Kinetics and mechanisms of the oxidation of hydrazinium ion (N2H5+) by aqueous Br-2, Cl-2, and BrCl. Electrophilicity scale for halogens and interhalogens

Very rapid oxidations of N2H5+ by Br-2, Cl-2, and BrCl are measured by stop ped-flow and pulsed-accelerated-flow methods in acidic solutions with exces s N2H5+. Second-order rate constants (M-1 s(-1)) at 25.0 degrees C, mu = 1. 0 M are 1.49 x 10(7), 1.01 x 10(8), and 5.6 x 10(8) for the reactions with Br-2, Cl-2, and BrCl, respectively. The reactions are postulated to proceed by nucleophilic reaction of N2H5C With XY electrophiles (XY = Br-2, Cl-2, BrCl) to form XN2H4+ With Y- and H+ release in the rate-determining step. I n the subsequent reactions, we propose that XN2H4+ eliminates X- and H+ rap idly to form N2H3+ and diazine, N2H2, which is oxidized by a second Br-2, C l-2, or BrCl to form N-2 in fast steps. The stoichiometries are measured an d confirmed to be 1:2 for the Cl-2 and BrCl reactions. The relative reactiv ities for the oxidation of N2H5+ by halogens and interhalogens (BrCl > Cl-2 > Br-2 > ICI >> IBr >> I-2) are used to establish an electrophilicity scal e (E-XY) for this type of reaction in aqueous solution.