Cation solvent influence

The propagation reactions of the growing cationic chain end with the monomer ethene have already been discussed in part 4.3. The reaction enthalpies of the corresponding propagation steps show different tendencies for the gas phase and solution, when the cationic chain end is lengthened. However, as the monomer is increased in size and the cationic chain end remains the same, then the tendencies for the gas phase and solution correspond to each other. This is an indication that the solvent influence on the cationic propagation reaction is determined by the nature of the cations in question and their solvation. 

Reaction with solvent - The solvent influences the course of cationic reactions not only through its dielectric constant, but also because many substances used as solvents are far from inert in these reactions [22, 23]. Although much more experimental material is required before a full treatment of the subject becomes possible, at least one example, the cationic polymerisation of styrene in toluene, is amenable to quantitative discussion. Experiment shows that polymerisation is rapid and complete, the molecular weight is low and the polymer contains para-substituted rings which are almost certainly tolyl endgroups [22]. Theoretically, a polystyryl carbonium ion can react with toluene in six different ways, only two of which (a.l and b. 1 below) can lead to tolyl endgroups in the first case the tolyl group is at the end of the terminated chain, in the second it is the start of a new chain. The alternative reactions can be represented as follows... 

The data in Table 1.1 allow one to estimate the position of the equilibrium for any of the other carbon acids with a given base. It is important to keep in mind the position of such equilibria as other aspects of reactions of carbanions are considered. The base and solvent used will determine the extent of deprotonation. There is another important physical characteristic which needs to be kept in mind, and that is the degree of aggregation of the carbanion. Both the solvent and the cation will influence the state of aggregation, as will be discussed further in Section 1.6. 

It was shown that the rate of olefin production was dependent on the size of the metal cation (Li+, Na+, K+, Rb+, and Cs+) of the base. The increase in olefin production with an increase in cation size was explained as an increase in cation solvation by solvent with the larger cations, making t-butoxide a stronger base and thus increasing the rate of proton abstraction. Possibly these differences in cation solvation influence the reactivity of the carbanion intermediate for the conversion of benzothiophene and account for increased reactivity with the larger cation, potassium. 

A closely related example, the equilibrium between tight and solvent-separated ion pairs of lithium lO-phenylnonafulvene-10-oxide, has already been given in Section 2.6 (formulas (2b) and (3b)). Depending on the solvent-influenced association with the lithium cation, the anion exists either as the aromatic benzoyl [9]annulene anion or as the olefinic nonafulvene oxide anion [208],... 

It should be mentioned that cation complexation by crown-type ligands can itself be solvent-dependent. For example, the dissociation rate of potassium [2.2.2]cryptate in EPD solvents increases with the donor number of the solvent [650]. Moreover, coronands themselves can interact with organic solvent molecules [651]. Such cation-solvent and ligand-solvent interactions can influence the formation of cation-ligand complexes. 

In analogy with ambident anions, mesomeric ambident cations do exist, but the solvent influence on their dual reactivity with nucleophiles has not been thoroughly investigated see reference [368] for a review. 

The rather narrow electrochemical window of water, limited by the discharge of hydrogen and oxygen, has stimulated the use of nonaqueous solvents for electrochemical reactions. Procedures for measuring and reporting electrode potentials in nonaqueous solvents are presented in reference [128]. The solvent influence on the redox properties of cations and anions has been reviewed [172], as have applications of ion-selective electrodes in nonaqueous solvents [129] and the influence of nonaqueous solvents on the polarographic half-wave potentials of cations [173]. 

The second-order kylation rates of carboxylate salts are influenced by the nature of the cation, solvent and the alkyl halides. In general, carboxylate anions with soft (large) counterions are more reactive than those with hard counterions because of higher charge separation in the former. Thus, the anticipated order of increasing reactivity Li (l) < Na (l.l) < K Cl.S) < Cs+(2) was observed in the reaction of 2-methyl-2-propylpentanoate with 1-iodopentane at (Xi C in HMPA-EtOH (1 1 v/v). ... 

The solvent influences initiation, propagation, transfer and termination reactions in cationic polymerizations initiated with alkyl-aluminum/coinitiator systems. The solvent affects the dielectric constant, is involved in solvation (particularly of ions) and can act as a transfer agent, terminating agent, and in some instances even as coinitiator. 

Solvents influence the rate of free-radical homopolymerization of acrylic acid and its copolymerization with other monomers. Hydrogen-bonding solvents slow down the reaction rates. Due to the electron-withdrawing nature of the ester groups, acrylic and methacrylic ester polymerize by anionic but not by cationic mechanisms. Lithium alkyls are very effective initiators of a-methyl methacrylate polymerization yielding stereospecific polymers.Isotactic poly(methyl methacrylate) forms in hydrocarbon solvents. Block copolymers of isotactic and syndiotactic poly(methyl methacrylate) form in solvents of medium polarity. Syndiotactic polymers form in polar solvents, like ethylene glycol dimethyl ether, or pyridine. This solvent influence is related to Lewis basicity in the following order ... 

The deviations will then depend on a combination of factors the hydrogen-bonding donor strength of the solvent, the strength of the cation-solvent interaction, and the steric requirements in the inner sphere. These factors may reinforce or oppose one another, and so produce the observed variety of behaviour in different solvents(3l). The solvent structure is also influenced by such factors, however, and it is not easy to distinguish their effects on outer-sphere complexation from those on solvent structure. 

The choice of solvent influenced both the activity and the selectivity of the catalytic system. Consequently, the reactions with the best cationic complexes phosphine gold(I) 6, phosphite gold(I) 20, and IPr gold(I) 16 were tested in different solvents (Table 2.3). In CH2CI2, phosphine complex 6 gave a similar ratio to phosphite complex 20 (Table 2.3, entries 1 and 6), but in Et20 the major product was the hydroxycychzation product 1-19, due to traces of residual water (Table 2.3, entry 2). DMF completely inhibited the reaction of all the complexes (Table 2.3, entries 4, 9, and 13). 

Donescu, D., lanchis, R., Petcu, C., Purcar, V., Nistor, C.L., Radovici, C., Somoghi, R., Pop, S.F., Perichaud, A., 2013. Study of the solvents influence on the layered silicates-cation polymer hybrids properties. Digest Journal of Nanomaterials and Biostructures 8, 1751-1759. 

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