''Ipso'' aromatic nitration in the gas phase

Nitration of selected dialkylbenzenes, including o- and p-cymene, p-tert-butyltoluene, p-diisopropylbenzene, and p-di-tert-butylbenzene, by protonated methyl nitrate, (CH3OH-NO2)(+), has been investigated in the gas phase in the pressure range from 10(-8) to 720 Torr by Fourier transform ion cyclotron resonance (FT-ICR) and collisional activated dissociation (CAD) mass spectrometry and by the radiolytic technique. The results provide the first demonstration of gas-phase electrophilic aromatic nitrodealkylation and a quantitative evaluation of its efficiency relative to the competing nitration process. Dealkylation occurs exclusively following attack of the nitrating cation at the ''ipso'' position, which promotes intramolecular migration of the alkyl substituent 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, which in the gas phase is unaffected by the many complicating factors, e.g., solvation, ion pairing, etc., operative in solution. The results are compared with those concerning condensed-phase nitrodealkylation and the related nitrodesilylation reaction.