Studies On The Structure, Spectrum Of Nitroium And The Electron Transfer In Aromatic Nitration

The mechanism of aromatic nitration continues to be a subject of active research and some controversy. In this dissertation, based on the structural and reactive specialty of notronium ion, which is the direct attack agent in aromatic nitration, some new opinion on aromatic nitration theory have been advanced.Some accurate quantum calculation indicate that the geometry of nitronium is symmetric linear with a relatively short N-O bond. The calculated N-O symmetric stretch fundamental frequency occurs at 1396cm'1, which shows great Raman activity. Nitronium has a large value of formation enthalpy. Compared with nitrogen dioxide and nitrite ion, nitronium has a notable electron-capturing power, which would be stronger when it is bending. As a kind of Lewis acid, nitronium can react with solvent molecules or negative ions to form N02+-carrier complexes, which decreases the free nitronium concentration. Strong acid could prevent nitronium from reacting with carriers. Various nitrating agents are all NO2+-carrier complexes actually. The formation of free nitronium is the replacement of nitronium from NO2+-carrier complexes by H+.When nitronium meets benzene molecule, firstly a benzene-to-nitronium electron-transfer process occurs and a radical-pair complex forms, which will transform to o - complex via intra-molecular rearrangement, o - complex lose H+ to yield nitro compound. When nitronium reacts with chlorobenzene or toluene molecule, four kinds of radical-pair complexes would be formed via electron-transfer. The stabilities and reacting activities of these radical-pair complexes would determine the product distribution. The properties of the substituents on aromatic ring would influence whether the product distribution could be controlled conveniently. If the nitrating agent is the O3- NO2 system, free aromatic cationic radicals would be formed and the spin density of each carbon atom is the determining factor of product distribution.Quantitative Raman spectroscopy was employed to determine the free nitronium concentration and the dissociation rate of nitric acid in HNO3 - H2SO4H2O mixed acid system with the temperature ranging from 20 to 40 . The results were shown in tertiary diagrams. The 100% and 0% iso-dissociation rate curves were analyzed theoretically. At the low acid concentration region, HNO3 react with H2SO4 by 1:1 molar ratio, while at the high acid concentration region, the molar ratio is 1:2. From these tertiary diagrams, the change of material system point during nitration reaction could be seen easily. According to some analysis, it was recommended that nitric and sulfuric acid should be introduced to the reacting system separately. One of them acts as the solvent; the other acts as the dropping agent.