Bromination of phenol red mediated by vanadium(V) peroxo complexes at pH 6.5

The kinetics of bromination of phenol red (HPhR) to yield bromophenol blue (BrPhB) was studied at pH 6.5, in the presence of peroxovanadium(V) species generated by acid decomposition of [VO(O-2)(2)(NH3)](-) and of [O{VO(O-2)( 2)}(2)](4-). In the concentration ranges 10(-6)-10(-7) (HPhR), (1.5-8.0)x10 (-4) (vanadium complexes) and 0.004-0.12 mol dm(-3) (bromide), the rate law is R=k[V](T) [Br-][HS], where HS is the substrate undergoing bromination i n the rate determining step, with k=2.49x10(5) dm(6) mol(-2) s(-1). Acid tr eatment of the precursor complexes yields a mixture of [VO(O-2)L](n) comple xes, with L=H2O, [VO(O-2)(H2O)](+), or O-2(2-), [VO(O-2)(2)](-). Alkaliniza tion leads to active species that react with bromide to yield a brominated vanadium complex (e.g. [VO(O-2)Br]), which is postulated to be the active b romination agent. Kinetic data rule out the mediation of hypobromous acid. The results support the idea that five-co-ordinated vanadium species are re quired in the bromination reaction.