A study of substituent effect on the oxidative strengths of sodium salts of N-bromo-arylsulphonamides:Kinetics and mechanism of oxidation of D-fructose and D-glucose in alkaline medium

摘要

N-Bromo-arylsulphonamides of different oxidizing strengths are used for studying the kinetics of oxidation of D-fructose and D-glucose in aqueous alkaline medium.The results are analysed and compared with those from the sodium salts of N-bromo-benzenesulphonamide and N-bromo-4-methyl-benzenesulphonamide.The reactions show zero-order kinetics in [oxidant],fractional order in [Fru/Glu] and nearly first order in [OH~-].Rates of oxidation of fructose are higher than those for glucose with the same oxidant.Similarly,E_a values for glucose oxidations are higher than those for fructose.The results are explained by a suitable mechanism and the related rate law is deduced.The effective oxidising species in the reactions of N-bromo-arylsulphonamides is Br~+.The oxidative strengths of the latter therefore depend on the ease with which Br~+ is released from them.The ease with which Br~+ is released from N-bromo-arylsulphonamides depends on the electron density on the nitrogen atom of the sulphonamide group,which in turn depends on the nature of the substituent on the benzene ring.The validity of the Hammett equation has also been tested for oxidation of both fructose and glucose.Enthalpies and entropies of activations of the oxidations by all the N-bromo-arylsulphonamides correlate well.The effect of substitution on E_a and log A of the oxidations is also considered.