Etude Quantitative de la Reactivite–XXI: Comportement sous l'Impact Electronique de Bromures de Cyclopentyle et Cyclohexyle α-substitutes cis et trans

Mass spectra of ten pairs of cis and trans 2-substituted cyclopentyl and cyclohexyl bromides were recorded for eleven separate energies located between 10 and 20 eV. For each isomer, a calculation of the mass spectrum and of its variations with electronic voltage from the quasi-equilibrium theory, allowed us to determine the activation energies of the main [M – Br] fragmentation. These numeric data show that the observed differences between the mass spectra of diastereoisomers essentially depend on: (1) the enthalpy difference between isomeric molecular ions in their ground states for fluorine and oxygen substituted compounds and (2) the enthalpy difference between the isomeric transition states of the [M – Br] cleavage for chlorinated and brominated compounds. In the latter case, this can be explained by the halide participation in the expulsion of the bromine substituent from the trans isomers.