Acid strength isotope effects

The log rate versus acid strength curve for the latter compound is of the exact form expected for reactions of the free base, whilst that of the former compound is intermediate between this form and that obtained for the nitration of aniline and phenyltrimethylammonium ion, i.e. compounds which react as positive species. That these compounds react mainly or entirely via the free base is also indicated by the comparison of the rate coefficients in Table 8 with those in Table 5, from which it can be seen that the nitro substituent here only deactivates weakly, whilst the chloro substitutent appears to activate. In addition, both compounds show a solvent isotope effect (Table 9), the rate coefficients being lower for the deuterium-containing media, as expected since the free base concentration will be lower in these. 

The relation between the acid strength of the catalysts and the mechanism has also been demonstrated by correlations [55,123] of the reaction parameter, p, of the Taft equation for the dehydration of secondary alcohols on A1203 + NaOH, Zr02, Ti02 and Si02 (see Table 4) with the sensitivity to pyridine poisoning, the heat of adsorption of water and diethylether and the kinetic isotope deuterium effects (Table 3) on the same catalysts (Fig. 5). The parameter p reflects the mechanism being... 

The isotope effect can be characterized by the ratio of the dissociation constants (KhIKo). For example, it was found experimentally that for OH-, NH-, SH-, and CH-acids is less than Kh Kh/Ku 2.5-3.9), that is, the strength of these acids is less in D2O than in H2O and the weaker acids exhibit higher Xh/Xd values. 

The kinetics of oxidation of dopamine (DPM) bybromamine-B (BAB)has been studied in HCl medium catalyzed by Ru(III) chloride and in NaOH medium with Os(VIIl) as catalyst at 298 K. Despite the fact that the stoichiometry and oxidation products are same in both catalyzed reactions, the kinetic patterns are different. In acid medium the rate shows a first order dependence each on BAB, DPM and Ru(in) and fiactional order on iC concentrations. In alkaline medium, the rate is first order each on [BAB]o and [DPM]o, a firactional order on [Os(VIII)] and an inverse first order on [OH ]. The effects of addition of benzene sulfonamide and halide ions as well as variation of ionic strengths and dielebic constant of the medium did not show significant effect on the rate of the reaction. Solvent isotope effects has been studied in D2O medium. Activation parameters have been calculated. The catalytic effeciencyis Os(VIII) Ru(III). The proposed mechanisms and the derived rate laws are in consistent with the observed kinetics. 

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