Salt effect cation

The concentration of salt in physiological systems is on the order of 150 mM, which corresponds to approximately 350 water molecules for each cation-anion pair. Eor this reason, investigations of salt effects in biological systems using detailed atomic models and molecular dynamic simulations become rapidly prohibitive, and mean-field treatments based on continuum electrostatics are advantageous. Such approximations, which were pioneered by Debye and Huckel [11], are valid at moderately low ionic concentration when core-core interactions between the mobile ions can be neglected. Briefly, the spatial density throughout the solvent is assumed to depend only on the local electrostatic poten-... 

Salt effects on the reaction of 2,4-dinitrochlorobenzene with amines or alkoxides have been investigated.Reinheimer et al. have studied decelerative ion pairing of alkali metal methoxides in reaction with this substrate cations and anions in added salts have specific effects on ion pairing. 

The reader can show that a third scheme also gives the same answer. In it the two cations first associate (however unlikely), and this dinuclear complex reacts with Cl-. To summarize any reaction scheme consistent with the rate law is characterized by the same ionic strength effects. In other words, it is useless to study salt effects in the hopes of resolving one kinetically indistinguishable mechanism from another. 

A unimolecular ionization was shown to be the mechanism of solvolysis by means of rate studies, solvent effects, salt effects, and structural effects (179,180). The products of reaction consist of benzo [bjthiophen derivatives 209 or nucleophilic substitution products 210, depending upon the solvent system employed. By means of a series of elegant studies, Modena and co-workers have shown that the intermediate ion 208 can have either the open vinyl cation structure 208a or the cyclic thiirenium ion 208b, depending... 

Winstein Robinson (1958) used this concept to account for the kinetics of the salt effects on solvolysis reactions. They considered that carbonium ions (cations) and carbanions could exist as contact ion-pairs, solvated ion-pairs and as free ions and that all these forms participated in the reactions and were in equilibrium with each other. These equilibria can be represented, thus ... 

Such competition between ion-pair collapse of MT+, C(N02)f and the radical-pair collapse of MT+, NO is also readily modulated by the addition of inert salt.14 The description of the solvent and salt effects in equations (82) and (83) is further confirmed by direct kinetics analysis of the decay of the cation radical MT+ on the nanosecond/microsecond timescale. 

Other reactions in which cations other than protons are catalyti-cally effective are esterification and acetal formation, catalyzed by calcium salts,277 and the bromination of ethyl cyclopentanone-2-carboxylate, catalyzed by magnesium, calcium, cupric, and nickel, but not by sodium or potassium ions.278 One interpretative difficulty, of course, is the separation of catalysis from the less specific salt effects. The boundary line between salt effects (medium effects) and salt effects (catalysis) is not sharp either in concept or experimentally. 

Re (ii). The "salt effect" is more intriguing. At low lithium concentrations (lithium is the most effective cation) the reaction is first order in the salt concentration and zero order in rhodium, methyl iodide, and carbon monoxide. The rate steeply increases with the lithium concentrations. At high lithium concentrations the rate dependencies equal the Monsanto process, i.e. first order in rhodium and methyl iodide, and zero order in CO. The metal salts are involved in two reactions ... 

Metal ions, in particular singly charged ions such as Na", K", Cs", and Ag are sometimes added to the matrix-analyte solution to effect cationization of the neutral analyte. [98] This is advantageous when the analyte has a high affinity to a certain metal ion, e.g., towards alkali ions in case of oligosaccharides. [6] Addition of a certain cation can also help to concentrate the ions in one species, e.g., to promote [Mh-K]" ions in favor of all other alkali ion adducts upon addition of a potassium salt. 

Both quaternary onium salts and cation complexes of lipophilic multidentate ligands (crown-ethers and cryptands) have been used as catalysts in two-phase systems in the presence of base (OH, F, etc.). However, under these conditions, the lack of chemical stability of quaternary salts and the very low complexation constants of multidentate ligands (especially crown-ethers) make all these systems barely effective in the activation of such anions. 

Surface-active crown ethers are distinctly differ from usual type of nonionics in salt effect on the aqueous properties, due to the selective complexing ability with cations depending on the ring size of the crown. As shown in Figure 3 (22), the cloud point of the crowns is selectively raised by the added salts. This indicates that the degree of cloud point increase is a measure of the crown-complex stability in water (23). 

When a constant ionic strength of the test solution is maintained and the reference electrode liquid bridge is filled with a solution of a salt whose cation and anion have similar mobilities (for example solutions of KCl, KNO3 and NH4NO3), the liquid-junction potential is reasonably constant (cf. p. 24-5). However, problems may be encountered in measurements on suspensions (for example in blood or in soil extracts). The potential difference measured in the suspension may be very different from that obtained in the supernatant or in the filtrate. This phenomenon is called the suspension (Pallmann) effect [110] The appearance of the Pallmann effect depends on the position of the reference electrode, but not on that of ISE [65] (i.e. there is a difference between the potentials obtained with the reference electrode in the suspension and in the supernatant). This effect has not been satisfactorily explained it may be caused by the formation of an anomalous liquid-junction or Donnan potential. It... 

The decomposition of the salt [(cp)Fe(CgHg) Na+] displaces the equilibrium with the participation of this salt to the right. Hence, the difference in stability of the (20e) anion (cp)Fe(CgHg) depending on the cation nature is the major factor responsible for the salt effect. The NaPFg salt can induce electron transfer between neutral organometallic species very efficiently. 

Salt effects (chlorides and bromides of alkali metal, alkaline earth, and ammonium cations) on rate constants for aquation of [Fe(bipy3] " " and [Fe(phen)3] " " in aqueous solution have been interpreted in terms of added cation effects on the activity of the water. 

The remarkable effect of hyperconjugation is seen in the structure of the 3,5,7-trimethyl-1-adamantyl cation (28), also obtained as its Sb2Fjj salt." ° The cation shows a signihcant flattening at Cl, with short C —Co. bonds (1.44 A) and relatively long Co.—Cp bonds (1.61 A). These point to signihcant C—C hyperconjugation, as shown in Eq. 9. 

Diets-Alder catalysis.2 This cation radical enhances the reactivity of a neutral or electron-rich eis-1,3-diene in Diels-Alder reactions. Thus 1,3-cyclohexadiene undergoes Diels-Alder dimerization only at temperatures around 200°. The presence of 5-10 mole % of this salt effects dimerization even at —78°, with the usual endo/ exo selectivity (5 1). It also permits facile condensation of 1,3-cyclohexadiene with a hindered dienophile such as 2,5-dimethyl-2,4-hexadiene (equation 1) the dimer of the former diene is a minor product (20% yield). 

We can now discuss the solvation number. In systems such as the metha-nol-water-CaCl2 system shown in Figure 5, the hydration number is the greatest, that is, 11 at x3 = 0.020. If the hydration number of ions is calculated from the hydration entropy, Ca2+ is seven and Cl is two (3). If it is assumed that CaC is completely dissociated and both the cation and anion forms hydrate, the hydration number becomes 7 + 2X2 = 11, which agrees with the value obtained from the salt effect. 

In Table XVIII, there are several trends that can be noted in k if one proceeds through the R4NBr series. First of all, k tends to decrease as the size of the TAA cation increases and, in fact, tetra-n-butylammonium bromide shows a large salting-in effect. This trend is emphatically demonstrated by Figure 13, which shows the smoothed salt effects of the various salts studied in the ethanol-water system at x = 0.206. Secondly, it appears that there is a larger salting-out effect as the mole fraction of ethanol increases in the binary solvent mixture. 

The solubility and surface tension results itemized in Table XVII confirm that there is a larger interaction between ethanol and the TAA salts as the size of the cation or organic portion increases. The data show that in spite of the R4NBr salts becoming more soluble in water as the cation size increases, their solubility increases much more rapidly in ethanol, in fact by a factor of 10 greater in ethanol than in water as the salt series of the present investigation is ascended. As a result, the two highest members of the series, the tetrapropyl and tetrabutyl salts, are actually more soluble in ethanol than in water, while the reverse is true for the lower three. Consequently, on the basis of relative solubilities of the salts studied in both ethanol and water, trends in the salt effect parameters similar to those of this work, based on the vapor-equilibrium studies listed in Table XVIII would be observed. 

As previously stated, Gross (51) and Larsson (52) suggested that the salt effect is an additive function of two constants characteristic of the cation kc, and the anion, k i.e., log So/S = kscs = (k + kc)cs- In these studies Larsson assumed K+ = kc - Individual ion contributions have also been devised in volume studies with the additivity often extending to moderate concentrations (53) and enthalpy studies (54). 

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