THORNTON RULE FOR HOMOLYTIC REACTIONS

Numerous free radical reactions involve polar transition state (TS), which can be responsible for the excellent Hammett rho(+)-sigma(+) correlations where the value of rho(+) is negative. The structure of polar TS can not be the resonance hybrid of the reactant and product of the radical reaction (the intermediate configuration). The deviation of the TS from the intermediate configuration (polarization of the TS) can be derived from the perpendicular effect (Thornton rule). The polar TS may accordingly invite nonlinear relations between the rates and the equilibria and the reactivities are controlled by the entropies. The perpendicular effects may invalidate Leffler-Hammond postulate and reactivity/selectivity principle.