Effect of Solvation on Nucleophilicity

In the absence of the solvation by protic solvents, the relative nucleophilicity of anions changes. Hard nucleophiles increase in reactivity more than soft nucleophiles. As a result, the relative reactivity order changes. In methanol, for example, the relative reactivity order is I CN Br Cr. In DMSO the order becomes CN cr Br The reactivity order in methanol is dominated by solvation and the more weakly solvated and I ions are the most reactive nucleophiles. The iodide ion is large and very polarizable. The anionic charge on the azide ion is dispersed by delocalization. When the effect of solvation is diminished in DMSO, other factors become more important, including the strength of the bond being formed, which accounts for the reversed order of the halides in the two series. There is also evidence that Sf 2 TSs are better solvated in aprotic dipolar solvents than in protic soivents. 

In interpreting many aspects of substitution reactions, particularly solvolysis, it is important to be able to characterize the nucleophilicity of the solvent. Assessment of solvent nucleophilicity can be done by comparing rates of a standard substitution process in various solvents. One such procedure is based on the Winstein-Grunwald 

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Effects of Solvation on Nucleophilic Reactivity in Hydroxylic Solvents... 

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