Metastable Ions as a Guide to Reaction Mechanisms. Loss of ’OH from Substituted Nitrobenzene Molecular Ions

Abstract

Loss of ‘OH from the molecular ions of nitrobenzenes having various substituents in the m- and p-positions does not involve substituent randomization and reaction through the ortho-isomer; the kinetic energy release accompanying metastable ion fragmentation shows this latter to be an entirely distinct process. The origin of the hydrogen lost as 'OH in the m- and p-compounds is the ring (mainly the position ortho to the nitro group in methyl p-nitrobenzoate). However, the hydro­gens of the substituent do play a direct role in the reaction as shown by an isotope effect upon the abundances of metastable peaks. Hydrogen atom abstraction by the ionized nitro group apparently occurs independently of participation by the substituent on the nitrobenzene but a second hydrogen transfer from this substituent to the ring precedes fragmentation. Direct interaction between groups o ' the aromatic ring that are not ortho to each other therefore apparently does not occur. Inversion of the rate constants for two reactions, NO' loss and CH2O loss from the methyl p-nitrobenzoate molecular ion in the metastable ion time range, has been observed following isotopic substitution.