Kinetics and mechanisms of the oxidation of hydrazinium ion (N2H5+) by ICIand IBr; Determination of the protonation constant of N2H5+

The kinetics of N2H5+ oxidation by ICI and IBr in 0.1-1.0 M [H+] is investi gated by following the formation of I2Cl- and I2Br- with excess ICl2- and I Br2-, respectively, when total hydrazine is the limiting reactant. Highly a cidic solutions are needed to suppress the rates of N2H4 reactions and to a void hydrolysis of the interhalogens. Protonation constants of N2H5+, K-P2 = [N2H62+]/[N2H5+][H+], measured spectrophotometrically at 25.0 degrees C w ith picric acid as an indicator, are 1.61 M-1 (mu = 0.50 M) and 1.72 M-1 (m u = 1.00 M). In the proposed mechanisms, ICl and IBr react by an IC transfe r process to N2H5+ with loss of H+ (k(1)) to form a steady-state species, I N2H4+, that eliminates I- and H+ to give N2H3+ (k(2)) Subsequent reactions consume a second interhalogen, as diazine (N2H2) is rapidly oxidized to N-2 . Rate constants (25.0 degrees C, mu = 1.00 M) for the ICl/N2H5+ reaction a re k(1) = 4.12 x 10(6) M-1 s(-1), k(-1)/k(2) = 1.9 M-2 and for the IBr/N2H5 + reaction are k(1) = 7.6 x 10(4) M-1 s(-1), k(-1)/k(2) = 28 M-2 The relati ve rates of oxidation of N2H5+ are ICl much greater than IBr much greater t han I-2.