Kinetics of hydrolysis of PCl₅ in situ as evaluated from the partial hydrolysis products formed in [¹⁸O]water

Abstract

The in situ hydrolysis products of PCl₅1, namely POCl₃, HPO₂Cl₂, H₂PO₃Cl and H₃PO₄5, were identified and their mol proportions measured at different extents of overall hydrolysis (from 7.0 to 93%). Partial hydrolysis in 95 atom % [¹⁸O]H₂O, step 1, was followed by complete hydrolysis in non-enriched water, step 2. The phosphoric acid isotopomers obtained at step 2 were derived from mixtures that contained from 0 to 4 ¹⁸O atoms per phosphate incorporated at step 1, depending upon the composition of the mixtures formed at step 1. The relative mol proportions of mixtures were deduced by regression analysis of the normalized peak intensities of the trimethyl phosphate isotopomer cluster at m/z = 140, 142 . . . 148 (obtained by electron-impact GC analysis). The mixture compositions were used to evaluate kinetically a sequential, irreversible model for hydrolysis of PCl₅. The relative values obtained for apparent hydrolysis constants and the species to which they were attributed were: PCl₅, 1.0; POCl₃, 0.24; PO₂Cl₂^-, 0.02; with that for HPO₃Cl^- being too great to measure. These values accurately predicted the composition of mixtures in all but a narrow range of overall hydrolysis between 60 and 70% where the hitherto undetected monochloro acid accumulated in yields up to 30%. As the water content becomes small with respect to the acid components in this region, the equilibrium between the monoanion and undissociated forms of the acids is shifted in favour of the undissociated acids (the final end products as the mixture becomes anhydrous). The relative rate constants in this region were estimated to be 0.02 and 0.08 for HPO₂Cl₂ and H₂PO₃Cl, respectively, thereby accounting for accumulation of the monochloro acid in this range. It is proposed that under these conditions the rates of hydrolysis of the mono and dichloro components are comparable because both components are present largely in their free acid forms which undergo hydrolysis by an associative mechanism.

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