The Swain-Scott and Edwards Equations

A nucleophilic reaction involves the donation of electron density from a nucleophile to a substrate. As a result, in the transition state, a partial bond forms between them  

An LFER correlation for nucleophilic reactions is described by this equation 13 

The parameter 5 is a measure of the susceptibility of the substrate to nucleophilic attack, and n a measure of the nucleophilic reactivity as defined by a reference reaction. The rate constants for attack at saturated carbon are used to define the values of n.14 Table 10-4 lists the values of n for certain nucleophiles. This particular compilation lists the 

The LFER correlation according to the Swain-Scott equation for nucleophilic attack on /3-propiolactone. Data are from Ref. 13. 

Attempts to use n values for one class of reactions (e.g., attack at saturated carbon) for another (e.g., attack at carbonyl carbon) are not entirely successful. Edwards proposed a two-term equation,15 

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