Primary kinetic salt effect

Quantitative relations of the primary kinetic salt effect are well explained by the Debye-Hiickel theory of strong electrolytes and the transition state theory. 

The reaction rate is sensitive to the net force between the reacting ions, and the interionic force is sensitive to the ionic strength determining the extent of the ionic atmosphere around the ion. 

It follows from the Debye-Hiickel theory that, at low concentrations, the activity coefficient y, of an ion i of charge number z, is given by  

Peter Debye, 1884-1966, a Dutch physicist and chemist, Nobel Prize for chemistry for 1936  

Thermod) amic equilibrium constant of activation (expressed in terms of the activities of reactants, A and B, and the transition state, X ) is defined as  

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If the ions have like signs, ZaZb is positive and the rate constant increases with increase in ionic strength. If the ions are oppositely charged, the rate constant decreases with increase in ionic strength. Equation (33.56) is a description of the primary kinetic salt effect or, more simply, the primary salt effect. Figure 33.3 shows a verification of this equation by... 

The hydrogen-ion catalysed hydrolysis of dimethyl acetal is a particularly suitable reaction to use because of the small primary kinetic salt effect. The assumption is made, however, that because the salt effect is negligible in perchloric acid solution within the range of concentration studied, this is also true in chloracetic acid. 

Equation 40 occurs to only a minor extent. The reactions of Eqs. 13 and 16 both appear to be independent of the primary kinetic salt effect, but the contribution of Eq. 16 increases with an increase in the chloride ion concentration. 

All the kinetic consequences of this relation, known under the name of primary kinetic salt effect, have been verified qualitatively and quantitatively. In particular ... 

Mercuric perchlorate has been shown to attack cyclohexanone, the reaction being zero-order in the salt and the rate being that of enolisation with a primary kinetic isotope effect of the same magnitude, to give a mercurated ketone ... 

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... 

The rate constants of chemical reactions the yield and the selectivity of a reaction, as well as the conditions for refining or recycling of products can be optimized by the choice of appropriate solvents. Discussion in this section is restricted to reaction mechanisms involving electrolytes or single ions. The role of electrolyte solutions in primary and secondary kinetic salt effects is not considered. For this problem see Refs. s. 

Formaldehyde reacts with nitrosobenzene (PhNO) to yield iV-phenylformohydrox-amic acid, PhN(OH)CHO. The primary kinetic isotope effect in mixed solvents is altered on addition of small amounts of salt, an effect also seen in water at higher salt concentrations, suggesting the involvement of chloride ion in C-H bond breaking. 

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-... 

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). 

Kinetic results which apparently do not fit the above treatment of the primary salt effect do so when the observed rates are correlated with the actual ionic strengths rather than the stoichiometric values. The actual concentrations in the reaction solution are calculated using the known value of the equilibrium constant describing the ion pair. This is discussed in Problem 7.5. 

PtLX+(aq) + Y (aq) - analysis of kinetic data demonstrating the primary salt effect,... 

Transition-state theory (Gardiner, 1969) Lasaga, 1981) predicts that rate coefficients for second-order reactions in solution depend on the activity coefficients of the reactants and activated complex and therefore vary with ionic strength (the primary salt effect), and this has been found to be the case. However, the dependence of rate coefficients of kinetic reactions in soils on ionic strength has apparently not been studied. 

Treatment of nitroxides with strong acids such as toluenesulfonic acid or perchloric acid facilitates disproportionation to form one oxoammonium salt in situ for every two equivalents of starting nitroxide. Under strongly acidic conditions, secondary alcohols are efficiently oxidized to ketones, whereas primary alcohols are much slower to react [33]. The reaction mechanism [31] is most likely that shown in Scheme 15. A kinetic isotope effect [kn/ku = 3.1) supports deprotonation of the alpha hydrogen as the rate limiting step [34]. The use of an additional oxidant such as bleach (NaOCl) or hypobromous acid (HOBr) or hypochlorous acid (HOCl) generated in situ from bromide or chloride ion [35] can facilitate the reaction by rapidly reforming the oxoammonium species under the reaction conditions. This allows the nitroxide to be utilized in catalytic amounts. Recently, Bobbitt [36] has... 

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