The oxidation of carbohydrazide by the 12-tungstocobaltate(III) ion in acidic medium: Kinetics and mechanism☆
Graphical abstract
The redox reaction between the 12-tungstocobaltate(III) ion and carbohydrazide is first order with respect to both the oxidant and the substrate. The observed pseudo first-order rate constant, kobs, is retarded by increasing the concentrations of H+ and alkali metal ion (Li+, Na+ and K+). There is a linear correlation between the kobs and the concentrations of carbohydrazide and H+ ion, but the plots of kobs against the concentrations of the alkali metal ions is non-linear. However, the same data is applicable to the Davies equation for the effect of the ionic strength on the kobs.
Complex formation between the 12-tungstocobaltate(III) ion and carbohydrazide is indicated by optical density measurements, which increased with increasing H+ concentration in the reaction mixture. This indicated that +H3NNHCONHNH2 probably formed a complex with the 12-tungstocobaltate(III) ion. A suitable mechanism involving deprotonation of the complex and subsequent decomposition of the deprotonated complex as the rate limiting step is suggested.
Introduction
The chemistry of hydrazine and its derivatives is of interest due to their wide application in biological activities [1], [2], [3], [4]. The investigations with these potential reagents have been mostly focused on structural aspects of their metal complexes in the solid state. The mechanistic aspects of their reactions in solution are not well established. In recent years some efforts have been directed towards the latter aspects. The oxidation kinetics of carbohydrazide(CHZ) by chloramine-T [5], bromamine-T [6], N-chloro and N-bromo benzenesulfonamide [7] have been reported.
Section snippets
Materials
The solutions were generally prepared fresh in twice distilled water. The solutions were standardised by known analytical methods. The solutions of LiCl (Sigma, LR), NaCl (Sarabhai M, GR) and KCl (Ranbaxy, AR) were prepared by weighing the respective sample used as received. The stock solution of LiClO4, used to adjust the ionic strength (μ), was prepared by neutralising lithium hydroxide(Sigma, LR) by perchloric acid (Merck, GR) and standardised as described [8]. The standard solution of
Spectral changes with [H+]
In another series of experiments, several reaction mixtures having 105[Co(IIIW)5−] = 9.4 and 103[H2NHNCONHNH2] = 1.0 mol dm−3 were prepared at different [H+] (between 0.001 and 0.07 mol dm−3) in a manner analogous to that described above. The optical densities of these reaction mixtures are shown in Fig. 4 wherein it is noted that the optical density increases with increasing H+ concentration, indicating that the species +H3NHNCONHNH2 forms the complex, which is consistent with the protonation constant
Acknowledgements
The financial grants from UGC (F.12-59/1997) and (F.12-147/2001) are acknowledged with thanks. Thanks are also due to the Department of Science and Technology (DST) for sanctioning the grant (2/17/83/STP-III) for the purchase of the stopped-flow instrument. Thanks are also due to Kriti Mehrotra, my grand daughter, at Cornell University, in helping with certain references.
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This work is supported by the UGC vide Grant F.12-59/1997 and F.12-147/2001.