74309-55-0Relevant articles and documents
Kinetic Energy Release and Position of Transition State during Intramolecular Aromatic Substitution in Ionized 1-Phenyl-1-(2-pyridyl)ethylenes
Schubert, Ralf,Gruetzmacher, Hans-Friedrich
, p. 5323 - 5328 (1980)
The loss of substituents (X = H, CH3, Cl, Br, I) from the molecular ions of ortho-substituted 1-phenyl-1-(2-pyridyl)ethylenes 1a-f and of the isomeric 1-phenyl-1-(3-pyridyl)- and 1-phenyl-1-(4-pyridyl)ethylenes 2 and 3 has been investigated.Cyclic fragment ions a are formed from the ortho-substituted 1-phenyl-1-(2-pyridyl)ethylene molecular ions by an intramolecular aromatic substitution reaction.The energetic requirements of this reaction have been studied in dependence from the dissociation energy of the C-X bond by measurements of the ionization energies, appearance energies, and kinetic energies released during the reaction.The activation energy εh of the process varies only slightly with the dissociation energy of the C-X bond cleaved during the reaction, whereas the entalpy of reaction changes from positive (endothermic) to very negative (exothermic) values in the reaction series 1a-f.Consequently the reverse activation energy εr ranges from small to very large values in this series.This trend in εr is not followed by the kinetic-energy release.A large kinetic-energy release and energy partitioning quotient q = 0.7 - 1.0 is only observed for endothermic or thermoneutral processes, while a small kinetic-energy release and q ca. 0.2 is associated with exothermic reactions in spite of a large εr.This behavior has been correlated to the position Xo* of the transition state on the reaction coordinate according to Miller's quantification of the Hammond postulate.The release of εr as kinetic energy is only observed for reactions with "symmetrical" or "late" transition states (Xo* > 0.4) while most of εr remains as internal energy in the products of reactions with "early" transition states (Xo* 0.4).
