Ionic reactions leaving groups

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). 

Effect of Solvent on El versus E2 versus ElcB. With any reaction a more polar environment enhances the rate of mechanisms that involve ionic intermediates. For neutral leaving groups, it is expected that El and ElcB... 

TABLE 17. The secondary alpha deuterium and primary leaving group nitrogen kinetic isotope effects and the relative transition state structures for the ion-pair S 2 reactions between sodium thiophenoxide and benzyldimethylphenylammonium nitrate in DMF at different ionic strengths at 0°C... 

Within the past 20 years, ferrates, i.e. anions possessing iron as the center atom, have found increasing application as nucleophilic complexes in substitution chemistry. In these reactions, the ferrate replaces the leaving group X in a first nucleophilic substitution event. A transfer of one ligand from the metal atom (i.e. a reductive elimination, path A, Scheme 7.2) or substitution of the metal atom via external attack of the nucleophile (path B) concludes this mechanistic scenario. However, the exact mechanism in ferrate-catalyzed nucleophilic substitutions is still under debate. Apart from the ionic mechanism, radical processes are also discussed in the literature. 

We now explore whether the pattern of reactivity predicted by the Marcus theory is found for methyl transfer reactions in water. We use equation (29) to calculate values of G from the experimental data where, from (27), G = j(JGlx + AG Y). The values of G should then be made up of a contribution from the symmetrical reaction for the nucleophile X and for the leaving group Y. We then examine whether the values of G 29) calculated for the cross reactions from (29) agree with the values of G(27) calculated from (27) using a set of values for the symmetrical reactions. The problem is similar to the proof of Kohlrausch s law of limiting ionic conductances. 

The equilibrium reaction between neutral hexacoordinate chelates and pentacoordinate siliconium halide salts is discussed in Section III.A.4 (Eq. 17). This reaction can be driven completely to the ionic side by replacement of the chloro ligand by better leaving groups triflate and bromide (Eq. (18), listed again with compound labels see Section III.A.4.iv). The products of this counterion-exchange reaction are stable siliconium salts 90(OTf)-92(OTf), 90(Br)-92(Br), which no... 

---