Structure of macrocyclic polyether 18-crown

Various explanations have been put forth for the alpha effect. One view is that the ground state of the nucleophile is destabilized by lone pair-lone pair repulsions, which decrease in going to the transition state. Another view is that the transition state is stabilized by electron release from the hetero atom. According to this reasoning, the process of bond formation is akin to that of a two-electron transfer  

The alpha effect may have a different origin for structurally distinct nucleophiles and it is likely that both electron-pair repulsion and stabilization of the transition state contribute to the overall effect.  

A significant advance in overcoming this problem is the use of macrocyclic polyethers in catalytic quantities in such media. Macrocyclic polyethers, such as 18-crown-6, the structure of which is shown in Figi 5.3, have a pronounced ability to coordinate metal ions. This ability allows for more salt to be dissolved in the organic solvent equally important, coordination of the metal ion leaves a relatively poorly solvated anion behind. The anions behave under these conditions as highly reactive species, sometimes termed naked anions. A study of the relative rates of 

Correlations of nucleophilicity to this point have emphasized reactions of the Sn2 type. Data pertinent to the SnI mechanism are also available from rate studies utilizing stable carbonium ion salts as substrates. These results indicate that the order of nucleophilicities of Lewis bases toward carbonium ions is substantially different from that exhibited in direct-displacement reactions. Ritchie has found that nucleophilicity toward cationic species can be correlated by the equation 

The order of reactivity observed, azide hydroxide cyanide, is opposite to that observed for these nucleophiles toward methyl iodide (see Table 5.3, p. 208). Since N+ for a nucleophile is independent of the cation and does not correlate either with basicity of the nucleophile or with the equilibrium constant for bonding to the cation, Ritchie believes the most important determinant of nucleophilicity toward cations to be the solvation energy of the nucleophile. The overall reaction appears to 

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