Bromination of phenol red mediated by vanadium(V) peroxocomplexes at pH 6.5.

TÓTARO R. M.; WILLIAMS P. A. M.; APELLA , M. C.; BLESA M. A.; BARAN E.J.

JOURNAL OF THE CHEMICAL SOCIETY, DALTON TRANSACTIONS

Dalton Transactions

Lugar: Osaka; Año: 2000 vol. 23 p. 4403 - 4406

0300-9246

The kinetics of the 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 the acid decomposition of [VO(O2)2(NH3)]- and of [O{VO(O2)2}2]4-. In the molar concentration ranges 10-6 - 10-7 (HPhR), (1.5 - 8.0) 10-4 (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 in the rate determining step, with k = 2.49 x 105 dm6 mol-2 s-1. Acid treatment of the precursor complexes yields a mixture of [LVO(O2L]n complexes, with L = H2O, [VO(O2)(H2O)]+, or O2 2-, [VO(O2)2]-. Alkalinization leads to the active species, that reacts with bromide to yield a brominated vanadium complex (e.g. [VO(O2)Br]), which is postulated to be the active bromination agent. Kinetic data rule out the mediation of hypobromous acid. The results support the idea that penta-coordinated vanadium species are required in the bromination reaction.