The Behavior of 4-Alkyl-4-bromo-2,5-cyclohexadienones Formed during the Aqueous Bromination of p-Alkylphenols

Article abstract of DOI:10.1021/jo00363a034

The title compounds ("ipso-dienones") 5 have been observed during the reaction of bromine with six p-alkylphenols 4 (R = Me, Et, n-Pr, i-Pr, t-Bu, 3,4-Me2) in aqueous solutions of pH 0-3.Their formation by ipso bromine attack on 4 accounts for about 10percent of the initial consumption of bromine.The decomposition of 5, which is catalyzed by H⁺ and by Br^-, is attributed to debromination.The rates of this reaction and of the attack of bromine on 4 are not very sensitive to the nature of the alkyl substituents.Studies of the behavior of 5 (R = Me) in buffers give curved buffer plots which provide additional support for the debromination mechanism and also demonstrate general acid catalysis.Decomposition of 5 (R = Me) in the presence of a trap for liberated bromine give straight buffer plots from which a Broensted α ca. 0.27 is deduced.The ipso-dienone 8, derived from 5-methylsalicylic acid, shows intramolecular catalysis by the carboxyl group (EM = 58 M) and no catalysis by buffer acids.