After use, drugs are eliminated from the body and as such end up in wastewater and disinfection pools. When treating such waters, the most used method is chlorination with chlorinating spices, among which hypochlorous acid and chlorine gas are most common. In reactions with chlorinating species, drug metabolites, among which the most interesting are those containing an amino group, N-chloramines are formed which are more reactive, toxic and persistent forms. The fate of N-chloramine species was studied by Hoffman-Löffler-Freytag (HLF) reactions. It was calculated which products are more likely given the barrier energy response and the stability of products. The mechanisms of these reactions have been studied by quantum chemical models that calculate the energy of products, reactants and transition states, and structures are visualized through the IQmol program. Quantum chemical calculations were performed using the Orca software package. Orca offers state-of-the-art methods in the field of functional density theory (DFT - Density Functional Theory) as well as methods based on wave functions. The results obtained in this paper show that N-chlorinated derivatives are converted into Nradicals which are converted into more stable C-radicals by intramolecular hydrogen transfer which are then chlorinated and cyclized under the influence of base into piperidine and pyrrolidine systems giving compounds of new characteristics. At the end of the reaction we are left with a pyrrolidine (or piperidine) ring and trimethylamine hydrochloride salt.