The stopped-flow apparatus has been used to determine the kinetics of reactions between manganese(III) and hydrazine and the methylhydrazines in acid (0.50-3.60 M) perchlorate media at 25°. The stoichiometries of these reactions were shown, using standard analytical techniques, to be 2Mn(III) + 2N2H5+ → N4H82+ (or N3H52+ + NH4+) + 2Mn(II) + 2H+ (or + H+), 4Mn(III) + 2CH3N2H4+ → (CH3)2N4H42+ + 4Mn(II) + 4H+, 2Mn(III) + 2NN(CH3)2N2H3+ → (CH3)4N4H42+ + 2Mn(II) + 2H+, 4H2O + 8Mn(III) + 2NN′(CH3)2-N2H3+ → 2CH3N2H3O2+ + 8Mn(II) + 6H+ + 2CH3OH, 2Mn(III) + 2(CH3)3N2H2+ → (CH3)9N4H22+ + 2Mn(II) + 2H+, and 2H2O + 2Mn(III) + 2(CH3)4N2H+ → (CH3)6N4H22+ + 2Mn(II) + 2H+ + 2CH3OH in the concentration range used for kinetic studies. Determinations of stoichiometry outside the kinetic range showed that in some cases the ratio Δ[Mn(III)]/Δ[hydrazine] is a function of the ratio of the initial concentrations of reactants. All reactions were second-order overall and first-order in each reactant. The reactions were also independent of initial concentration of reactants, [Mn(II)], [NaClO4], [NaNO3], ionic strength, wavelength, and the presence of O2 with excess of hydrazine. The observed rate constant was dependent on acidity. A mechanism consistent with these results was postulated in which the rate-determining steps involve the formation of protonated hydrazoyl radicals. The predominant rate-determining step is the reaction between MnOH2+ and the monoprotonated hydrazine molecules for all substrates except tetramethylhydrazine. Letting the primed rate constants designate reaction with MnOH2+, the rate constants for the rate-determining steps are NH3NH2+, k1 ≤ 5.2 × 102 M-1 sec-1, k1′ = 10.3 × 103 M-1 sec-1; CH3-NH2NH2+, k1 ≤ 8.0 × 101 M-1 sec-1, k1′ = 3.0 × 103 M-1 sec-1; (CH3)2NHNH2+, k1 ≤ 1.3 × 102 M-1 sec-1, k1′ = 4.0 × 103 M-1 sec-1; CH3NH2NHCH3+, k1 ≤ 4.0 × 101 M-1 sec-1, k1′ = 7.3 × 102 M-1 sec-1; (CH3)2NHNHCH3+, k1 ≤ 2.0 × 101 M-1 sec-1, k1′ = 4.3 × 102M-1 sec-1; (CH3)2NHN(CH3)2+, k1 = 2.8 × 102 M-1 sec-1, k1′ = 3.0 × 101 M-1 sec-1, based on the known value of the acid-dissociation constant of Mn3+. The relative error in these rate constants is estimated at ±10%. The acid-dissociation constant of Mn3+ derived from a graphical kinetic analysis (KH = (1.1 ± 0.3) M) is in reasonable agreement with previous estimates. No nitrogen is formed as a product for any of the reactions studied; the reaction products could not be characterized, but may be inferred from a consideration of observed stoichiometry variations.
