Arene Electrophilic Substitution Of The Theoretical Study On Nitration Mechanism

Aromatic nitration is an important and typical example of electrophilic substitution in Organic Chemistry. The nitration mechanism is also a very important and concerned topic for synthesis of explosives, since benzene molecule is a basic unit to build up into the energetic materials. The theoretical investigation on the co-exist system of benzene and nitro group is dependent on the high level of computational condition for the strong electron correlations in this conjugated system. The theoretical research on the aromatic nitration mechanism was progressing slowly in the past decades because of the low level of both computer and program at the time, and was inevitably behind the synthesis or macrokinetics study of the nitration. From the middle of 80's to the beginning of 90's in the last century, the theoretical studies on the aromatic nitration mechanism usually dealt with the optimization of a minimum on a potential surface by restricting partial parameters of the sensitive nitro group sometimes. The computational method employed there was almost at the HF (Hartree-Fock) level without respect to the electron correlations in the system. In 1992, an influential article (J Am Chem Soc, 1992, 114: 6827) showed that "no activation barrier was found along the chosen reaction coordinate". The theoretical research in this field seemed to come to a halt since then. And the location of transition state on a potential surface for the aromatic nitration had long been a difficult question until the beginning of the new century.The hybrid density functional scheme B3LYP and MP2 with electron correlations are employed in the present thesis to gave a explicit illustration on the nitration mechanisms of benzene and its derivatives including toluene, halobenzenes (PhX, X=F, Cl), a -fluorotoluenes (PhCH3-nFn, n=1, 2, 3) and benzyl alcohol (PhCH2OH) for the first time. The solvent effects of the nitration of benzene and the orientations of methyl, halogen and hydroxyl group in the nitrations are revealed and concluded to provide a solid base for building a new quantum chemical theory on the electrophilic nitration.1. The nitration mechanism of benzene and the solvent effectsThe transition state of the nitration of benzene with nitronium ion (NO2+) has been obtained by full optimizations of MP2 with 6-311G**, and B3LYP with 6-311++G**, 6-311G** and 6-31G** respectively. And the molecular geometries, electronic structures, IR spectra and thermodynamic properties, of reactant (o -R) and intermediate (o -INT) areconsequently obtained by the intrinsic reaction coordinate (IRC) calculation. The experimental observation of the absence of kinetic isotopic effect in the nitration process has been explicitly explained and the reaction pathway has been confirmed. The activation energy is calculated as 8.37kJ/mol by B3LYP/6-311G**. The order of rate constant of 1010mol-1s-1 is in a good agreement of an experimental observation. The results both in thermodynamics and kinetics support that the electrophilic substitution mechanism is more preferable than the electron transfer mechanism of radical pairs.The solvent effect on the geometries of stationary points and the reaction mechanism were systematically studied for the nitration of benzene with nitronium by self-consistent reaction field (SCRF) technique with different dielectric constants of 5.0, 25.0, 50.0 and 78.5. It was then found that the solvent effect would depress the activation energy and finally make the formation of a-TS without energy barrier in aqueous solution. Furthermore, the linear correlations given by charge migrations of NO2 group, dipole moments of solute, gaps of HOMO and LUMO and solvent stabilization energies in different solvents were demonstrated for the both theoretically and experimentally concerned Wheland intermediate.2. The positional selectivity of the nitration of tolueneThe isomeric nitration complexes of toluene with NO2+ given by the attack on ortho, meta, and para position have been fully optimized at the B3LYP/6-31G** level to get the info...