Density functional theory study on the reaction mechanism of 1-nitroanthraquinone synthesis by nitration of anthraquinone mixed acid

The nitration of anthraquinone (AQ) to form 1-nitroanthraquinone (1-NAQ) is a key step of significant industrial value in aromatic nitration reactions. However, its microscopic mechanism and the formation mechanism of regioselectivity have long lacked clear theoretical explanations. This study employs density functional theory calculations combined with an implicit solvent model to reveal the complete reaction pathway of AQ nitration under mixed acid conditions at the atomic scale. Both kinetic and thermodynamic results consistently indicate that this reaction follows a typical two-step electrophilic aromatic substitution mechanism: the attack of the nitroyl ion (NO2+) on the C1 site constitutes the rate-determining step (energy barrier: 7.327 kcal/mol), while the sulfite ion abstracts the intermediate H to form 1-NAQ (energy barrier: 2.896 kcal/mol). The highly regional selectivity of the reaction toward C1 is elucidated through the local electronic structure characteristics, charge distribution, and the formation and cleavage behavior of key bonds. Thermodynamic analysis further reveals that the Gibbs free energy of both transition states decreases significantly with increasing temperature, reflecting strong thermal driving forces. The variation in heat capacity with temperature indicates the electronic restructuring accompanying the destruction and restoration of aromaticity during the reaction process. This study elucidates the mixed-acid nitration reaction mechanism of AQs, providing a comprehensive quantitative description of the nitration mechanism for AQ compounds. It establishes a theoretical foundation for designing safe, efficient, and continuous nitration processes.