Correlation Analyses of Solvolytic Rate Data with Single‐ and Dual‐Parameter Hammett Equations

The results of detailed examinations on the comparison of Hammett‐type correlation analysis of solvolytic rate constants with single‐ and dual‐parameter equations are summarized. From the difference of ρ values, the variation of γ values with numbers of data points, the solvent intervention in the standard reactions for defining σ + constants, the influence of o‐methyl group on the reactivity of the reference substrate ( tert ‐cumyl derivatives), and the behavior of the α‐aryl group in highly congested systems, it could be concluded that both the Hammett‐Brown and the Yukawa‐Tsuno treatments of solvolytic rate constants are only approximations. Although in some cases the use of the dual‐parameter Yukawa‐Tsuno equation (Eq. 4) could give a better correlation and a more proper explanation than the use of the single‐parameter Hammett‐Brown equation (Eq. 2), in other cases the excellent correlation observed from the former might be merely a computational artefact. Therefore, the result obtained from the Yukawa‐Tsuno equation should be interpreted with caution.