PK a of the hydrazinium ion and the reaction of hydrogen atoms with hydrazine in aqueous solution

Abstract

The techniques of electron pulse radiolysis and paramagnetic resonance free induction decay (FID) attenuation measurements have been used to determine Arrhenius parameters for the reaction of the hydrogen atom with N2H5+ and N2H4 in aqueous solution. At 41.6 °C, a scavenging rate constant of (5.8 ± 1.9)× 106 dm3 mol–1 s–1 was directly measured for the N2H5+ reaction, with a corresponding activation energy of 61.4 ± 1.2 kJ mol–1 over the temperature range 41.6–82.1 °C. The pKa of the hydrazinium ion was determined by absorption spectroscopy, and over the temperature range 0–80 °C, this equilibrium was well described by pKa=(2600 ± 30)/T–(0.771 ± 0.099). Using these values, and FID attenuation measurements in borax buffered solution over the temperature range 0.2–83.0 °C, the hydrogen atom rate constant for N2H4 reaction was calculated to be (6.74 ± 0.23)× 107 dm3 mol–1 s–1(24.5 °C), with an activation energy of 16.28 ± 0.80 kJ mol–1. The reaction rate of ˙H with N2H4 is consistent with a hydrogen atom abstraction, giving H2 and ˙N2H3 radical as products, as in the gas phase. Based on the large activation enthalpy and entropy obtained for the reaction of hydrogen atoms with N2H5+, however, we suggest that this is more likely an addition–fragmentation process, to give ˙NH2, NH3 and (H+)aq as products.