Factors Affecting the Relative Efficiency of General Acid Catalysis

Abstract

Specific acid catalysis (SAC) and general acid catalysis (GAC) play an important role in many organic reactions. Elucidation of SAC or GAC mechanisms requires an understanding of the factors that affect their relative efficiency. A simple model reaction in which SAC and GAC occur concurrently is analyzed to provide guidelines for such analyses. Simple rules that predict the effect of pH on the relative efficiency of GAC are discussed. An explicit expression for determining the optimal pH at which the effects of GAC are maximized is provided as well as an analogous expression for the general base case. The effects of the pKa of the general acid and the total catalyst concentration on GAC efficiency are described. The guidelines are graphically illustrated using a mixture of theoretical data and literature data on the hydrolysis of di- tert-butyloxymethylbenzene. The relationship between the model and the Brønsted equation is developed to explain the common observation that GAC is most easily observable when the Brønsted coefficient a is near 0.5. The ideas presented enable systematic evaluations of experimental kinetic data to provide quantitative support for SAC or GAC mechanisms. This material is suitable for the curriculum of a senior-level undergraduate course in physical-organic chemistry.