Counterions reactions involving

Specific-ion electrodes are expensive, temperamental and seem to have a depressingly short life when exposed to aqueous surfactants. They are also not sensitive to some mechanistically interesting ions. Other methods do not have these shortcomings, but they too are not applicable to all ions. Most workers have followed the approach developed by Romsted who noted that counterions bind specifically to ionic micelles, and that qualitatively the binding parallels that to ion exchange resins (Romsted 1977, 1984). In considering the development of Romsted s ideas it will be useful to note that many micellar reactions involving hydrophilic ions are carried out in solutions which contain a mixture of anions for example, there will be the chemically inert counterion of the surfactant plus the added reactive ion. Competition between these ions for the micelle is of key importance and merits detailed consideration. In some cases the solution also contains buffers and the effect of buffer ions has to be considered (Quina et al., 1980). 

Literature on reactions involving micellar counterions is particularly rich and for good reasons. The local concentration of counterions in the micellar Stern region is extremely high compared to typical aqueous solutions. As a result, bimolecular reactions involving bases such as hydroxide and acetate or oxidants such as perchlorate can be accelerated significantly by using these as a counterion for cationic surfactants. Discussion here will be restricted to a selected number of relatively recent examples of particular interest. This should not, however, distract from the merit of many of the other publications in this field. 

The kinetic profile of reactions involving the micelle s counterions is frequently analyzed in terms of the PIE model. Despite the known shortcomings of this model, it nevertheless typically reproduces kinetic data rather well - though one should remain conscious of the potential problems related to parameter covariance (vide supra). 

Despite the sometimes impressive reaction rate enhancements, second-order rate constants for most bimolecular reactions involving counterions actually decrease,with just a few remaining virtually constant or increas-ing. As discussed (vide supra), micellar rate constants for (pseudo) unimole-cular reactions are frequently lower than rate constants in water. Many of the... 

Compared to micellar bimolecular reactions involving reactive surfactant counterions, considerably less work has been done on micellar bimolecular reactions involving two neutral reactants. We will discuss here micellar effects on cycloaddition reactions though this is by no means the only system for which micellar catalysis has been investigated (see, e.g., Bonollo et al. °). 

Reactions with the counterion may involve spontaneous acid expulsion [Eq. (13.28)] or ion combination, which is a chain termination reaction [Eq. (13.29)] ... 

Corrected for an additional termination reaction involving counterion dissociation. 

These structures might of course be regarded as the first propagating species of the polymerisation of 23 initiated by methyl iodide. The ring closure of the open chain form is a process similar to a back-biting reaction, and as such is likely to be thermodynamically favoured by the presence of a ring substituent at the point of closure (157,158). Thus the polymerisation of 23 with I" counterion must involve a similar equilibrium in propagation and any data for rate constants will therefore be of a composite nature. 

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). 

Diazotization procedures. Widely used for the production of aromatic fluorine is the Balz-Schiemann reaction. The approach involves diazotization of the aniline and isolation of the insoluble tetrafluoroborate salt, followed by decomposition under heating conditions (Fig. 32). Initially introduced in 1927 [137,138], it did not achieve commercial utility until the mid-1980s. A modification of the Balz-Schie-mann reaction involves replacing the tetrafluoroborate with other counterions such as a fluorine anion [139],... 

Apart from the relevance to the radiation-induced polymerizations, the pulse radiolysis of the solutions of styrene and a-methylstyrene in MTHF or tetrahy-drofuran (THF) has provided useful information about anionic polymerization in general [33]. Anionic polymerizations initiated by alkali-metal reduction or electron transfer reactions involve the initial formation of radical anions followed by their dimerization, giving rise to two centers for chain growth by monomer addition [34]. In the pulse radiolysis of styrene or a-methylstyrene (MS), however, the rapid recombination reaction of the anion with a counterion necessarily formed during the radiolysis makes it difficult to observe the dimerization process directly. Langan et al. used the solutions containing either sodium or lithium tetrahydridoaluminiumate (NAH or LAH) in which the anions formed stable ion-pairs with the alkali-metal cations whereby the radical anions produced by pulse radiolysis could be prevented from rapid recombination reaction [33],... 

Tetracoordinate Si+ complexes may undergo transformations in the mobile ligand, since nucleophilic attack of the counterion may be directed toward electrophilic centers located in the ligand group. Some reactions involving these transformations are of importance in synthesis (86,272-274). For example, the silylation of phosphorus alkyl esters has been broadly explored by bioorganic chemists as a convenient method of generation of phosphorus acids [Eq. (57)] (273). The mechanism of the silylation reaction [Eq. (57)] has been well documented for the cases where X is... 

Consequently, for polymerization of N-substituted aziridines with small substituents like N-methylaziridine, limited conversions to relatively low molecular weight polymers were observed [165,166]. On the other hand, in the polymerization of N-f-butylaziridine chain transfer to polymer is practically eliminated [167]. No other transfer or termination reaction involving quaternary ammonium active species with, e.g., BF4-counterion has been detected ... 

The characteristics of the active centers in free-radical polymerizations depend only on the nature of the monomer and are generally independent of the reaction medium. This is not the case in ionic polymerizations because these reactions involve successive insertions of monomers between a macromolecular ion and a more or less tightly attached counterion of opposite charge. The macroion and counterion form an organic salt which may exist in several forms in the reaction medium. The degree and nature of the interaction between the cation and anion of the salt and the solvent (or monomer) can vary considerably. 

The dependence of the propagation rate on initiator concentration is more complex, however, and can be explained as reflecting the existence of more than one kind of active center in media that can solvate the counterion. The simplest situation, which is used here for illustration, corresponds to an equilibrium between free ions and ion pairs. [It is likely that various kinds of ion pairs exist (cf. Eq. 9-1) but these ramifications can be neglected in this simple treatment.] The reactions involved in the actual propagation steps in the polymerization of a monomer M by an alkyl lithium compound RLi can then be represented as... 

The Tsuji-Trost reaction involves Pd-catalyzed allylation of stabilized enolates and related compounds, such as malonate and acetoacetate anions. Since most of the counterions have been alkali metals, such as Li and Na, this vast topic is mostly outside the scope of this... 

Most recently the polymerization of methyl methacrylate with Ba counterion has been reported. In THF at —70 °C the rate constant for propagation is independent of the active centre concentration and growth seems to occur via ion pair species only. The kinetic data obtained compare favourably with those for polymethylmethacryl sodium and caesium.As before, active centres are terminated by side-reactions involving the ester group. 

Perhaps the most important chain configuration is the cis-, 4 structure favoured by reactions involving Li" counterion in non-polar solvents. In this instance a lithium compound modelling the active centre exists in both cis and trans forms in the ratio c/t, 35/65, and there seems no reason to suppose that... 

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