It is often difficult to distinguish between hydrogen-bond catalysis and general acid catalysis. Hydrogen-bond donors can have varying acidity, from mild to essentially strong Brønsted acids like phosphoric acids. Looking at the extent of proton transfer over the course of the reaction is challenging and has not been investigated thoroughly in most reactions. Nevertheless, strong acid catalysts are often grouped with hydrogen-bond catalysts as they represent an extreme on this continuum and their catalytic behaviors share similarities. The mechanism of activation for these reactions involves initial protonation of the electrophilic partner. This has the effect of rendering the substrate more electrophilic and creating an ion pair, through which it is possible to transfer stereochemical information.
