Solvent reaction, structural change

Probably the most useful data obtained kinetically are the rate constants themselves. They are important since they can tell us the effect on the rate of a reaction of changes in the structure of the reactants (see Chapter 9), the solvent, the ionic strength, the addition of catalysts, and so on. 

Having in mind the various forms of the isokinetic relationship, we can also show its physical meaning in kinetics more clearly. Let us consider a reaction series with a variable substituent, solvent, or other factor. The term reaction series was discussed by Bunnett (14), with the conclusion that the common mechanism of all reactions is a necessary condition (12). However, this condition can seldom be ascertained, and best after finishing the whole analysis. At the beginning, it may be sufficient that the reaction products are invariable and the kinetic order equal. In addition, the structural changes should not be too large of course, this condition cannot be defined precisely. 

Structural change of solvents and coals after the liquefaction reaction. To analyse structural change of solvents and coals in the liquefaction reaction, the solvent and coal should be separated. 

An explanation not easily distinguishable from the one involving resonance with a carbonium ion structure in the transition state is that the reactive species is an ion pair in equilibrium with the covalent molecule. This is quite likely in a solvent insufficiently polar to cause dissociation of the ion pairs. Examples of second order nucleophilic displacements accelerated by the sort of structural change that would stabilize a carbonium ion are of fairly frequent occurrence. Allyl chloride reacts with potassium iodide in acetone at 50° seventy-nine times as fast as does -butyl chloride.209 Another example is the reaction of 3,4-epoxy-1 -butene with methoxide ion.210... 

The source of some of the difficulties encountered in trying to explain the effects of structural changes on ionization rates may be due to the different parts played by the solvent, as for example, the sulfur dioxide of the trityl chloride equilibrium experiments and the aqueous acetone of the benzhydryl chloride rate data. The solvent is bound to modify the effect of a substituent, and although the solvent is usually ignored in discussing substituent effects this is because of a scarcity of usable data and not because the importance of the solvent is not realized "... solvation energy and entropy are the most characteristic determinants of reactions in solution, and... for this class of reactions no norm exists which does not take primary account of solvation. 220 Precisely how best to take account of solvation is an unanswered problem that is the subject of much current research. 

The reaction mixture is often complicated by condensation of mono-saccharidic molecules yielding unwanted homodisaccharides. This problem is also reduced by the minimum water approach (water activity of ca. 0.7-0.8). The organic solvent at low concentration probably deactivates the enzyme due to structural changes whereas high solvent concentrations with the necessary minimum water cause fixation of the enzyme structure in its active conformation. Unfortunately, glycosidases are rather instable in low-water media. This is a big drawback compared to highly solvent-resistant lipases.94... 

Once thermal reactions begin, the coal undergoes structural changes through spontaneous bimolecular reactions between coal constituents or solvent species (75) or through catalytic reactions accelerated by added catalysts or the inherent mineral components (76). Hence, the reactivity changes with the progress of these reactions. 

For an outer-sphere reaction there are three factors which play a role in determining the rate of electron transfer. The first is the approach of the reactants to be in sufficiently close proximity to create an electronic interaction which provides a basis for the delocalization of the exchanging electron. The second is a barrier to electron transfer that is created by the equilibrium structural differences between reactants and products. The third is an additional barrier that is created in the surrounding solvent by the change in charge distribution associated with the electron transfer act. 

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