Secondary salt effects

If the rate equation contains the concentration of a species involved in a preequilibrium step (often an acid-base species), then this concentration may be a function of ionic strength via the ionic strength dependence of the equilibrium constant controlling the concentration. Therefore, the rate constant may vary with ionic strength through this dependence this is called a secondary salt effect. This effect is an artifact in a sense, because its source is independent of the rate process, and it can be completely accounted for by evaluating the rate constant on the basis of the actual species concentration, calculated by means of the equilibrium constant appropriate to the ionic strength in the rate study. 

Buffer salts also can exert a secondary salt effect on drug stability. From Table 5 and Fig. 5 it is clear that the rate constant for an ionizable drug is dependent on its pKa. Increasing salt concentrations, particularly from polyelectrolytes such as citrate and phosphate, can substantially affect the magnitude of the pKa, causing a change in the rate constant. (For a review of salt effects, containing many examples from the pharmaceutical literature see Ref. 116.)... 

Electrostatic effects other than ionization are also important. Interactions between reacting ions depend on the local electrical environment of the ions and thus reflect the influence of the dielectric constant of the solvent and the presence of other ions and various solutes that may be present. In dilute solutions the influence of ionic strength on reaction rates is felt in the primary and secondary salt effects (see below). 

The secondary salt effect is important when the catalytically active ions are produced by the dissociation of a weak electrolyte. In solutions of weak acids and weak bases, added salts, even if they do not exert a common ion effect, can influence hydrogen and hydroxide ion concentrations through their influence on activity coefficients. 

In the most common case z = 0, and the secondary salt effect implies that the hydrogen ion concentration will increase with increasing ionic strength. However, the direction of the effect is determined by the sign of the quantity (z - 1). 

The existence of the primary and secondary salt effects indicates the importance of maintaining control over ionic strength in kinetics studies. One may choose to keep the ionic strength low so as to minimize its effects, or one may make a series of measurements at various ionic strengths in order to permit extrapolation to the limit of infinitely dilute solution. Another useful alternative is to maintain the ionic strength constant at a value that is suffi-... 

Take into account both primary and secondary salt effects, but neglect the contributions to the ionic strength of species resulting from the dissociation of NH4OH. 

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. 

In primary salt effect, addition of an electrolyte (salt) or variation of ionic strength affects the activity coefficients and hence the rate of reaction. However, in a reaction where H+ or OH ions produced from a weak acid or weak base act as catalyting agent, the addition of salt influences the concentration of H+ or OH ions. Since the rate of reaction depends upon the concentration of H+ or OH, it will be affected by the salt concentration. This phenomenon is known as secondary salt effect. 

Salt effects in kinetics are usually classified as primary or secondary, but there is much more to the subject than these special effects. The theoretical treatment of the primary salt effect leans heavily upon the transition state theory and the Debye-Hii ckel limiting law for activity coefficients. For a thermodynamic equilibrium constant one should strictly use activities a instead of concentrations (indicated by brackets). 

The secondary salt effect originates in the effect of salts upon equilibria that may be of importance to the rate, e.g., changing the activity of catalytically functioning species. 

Although these effects are often collectively referred to as salt effects, lUPAC regards that term as too restrictive. If the effect observed is due solely to the influence of ionic strength on the activity coefficients of reactants and transition states, then the effect is referred to as a primary kinetic electrolyte effect or a primary salt effect. If the observed effect arises from the influence of ionic strength on pre-equilibrium concentrations of ionic species prior to any rate-determining step, then the effect is termed a secondary kinetic electrolyte effect or a secondary salt effect. An example of such a phenomenon would be the influence of ionic strength on the dissociation of weak acids and bases. See Ionic Strength... 

Secondary Salt Effect This arises when one of the reaciants is a weak electrolyte, the extent of ionization and hence the concentration of ions are then affected by the presence of salts. The secondary salt effect is involved in catalytic reactions. 

Triazenes have been prepared by the treatment of resin-bound aromatic diazonium salts with secondary amines (Figure 3.27). Regeneration of the amine can be effected by mild acidolysis (Entry 1, Table 3.23). Triazenes have been shown to be stable towards bases such as TBAF, potassium hydroxide, or potassium tert-butoxide [454], and under the conditions of the Heck reaction [455]. Primary amines cannot be linked to supports as triazenes because treatment of triazenes such as R-HN-N=N-Ar-Pol with acid leads to the release of aliphatic diazonium salts into solution [373]. Triazenes derived from primary amines can, however, be used for the preparation of amides and ureas (see Section 3.3.4),... 

One should mention here that in reactions involving ions, the effects of electrolytes can be pul into two principal categories (a) primary salt effect and lb) secondary salt effects. 

Primary sail effects refer to the effects of electrolyte concentration on the activity coefficients. Secondary salt effects are those concerned with the actual chances in concentration of the reacting species resulting from the addition of electrolytes. 

Aroyl esters of anthracene-9-methanol are photolysed in methanol to give products consistent with the anthracene-9-methyl cation as an intermediate.41 Rate constants for the solvolyses of secondary alkyl tosylates in fluorinated solvents were analysed in terms of the possible involvement of very short-lived carbocation-tosylate ion pair intermediates.42 The effect of added electrolytes on the rate of solvolysis of cumyl chloride and its -methyl derivative was studied in 90% aqueous acetone and 80% aqueous DMSO, with the results revealing a combination of a special salt effect and a mass law effect.43 Kinetic parameters obtained for the solvolysis of (8) (R1 = R2 = Me and R1 = Ar, R2 = H) show that there is substantial n, n participation in the transition state [e.g. (9). 44... 

Oxidation. The salt effects rapid cleavage of glycols in high yield. It can be used also for the oxidation of secondary nitro compounds to ketones (Nef reaction).- An excess of the guanidine base is used to form the nitronatc, which is then oxidized to the ketone by the salt. Yields are —80-95%, and are higher than those obtained with iodosylbenzene, C HjIO. Oxidation of primary nitro compounds by his method gives only low to moderate yields of aldehydes. 

Eq. (5-99) was checked in detail in a large number of studies, but an extensive treatment of primary and secondary salt effects on reaction rates is beyond the scope of this Section. The reader is referred to references [2-5, 11, 12, 19-21, 28], particularly to the excellent comprehensive reviews of Davies [260], Blandamer et al. [828], and Loupy et al. [829]. 

In comparisons of rate coefficients measured at different pH values, it is necessary to keep a constant ionic strength by addition of a neutral salt such as NaCl, KC1, NaNOs, or NaC104, as the pH of a buffer system may be altered by a change of ionic strength (secondary salt effect [1]). Furthermore, the catalytic rate coefficient of a hydrogen ion or hydroxide ion catalyzed reaction is dependent on the ionic strength also (primary salt effect [1], see also Vol. 2, p. 337). 

A confirmation of the fact that the catalytic action is caused by NH2" alone has been found by experiments on common ion effects and secondary salt effects. If potassium bromide is added to a solution of potassium amide, the concentration of amide ions changes for two reasons. By secondary salt effect, the ionic strength increases and f decreases according to the Debye-Hiickel equation (Equation 8). In Equation 7 of the... 

Kilocalories, 14 conversion of, 246 Kilojoules, 14 conversion of, 246 Kinetically equivalent terms, 123 Kinetically indistinguishable terms, 123 Kinetic equivalence, 123, 136, 217, 349 in intramolecular catalysis, 267 salt effect and, 411 Kinetic isctdpe effects, 292 primaiy, 293 secondary, 298 solvent, 300... 

The salt effect on the reaction is simply a secondary effect on the various equilibria, and there is no detectable kinetic salt effect on the decomposition of H2C-. 

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