Hydrolysis of Phosphoryl Trichloride (POCl₃): Characterization, in Situ Detection, and Safe Quenching of Energetic Metastable Intermediates

The accumulation of metastable intermediates resulting from the incomplete hydrolysis of phosphoryl trichloride-containing mixtures carries the risk of latent exothermic events. Significant accumulation of two P−Cl species containing reactive phosphorus−chlorine bonds was detected spectroscopically (31P NMR) during inverse quench of POCl3−MeCN mixtures under typical literature conditions. The dominant reactive intermediate was unequivocally assigned as phosphorodichloridic acid (X-ray, 31P NMR). Quantitative 31P NMR time-course experiments allowed for the determination of kinetic parameters of POCl3 hydrolysis under synthetically relevant concentration and temperature conditions in batch settings. Development of an in situ Raman method allowed to further expand these studies to semibatch conditions under different pH regimes. Furthermore, we hereby describe an in situ Raman method suitable to ascertain completeness of the quench for large-scale preparations involving POCl3. These analytical techniques can be supported by differential scanning calorimetry (DSC) and accelerated rate calorimetry (ARC) in order to confirm absence of reactive species.