Kinetic energy release and position of transition state during the intramolecular substitution of ionized 2‐benzoyl pyridines

Abstract

The loss of substituents X from molecular ions of ortho substituted 2-benzoyl pyridines has been investigated as a function of the dissociation energy of the C-X bond. Comparison of unimolecular and collisional induced decompositions of the resulting [M-XI' ions and reference ions arising from 3-hydroxypyrido[l,2-alindole shows that cyclic fragment ions are formed in every case by an intramolecular substitution reaction with the exception of the parent compound (X=H), which gives rise to a mixture of [M-HI' ions with different structures. The heat of formation of the cyclic ion has been estimated experimentally and by calculation using thennochemical data, and from this value and the appearance energies, the activation energies of the reverse reactions have been evaluated for the different reaction systems. Measurement of the kinetic energy release during the substitution readions shows that only part of the reverse activation energy is released as kinetic energy. The energy partitioning quotient varies from 0.37 to 0.08 depending on the dissociation energy of the C-X bond or the reaction enthalpy. A sudden change in the energy partitioning quotient is observed with increasing exothermicity of the reaction, paralleling the behaviour of similar reaction systems. These results are interpreted as a demonstration of the influence of the variation of transition state position on the energy partitioning quotient.It has been shown recently2 that the partitioning of the excess potential energy of the transition states (or the activation energy of the reverse reactions F:) between the kinetic energy T, released during the reaction, and the internal energy F * of the products during the loss of ortho substituents from the molecular ions of substituted benz.alacetones depends strongly and systematically on the thermochemistry of the reaction or the position3 of the transition state on the reaction coordinate. Thus it appears that an investigation of the variation of the kinetic energy release and the energy partitioning quotient q = T/&f with structural changes of the reactant ions gives some insight into fundamental properties of elementary reactions of organic ions, which will be useful in testing mechanistic models of organic chemistry and linking them more closely to physical theories of chemical reactions. The loss of substituents from molecular ions of benzalacetones with formation of 2-rnethylbenzpyrylium ions4 can be regarded as an intramolecular aromatic substitution reaction,5 a well known reaction in organic chemistry. As a further example of this type of reaction the results of a detailed investigation of the loss of ortho substituents from the molecular ions of substituted 2-benzoyl pyridines la-lg (Scheme 1) are reported.

Interplay of thermochemistry and Structural Chemistry, the journal (volume 18, 2007) and the discipline

“…There are no enthalpies of formation data for 2-benzoylpyridine nor for any of its derivatives. However, a study of the energetics of C-X bond cleavage in the parent ketone and 2 0 -(benzene)-substituted derivatives has been reported [275], which gives us information about substituent effects on these species. In addition, enthalpy of vaporization and heat capacity measurements have been reported for 2-benzoylpyridine [276].…”

Dissociative Ionization and Metastable Ions of Nitrobenzene and Nitrobenzene-d5

“…(5) Most applications of the MIKES to thermochemical problems attempt to use measured values of the kinetic energy FB [17][18][19] (as determined from the peak base-width) to correct AHf(F+) for the frag mentation reaction of interest.…”