Abstract
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) + 2 2 + -*- N4HS2+ (or 3 2+ + NH4+) + 2Mn(II) + 2H+ (or + H+), 4Mn(III) + 2CH3N2H4 + - (CH3)2N4H42+ + 4Mn(II) + 4H+, 2Mn(III) + 2NN(CH3)2N2H3+ - (CH3)4N4H42+ + 2Mn(II) + 2H+, 4H20 + 8Mn(III) + 2NN'(CH3)2- N2H3+ -* 2CH3N2H302+ + 8Mn(II) + 6H+ + 2CH3OH, 2Mn(III) + 2(CH3)3N2H2+ — (CH3)6N4H22+ + 2Mn(II) + 2H+, and 2H20 + 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 [Mn(III)]/ [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)], [NaC104], [NaNOs], ionic strength, wavelength, and the presence of 02 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 tetra- methylhydrazine. Letting the primed rate constants designate reaction with MnOH2+, the rate constants for the rate-determining steps are NH3NH2+, ki £; 5.2 X 102 M~l sec-1, hi = 10.3 X 103 M~l sec-1; CH3- NH2NH2+, h < 8.0 X 101 sec-1, h' = 3.0 X 103 Hi-1 sec-1; (CH3)2NHNH2+, h 1.3 X 102 Af-1 sec-1, h' = 4.0 X 10= M~l sec-1; CH3NH2NHCH3+, h 4.0 X 101 If-1 sec-1, h1 = 7.3 X 102 M~l sec-1; (CH3)2NHNHCH3+, h ^ 2.0 X 1011¥-1 sec-1, h' = 4.3 X 102M-1 sec-1; (CH3)2NHN(CH3)2+, h = 2.8 X 102 M~l sec-1, k\ = 3.0 X 101 M~l sec-1, based on the known value of the acid-dissociation constant of Mn8+. The relative error in these rate constants is estimated at ±10%. The acid-dissociation constant of Mn3+ derived from a graphical kinetic analysis (Ku = (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 maybe inferred from a consideration of observed stoichiometry variations.