The role of reaction energy and transition state bond order on the reactivity of ambifunctional compounds in solution

Abstract

The ambident behaviour of some reagents, NO₂^–, SCN^– and ambident bases, in liquid solutions can be accounted for, quantitatively, in terms of the intersecting-state model. For many reactions changes in reactivity are dominated by the reaction energy, ΔG°, and the transition state bond order, n^‡. A gain in polarity of the reactive bonds during the course of reaction leads to more negative ΔG° and to a decrease in n^‡. This reveals that for the reaction energy barrier, ΔG° and n^‡ can work in opposite directions as a function of the electronegativity of the reaction sites and be responsible for ambident behaviour in ‘charge control’ reactions (ΔG°) and ‘frontier-orbital control’ reactions (n^‡). These concepts are employed to interpret the role of reagent, solvent, cation, and reaction mechanism on ambident reactivity.