Kinetic energy release and position of the transition state during the intramolecular substitution of ionized benzalacetones

Abstract

The formation of 2-methylbenzopyrylium ions 3 + by intramolecular substitution of various ortho substituents in the molecular ions of benzalacetones 1 requires a constant activation energy of 0.520.6 eV. Consequently the reverse reactions have different activation energies ε r ‡, which depend on the nature of the substituent. The heat of formation of 3 + was determined and ε r ‡ evaluated. Measurement of the kinetic energy release T B gives the energy-partitioning ratios q = T B/ε r‡, which form two groups in the ranges 0.20.3 and 0.91.0. The group with q = 0.20.3 corresponds to the class of exothermic reactions and that with q = 0.91.0 to the class of endothermic (thermoneutral) processes. The large change in q, paralleling the change in reaction enthalpy, is interpreted as an alteration in the position of the transition state. The result show that some of Polanyi's concepts in the field of reaction dynamics as well as a recently developed “quantitative Hammond-postulate” are also followed by complicated organic species.