Aromatic substitution with rearrangement. Part III. Isotope effects in the prototropic rearrangement of 4-bromo-2,6-di-t-butylcyclohexa-2,5-dienone

Abstract

The prototropic rearrangement of 4-bromo-2,6-di-t-butylcyclohexa-2,5-dienone to 4-bromo-2,6-di-t-butylphenol has been studied in acetic acid and in aqueous acetic acid, with and without deuterium in the 4-position of the dienone and in the solvent. With perchloric acid as catalyst, the first-order rate-coefficient is linearly correlated with the protonating power of the medium (i.e. with H_o, as calculated from the extent of protonation of o-nitroaniline in the same solvent). The reaction is subject to a relatively large primary isotope effect k^4–H(/k^4–D= 4·2), which decreases slightly with increased water content of the solvent. The solvent isotope effect is nearly the same as that on the protonation of o-nitroaniline. The mechanistic implications of these results are discussed.