Reactivity and selectivity, an intersecting-state view

A recently developed intersecting-state model which estimates energy barriers of chemical reactions in terms of thermodynamic, geometric, and electronic factors is employed to discuss the validity of several reactivity relations, namely the postulates of Hammond and Leffler, various linear free energy relationships, and the reactivity–selectivity principle. It is shown that such rules and principles are obeyed when all the changes in reactivity are dominated by changes in the reaction energy. The breakdown of such rules can occur when other factors such as the transition-state bond order, bond lengths, and force constants of the reactive bonds play a dominating role in chemical reactivity. Examples are provided to illustrate the practical limitations of the current reactivity–selectivity relations.