Ionic reactants

Phase-transfer catalysis (Section 22.5) Method for increasing the rate of a chemical reaction by transporting an ionic reactant from an aqueous phase where it is solvated and less reactive to an organic phase where it is not solvated and is more reactive. Typically, the reactant is an anion that is carried to the organic phase as its quaternary ammonium salt. 

The polyelectrolyte catalysis of chemical reactions involving ionic species has been the subject of extensive investigations since the pioneering studies of Morawetz et al. [12] and Ise et al. [13-17]. The catalytic effect or the ability of poly-electrolytes to enhance or retard reaction rates is mainly due to concentration or exclusion of either or both of the ionic reactants by the polyions added to the reaction systems. For example, the chemical reaction between ionic species carrying the same charge is enhanced in the presence of polyions carrying the opposite charge. This enhancement can be attributed to an increase in the local concentration... 

It follows that for ionic reactants in binary solutions, the limiting current is given not by Eq. (4.10) but by the equation... 

Generally, an electrolyte may contain several ionic reactant species but no obvious excess of a foreign electrolyte. Then, as already mentioned, a calculation of the migration currents [or coefficients a in equations of the type (4.22)] is very complex and requires computer use. 

When the gas-phase reactions, such as the relative acidities or basicities were compared with their counterparts in solution (in a solvent such as water) it was generally found16,17 that the energetics in the solvent were strongly affected by solvation effects and particularly the solvation of the ionic reactants. Relationships between the gas-phase and solution-phase reactions and the solvation energies of the reactants are generally obtained through thermodynamic cycles. From the cycle,... 

Adding a phase-transfer catalyst solves this problem by transferring the ionic reactant into the organic phase. 

We need to know which ions are available. To do this, it will be helpful to break apart any ionic reactants into their constituent ions ... 

When a neutral olefin forms covalent adducts with ionic reactants at one of its C—C double bonds, a new single C—C bond is formed which, owing to the influence of the substituents added during the ion/molecule reaction, may undergo facile dissociation. The... 

A comparison is made between the gas phase and solution phase reaction pathways for a wide range of organic reactions. Examples are presented in which the gas phase and solution phase mechanisms are the same for a given set of reactants in which they differ, but attachment of the first molecule of solvent to the bare gas phase ionic reactant results in the solution phase products and in which the bare, monosolvated, and bulk-solvated reactions proceed by three different pathways for the same reactants. The various tools available to the gas phase ion chemist are discussed, and examples of their use in the probing of ionic structures and mechanisms are reported. 

A compound whose addition to a two-phase organic water system helps to transfer a water soluble ionic reactant across the interface to the organic phase where a homogeneous reaction can take place is called a phase transfer catalyst. These catalysts enhance the rate of a reaction. A quaternary ammonium halide R4N+ X- e.g., tetrabutylammonium halide is phase transfer catalyst. It can cause the transfer of the... 

A reaction of particular relevance with respect to applied catalysis is the oxidative dehydrogenation (ODH) of hydrocarbon by VmOn ions according to reaction 2, which involves a two-electron reduction of the cluster. By means of a systematic study of the reactions of various YmOn ions as well as the related oxo-vanadium hydroxides VmO H+ ions with a set of C4-hydrocarbons, it was demonstrated recently that the ODH activity of the cluster ions shows a clear correlation with the formal valence of vanadium in the cluster ions with a maximum reactivity for formal vanadium (V) (Fig. 3) [84]. In such a kind of reactivity screening, it is essential to include more than a single reagent as a probe for the reactivity of the different ions in order to reduce interferences by kinetic barriers of one particular combination of neutral and ionic reactants [85]. Accordingly, the sums of the relative rate constants for the ODH reactions of the four different butenes are considered and normalized to the most reactive ion studied, which turns out to be the formally pure vanadium (V) compoimd In addition to isomeric... 

The Equations. It is possible to view the effects of pressure on electrochemical reaction rates in two ways. On the one hand, the partial pressure of a gaseous reactant (e.g 02) takes its place in kinetic equations and has an effect on the reaction rate similar to that of the concentration of an ionic reactant. 

As in the previous chapter on the Smoluchowski theory and its extensions, similar boundary and initial conditions may be used. The reaction of a species A with a vast excess of B (yet still sufficiently dilute to ensure that Debye- Hiickel screening is unimportant) can be considered as one where the A species are statistically independent of each other and are surrounded by a sea of B species. An ionic reactant A has a rate of reaction with all the B reactants equal to the sum of the rates of reaction of individual A—B pairs. This rate for large initial separations of A and B is... 

So far, very dilute solutions have been considered such that the interaction between ions is only coulombic. When other (unreactive) ions are nearby, the direct interaction between ionic reactants is partially screened and was first developed by Debye and Hiickel [91]. They showed that the potential energy, eqn. (39), is modified and becomes... 

In the absence of excess of inert electrolyte, the concentration of an ionic reactant in the pre-electrode plane and the potential at this plane are interconnected and depend on the structure of the double layer. Thus, the apparent electrode reaction order will also be influenced by the double layer. 

The second term in eqn. (110) is the double layer correction to the observed reaction order due to the changes in the interfacial potential distribution with the bulk concentration of the ionic reactant. When 9A02/9 In [O] = 0, eqn. (110) becomes identical to eqn. (89) for concentrations instead of activities. This occurs in the presence of large excess of supporting electrolyte, since the concentration of the reacting ion 02o does not determine the interfacial potential distribution and the true reaction order is obtained in eqn. (110). 

Other endoergic reactions for which an increased cross section has been observed when the internal vibrational energy of the ionic reactant... 

The catalytic effect for reactions involving an ionic reactant usually shows a strong dependence on the total amphiphile concentration. The maximal effective rate constant is attained at concentrations just over the CMC. Romsted284 showed that this occurs due to the competition between the ion binding of the reactive ions (OH- in the example above) and the counterions of the amphiphile. Recently, Diekman and Frahm285 286 showed that it is possible to rationalize the kinetic data by describing the ion distribution through a solution of the Poisson-Boltzman equation. (See Fig. 5.1). 

Generally, PTC involves the transfer of an ionic reactant from an aqueous or solid phase into an organic phase across an interfacial area, where it reacts with a non-transferred reactant. Once reaction is complete, the catalyst must transport the ionic product back to the aqueous or solid phase to run a new catalytic cycle. The classical description of the PTC cycle between an aqueous or solid phase and an organic phase is illustrated in Scheme 3.7. 

The decrease of the concentration of the electroactive species with increasing potential has to be attributed to double layer effects. As first pointed out by Frumkin [58], in dilute solutions the electron transfer rate is affected by variations of the potential in the double layer in two ways. The potential in the outer Helmholtz plane, fa, is due to the extension of the double layer not identical to the potential in the solution (at the end of the double layer), so that the effective driving force of the reaction is DL — fa. Furthermore, the concentration of ionic reactants in the reaction plane, c, is influenced by electrostatic effects and differs from the concentration just outside the double layer, c0, by a Boltzmann term ... 

Either the ionic reactant or product of equation 13, indicated as Mn(L )+ in equation 14, were used to determine relative binding energies, D(M+—L), of various organic molecules (L2) to Mn+ through ICR determination of the ligand-exchange equilibrium of equation 1458. 

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