Kinetics and mechanism of the complex bromate-iodide and bromate-iodine reactions in acidic medium

Mixtures of bromate, iodide and iodine exhibit some of the most complex and yet fascinating examples of interhalogen chemistry. The kinetics and mecha nism of the bromate-iodide system is well documented. Little work has been done on the bromate-iodine reaction. In this paper we report mechanistic st udies of the bromate-iodine reaction and mixtures of bromate, iodide and io dine. In excess bromate the stoichiometry of the bromate-iodide reaction ha s been previously shown to be BrO3- + 6I(-) + 6H(+) --> Br- + 3I(2) (aq) +3 H(2)O, while that of the bromate-iodine reaction is 2BrO(3)(-) + I-2 (aq) - -> 2IO(3)(-) + Br-2 (aq). Although the above reactions are both thermodynam ically feasible, in mixtures of the three reactants, it appears that there is some switching mechanism which allows only one of the reactions to proce ed at a time. The bromate-iodide reaction goes to completion first. The bro mate-iodine reaction then takes over. The consumption of iodine is accompan ied by the rapid formation and depletion of the interhalogen intermediate i odine bromide, IBr and then formation of bromine. We present a 17-step mech anism which encompasses both reactions. We have extensive experimental data to support the proposed mechanism.