Competing bromination and oxidation pathways in acid bromate solutions: an experimental and theoretical study

Abstract

The qualities that render concentrated H₂SO₄–BrO₃ mixtures as powerful bromination and oxidation reagents are examined by following the reactions of acetophenone, cyclohexanol and benzaldehyde, and by using density functional theory (DFT) to study the dissociation of bromic acid (HBrO₃) and of bromate ions. The experimental results indicate that acid concentration and substrate type dictate the selectivity between the bromination and oxidation pathways. Supporting this, the computational studies show that the HBrO₃ molecule is metastable, its exothermic dissociation to {HOBr + O₂} being opposed by a significant barrier, higher than what is expected for intersystem crossing. The energetics of this dissociation are compared with O-atom transfer in the oxidation of methanol to formaldehyde, and the similar values (ΔH° = −40.2 and −44.6 kcal mol⁻¹, respectively) are consistent with the mixture of bromination and oxidation products observed in some of the experiments (e.g. with benzaldehyde). In the case of acetophenone, ring bromination is superseded by bromination of the side chain and debromination of the ring. The hypothesis that this is a rearrangement reaction induced by bromate is examined using substituted bromo- and chloroacetophenones as substrates. The general tendency of organic substrates to undergo fast oxidation and/or bromination in the presence of acid bromate is discussed.