A THERMODYNAMIC HYDROGEN-BOND ACIDITY SCALE FOR MONOSUBSTITUTED ACETYLENES

A scale of solute hydrogen-bond acidity has been derived from measured equilibrium constants (as log K values) for hydrogen-bond complexation of a series of monosubstituted acetylenes against the reference base triphenylphosphine oxide in dilute CCl4 solution at 20-degrees-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 alpha2H values suitable for use in linear solvation energy relationships.