The linear free-energy relation of the form: \[\log _{10}(\frac{k}{k_{0}})=s\ n\] applied to the variation of reactivity of a given electrophilic substrate towards a series of nucleophilic reagents. \(n\) is characteristic of the reagent (i.e. a measure of its nucleophilicity) and \(s\) is characteristic of the substrate (i.e. a measure of its sensitivity to the nucleophilicity of the reagent). A scale of \(n\) values is based on the rate coefficients \(k\) for the reaction of methyl bromide with nucleophiles in water at \(25\ ^{\,\unicode{x26ac}}\rm{C}\), \(s\) being defined as 1.00 for these reactions and \(n\) being defined as 0.00 for the hydrolysis of methyl bromide. (Other scales have been devised.)