Aggregation of Phosphoric Acid, Phenyl Dihydrogenphosphate, and Related Organophosphorus Acids in Conductometric Titration with Triethylamine in DMF

Abstract

Abstract The conductance increased linearly up to the equivalence point in the conductometric titration of diphenyl hydrogenphosphate ((PhO)2PO(OH)) with triethylamine (Et3N) in DMF at 25 °C. However, it was found that the conductometric titration of phenyl dihydrogenphosphate (PhOPO(OH)2, 1.0 × 10−3—1.0 × 10−2 mol dm−3) gave a maximum at the half-equivalence point (CHA : [Et3N] = 1 : 1/2) and not the linear curve to the equivalence point. Phosphoric (PO(OH)3), phenylphosphonic (PhPO(OH)2), and vinylphosphonic (CH2CHPO(OH)2) acids also gave maxima at the half-equivalence points. The appearance of maxima was explained by homoconjugation between the acid (HA) and the conjugate anion (A−) upon neutralization with the base (B) in DMF : B + 2 HA BH+···HA2−. Dibutyl hydrogenphosphate ((n-BuO)2PO(OH)) of higher concentrations also gave a (small) maximum at the half-equivalence point, while dimethylphosphinic acid ((CH3)2PO(OH)) gave no maximum in DMF. In less basic solvents, acetonitrile and benzonitrile, even diphenyl hydrogenphosphate, gave a maximum. The cause for the appearance of maxima at the half-equivalence point in DMF is discussed from the viewpoint of the basicities of the conjugate anion (A−) and the number of hydrogen bondings involved in the homoconjugation reaction. The ion-pair and quadrupole (the dimer of ion pairs) formation constants of the organophosphorus acids in DMF were obtained by conductometry. The equilibrium concentration of the dimer (H2A2) of PhOPO(OH)2 was found to exceed that of the ion pair (HA) at >1.6 × 10−3 mol dm−3 of the acid.