Experimental study on the mechanism of gas-phase aromatic nitration by protonated methyl nitrate

Article abstract of DOI:10.1021/ja00100a018

The mechanism of gas-phase aromatic substitution by (CH₃ONO₂)H⁺ ions has been studied by a combination of FT-ICR mass spectrometry and atmospheric pressure radiolytic techniques. Clarifying a long-standing ambiguity, the ICR results characterize the CH₃OH-NO₂⁺ complex (1), in essence a nitronium ion solvated by a methanol molecule, as the nitrating agent, whereas the CH₃NO₂H⁺ isomer (2) is devoid of nitrating properties and reacts with benzene exclusively as a Br?nsted acid. Indeed, the reaction with benzene has been exploited as an ICR "titration" technique to evaluate the relative abundances of 1 and 2 in mixed populations of (CH₃ONO₂)H⁺ ions from different preparative procedures. Radiolytic nitration of p-H-toluene-d₇ and p-D-toluene-h₇ leads to intraannular hydron migration from the ipso nitrated position, whose rate has been estimated to be ca. 1.6 × 10⁶ s^-1 at 315 K. The mutually supporting evidence from the ICR and the radiolytic experiments outlines a reaction mechanism involving preliminary formation of a Wheland intermediate from the attack of 1 on the arene, followed by its isomerization into the more stable O-protonated nitrobenzene structure via a proton shift whose rate is estimated to be ca. 3.6 × 10⁷s^-1 at 315 K. The results are compared with those of a recent theoretical analysis of the mechanism of aromatic nitration by isomeric (CH₃ONO₂)H⁺ ions, and their correlation with condensed-phase nitration is briefly discussed.