关键词: 苄基氟, 苄醇, α-取代, 取代基效应, 速控步骤

The rate-determining steps of isomeric ortho, meta and para nitrations of benzyl alcohol and benzyl fluoride have been theoretically investigated at B3LYP/6-311G** level. Stationary points of the step involving reactant, transition state and intermediate complexes were successfully located and characterized without any restriction on the internal coordinates. Their molecular geometries, electronic structures, IR spectra, and the FMO symmetries of two initial aromatic compounds and the nitronium ion NO₂⁺ have been studied. The vibrational shifts of C—N and C—H stretches from TS to INT at the rate-determining step of target aromatic nitrations show up that the formation of C—N and the cleavage of C—H are not concerned but stepwise to provide an explanation, on a microscopic scale, for the experimental fact of the absence of kinetic isotopic effect in the nitration process for the first time. Also, the influence of introduced fluorine on the orientation effect was figured by calculating the microscopic and kinetic properties of the rate- determining step of concerned aromatic nitrations. The NO₂⁺ attack to the benzyl fluoride was deactivated because of high electronegativity and strong electron withdrawal of fluorine atom. The activation energy after α-substitution of methyl group by a fluorine atom became higher. Yet the CH₂F group was still an ortho-para directive, since the activation energies of ortho and para nitrations were relatively lower than that of meta nitration. Contrarily, the introduction of electron-donating substituent OH onto the methyl accelerated the NO₂⁺ attack. Consequently the stabilization energies of complexes were droped even lower than before the α-substitution occurred. The activation energies of ortho and para nitrations, however, were also quite lower than the energy of meta nitration. Therefore, the CH₂OH is an ortho-para directing group. Compared to the ortho, the para position is the most favorable due to the NO₂⁺ to approach, for its low steric hindrance effect and high exothermicity. Attack at ortho, however, became even more crowd since one more O atom has been introduced after the α-substitution onto the methyl group. Hence, the energy of ortho nitro benzyl alcohol complex is higher than the energy of para one.

Key words: benzyl fluoride, benzyl alcohol, α-substitution, substituent effect, rate-determining step