What Makes for a Good Nucleophile

TABLE 1.1 Swain-Scott Nucleophilicity Constants and Conjugate Acid pK Values of Some Common Nucleophiles  

TABLE 1.2 Qualitative Classification of Nucleophiles, Based on the Swaln-Scott Nucleophilicity Constants 

Nucleophilicity and electrophilicity are closely related to Lewis basicity and acidity, respectively. Nucleophiles are Lewis bases (electron-pair donors) and electrophiles are Lewis acids (electron-pair acceptors). Now, as discussed previously, nucleophilicity is measured in terms of the rate of a nucleophilic attack, so it s a kinetic concept. Basicity, on the other hand, is measured in terms of the equilibrium constant for protonation (or for association with some Lewis acid), so it is a thermodynamic concept. Another difference is that. 

The more electronegative elements hold on to their electrons more tightly. 

The larger atoms are less electronegative and the anions derived from them are more polarizable, which translates to increasing nucleophilicity as one goes down a group. 

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