Hydrophobic Quinoneimine Dye in Mixed Aqueous Solvents

We found recently that azo dyes which exist in two tauto­ meric forms show a pronounced shift in the tautomeric equili­ brium when organic solvents are added in small amounts to aqueous solutions of the dyes. 4 The greatest shift in the equilibrium occurs in the predominantly aqueous compositions where the solvent retains three-dimensional structure, and we interpreted the results in terms of selective solvation of the dyes by the organic solvent within the water structure. The exchange of a water-solute interface by an organic solvent-solute interface in a predominantly aqueous medium should have a pronounced effect on the kinetics of reactions of a hydrophobic solute. . The effect should be different for bimole­ cular and unimolecular reactions. Observation of a dramatic change in rate upon addition of small amounts of organic solvent to an aqueous medium that could not be accounted for in terms of changes in bulk solvent properties would indicate that selective solvation of the solute or of the activated complex, or both, was occurring.