A thermodynamic hydrogen-bond acidity scale for monosubstituted acetylenes

Abstract

A scale of solute hydrogen-bond acidity has been derived from measured equilibrium constants (as log K values) for hydrogen-bond complextion of a series of monosubstituted acetylenes against the reference base triphenylphosphine oxide in dilute CCl₄ solution at 20°C. The effects of the substituents Pent, SiEt₃, OEt, Ph, SnPh₃, SEt, p-BrC₆H₄, ClCH₂, p-NO₂C₆H₄, COMe, COOEt, CF₃ and CN on the complexation reaction are discussed in terms of their field and resonance substituent constants. The relation between log K and the charge on the acetylenic proton indicates that electrostatic interaction predominates for these weak hydrogen-bond donors. Hydrogen-bond acidity is closely related to kinetic acidity. The log K values may be transformed into αH₂ values suitable for use in linear solvation energy relationships.