General salt-effect

It is speculated also that the promotional effect observed for LiOAc in the model studies is rather a general salt effect from the formation of Lil [9c]. This is clearly not the case. In model studies, no CH3OAC is detected when LiOAc is added to CH3I under the IR conditions. The reaction rate to Lil from LiOAc and CH3I under these conditions is very slow relative to the oxidative addition reaction. 

In conclusion ionic liquids can be considered a mixing of general salt effects and ion-specific effects, often only partially investigated and understood, which make them fascinating media having new and improved properties. 

Summarizing available evidence at present, specific ion effects are hardly separable from general salt effects in alkali halides or nitrates, but may easily become dominant due to short-range interactions in the presence of larger ions. 

The rate of oxidation of o- and p-nitrotoluenes by [FeCChOeP" in 50% DMSO is retarded by the presence of edta. Copper catalysis is discounted on the grounds that the nitrotoluenes would not form complexes with copper(ii). The retardation may be explained by a general salt effect. Base catalysis and a specific solvent effect of DMSO suggest that the reaction proceeds by an initial rapid proton abstraction (Scheme 6). 

The continuum treatment of electrostatics can also model salt effects by generalizing the Poisson equation (12) to the Poisson-Boltzmann equation. The finite difference approach to solving Eq. (12) extends naturally to treating the Poisson-Boltzmann equation [21], and the boundary element method can be extended as well [19]. 

If (A i[X ]/A 2[Y ]) is not much smaller than unity, then as the substitution reaction proceeds, the increase in [X ] will increase the denominator of Eq. (8-65), slowing the reaction and causing deviation from simple first-order kinetics. This mass-law or common-ion effect is characteristic of an S l process, although, as already seen, it is not a necessary condition. The common-ion effect (also called external return) occurs only with the common ion and must be distinguished from a general kinetic salt effect, which will operate with any ion. An example is provided by the hydrolysis of triphenylmethyl chloride (trityl chloride) the addition of 0.01 M NaCl decreased the rate by fourfold. The solvolysis rate of diphenylmethyl chloride in 80% aqueous acetone was decreased by LiCl but increased by LiBr. ° The 5 2 mechanism will also yield first-order kinetics in a solvolysis reaction, but it should not be susceptible to a common-ion rate inhibition. 

It is important to note that the solubility product relation applies with sufficient accuracy for purposes of quantitative analysis only to saturated solutions of slightly soluble electrolytes and with small additions of other salts. In the presence of moderate concentrations of salts, the ionic concentration, and therefore the ionic strength of the solution, will increase. This will, in general, lower the activity coefficients of both ions, and consequently the ionic concentrations (and therefore the solubility) must increase in order to maintain the solubility product constant. This effect, which is most marked when the added electrolyte does not possess an ion in common with the sparingly soluble salt, is termed the salt effect. 

Bronsted s salt effects, see Zollinger, 1953a for a general review of salt effects see Loupy and Tchoubar, 1992). 

We will first explore what salt effect is expected for k and k2, and then examine the general situation. It is convenient to proceed by defining the net activation process.15 This is the chemical equation for the hypothetical process in which the transition state is formed from the predominant forms of the reagents, and not from the reactive entities. For the two pathways implicit in Eq. (9-47), these are the net activation processes ... 

One can test for general acid-base catalysis by varying [BH+] and [B] at constant pH. An easy test is to dilute the buffer progressively at a constant ratio of [BH+]/[B], making up any ionic strength change so as not to introduce a salt effect. If the rate is invariant with this procedure, then general acid-base catalysis is absent under the circumstances chosen. 

Clearly, the intermediate value of a can readily be evaluated, but the value near 0.9 cannot. In fact, one would need [HOAc] = 0.7 M to see even a 10 percent effect on the rate at pH 5 if a were 0.9. This would be difficult to detect, especially since the accompanying [OAc ] would then be 1.3 M, leaving the system open to salt effects at this high an ionic strength. In general, the measurable range is about 0.2 < a < 0.8. 

We have seen how the polarity of the solvent influences the rates of Sn 1 and Sn2 reactions. The ionic strength of the medium has similar effects. In general, the addition of an external salt affects the rates of SnI and Sn2 reactions in the same way as an increase in solvent polarity, though this is not quantitative different salts have different effects. However, there are exceptions though the rates of SnI reactions are usually increased by the addition of salts (this is called the salt effect), addition of the leaving-group ion often decreases the rate (the common-ion effect, p. 395). 

Effectiveness in Salt Solutions. Effectiveness in calcimn and magnesium salt solutions is different from that in sodium salt solutions [162]. In general, the effectiveness is lower at zero salinity. 

Srinivasan J, Trevathan MW, Beroza P, Case DA (1999) Application of a pairwise generalized Born model to proteins and nucleic acids Inclusion of salt effects. Theor ChemAcc 101 426-434. 

The high ionic concentration at the micellar surface may result in an ionic strength effect on reaction rate. Salt effects in water, however, are generally smaller for ion-molecule reactions than for reactions which involve an increase or decrease of charge, and they should be approximately zero in the... 

An extensive study of the effect of salts on the pH-activity curves of PM was made on alfalfa49 and on orange21 and tomatoes.44 In general, the effect of salts is to lower the pH at which maximum activity is attained and to extend the activity into lower pH regions. At the higher pH values (7-8), salts have practically no activating effect. The main usefulness of salt activation of PM seems to lie in counteracting adverse pH conditions. 

It is generally observed that the rate of reaction can be altered by the presence of non-reacting or inert ionic species in the solution. This effect is especially great for reactions between ions, where rate of reaction is effected even at low concentrations. The influence of a charged species on the rate of reaction is known as salt effect. The effects are classified as primary and secondary salt effects. The primary salt effect is the influence of electrolyte concentration on the activity coefficient and rate of reaction, whereas the secondary salt effect is the actual change in the concentration of the reacting ions resulting from the addition of electrolytes. Both effects are important in the study of ionic reactions in solutions. The primary salt effect is involved in non-catalytic reactions and has been considered here. The deviation from ideal behaviour can be expressed in terms of Bronsted-Bjerrum equation. 

The overall steric demands of the catalyst and the substrate are important in the spatial arrangement of the H-bonded complex. Consequently, although the less rigid ephedrinium salts have been used with some success, they are generally less effective than the derivatives of the cinchona alkaloids, the rigidity of which imposes a greater stereochemical restraint on the structure of the H-bonded complexes. 

Advantages of the carbonate-exchange technique are (1) experiments up to 1,400°C, (2) no problems associated with mineral solubility and (3) ease of mineral separation (reaction of carbonate with acid). Mineral fractionations derived from hydrothermal and carbonate exchange techniques are generally in good agreement except for fractionations involving quartz and calcite. A possible explanation is a salt effect in the quartz-water system, but no salt effect has been observed in the calcite-water system (Hu and Clayton 2003). 

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