Medium effects in protonation equilibrium studies. Accurate acidity constants using the excess acidity method

Abstract

Ultraviolet spectra of N-tert-butylbenzamide taken in 0–92% sulfuric acid solutions clearly show that two processes, having roughly equal effects on the spectra, take place over this acidity range. One is protonation of the molecule, and the other is either tautomerization from N- to O-protonated forms, or a pronounced medium effect on the latter. Attempted separation of these two processes using characteristic vector analysis fails. However, separation is possible using an analysis based on the excess acidity (X-function) method. This shows that the second process is a medium effect. The analysis permits accurate calculation of [Formula: see text] and m* values in the presence of medium effects on the acid form, the base form, or both.