is as much as 22.5 kJ mol-1). The kA values of the single... 

It is seen that the values of kd are very close. Hence, the reaction of POOH with the C—H bond is not the main initiation reaction. If the breakdown is a monomolecular process, the rate of O—O bond homolysis in polymer must be close to that in the gas phase. 2,2-Dimethylethyl hydroperoxide breaks down in the gas phase with a rate constant of 1.6 x 1013 exp(— 158/i 7) = 5.3 x 10 x s 1 (398 K, [4]), that is, by four orders of magnitude more slowly than in polymer. Hence, the decomposition reactions in the polymers are much faster than the monomolecular homolysis of peroxide. Decomposition reactions may be of three types (see Chapter 4), such as the reaction of POOH with a double bond... 

Chloryl fluoride is stable at ambient temperature in well-passivated and dry containers. Its thermal decomposition in quartz was studied by Schumacher et al. 24, 137). It reaches a measurable rate only above 300°C. The decomposition reaction is monomolecular and its rate is pressure-dependent. The actiyation energy was calculated to be 45 2 kcal mole and the rate constant was determined as = 2.3 x 10 X sec . The following decomposition mechanism was... 

The thermal decomposition of FCIO 2 in Monel was studied by Macheteau and Gillardeau (183). Decomposition to CIF and O2 was observed at 100°C (2.5% in 144 hr) and 200°C (10% in 235 hr), but a temperatme >250°C was required for rate measurements. It was found that the decomposition is of first order and monomolecular at temperatures up to 285°C. At 300°C the reaction becomes second-order. The calculated rate constants and half-life times are summarized in Table XVI. The... 

Elementary reactions (also termed monomolecular reactions) that involve only a single entity in the formation of an activated complex. Unimolecular rate constants, k, are concentration-independent and are typically expressed in units of sUnimolecular reactions are expected to be first order (i.e., -dc/dt = kc where c is the concentration and t is time). Examples of unimolecular processes include radioactive disintegrations, isomeriza-tions, disassociations, and decompositions. Reactions in solution are unimolecular only if the solvent is not covalently incorporated into the product(s). 

According to A. J. B. Robertson [23] decomposition of octogen at temperatures above 280°C occurs as a monomolecular reaction. Activation energy =52.7 kcal, log = 19.7. 

The mechanisms of stepwise monomolecular thermal decomposition of 1,5- and 2,5-disubstituted tetrazoles feature nitrogen evolution by rate-limiting breakdown of intermediate azidoazomethines and azodiazo compounds, respectively 67 the activation parameters have been reported. 

The thermochemical characteristics of l,3,5-trinitro-l,3,5-triazepane, such as energies of dissociation of N-NOz bonds, enthalpies of formation, vaporization, and combustion, as well as enthalpy of formation of amine radicals, have been determined <2004MI92>. The rate constants of initial monomolecular stages of thermal decomposition in the solid phase were measured for its furazano-fused analog 20 <1999RCB1250> and the ratio of the rate constants of decomposition in the melt and solid states, characterizing the reaction retardation in the crystal lattice, was determined. The kinetics of the thermal decomposition of 20 has also been studied <1995MI885>. 

Strange as it seems, there are one-step reactions in nature, e.g. first-order reactions of monomolecular decomposition... 

The atomic mechanism responsible for monomolecular reactions, including thermal decompositions, was first discussed by Polanyi and Wigner.26 Their model assumes that decomposition occurs when, due to energy fluctuations in the bonds of the molecule, the bond strength is exceeded or more precisely, the bond energy resides in harmonic vibrations and that decomposition occurs when their amplitude is exceeded. The resulting expression for the first-order Polanyi-Wigner rate constant is... 

As an example of a unimolecular decomposition reaction, we study the monomolecular catalytic cracking reaction of //-paraffins in high-silica acid zeolites or other crystalline or ordered acid porous materials, in this section [97-102],... 

The latter reaction which has the activation energy about 85 kJ/mol seems to be more probable at higher temperatures at lower temperatures hydroxy radicals may appear in other reaction pathways as in the monomolecular decomposition of hydroperoxides, etc. Hydroxyl which are very mobile and very reactive propagate the reaction site only to a short distance (the rate constant of transfer reaction of HO radicals with CH3 group of ethane is equal 108 dm3 mol-1 s approximately at 20 °C). 

The kinetic expressions derived by Antipina and Frost have general applicability to monomolecular heterogeneous catalytic reactions which occur on a uniform surface. The expression can be made to describe the cracking of synthin or decomposition of octene over silica-alumina as well as hydrogen disproportionation of gasoline and cracking of gas oils over the silica-alumina. Numerous other applications are discussed. 

In addition to the foregoing reaction, potassium chlorate undergoes decomposition into oxygen and potassium chloride. This is a monomolecular reaction 1 and proceeds according to Hie equation... 

Colloidal palladium eatalytically assists the decomposition of aqueous solutions of hydrogen peroxide, oxygen being evolved. The reaction is monomolecular, and the influence of as minute a quantity of palladium as one gram atom in 26,000,000 litres of solution can be detected with N/60NaOH and N/10H2O2 solution.2 The reaction is accelerated by the presence of caustic soda, the optimum concentration of which is about N/16. In acid solution the peroxide decomposes very slowly. 

Oxalic acid dinitrate ester, 02N—02C—C02—N02 is metastable with respect to decomposition into C02 and N02. The reaction barrier (transition state) for the monomolecular dissociation was calculated to be 37 kcal mol-1 at CBS-4M level of theory. 

Table 5-8. Solvent influence on rates of monomolecular decomposition of various free-radical initiators [164],...

The thermal decomposition of S5mrmetrical dialkyl peroxides such as diisopropyl peroxide in solution has been shown to involve a competition between monomolecular homolysis k ) and an electrocyclic reaction yielding acetone and hydrogen ( h) cf Eq. (5-59) [564]. 

Analogous to m-azoalkanes (1,2-diazenes), the thermolysis of 1,1-diazenes is also solvent-sensitive. The monomolecular decomposition rate of A-(2,2,5,5-tetramethyl-pyrrolidyl)nitrene decreases with increasing solvent polarity [568]. 

For further examples of dichotomous solvent-influenced radical/ionic perester decompositions, see the base-catalyzed perester fragmentation shown in Eq. (5-39) in Section 5.3.2 [110], as well as the decomposition of t-butyl heptafluoroperoxybutyrate, C3p7-C0-0-0-C(CH3)3 [691]. The relative extent of monomolecular and induced thermal decomposition of disubstituted dibenzyl peroxydicarbonate, ArCH2-0-C0-0-0-C0-0-CH2Ar, is also substantially influenced by the reaction medium [692]. The thermolysis of suitable dialkyl peroxides can also proceed by two solvent-dependent competitive reaction pathways (homolytic and electrocyclic reaction), as already shown by Eq. (5-59) in Section 5.3.4 [564]. 

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