Ionization constants of 3-substituted propiolic acids in water

Abstract

The pK_a of ten 3-substituted propiolic acids XCCCO₂H with X = OPh, Me, Ph, SiMe₃, SPh, H, I, Cl, Br, and CO₂Et have been measured at 25 °C in water by a conductance method. The effect of substituents has been compared in the propiolic, acrylic, and benzoic series by means of linear free energy relationships. It is found that the Hammett and the DSP equations are less precisely obeyed in the propiolic series. Significant deviations occur for hydrogen and iodine which confirm the positive character of these groups when bound to an ethynyl group.