Seayad et al. grouped organocatalysts in four general classes: Brønsted acids (proton donors), Brønsted bases (proton acceptors), Lewis acids (electron acceptors) and Lewis bases (electron donors). Brønsted acids can be further categorized according to their pKa value into actual Brønsted acids dissociating a proton in the catalytic process (e.g. phosphates ) and hydrogen bond donors keeping the bond with their proton intact and forming hydrogen bridges in the catalytic process instead (e.g. thioureas ).