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Kinetics and Mechanisms of the Reduction of Some Carbonyl Compounds with Sodium Dithionite in Aqueous Alkaline Medium

P. O. Ukoha, F. N. Okafor


Kinetics of the reduction of some carbonyl compounds (acetophenone, benzophenone, benzaldehyde, and salicylaldehyde) with sodium dithionite was studied at 28± 0.5oC and basic concentration in the range of 0.01 to 0.05 mol dm-3 (NaHCO3). Under these conditions, the reaction rates were found to be first order with respect to sodium dithionite as plot of log kobs versus log [S2O42-] showed linearity to about 95% extent of reaction, however, the overall reaction rate was second order at constant [OH-]. Furthermore, order of reactions with respect to acetophenone, benzophenone, salicylaldehyde, and benzaldehyde was unity as the plot of log(A∞-At) versus time showed about 90% linearity. Reaction rates increased with ionic strength and addition of chloride ions of varying concentration. Michaelis–Menten plot of 1/kobs versus 1/[S2O42-] were linear and have small intercepts on y-axis. Increase in pH (from 1-13) also showed remarkable increase in the reaction rates. Kinetic investigations showed that an outer-sphere reaction mechanism was operational and plausible mechanisms have been proposed for these reactions.
The rate laws for the reactions conform to the equations:
Rate = {k3K1 + k2 [OH-]2} [S2O42-] [RCHO] where RCHO = benzaldehyde and salicylaldehyde Rate = {k3K1 + k2 [OH-]2} [S2O42-] [RCOR1] where RCOR1 = benzophenone and acetophenone

Keywords: carbonyl compounds, dithionite ions, kinetics, mechanism, reduction

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