NO₂+ Nitration Mechanism of Aromatic Compounds: Electrophilic vs Charge-Transfer Process

Citations Over TimeTop 20% of 2000 papers

Abstract

The nitration of methylnaphthalenes with NO(2)BF(4) and NOBF(4) was examined in order to shed light on the controversial aromatic nitration mechanism, electrophilic vs charge-transfer process. The NO(2)(+) nitration of 1,8-dimethylnaphthalene showed a drastic regioselectivity change depending on the reaction temperature, where ortho-regioselectivity at -78 degrees C and para-regioselectivity at 0 degrees C were considered to reflect the electrophilic and the direct or alternative charge-transfer process, respectively, because the NO(+) nitration through the same reaction intermediates as in the NO(2)(+) nitration via a charge-transfer process resulted in para-regioselectivity regardless of the reaction temperature. The NO(2)(+) nitration of redox potential methylnaphthalenes higher than 1,8-dimethylnaphthalene gave a similar ortho-regioselectivity enhancement to 1,8-dimethylnaphthalene at lower temperature, thus reflecting the electrophilic process. On the other hand, the NO(2)(+) nitration of redox potential methylnaphthalenes lower than 1,8-dimethylnaphthalene showed para-regioselectivity similar to the NO(+) nitration, indicating the direct or alternative charge-transfer process. In the presence of strong acids where the direct charge-transfer process will be suppressed by protonation, the ortho-regioselectivity enhancement was observed in the NO(2)(+) nitration of 1,8-dimethylnaphthalene, suggesting that the direct charge-transfer process could be the main process to show para-regioselectivity. These experimental results imply that the NO(2)(+) nitration proceeds via not only electrophilic but also direct charge-transfer processes, which has been considered to be unlikely because of the high energy demanding process of a bond coordination change between NO(2)(+) and NO(2). Theoretical studies at the MP2/6-31G(d) level predicted ortho- and para-regioselectivity for the NO(2)(+) nitration via electrophilic and charge-transfer processes, respectively, and the preference of the direct charge-transfer process over the alternative one, which support the experimental conclusion

Related Papers

• → Electrophilic aromatic substitution. Part VI. Kinetics and mechanism of nitration of halogenobenzenes(1970)24 cited
• → Electrophilic aromatic substitution. Part X. The kinetics and mechanism of nitration of some anilides and aromatic ethers in sulphuric acid and in acetic anhydride, with particular reference to the existence of a special mechanism for ortho-nitration in the latter medium(1971)8 cited
• → An Inquiry-Based Approach to Electrophilic Aromatic Substitution: Quantifying and Identifying Nitration Products by1H NMR(2021)
• → Electrophilic Aromatic Substitution(2007)
• → 5.1.4. Regioselectivity in the Nitration of Acylanilines by Electrophilic Aromatic Substitution(2016)