MECHANISM OF THE BRONSTED ACID-CATALYZED HYDROLYSIS OF 2-ARYL-2-METHYL-1,3-DITHIANES IN AQUEOUS PERCHLORIC-ACID

The acid catalysed hydrolysis of five para-substituted 2-aryl-2-methyl-1,3-dithianes in aqueous perchloric acid (3-9 mol dm-3) has been studied kinetically. The effect of changes in substituents (rho = -3.1), temperature (DELTA-S double-ended-dagger = -38-60 J K-1 mol-1) and acidity (m double-ended-dagger = 0.54-0.74), and the solvent isotope effects (K(obs)D2O/K(obs)H2O < 1), are all compatible with an essentially AS(E)2 mechanism for the MeO, Me, H and Cl derivatives. For the p-NO2 compound the acidity dependence, and the values of DELTA-H-double-ended-dagger and DELTA-S-double-ended-dagger (ca. -200 J K-1 mol-1) suggest a change to an A2-like mechanism. Our results are discussed in the light of previous work on acetal hydrolysis.