Substituent Effects. 9. Two Charged Groups. Remarks on the Eucken/Kirkwood-Westheimer Model and the Reversed-Field Effect

Article abstract of DOI:10.1021/jo00133a017

Thermodynamic dissociation constants are given for 3,5-(NMe3⁺)_2-, 3,5-(SO3^-)_2-, and 3-NMe3⁺-5-SO3^-C6H3COOH in water and ethanol-water mixtures.These follow the extended Hammett equation Δ = ρ?^L + δ^B (δ^B is the electrostatic Bjerrum term).This equation also accounts for the phenolic dissociation of the zwitterionic form of tyrosine (VI) and N,N,N-trimethyltyrosine (VII).In the latter compound the "reversed field" is acid weakening; nevertheless, acid strengthening is observed.Some other reversed-field models are given.The data strongly favor the view that the electrostatic field is not the prevailing factor.Substituent effects in 0-4 M NaCl are also presented; the zwitterionic compounds (V and VII) conform to the behavior of dipole-substituted acids.