Kinetics and Mechanism of Bromination of Tris(Acetylacetates) of Co(III), Cr(III) and Bis(Acetylacetonato) Co(II) by Pyridinium Bromochromate in Aqueous Acetic Acid Medium

Abstract

The kinetics of bromination of Cr(acac)₃, Co(acac)₃ and Co(acac)₂ by pyridinium bromochromate(PBC) in aqueous acetic acid have been studied under pseudo-first order conditions Co(III), Cr(III) acetylacetonates undergo electrophilic γ-substitution yielding M(Bracac)₃ whereas Co(II) acetylacetonate undergoes nucleophilic bromination, yielding [CoBr₄][PyH]₂. The rate for the tris-chelates is dependent on the concentration of the reagent(PBC) and not the substrate. For the bis-chelate, the rate is dependent on both the substrate and the reagent. Probable mechanistic pathways for the bromination of the two cases are presented. A rate law is derived for the electrophilic γ-substitutions in metal(III) chelates.