Reactivity Bottlenecks in Alkaline Hydrolysis of Polychlorinated Polynitrobenzenes: Mechanistic Insights and Implications for Water Treatment

Polychlorinated polynitrobenzenes (PCPNBs), commonly present in explosive manufacturing waste and as industrial chemicals, pose significant environmental risks due to their persistence and toxicity. While alkaline neutralization is commonly used to treat PCPNB-containing waste acids, their hydrolysis behavior under such conditions remains poorly understood. Here, we investigate the alkaline hydrolysis of 1,2,3,5-tetrachloro-4,6-dinitrobenzene (T4), a representative PCPNB, through combined batch experiments and density functional theory (DFT) calculations. Our results show the reaction follows pseudo-first-order kinetics via a concerted bimolecular nucleophilic aromatic substitution mechanism. Dechlorination is kinetically favored over denitration, releasing ∼4.1 times more Cl^–than NO₂^–at 95 °C, attributed to chlorine’s superior leaving ability and the greater electrophilicity of its substituted carbon. Crucially, the resulting polychlorinated polynitrophenols (PCPNPs) readily undergo deprotonation (pK_a1≤ 4.5). This deprotonation significantly elevates the energy barriers for subsequent substitutions (e.g., from 9.8 to 17.2 kcal/mol) due to enhanced electrostatic repulsion and diminished resonance stabilization. This previously overlooked mechanistic bottleneck inhibits further degradation, leading to persistent PCPNP accumulation. By elucidating this critical protonation state control over hydrolysis reactivity, our work provides vital theoretical and molecular-level insights into the environmental fate of PCPNBs and PCPNPs in natural and engineered systems, informing the design of more effective remediation strategies.