Monomolecular process

For strictly monomolecular processes the general theory would now proceed by analysing the time-dependent... 

AKZ640). Although some data (00UK642) support a ready hydration of the activated triple bond in a weakly basic medium, the latter route seems less probable, since the cyclization of hydrazine 22 is a monomolecular process (70AKZ640) and the hydrazine group is much more nucleophilic than water. 

Adducts of the type Me2Si(NCMe3)2Sn H2N—R are unstable and cleaved to Me2Si(N(CMe3)H)2 and the intermediate SnN—R 176,177>. It is not yet clear whether this scission is a monomolecular process, as might be inferred from the structure... 

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... 

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... 

It is seen that the values of kA 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 — bond homolysis in polymer must be close to that in the gas phase. 

Due to the existence of two quite different distinctive distances (scale factors) - lo and l - the recombination kinetics also reveals two stages called monomolecular and bimolecular respectively. The defects survived in their geminate pairs go away, separate and start to mix and recombine with dissimilar components from other pairs. It is clear that the problem of kinetics of the monomolecular process is reduced to the time development of the probability w(f) to find any single geminate pair AB as a function of the initial spatial distribution of the pair components f(r), recombination law cr(r) and interaction Uab (r). The smaller the initial concentration of defects, n(0) —> 0, as lo —> oo, the more correct is the separation of the kinetics into two substages, whereas the treatment of the case of semi-mixed geminate pairs is a very difficult problem discussed below. 

Virtually all previously published opinions on thermally degraded poly (vinyl chloride) regarded the dehydrochlorination process as being in the first instance, a monomolecular process resulting in conjugated polyene structures, and that all oxidation and cross-linking reactions... 

Excited-state relaxation can proceed spontaneously in monomolecular processes or can be stimulated by a molecular entity (quencher) that deactivates (quenches) an excited state of another molecular entity, by energy transfer, electron transfer, or a chemical mechanism [lj.The quenching is mostly a bimolecular radiationless process (the exception is a quencher built into the reactant molecule), which either regenerates the reactant molecule dissipating an energy excess or generates a photochemical reaction product (Figure 4.1). 

For monomolecular processes of any complexity, this electrotechnic analogy makes it possible to determine the stationary rate of chemical reac tions with respect to the reactive species. To do this, one has to consider the Kirchhoff equation for the balance of current inflow and outflow at aU points of the electric circuit contacts. 

Here, index i and parameter are substituted for by index a and parame ter Ao(, which is always allowed for monomolecular processes. Thus, the functional O derivative with respect to thermodynamic rush of an inter mediate is proportional to the rate of this intermediate concentration changes everywhere, even far from thermodynamic equihbrium. For this reason, in the state stationary with respect to the intermediate concentra tion, this derivative turns zero (cf the case of the Rayleigh Onsager functional). 

Dissociation reactions of the general form AB —> A + B are monomolecular processes, in which the rate of decay of the complex is proportional to its concentration. The concentration dependence of cAB is given by the following differential equation ... 

The isomerization of open-chain alkanes with more than six carbon atoms gives isobutane as the main product, together with disproportionated materials, even though the reaction proceeds by the monomolecular pathway [144]. On the other hand, for cyclic alkanes the monomolecular process with preservation of the cyclic structure seems to be the most probable, judging from the results for cyclohexane. The absence of isobutane in the products indicates that the reaction path does not involve open-chain intermediate species. Therefore, it is of interest to try cycloalkanes larger than cyclohexane for clarification of the reaction mechanism along with the catalytic action of S04/Zr02. 

An idea that was tested experimentally is that if the bimolecular mechanism prevails 19), the products formed from n-butene reactants, on the one hand, and from any of the possible isomers formed by dimerization of n-butenes (such as 3,4-dimethylhex-l-ene), on the other hand, should be similar. Thus, the transformations of 2,2,4-trimethylpent-2-ene, 3,4-dimethylhex-2-ene, and methylheptenes were investigated with micro-porous catalysts such as MnAPO-11 and SAPO-11. The results are summarized in Fig. 11, in which the ratio (propene pentene) /n-butenes is plotted for different reactants. The data show that with a selective isomerization catalyst, this ratio is quite low (<0.1) for n-butene reactants in contrast, it is quite high (approximately 0.8) when 3.4-dimethylhex-2-ene or methylheptenes are the reactants, indicating that these compounds are not intermediates in the selective isomerization of n-butenes. Consequently, the isobutylene formed on selective catalysts results from a monomolecular process. Th ese results are considered to be good indirect evidence that the bimolecular reaction is not selective for isobutylene formation. 

The proposed pathway will be more favorable kinetically than that suggested for the true monomolecular process, whereby a primary carbenium ion is formed. To further test the idea that carbonaceous residues are the active and selective sites for the skeletal isomerization of n-butenes, the authors reported results showing that the rate of isobutylene formation catalyzed by ferrierite passed through a maximum as the conversion continuously decreased (Fig. 12) (51). 

The acyl chloride represents an activated growth centre which reacts with the strongly nucleophilic amine group either in the monomolecular process (139), (140) or in the bimolecular condensation reaction... 

Most feeds contain some olefin as an impurity moreover many sulfated zirconia catalysts contain traces of iron or other transition metal ions that are able to dehydrogenate hutane. In the presence of such sites, the olefin concentration is limited by thermodynamics, i.e a high pressure of H2 leads to a low olefin concentration. That aspect of the reaction mechanism has been proven in independent experiments. The isomerization rate over sulfated zirconia was dramatically lowered by H2. This effect is most pronounced when a small amount of platinum is deposited on the catalyst, so that thermodynamic equilibrium between butane, hydrogen and butene was established. In this way it was found that the isomerization reaction has a reaction order of +1.3 in -butane, hut -1.2 in hydrogen [40, 41]. The byproducts, propane and pentane, are additional evidence that a Cg intermediate is formed in this process. As expected, this kinetics is typical for butane isomerization only in contrast pentane isomerization is mainly a monomolecular process, because for this molecule the protonated cyclopropane ring can be opened without forming a primary carbenium ion [42]. 

However, a closer look at the details of the decomposition of a formate ion at the surface reveals a number of problems. It is not certain whether the decomposition is monomolecular or bimolecular. From their work on nickel, Schuit et al. (137) came to support the assumption of a monomolecular process, on the strength of energy considerations, and most other authors make this assumption more or less intuitively. The possibility of a bimolecular process can, in our opinion, not be definitively excluded, however. 

Scheme 4.10 (a) Monomolecular process using an alkoxyamine as an initiator and a controller and (b) bimolecular process using a nitroxide radical as a controller and radical initiator (I2) to generate radicals. 

The oxygen adduct of 8 was decayed to the corresponding protohemin [Fe(III)] complex with isosbestic points, and this decay obeyed first-order kinetics. The lifetime of the S-oxygen adduct was increased with the pH of the aqueous solution. Its life-time was independent of the heme concentration. These results indicate that the irreversible oxidation [to Fe(III)] of the heme-oxygen adduct with the polymeric ligand proceeds mainly via a monomolecular process caused by the attack of a proton on the heme-coordinated oxygen (proton-driven oxidation) rather than via a p-dioxo dimer. 

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