Photolytic decomposition of hydrochlorothiazide

Hydrochlorothiazide decomposes upon irradiation with near-UV light (lambda greater than 310 nm) both in methanol and aqueous solutions. In the photolysis the chlorine substituent is removed to be replaced by either--H or--OR from the solvent ROH. Hydrolysis of the thiadiazine ring is superimposed upon the dechlorination. The presence of oxygen inhibits the decomposition. The mechanism of the photolysis is suggested to involve cation radical formation which facilitates the hydrolysis step. 5-Chloro-2,4-disulphonamido-aniline, the normal hydrolysis product from hydrochlorothiazide, is also susceptible to photolytic dechlorination by a similar mechanism.