Alkaline hydrolysis of solid 1,3,5-trichloro-2,4-dinitrobenzene (T3): kinetics, mechanism, and overlooked deprotonation effects

The environmental risks associated with chloronitroaromatic compounds (CNAs) have raised significant concerns, yet their transformation through alkaline hydrolysis remains poorly understood, especially for hydrophobic polychlorinated polynitrobenzenes (PCPNBs). This study systematically investigates the hydrolysis of granular 1,3,5-trichloro-2,4-dinitrobenzene (T₃, 176 × 100 µm) through combined experimental and density functional theory (DFT) approaches. Granular T₃ hydrolysis follows pseudo-first-order kinetics (0.163 h⁻¹ at 95 °C), limited by solubility and intrinsic reactivity at ambient temperature and mass transfer at high temperature, while dissolved T₃ exhibits pseudo-second-order kinetics (1.074 L·mg⁻¹·h⁻¹ at 95 °C). DFT calculations confirm the thermodynamic feasibility of both chlorine (Cl) and nitro (NO₂) substitutions via the bimolecular nucleophilic aromatic substitution (S_N2Ar) mechanism. Dechlorination is kinetically favored over denitration due to the higher electrophilicity and lower steric hindrance of the Cl substitution site and the weaker C-Cl bond. Importantly, this study reveals a critical but previously overlooked factor in PCPNBs’ transformation: deprotonation. pK_a and energy barrier calculations indicate that deprotonation of the substituted products significantly increases the energy barrier for subsequent substitution (e.g., from 16.7 to 31.1 kcal·mol⁻¹). This kinetic suppression originates from enhanced electrostatic repulsion between the negatively charged aromatic ring and the nucleophile, coupled with diminished resonance stabilization due to reduced aromaticity in the transition states. Electrospray ionization mass spectrometry (ESI-MS) and UV–Vis analyses validate these predictions, identifying monosubstituted, deprotonated polychlorinated polynitrophenols as dominant products. These insights enhance the mechanistic understanding of CNAs hydrolysis and highlight the critical role of protonation state in determining their environmental fate.