On the Occurrence of an Electron-Transfer Step in Aromatic Nitration

Abstract

The possibility that aromatic nitration proceeds via the formation of an electron donor−acceptor complex and its possible evolution in a “contact” radical pair is discussed on the basis of ab-initio configuration interaction computations on benzene/toluene NO2 + systems. The analysis of the region of the potential energy hypersurfaces corresponding to the two reactants kept at van der Waals distances shows the existence of a conical intersection between the ground state and the first excited charge transfer singlet, leading to electron transfer from the aromatic substrate to the nitronium ion. The activated state for electron transfer (ET) might then be identified with the “early” transition state invoked by Olah to explain the high positional selectivity of substitution in spite of a low substrate selectivity. The retention of positional selectivity at encounter-limited rates may then be ascribed to the fact that the formation of a Wheland intermediate corresponds to a radical pair recombination and as such is spin density driven. The objection that ET is a kinetically difficult step is met, the computed upper limit of the barrier to ET being 13 kcal/mol for the toluene substrate.

An Experimentally Established Key Intermediate in Benzene Nitration with Mixed Acid

“…[13][14][15] Thereaction then gives nitrobenzene by removal of the ipso proton from ArHNO 2 + by either the sulfuric acid or the bisulfate anion, HSO 4 À ,w hich both act as ab ase.Asingle electron transfer (SET) mechanism, widely considered as an alternative reaction path, involves the transfer of an electron from the aromatic ring to the nitronium ion;r ecombination of the resulting radical pair produces the same Whelandintermediate (ArHNO 2 + ). [18][19][20][21][22][23] Early model gas-phase computations by Peluso and Del Re [21] on the interaction of benzene and NO 2 + established the possibility of ac onical intersection between the ground and the first excited singlet state,accompanied by charge-transfer,t hereby supporting the SET mechanism for the reaction. In 2010, Xu, Zilberg, and Haas [23] employed CASSCF computations with the cc-pVDZ basic set to investigate the interactions of benzene with several positively charged electrophiles,i ncluding NO 2 + .T he results suggested that benzene and the charged electrophile are in an electronically excited state in the gas phase initially,whereas the ratecontrolling step lies on the ground state potential energy surface.T he transition from excited to ground state is achieved by ac onical intersection.…”

“…[18][19][20][21][22][23] We estimated the electron transfer from the aromatic p-system to the nitronium unit by applying two alternative population analysis methods.F igure 1b compares computed atomic charges for the p-complex intermediates in isolation (gas phase), computed at B3LYP/6-311 + G(2d,2p) and within the stepwise route in mixed acid conditions.B oth the NBO method [41,42] and Hirshfeld population analysis [43] predict ar oughly 0.70 ec harge transfer from the aromatic ring to the NO 2 + ion for the complex in isolation. TheS ET mechanism of nitration is aw ell-established concept.…”

Nitração aromática: substituição eletrofílica ou reação com transferência de elétrons?

“…Contudo, um exame criterioso dos resultados revela inconsistências, pois posições com densidade de carga próximas possuem grande diferença nas velocidades relativas 67 . Em trabalho mais recente Peluso e Del Re 68 usaram a base 3-21G com interação de configuração para discutir a possibilidade de nitração via formação de um complexo doador-aceptor, com evolução para o par radical-cátion radical 68 . A análise da superfície de energia potencial na região correspondente aos dois reagentes mantidos à distância de van der Waals mostra a existência de uma interseção entre o estado fundamental e o primeiro estado excitado, levando à transferência de elétron do substrato aromático para o íon nitrônio.…”

“…Os autores concluem que a principal via de nitração envolve uma etapa de transferên-cia de elétrons mesmo na nitração ativada termicamente. Para a nitração do tolueno foi calculada uma barreira de apenas 13 kcal/mol para o processo envolvendo transferência de elétrons 68 . De uma forma geral os poucos cálculos teóricos que existem não são suficientes para resolver o problema.…”

An interpretation of the phenol nitration mechanism in the gas phase using G3(MP2)//B3-CEP theory

“…In addition to experimental observations performed in the condensed phase, theoretical studies [14][15][16][17][18][19][20][21][22][23] investigating the mechanism for the electrophilic nitration of benzene and monosubstituted derivatives in the gas phase have elucidated important details that support the existence of single-electron transfer steps.…”