Amel M. Ismail

Kinetics and mechanism of isatin ring opening in aqueous binary mixtures of methanol and acetonitrile cosolvents

Solvent effects on the kinetics of hydrolysis of isatin by sodium hydroxide have been investigated within the temperature range (30–55°C) in methanol-water and acetonitrile-water media of varying solvent compositions up to 70% (v/v) of the organic solvent component. The thermodynamic activation parameters were calculated and discussed in terms of solvation effects. The determined isokinetic temperatures, in both systems, revealed the existence of compensation effect arising from strong solute-solvent interactions; log k was correlated with both log [H2O] and the reciprocal of the dielectric constant. The first correlation was observed to be linear while the second was nonlinear. Finally a mechanism for the isatin ring opening was proposed, which accounts for the role and the effect of the solvent on the reaction rate.

The solvolysis oftrans-[Co(4-Mepy)4Cl2]ClO4 in water-methanol mixtures

SummaryThe solvolysis of thetrans-[Co(4-Mepy)4Cl2]ClO4 complex was studied in 0 to 70% v/v H2O: MeOH mixtures at 40, 45, 50 and 55 °C. The high negative ΔS* values found for the complex cation under investigation, relative to that oftrans- [Co(py)4Cl2]+ reported in the literature, were attributed to the substituent methyl groups. The free energies of transfer of both the ground and the transition states were calculated from which the dominant effect of the solvent on the transition state is apparent.

Solvent effects on the initial and transition states upon solvolysis oftrans-dichlorotetra(4-ethylpyridine) cobalt(III) perchlorate in water +t-butanol mixtures

SummaryThe solvolysis of the title complex was investigated in water and in water +t-butanol mixtures in the 40 to 55°C range. The activation thermodynamic parameters were calculated and the extrema observed for ΔH† and ΔS† were compared with data obtained from the measured physical properties of the same solvent mixtures. The ethyl substituent in the pyridine rings manifests its effect on the activation entropy values and on the relatively low activation energies compared with the respective values obtained previously from the solvolysis oftrans-[Co(4-MePy)4Cl2]+ in the same media. The free energies of transfer of the cation were calculated in the ground and transition states. In the ground state, the ethyl and methyl substituents in the pyridine rings provide the complex cation with nearly equal stabilities. However, the effect of solvent on the cation stability is more pronounced in the transition state for the ethyl substituent.

Synthesis and characterization of some diacetonitriletetraminecobalt(III) complexes and acid hydrolysis kinetics of cis-[Co(tn)2(CH3CN)2]3+ ion

Abstract A series of octahedral cis -diacetonitriletetraminecobalt(III) perchlorate complexes of the general formula [Co(N 4 )(CH 3 CN) 2 ](ClO 4 ) 3 , where N 4 = (NH 3 ) 4 , (en) 2 , (tn) 2 and tren, were prepared and characterized by elemental analyses and UV-vis spectra. The acid hydrolysis kinetics of cis -[Co(tn) 2 (CH 3 CN) 2 ] 3+ were studied spectrophotometrically in different aqueous perchloric acid concentrations and temperatures (35–55°C) in 0.1 M (NaClO 4 ) ionic strength. 13 C NMR spectra were used to identify the products of the reaction. The primary aquation had occurred during the time of dissolving the complex. The kinetics of aquation of the cis -[Co(tn) 2 (CH 3 CN)(H 2 O)] 3+ ion to the corresponding cis -diaqua species was found to be acid-inhibited. For the acid-independent pathway k 2 (25°C), Δ H 2 ‡ and Δ S 2 ‡ were evaluated as 1.5 x 10 −5 S −1 , 103 kJ mol −1 and 7.5 J mol −1 K −1 , respectively. The results are discussed and compared to other related systems. Evidence for an I d mechanism is given.

Influence of Solvent upon Rates of Ion Pair Reactions: Studies on the Aquation of Bromopentamminecobalt(III) Perchlorate in Binary/Water-Ethan-1,2-Diol Mixtures

The kinetics of the aquation of bromopentamminecobalt(III) ion have been investigated in malonate media containing different percentages of ethan-1,2-diol (up to 50%). The ion pair rate constant kip, has been determined over a wide range of malonate buffer compositions using the Wyat and Davies treatment. The thermodynamic and extrathemodynamic correlations existing between those of the association reaction (ΔG°ass, ΔH°ass, ΔS°ass) and of the ion pair aquation reaction (ΔG≠ip ΔH≠ip, ΔS≠ip) have been investigated. The kinetic solvent effects are examined in terms of the dielectric constant and water concentration effects on the ion pair rate constant. Also, the solvent effects are analysed in terms of Kamlet–Taft solvatochromic parameters and are applied successfully in mixed aqueous – ethan 1,2-diol mixtures. However, the extrema found in the change of ΔH≠ip and ΔS≠ip with the mole fraction of the co-solvent have been correlated well with that found in the change of the physical properties of the solvent–water mixtures with the solvent structure. Small changes of ΔG≠ip with the mole fraction of the solvent are attributed to a quasi-mirror image compensation between ΔH≠ip and ΔS≠ip.

The influence of solvent structure on the solvolysis of trans-dichlorotetra(4-ethylpyridine) cobalt(III) perchlorate

SummaryThe solvolysis oftrans-[Co(4-Etpy)4Cl2]ClO4, was followed spectrophotometrically in water/isopropanol at different temperatures. The activation energy varied nonlinearly with the mole fraction of the co-solvent,χ2. The plot of logk versus Ds−1 was also non-linear. These features were attributed to the differential solvation of the initial and transition states. On plotting ΔH† versus ΔS†, the points fall very close to straight line. The isokinetic temperature was found to be 334K, indicating that the solvolysis reaction is controlled by ΔS† and not ΔH†. The change in ΔH† and ΔS† with the mole fraction of the cosolvent shows extrema at the composition range where changes in solvent structure occur. The influence of the solvent structure on the complex ion in the transition state dominates over that in the initial state, where −ΔGt0[Co(4-Etpy)4Cl]2+>−ΔGt0[Co(4-Etpy)4Cl2]+.

The solvolysis oftrans-[Co(4-Mepy)4Cl2]ClO4 in water: acetonitrile and water: dioxan mixtures

SummaryThe solvolysis of the title complex is slower in 0 to 70% v/v aqueous dioxan than in aqueous acetonitrile at 40, 45, 50 and 55° C, a fact that is attributed to the higher basicity and ionizing power of the latter solvent mixtures. The observed non-linearity for logk (solvolysis) versus 1/D plots was interpreted in terms of the relative stability of ground and transition state species. The calculated Gibbs free energy for transfer of the species in the transition state is consistent with the extreme stretching of the chlorinecobalt bond, a result which confirms the interchange dissociative mechanismId for the complex under study.