Resumen de Factors That Influence Relative Acid Strength in Water: A Simple Model
The pKa's of diverse aqueous acids H–A correlate well with the sum of two gas-phase properties: the H–A bond-dissociation enthalpy (BDE) and the electron affinity (EA) of the A• radical. Acidity correlates poorly with each of these separately. It is suggested that rather than bond strength alone or bond polarity, the sum of the enthalpies of these two steps is a fairly good indicator of relative acidity, especially when comparison is made among acids of a given type. Electronegative groups attached elsewhere on the molecule have a greater effect on the acidity not in the weakening or polarization of the H–A bond, but rather in enhancing the electron affinity of the radical, or put another way, stabilizing the conjugate base.
