Some Studies of the Dienone Intermediate Formed in the Bromination of 2,6-Di-t-Butylphenol

Abstract

The bromination of 2,6-di-t-butylphenol by bromine in acetic acid at 25 has been studied. The reaction follows two pathways; one by addition leading to 4-bromo-2,6-di-t-butylcyclohexa-2,5-diemone and the other by direct substitution leading to 4-bromo-2,6-di-t-butylphenol. The kinetic solvent istotope effect on the overall bromination reaction and on both routes is also investigated. The acid-catalysed prototropic rearrangement of the bromodienone to bromophenol in acetic acid and in aqueous acetic acid with and without deuterium in the 4- position of the dienone and in the solvent has been studied. The first-order rate-coefficient is linearly correlated with the protonating power of the medium (i.e. with ho, as calculated from the extent of protonation of 0-nitro~iniline in the same solvent). With lithium bromide as the catalyst, the rate of rearrangement of the dienone proceeds ten times faster than the perchloric acid catalysed-rearrangement at a given acidity. Moreover, in the presence of lithium bromide, bromine is formed during the rearrangement. The mechanistic implications of these results are discussed.