In the first step (Fig. The first category is the zwitterionic mechanism for primary and secondary amines , which includes two steps: the first step is the formation of an intermediate zwitterion (Eq. (1)), and the second step is the deprotonation of the zwitterion by reacting with a base (Eq. (2)). In these steps, R 1 R 2 NH represents a primary amine or a secondary amine if R2 is a hydrogen atom or R1 and R2 are neither hydrogen atoms, respectively. The symbol B represents an amine, water, hydroxide or other substances that is used as a base.