An Experimentally Established Key Intermediate in Benzene Nitration with Mixed Acid

Abstract

Experimental evidence is reported for the first intermediate in the classic S E Ar reaction of benzene nitration with mixed acid. The UV/Vis spectroscopic investigation of the reaction showed an intense absorption at 320 nm (appearing as ab and shoulder) arising from ar eaction intermediate.O ur theoretical modeling shows that the interaction between the two principal reactants with solvent (H 2 SO 4 )m olecules significantly affects the structure of the initial complex. In this complex, al arger distance between the aromatic ring and nitronium ion precludes the possibility for electronic charge transfer from the benzene p-system to the electrophile.T he computational modeling of the potential energy surface reveals that the reaction favors astepwise mechanism with intermediate formation of p-a nd s-( arenium ion) complexes.

Solvation in nitration of benzene and the valence electronic structure of the Wheland intermediate

“…In the final step, we assumed the deprotonation of the scomplex by H 2 SO 4 molecules, which are the most abundant species in the solution system (s-complex + H 2 SO 4nitrobenzene + H 3 SO 4 + ). 16 Although the free energy change by deprotonation in the H 2 SO 4 solution was calculated as À23.8 kcal mol À1 as shown in the figure, deprotonation by other solvent molecules will be possible under the real experimental conditions. As an example, stabilisation due to the abstraction by HSO 4 À anions was À60.5 kcal mol À1 .…”

“…Following previous considerations on basis sets and functionals, 5,14,16,17 free energy surfaces for nitration of benzene were evaluated using the density functional method with the M06-2X 36 functional and 6-311G(d,p) basis set. At first, geometry optimisations in solution were performed with the conductor-like polarisable continuum model (CPCM) 37,38 of the H 2 O solution.…”

“…Our results agree with the fact that experimental observation of the stable p-complex is difficult. 16 In the gas phase, p-complexes were formed and considerably stabilised. This stabilisation can be attributed to the instability of nitronium ions.…”

“…Previous studies with a polarisable continuum model (PCM) 15 suggested that the solvation drastically changes the reaction profile of benzene nitration and affects the electronic structures. 5,14,[16][17][18] Despite these efforts, the reaction process as a chemical phenomenon in condensed molecular systems, especially the microscopic solvation, is still unknown. In the reaction process, the role of the typically mixed acid condition, or countless amounts of sulfuric acid molecules, has never been investigated.…”

Studies on the Origin of the Stabilizing Effects of Fluorinated Alcohols and Weakly Coordinated Fluorine-Containing Anions on Cationic Reaction Intermediates

“…5g This result indicates that none of the other polar solvents could stabilize the π-complexes, which previously had only been observed in a nitric and sulfuric acid mixture. 18 In 2020, our group found that in PFTB, intramolecular epoxide-opening cyclization of an epoxy alcohol proceeds with endo selectivity and that the selectivity can be increased by adding 1 equiv of [EMIM]BF 4 (Scheme 2e). Using the [EMIM]BF 4 /PFTB combination, we were able to construct the skeleton of marine ladder polyethers in one step by means of endo-selective intramolecular cascading epoxide-opening cyclizations of polyepoxy alcohols.…”