Kinetics salt effects

Winstein suggested that two intermediates preceding the dissociated caibocation were required to reconcile data on kinetics, salt effects, and stereochemistry of solvolysis reactions. The process of ionization initially generates a caibocation and counterion in proximity to each other. This species is called an intimate ion pair (or contact ion pair). This species can proceed to a solvent-separated ion pair, in which one or more solvent molecules have inserted between the caibocation and the leaving group but in which the ions have not diffused apart. The free caibocation is formed by diffusion away from the anion, which is called dissociation. 

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. 

The experimental values are shown in Fig. 9-3. They show essentially no kinetic salt effect at low and moderate ionic strengths, consistent with A but not with B. 

Salt effects. A third scheme was mentioned as being theoretically possible for the reaction of FeJ and Sn2+ in solutions containing Cr. It was said to be equivalent to the other two in kinetic form and in the exact magnitude of its kinetic salt effect as related to the kinetic term k2[Feu ][Sn21J[Cl ]. Prove both statements for this reaction scheme ... 

The rate of a reaction that shows specific acid (or base, or acid-base) catalysis does not depend on the buffer chosen to adjust the pH. Of course, an inert salt must be used to maintain constant ionic strength so that kinetic salt effects do not distort the pH profile. 

In preformulation one task is to establish the stability of the drug substance in both solid and dissolved state. In the latter case it is important, with small samples of drug substance, to assess (a) the effect of buffer type, (b) the effect of buffer concentration, (c) the effect of pH in a practical range, (d) the effect of temperature, and (e) the kinetic salt effect. 

On the experimental side, evidence was accumulating that there is more than one kind of reducing species, based on the anomalies of rate constant ratios and yields of products (Hayon and Weiss, 1958 Baxendale and Hughes, 1958 Barr and Allen, 1959). The second reducing species, because of its uncertain nature, was sometimes denoted by H. The definite chemical identification of H with the hydrated electron was made by Czapski and Schwarz (1962) in an experiment concerning the kinetic salt effect on reaction rates. They considered four... 

There are, however, many specific and anomalous kinetic salt effects, especially at higher salt concentrations, the origin of which lies in the effects on nonelectrolyte reactant activity coefficients. This is often the most interesting effect for enzymes, because the charge on the substrate is frequently zero, making the product ZiZ also zero. The exact charge on the enzyme molecules can be difficult to determine if one is working at a pH removed from the isoelectric point of the enzyme. 

Kinetics of Reduction of Toluidine Blue with Sulfite - Kinetic Salt Effect in Elucidation of Mechanism 244... 

Surface polarity can also be independently evaluated by physical means. deMayo and coworkers have assigned surface polarity of silica gel particles by observing shifts in the absorption spectra of absorbed spiropyrans which are sensitive to solvent polarity . Darwent and coworkers have shown that kinetic salt effects follow surface charge on colloidal titanium dioxide and, with zeta potential measurements, that surface area and charge could be separately evaluated... 

A study19 of the effect of added lithium perchlorate on the second-order rate coefficients for reaction (12) (R = Et, Pr", Bu") showed that all three substitutions, in solvent 96 % methanol-4 % water, were subject to marked positive kinetic salt effects. The effects were analysed in terms of the Bronsted-Bjerrum equation... 

Abraham and Spalding19 separately determined values of yEuSn, yPrlithium perchlorate, and combined these values with those of k/k0 to yield values of y for the three transition states. The magnitude of the observed kinetic salt effects may be seen from Fig. 1 in... 

Fig. 1. Kinetic salt effects in the bimolecular substitution of tetraalkyltins (R4Sn) by mercuric iodide in solvent 96 % methanol-4 % water. Et = Et4Sn, Pr" - Pr4"Sn, Bu — Bu"4Sn.

The effect of added tetra-n-butylammonium perchlorate on the rate of reaction (21) (X = Cl) was also studied. It was found30 that the perchlorate greatly increased the value of the second-order rate coefficient, and that this positive kinetic salt effect was more marked the lower was the solvent dielectric constant. The salt effects were analysed in terms of the equation... 

ANALYSIS OF THE KINETIC SALT EFFECT OF TETRA-fl-BUTYLAMMONIUM PERCHLORATE ON THE REACTION OF TETRAETHYLTIN WITH MERCURIC CHLORIDE30... 

Second-order rate coefficients for reaction (21) (X = I and OAc) were also reported by Abraham and Behbahany30 and are given in Table 20. Kinetic salt effects of added tetra-n-butylammonium perchlorate were studied for reaction (21) (X = I and OAc) both with solvent methanol and solvent tert.-butanol. Reaction (21) (X = 1) was accelerated in both solvents to about the same extent as was reaction (21) (X = Cl), and mechanism SE2(open) was therefore suggested. The reaction of tetraethyltin with mercuric acetate was subject to very large positive salt effects in methanol, perhaps due to anion exchange, but was unaffected by the electrolyte in solvent /er/.-butanol. Abraham and Behbahany30 considered that it was not possible to deduce the mechanism of the acetate reaction and that further work was necessary to decide between mechanism SE2(open) and mechanism SE2(cyclic). 

The addition of sodium perchlorate considerably increases the value of k°2s for the iodinolysis of tetramethyllead in solvents methanol, ethanol, and acetonitrile50. Since values of K do not vary very much with ionic strength (at least with solvent methanol13), it seems fairly clear that k2 itself must increase with increase in ionic strength. Kinetic salt effects were not studied with the other solvents listed in Table 26, but it seems reasonable to suggest that in all these cases the substitution proceeds by mechanism SE2(open) through a transition state such as (XVII) or (XVIII). 

Positive kinetic salt effects indicate that these reactions proceed by mechanism SE2(open). Product analyses indicate that these reactions proceed by mechanism SE2(open). 

An examination of Table 2 reveals that although mercuric acetate and mercuric nitrate have often been used as electrophilic reagents, there are but few instances in which independent evidence as to their mechanism of reaction has been put forward. Positive kinetic salt effects have been observed in the substitution of sec.-butylmercuric acetate by mercuric acetate (with lithium nitrate in solvent ethanol)2, the substitution of di-sec.-butyl mercury by sec.-butylmercuric nitrate (with lithium nitrate in solvent ethanol)11, and the substitution of tetraethyltin by mercuric acetate (with tetra-n-butylammonium perchlorate in methanol)7. In the latter case, it was suggested7 that the observed very large positive kinetic salt effect was possibly due to anion exchange between mercuric acetate and the perchlorate ion. 

Abraham and Behbahany7 have discussed kinetic salt effects on reactions involving neutral molecules in terms of the equation... 

Most of the substitutions in Table 6 refer to metal-for-metal exchanges (Nos. 1-15). Of these fifteen reactions, fourteen are subject to positive kinetic salt effects ... 

KINETIC SALT EFFECTS IN SOME BIMOLECULAR ELECTROPHILIC SUBSTITUTIONS INVOLVING NEUTRAL REACTANTS... 

The observation that added sodium acetate gives rise to no kinetic salt effect at all in the acetolysis of dineophylmercury has already been put forward32 as evidence that the acetolysis proceeds by mechanism SE2(cyclic). 

Kinetic salt effects therefore represent an extremely valuable method of distinguishing mechanism SE2(open) from mechanism SE2(cyclic) for the case in which the reactants are neutral molecules. Naturally, since these two mechanisms may shade into each other, it is possible that in some cases only small positive kinetic salt effects will be observed if the mechanism is in the SE2(open)-SE2(cyclic) border line region. 

Co-solvent effects are generally more difficult to interpret than are kinetic salt effects, largely because the addition of a new solvent produces much more violent changes in the reaction medium than does addition of an inert salt. The mechanism of substitution may therefore be affected by addition of a co-solvent this is es-picially true of reactions proceeding by mechanism SE2, where the SE2(open) and SE2(cyclic) transition states may be regarded as the two extremes of a whole range of transition states. 

Two questions are inseparable how to optimize ion radical reactions, and how to facilitate electron transfer. As noted in the preceding chapters, electron transfers between donors and acceptors can proceed as outer-sphere or inner-sphere processes. In this connection, the routes to distinguish and regulate one and another process should be mentioned. The brief statement by Hubig, Rathore, and Kochi (1999) seems to be appropriate Outer-sphere electron transfers are characterized by (a) bimolecular rate constants that are temperature dependent and well correlated by Markus theory (b) no evidence for the formation of (discrete) encounter complexes (c) high dependence on solvent polarity (d) enhanced sensitivity to kinetic salt effects. 

Inner-sphere electron transfers are characterized by (a) temperature-independent rate constants that are greatly higher and rather poorly correlated by Marcus theory (b) weak dependence on solvent polarity (c) low sensitivity to kinetic salt effects. This type of electron transfer does not produce ion radicals as observable species but deals with the preequilibrium formation of encountered complexes with the charge-transfer (inner-sphere) nature (see also Rosokha Kochi 2001). 

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