Effect of β-cyclodextrin on the bleaching of a triarylmethane dye by hydrogen peroxide and alkali

Bleaching of the triarylmethane dye, Green S, by hydrogen peroxide, involving nucleophilic attack at the electrophilic central carbon of the dye is first accelerated and then slowed down with increasing cyclodextrin concentration. This rate maximum is clear evidence for reaction pathways involving both one and two molecules of cyclodextrin. Because precise values of the 1:1 and 2:1 binding constants of the cyclodextrin and dye by an independent method could not be obtained, curve fitting is carried out using lower and upper limits of the product of the binding constants. This proves to be a useful approach in these circumstances. Alkali bleaching of the dye, also involving nucleophilic attack at the central carbon, is accelerated by cyclodextrin and the reverse reaction is slowed down, and no rate maxima or minima are observed. These results are consistent with pathways involving both one and two molecules of cyclodextrin provided the second cyclodextrin stabilises Green S and its alkali bleaching product to the same extent. The kinetic data are interpreted using the transition state pseudo-equilibrium constant approach, which has been extended to include reversible reactions. The results are discussed in terms of a simple cyclodextrin field effect.