Sulfonamide antibiotics are an important class of organic pollutant in the
aquatic environments. To understand the hydrolysis behavior of sulfonamides, the
hydrolysis mechanisms of a typical sulfonamide sulfachloropyridazine (SCP) were
investigated using the density functional theory (DFT) at the B3LYP/6-31+G (d, p) level.
SCP hydrolysis resembles nucleophilic substitution by water molecule attacking
sulfonyl group (pathway 1) and heterocyclic aromatic ring (pathway 2) respectively.
Due to the electrophilic center sulfur atom in pathway 1 carrying much larger positive
charge than the carbon atom in the pathway 2, the sulfonyl group can be easily
attacked by water molecule, and thus the pathway 1 can be dominant. By comparing
the hydrolysis energy barrier of different forms of SCP, it was found that the SCP
hydrolysis in neutral and once-protonated state are much more energetically favorable
to proceed than the double protonated form. In addition, the hydrolysis path is not
found for the dissociated anionic SCP. As the pH values in solution decreases, the
corresponding neutral and once-protonated SCP increases, then the hydrolysis rate
becomes faster, which is consistent with the experimental observations that the
hydrolytic degradation rate at pH=4 is much faster than those of pH=7 and 9.