Swain-Scott and Edwards relationship

Swain and Scott [87] investigated a nucleophilicity scale using as the standard reaction attack of nucleophile on methyl bromide (Eqn. 94)  

Representative s values are collected in Table 24 and n values in Table 25. The order of nucleophhcity (n) in the Swain-Scott approach bears no relationship to the pK of the conjugate acid of the nucleophile. Since in a given constant series of nucleophiles basicity and nucleophiUcity are related there have been attempts to correlate nucleophiUcity with one or more parameters. If the relative importance of these parameters is the same as in the methyl bromide reaction then the Swain-Scott equation will hold. 

Edwards and Pearson [88] indicated that a highly polarisable reagent is a better nucleophile in relation to its base strength than a less polarisable reagent. Edwards [89] formulated a reactivity Eqn. 96 

Some representative Swain s values for reaction with nucleophiles 

The influence of two parameters is necessary to explain such problems as the differential attack of methoxide and thiophenoxide ion on neopentyl 4-toluene sulphonate (Eqn. 98). 

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