Molecular Factors that Modify pK a Values

Abstract

Qualitatively, factors that modify pK a values are well understood. They comprise mainly inductive, electrostatic and electron-delocalization (mesomeric) effects, together with contributions from hydrogen bonding, conformational differences and steric factors. Where a substituent tends to stabilize a cation more than the corresponding neutral molecule, or a neutral molecule more than the anion derived from it, the pK a of the acid will be raised. That is, the substituent will be acid-weakening or base-strengthening. Conversely, increased stabilization of an anion relative to the neutral species is acid-strengthening or base-weakening. Consideration of factors that modify the relative stabilities of these species can thus provide insights as to why pK a values vary but they do not permit calculations of the differences involved. It is nonetheless, useful to have even a qualitative indication of how substituents affect pK a values.