The analysis of substituent effects for reactions not following the Hammett relation

An extended Hammett equation based on separation of the enthalpic and entropic contributions to the total substituent effect is applied to systems where substituent–reaction site interactions are strong and non-uniform in nature, and therefore, the classical Hammett relation is not obeyed. Enthalpic and entropic substituent constants (σH and σS) are defined from independently determined values of standard enthalpy and entropy for the reference Hammett reaction in water. The extended equation Q=Q0+ρHσH+ρSσS was found to describe successfully substituent effects fora collection of systems containing ortho-substituents and/or strong substituent–reaction site interactions. The results of analysing 42 selected sets of equilibrium and kinetic data are summarized and discussed.