IPSO AROMATIC NITRATION IN THE GAS-PHASE

Nitration of selected dialkylbenzenes, including o- and p-cymene, p-te rt-butyltoluene, p-diisopropylbenzene, and p-di-tert-butylbenzene, by protonated methyl nitrate, (CH3OH-NO2)(+), has been investigated in th e gas phase in the pressure range from 10(-8) to 720 Torr by Fourier t ransform ion cyclotron resonance (FT-ICR) and collisional activated di ssociation (CAD) mass spectrometry and by the radiolytic technique. Th e results provide the first demonstration of gas-phase electrophilic a romatic nitrodealkylation and a quantitative evaluation of its efficie ncy relative to the competing nitration process. Dealkylation occurs e xclusively following attack of the nitrating cation at the ''ipso'' po sition, which promotes intramolecular migration of the alkyl substitue nt to the nitro group, yielding O-alkylated or O-protonated ions. The positional selectivity of the nitration, and the relative rate of the ''ipso'' attack by (CH3OH-NO2)(+) are discussed, especially as regards the role of the intrinsic steric requirements of the alkyl group, whi ch in the gas phase is unaffected by the many complicating factors, e. g., solvation, ion pairing, etc., operative in solution. The results a re compared with those concerning condensed-phase nitrodealkylation an d the related nitrodesilylation reaction.