Nucleophiles nucleophilicity and basicity

The SN2-type reaction can be considered simply as being initiated by attack of a nucleophile onto the electron-deficient end of a polarized bond X-Y. 

If X = H, then this equates to removal of a proton and we would consider the nucleophile to be a base (see Section 4.1). It follows that there is going to be 

In many cases, nucleophihcity can be correlated with basicity, and this forms a helpful way of predicting how good a potential nucleophile may be. The sequences of relative basicity given in Table 6.2 

The correlation is useful but not exact. This is because basicity is a measure of the position of equilibrium between a substrate and its conjugate acid (see Section 4.4), whereas nucleophilicity relates to a rate of reaction. The above relationship breaks down when one looks at atoms in the same column of the periodic table. As atomic number increases, basicity decreases, whilst nucleophilicity actually increases (Table 6.3). This originates from the size of the atom, so that electrons associated with larger atoms become less localized, consequently forming weaker bonds with protons (see acidity of HX, Section 4.3.2). On 

Despite these inconsistencies, there are two important features worth remembering 

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