The Edwards Equation for Nucleophilic Aliphatic Substitution

Reactions of nucleophiles of similar structure (such as all phenolate 

The Edwards equation sums component processes modelling the transfer of the pair of bonding electrons from substrate to product. The component model processes are proton transfer to the nucleophile (the proton affinity) (Equation 27) and the polarisability of the nucleophile as judged by its standard electrode potential (E ) (Equation 28). 

The Edwards equation (Equation 29) sets E = E3 + 2.60 and = + 1.74 and has been successful in correlating a wide range of data for displacements at tetrahedral carbon but a major disadvantage is 

The Edwards equation may be modified using molecular refractivity as a measure of polarisability instead of ionisation potentials (Equation 30)  

The parameter is the same as in Equation (29) and P is log(/ /7 , o where K is the molecular refractivity. There are even fewer values of R compared with so that Equation (30) loses much of its interest as a correlating tool. Moreover, as we saw in Chapter 2, molecular refractivity possesses a substantial molar volume component. 

---