Redistribution reactions kinetics

The redistribution reaction of MeSnCl3 to give Me2SnCl2 and S11CI4 in solution at 50 °C follows second-order kinetics, and is faster in coordinating solvents and in the presence of alcohols or amines, suggesting a nucleophile-assisted electrophilic mechanism.330... 

Reaction dynamics as opposed to reaction kinetics strives to unravel the fundamentals of reactions—just how they transpire, how intramolecular vibrational energy redistributions provide energy to the modes most involved along the reaction coordinate, how specihc reaction states progress to specihc product states, why product energy distributions and ratios of alternative products are as they are, and, of course, how fast the basic processes on an atomic scale and relevant timeframe occur. 

This chapter is concerned with the influence of mechanical stress upon the chemical processes in solids. The most important properties to consider are elasticity and plasticity. We wish, for example, to understand how reaction kinetics and transport in crystalline systems respond to homogeneous or inhomogeneous elastic and plastic deformations [A.P. Chupakhin, et al. (1987)]. An example of such a process influenced by stress is the photoisomerization of a [Co(NH3)5N02]C12 crystal set under a (uniaxial) chemical load [E.V. Boldyreva, A. A. Sidelnikov (1987)]. The kinetics of the isomerization of the N02 group is noticeably different when the crystal is not stressed. An example of the influence of an inhomogeneous stress field on transport is the redistribution of solute atoms or point defects around dislocations created by plastic deformation. 

In addition, this procedure was quite tedious and time consuming. Therefore, in recent years when physical methods for assaying molecules in mixtures—methods such as nuclear magnetic resonance (NMR), gas chromatography, and others—have become available, a renaissance in the study of redistribution reactions has taken place. These methods allowed a rapid, quantitative, and precise determination of all of the reaction products present in a mixture. Also, equilibrium reactions could be carried out in much smaller sample sizes, thus permitting the study of exotic, hard-to-obtain compounds. Redistribution reactions—the kinetics as well as the equilibria—can now be measured directly in sealed NMR tubes. Furthermore, the relatively recent widespread availability to chemists of high-speed computers, in addition to these modern analytical tools, has facilitated the use of the appropriate mathematics even when highly complicated. 

In the last decade, an immense amount of experimental material has been generated describing the preparation and the chemical and physical properties of transition metal n complexes and coordination compounds. Recently great emphasis has been placed on the study of the kinetics and the reaction mechanisms involving such compounds. Although redistribution reactions as defined earlier in this review and as exemplified specifically by the reaction of Eq. (168) (M = transition metal, L=coordinated ligand)... 

The mechanism snfficiently explains how prodnctive condensation metathesis occms since terminal alkenes are contained in every step of the productive cycle (Scheme 5). In this kinetically controlled regime (high concentration of terminal alkene present), alkylidene reactions with unsubstituted (terminal) alkenes are favored over interchain redistribution reactions or other degenerate eqnilibria. However, when terminal alkene concentration decreases near the end of the polycondensation reaction, competing... 

However, one difficulty of the proposed classification is the fact that reactions of Type III probably overlap with those of Type II. One good example of this is redistribution reactions, presumably a reaction of the organic group. However, Maher and Evans 19) find that intramolecular exchange of methyl groups of monomeric trimethylthallium follows second-order kinetics in deuterobenzene and suggest a dimeric transition state. 

In this section and in 5.7.4 redistribution reactions are referred to that involve the breaking and making of Si—Hg and Ge—Hg bonds. In their simplest form these are selfexchange reactions that show second-order kinetics with large negative entropies of activation consistent with an electron-deficient transition state, e.g. ... 

Chojnowski and Mazurek16 have studied the reaction of phenyldimethyl silanolates with cyclosiloxanes under conditions where siloxane bond redistribution reactions involving the polymer and the catalyst are completely suppressed. For the reaction of sodium phenyldimethyl silanolate (I) with 2,2,5,5-tetramethyl-l-oxa-2,5-disilacyclopentane (II) they find that the rate of disappearance of I in an excess of II follows first-order kinetics. The observed first-order rate constant k varies with the initial... 

The process of synthesizing high-molecular-weight copolymers by the polymerization of mixed cyclics is well established and widely used in the silicone industry. However, the microstructure which depends on several reaction parameters is not easily predictable. The way in which the sequences of the siloxane units are built up is directed by the relative reactivities of the monomers and the active chain-ends. In this process the different cyclics are mixed together and copolymerized. The reaction is initiated by basic or acidic catalysts and a stepwise addition polymerization kinetic scheme is followed. Cyclotrisiloxanes are most frequently used in these copolymerizations since the chain growth mechanism dominates the kinetics and redistribution reactions involving the polymer chain are of negligible importance. Several different copolymers may be obtained by this process. They will be monodisperse and free from cyclics and their microstructure can be varied from pure block to pure random copolymers. 

This model for the thermal rearrangement of l-phenylbicyclo[2.2.1]hex-ene provides an explanation for the stereochemistry observed in the [1,3] methyl shift. More important, it also brings into question some of the common assumptions of reaction kinetics. Transition state theory is based on the premise that the redistribution of internal kinetic energy is faster than is the progress of a collisionally activated reactant over a potential energy surface to a transition state and then to a product. If this assumption is not valid, then the... 

Germanium.— The redistribution reactions in mixtures of tetra-alkylgermanes with germanium tetrachloride or tin tetrachloride follow second-order kinetics and occur... 

Introduction of fillers into the reaction system may have an accelerating action during formation of both linear and crosslinked polymers because of redistribution of intra- and intermolecular interactions. This means that filler affects the very structure of the reaction mixture (solution or melt) in the same way as the formation of some aggregates or the development of entanglements affect the adsorption from solutions. These effects may be responsible for change in the reaction kinetics and changing the properties of the filled polymers. 

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