Ozone fading of natural organic colorants: mechanisms and products of the reaction of ozone with indigos

Indigo, dibromoindigo, and colorants containing thioindigo and tetrachlorothioindigo were exposed in the dark to dry, purified air containing ozone (10 ppm) for 4 days, and the exposed samples were analyzed by mass spectrometry. Under the conditions employed, indigo and dibromoindigo were entirely consumed, and the major reaction products were isatin and isatoic anhydride from indigo and bromoisatin and bromoisatoic anhydride from dibromoindigo. Thioindigo and its chloro derivative also reacted with ozone, though at a slower rate; the corresponding substituted isatins and anhydrides were tentatively identified as reaction products. These results can be rationalized in terms of a mechanism involving electrophilic addition of ozone onto the unsaturated carboncarbon bond. This mechanism adequately describes the observed loss of chromophore (fading) for all indigos studied and presumably applies to other indigo compounds as well. The reaction products of indigo, isatin and isatoic anhydride, were not ozone fugitive under our conditions