Approaching the Solid-Phase Reaction Limit: Nitrosation of 4-Phenylpiperidine Hydrochloride in Dense Slurries

In this work, we experimentally studied the nitrosation of 4-phenylpiperidine hydrochloride in the presence of sodium nitrite under acidic conditions while systematically decreasing water content, from fully homogeneous aqueous solutions to highly concentrated slurries and, ultimately, to a humidified, solid-like environment. The formation rate of 1-nitroso-4-phenylpiperidine was quantified and used to parameterize a novel kinetic model. The proposed model explicitly accounts for the effect of limited solubilities of the reacting species and incorporates a correction to describe mobility limitations anticipated in highly dense slurries and solid-like systems. The model successfully reproduces the experimentally observed bell-shaped dependence of the initial nitrosation rate on water content and quantitatively captures the progressive departure from solution-like behavior as water content decreases. These results define practical boundaries for deviations from solution-phase kinetics and support the conclusion that applying solution-phase models to solid-phase systems leads to a systematic overestimation of nitrosamine formation rates, thereby providing a substantial safety margin in nitrosamine risk assessment.