Reaction of Polycylic Aromatic Hydrocarbons (PAH) with Nitrogen Dioxide in Solution. Support for an Electron-Transfer Mechanism of Aromatic Nitration Based on Correlations Using Simple Molecular Orbital Theory

Article abstract of DOI:10.1021/jo00200a035

Eight unsubstituted, polycyclic aromatic hydrocarbons (PAH) were allowed to react with nitrogen dioxide in dichlormethane at 25 deg C, and relative rate constants were obtained by direct competition techniques.The rate constants depend markedly on substrate structure, with over a 10⁴ difference in rate constants between the least reactive (benzene) and most reactive (perylene) compounds studied.The major products formed from most substrates are nitroaromatics.Anthracene, however, also reacts with nitrogen dioxide to form appreciable amounts of 9,10-anthraquinone.Linear free energy relationships were determined between rate data and molecular orbital parameters based on models involving rate-determining ?-complex formation or electron-transfer (ET) reactions.Based on the better correlations obtained using the latter model, it is suggested that the more easily ionized PAH undergo nitration by an ET mechanism.Values of absolute rate constants for the nitration of three of the PAH (as measured by stopped-flow) also are reported and correlate well with our relative rate constants.The formation of 9,10-anthraquinone is suggested to result from the trapping of the intermediate anthracene radical-cation by water.