Polarizability, Basicity, and Solvation Interplay

A comparison of nucleophiles in which the nucleophilic atoms bear the same charge, and are in the same column of the periodic table, indicates that the nucleophilicity increases as one proceeds down the column. For example, the order of nucleophilicities for the halide anions is 1 Br Cl F in polar protic solvents. As discussed above, in these solvents, the anions are well solvated, and fluoride makes stronger bonds with the solvent because the anionic charge is more localized. This trend is opposite of that expected if there was a strict correlation between basicity and nucleophilicity (the order of basicity is F Cl Br I ). Similarly, thiolate anions are consistently better nucleophiles than analogous alkox-ides, even though the alkoxides are significantly more basic. The increased nucleophilicity 

When polar aprotic solvents are used, anionic nucleophiles are not well solvated. In these cases, the nucleophilicity is more often foimd to track basicity. For example, in DMSO it can be found that F Cl Br 1 is the order of nucleophilicity for various reactions. 

As a last example of a factor that influences nucleophilicity, we examine molecular shape. A nucleophile is required to penetrate the solvent shell around an electrophile, and often to hone in on an electrophilic center that may be sterically congested. In this regard, large bulky nucleophiles are not very effective, but small, bullet (linear) shaped nucleophiles can be quite effective. For example, the nucleophiles cyanide (NC ) and azide (N3 ) are particularly good. They are much more nucleophilic than would be predicted based upon their basicity. 

In summary, shape, charge, basicity, polarizability, and solvation are all factors that determine the nucleophilicity of compoimds. Understanding these factors allows us to use the differences to our advantage when probing mechanisms. 

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