The kinetics of the hydride-transfer reactions from bis [4-(dimethylamino)phenyl] methane bis[4-(MH 2 ), (dimethylamino)phenyl] methoxy methane (MHOMe), Leuco Malachite Green (MGH) and Leuco Crystal Violet (CVH) to 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ) have been investigated spectrophotometrically by means of the stopped-Ñow technique. The negative activation enthalpies were obtained for the hydride-transfer reactions from MHOMe and MGH to DDQ in dichloromethane. MH 2 , These negative activation enthalpies suggest that the chargeÈtransfer (CT) complexes are true intermediates in the hydride-transfer reactions from these donors to DDQ, as previously pointed out for system in MH 2 ÈDDQ trichloroethane. The formation of chargeÈtransfer complexes between MHOMe, MGH and CVH and MH 2 , p-chloranil (CA) were observed in EPA (ethyletherÈpentaneÈethanol) at low temperatures. The oxidation potential of MHOMe, MGH and CVH were estimated using the linear correlation between the CT (E ox ¡ ) transition energy and the value for the known CT complexes between CA and N,N-dimethylaniline E ox ¡ derivatives. The rates were found to be correlated to the values of the donors.

The reaction kinetics and mechanism of the oxidations of leucomethylene blue (MBH) and leucothionine (TH) by iron (III) sulfate in aqueous solution were studied spectrophotometrically by the stopped-flow technique. MBH and TH, which were freshly prepared by photoreduction of methylene blue and thionine, respectively, with ascorbic acid were used in the kinetic measurements. The pseudo-first-order rate constants (k obsd ) show kinetic saturation at high initial concentrations of iron (III) sulfate for MBH and TH. It was found that the reciprocal of k obsd increases linearly with increase in the reciprocal of [Fe 3 ] 0 . A broad absorption band was observed on mixing MBH and Fe 3 solutions at low temperatures, and this was attributed to a charge-transfer complex between MBH and Fe 3 . The effects of Fe 2 ion and L-()-ascorbic acid on the rates of oxidation were also investigated. A small kinetic isotope effect on the oxidation rate for MBH was observed. The results can be explained by a general mechanism with stepwise electron-proton-electron transfers with the formation of a complex between reactants.

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The kinetics of the hydride transfer reaction between leuco methylene blue and thionine was investigated spectrophotometrically by means of the stopped-Ñow technique. Leuco methylene blue and leuco thionine were produced by photoreduction of methylene blue and thionine with triethylamine in ethanol in the drive syringe of the stopped-Ñow apparatus and their solutions were used as such in the measurement of the reaction rate. This reaction was found to be reversible, and the rate constants for forward and backward reactions were obtained. The ratio of is consistent with the value of the equilibrium constant (K) which was obtained k f /k b from the absorbance at 655 nm of methylene blue at equilibrium using the stoichiometric relationships and conservation conditions. The mechanism of this reversible hydride transfer reaction was discussed.

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