Volume 3, Issue 1

The volumes of activation for the spontaneous, base-, and acid- catalysed path of the hydrolysis reaction of a series of trans-[Co(MeNH 2 )(NH 3 ) 4 X]( 3-n)+ ions (X = Cl - , Br - , (ONO 2 ) - (OSO 3 ) 2- ) have been determined in order to establish analogies with the dissociative trends found in previous work with the spontaneous hydrolysis of neutral ligands from the same cores. While for the base catalysed path a significant decrease in the activation volume is found on going from the {Co(NH 3 ) 5 } to the trans- {Co(MeNH 2 )(NH 3 ) 4 } inert skeleton (i.e. 9.8, 12.5, 4.0 and 9.1 cm 3 mol -1 for the chloro, bromo, nitrato and sulfato derivatives), no significant changes are observed for the same complexes in the spontaneous reaction. The trends are rationalized in terms of the important changes occurring in electrostriction factors for the D CB and I d intimate mechanisms operating and the important increase in the degree of dissociativeness due to the presence of a trans-methylamino ligand. For the acid catalysed path the differences are much more difficult to assess, specially taking into account the limited information available as well as the inherent errors involved in the rate constant determination.

Acid dissociation constants for the aminoglycosides neomycin and neamine and of 2-deoxystreptamine are reported. Solution NMR studies indicate that complexation with copper(II) and zinc(II) involves principally the primary amine donors, and formation constants with both neamine and 2-deoxystreptamine (2-DOS) and these metal ions are reported (at 25°C, I = 0.5 mol dm -3 ). Complexation studies probed by electrospray mass spectrometry identify both 1:1 and 1:2 species for Cu(II) and Zn(II) with 2-DOS, but only 1:1 species with neamine, consistent with behaviour in bulk solution. Preliminary formation kinetics of Cu(II) with neamine indicate rapid complexation at high pH but slower two-step processes near pH 7, consistent with complications in the thermodynamic determinations in the intermediate pH regime. Reaction of 2-DOS and neamine with Cu(II) in methanol appears as a single process, with the former occurring ~ 103-fold faster

The kinetics of oxidative deamination and decarboxylation of L-asparagine (L-Aspg) by aqueous alkaline permanganate at constant ionic strength of 0.50 mol dm -3 were studied spectrophotometrically. The reaction exhibits first-order kinetics in [permanganate ion] and fractional-order dependences in [L-Aspg] and [alkali]. Initially added products such as aldehyde, ammonia and manganate have no significant effect on the rate of reaction. An increase in ionic strength and a decrease in dielectric constant of the medium increase the rate. The oxidation process in alkaline medium has shown to proceed via two paths: One a substrate dependent path, the other a substrate independent path. The constants involved in the mechanism were evaluated. There is a good agreement between observed and calculated rate constants. The activation paramaters with respect to slow step of path I and path II were calculated.

The kinetics of Mg 2+ incorporation into the three water soluble porphyrin isomers of the relatively planar cationic tetrakis(N-Methyl-X-pyridyl)porphyrins (X = 2,3, or 4) were studied from pH 6.5 to 8.9 at 25 °C, I = 2.6. The reactions were first order in both porphyrin and magnesium ion concentrations, and the specific rate constants increased with an increase in pH. The proposed mechanism involves Mg 2+ reacting with the differing centrally protonated forms of these porphyrins, in the order Ρ 2- > H-P - " > H 2 -P. Various other porphyrins were substantially less reactive. In contrast, Mg reacts orders of magnitude faster with the non-planar ß-octabromo-tetra(N-methyl-4-pyridyI) porphyrin, where the reaction proceeds almost exclusively through the Mg 2+ + P 2- pathway. As compared to other metal ions, the relatively slow incorporation of Mg 2+ into porphyrins is in part a consequence of the comparatively slow water exchange rate constant for the aquo Mg 2+ ion.

The kinetics and mechanism of Cr(VI) oxidation of D-glucose in the presence and absence of picolinic acid (PA) in aqueous acid media have been carried out under the conditions, [D-glucose] T ⋟ [Cr(VI)] T at different temperatures. Under the kinetic conditions, HCrO 4 - has been found kinetically active in the absence of PA while in the ΡΑ-catalysed path Cr(VI)-PA complex has been established as the active oxidant. In the ΡΑ-catalysed path, Cr(VI)-PA complex receives a nucleophilic attack by the substrate to form a ternary complex which subsequently experiences a redox decomposition (through 2e transfer) leading to lactone (oxidised product) and Cr(IV)-PA complex. Then Cr(IV)-PA complex participates further in the oxidation of D-glucose and ultimately is converted into Cr(III)-PA complex. In the uncatalysed path, Cr(VI)-substrate ester experiences an acid catalysed redox decomposition (2e transfer) at the rate determining step. The uncatalysed path shows a second-order dependence on [H + ], Both the paths show first-order dependence on [D-glucose] T and [Cr(VI)] T . The ΡΑ-catalysed path is first-order in [PA] T . These observations remain unaltered in the presence of externally added surfactants. Effect of cationic surfactant (i.e. cetylpyridinium chloride, CPC) and anionic surfactant (i.e. sodium dodecyl sulfate, SDS) on both the uncatalysed and ΡΑ-catalysed path has been studied. CPC inhibits both the uncatalysed and ΡΑ-catalysed path while SDS catalyses the reactions. The observed micellar effects have been explained by considering the hydrophobic and electrostatic interaction between the surfactants and reactants. Applicability of different kinetic models, e.g. pseudo-phase ion exchange (PIE) model, Menger-Portnoy model, Piszkiewicz cooperative model, has been tested to explain the observed micellar effects. Effect of [surfactant] T on the activation parameters has been explored to rationalise the micellar effect.

The oxidation of antimony(III) by tetrachloraurate(III) has been studied in aqueous hydrochloric acid medium at 27°C. The reaction follows the rate law ks = k 1 K 1 K 2 K 3 K 4 [Cl]/[H 3 O + ]([H 3 O + ] + K 1 ) (i) (i) where K 1 ;K 2 ,K 3 and K 4 are the respective equilibrium constants of the reactions (ii) to (v) and kj is the rate constant of the slow complementary two HAuCl 4 + H 2 O ⇄ AuCl 4 - + H 3 O + K 2 (ii) AuCl 4 - +H 2 O⇄ AuCl 3 (OH) - + H 3 O + + Cl - K 2 (iii) SbCl 4 - + Cl - ⇄ SbCl 52- K 3 (iv) SbCl 5 2- + Cl - ⇄ SbCl 6 3- K 4 (v) electron transfer step of the reaction between the active species AuCl 3 (OH) - and SbCl 6 3- of the reactants. The enthalpy and entropy of activation were found to be 19.2 ± 1 kj mol -1 and - 182.5 ± 4 JK -1 mol -1 respectively,suggesting a chorobridged electron transfer.