Kinetics and mechanisms of the oxidation of hydrazinium ion (N2H5+) by aqueous Br2, Cl2, and BrCl. Electrophilicity scale for halogens and interhalogens.

Very rapid oxidations of N2H5+ by Br2, Cl2, and BrCl are measured by stopped-flow and pulsed-accelerated-flow methods in acidic solutions with excess 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 Br2, Cl2, and BrCl, respectively. The reactions are postulated to proceed by nucleophilic reaction of N2H5+ with XY electrophiles (XY = Br2, Cl2, BrCl) to form XN2H4+ with Y- and H+ release in the rate-determining step. In the subsequent reactions, we propose that XN2H4+ eliminates X- and H+ rapidly to form N2H3+ and diazine, N2H2, which is oxidized by a second Br2, Cl2, or BrCl to form N2 in fast steps. The stoichiometries are measured and confirmed to be 1:2 for the Cl2 and BrCl reactions. The relative reactivities for the oxidation of N2H5+ by halogens and interhalogens (BrCl > Cl2 > Br2 > ICl >> IBr >> I2) are used to establish an electrophilicity scale (EXY) for this type of reaction in aqueous solution.