2-Aroylbenzoic acid was allowed to react with hydrazine hydrate, hydroxyl amine and methylhydrazine to afford the corresponding 5,6,7,8-tetrachlorophthalazin-1-one. The antimicrobial effect of the phthalazin-1-one in the water of the swimming pool and river can be discussed. DFT based on quantum chemical computational of the new compounds were synthesized with the objective of studying their antimicrobial activity. The newly synthesized compounds were characterized on the basis of their spectral (1H-NMR, Mass spectrum, IR and Elementary analysis