Kamal H. Halawani

Ionic solvation in water + co-solvent mixtures: Part 19. Free energies of transfer of single ions from water into mixtures of water with polyhydroxy compounds: an examination of the assumptions used in determining ΔGt⊖ (i)

Abstract The assumptions underlying the spectrophotometric solvent-sorting method for determining Δ G t XXX ( i ) for individual ions from water into water-rich water + co-solvent mixtures have been critically examined. Additions of the hydrophilic co-solvents glucose, mannitol, sorbitol and inositol to water support the original view, derived from the effect of additions of glycerol, that the main assumption holds at least for x 2 ≲ 0.25 and for some with x 2 ≲ 0.35. Errors in criticisms of this method are specified and the extent of the applicability of the TATB method for producing Δ G t XXX ( i ) is examined. Free energies of transfer for H + , Cl - , Na + , K + and OH - have been calculated from E XXX data and vapour pressures for these hydrophilic co-solvents where available. These are compared with values obtained using additions of hydrophobic alcohols and aprotic cosolvents.

Ionic solvation in water–Co-solvent mixtures. Part 18.—Free energies of transfer of single ions from water into water–2-methoxyethanol mixtures

ΔG°t(H+), the free energy of transfer of the proton from water into water–co-solvent mixutres, has been determined experimentally for the co-solvent 2-methoxyethanol keeping the conditions water-rich. Specific criticisms of the method used are discussed and rejected . ΔG°t(X–) for simple anions has been calculated from ΔG°t(HX) leading to the calculation of ΔG°t(M+) from ΔG°t(MX). ΔG°t(i)e, the free energy of transfer of the charge alone, has been determined for organic cations and anions using ΔG°t(H+) found here, combined with values for the pK of acids and bases in water–2-methoxyethanol. ΔGintt, the difference in the partial molar free energy of interaction of a molecule with the solvent in transferring the molecule from water into the mixture has been determined for two electrically neutral organic bases. The values for ΔG°t(i), ΔG°t(i)e and ΔGintt(i) are compared among themselves and with values found in mixtures of water with other co-solvents.

Ionic solvation in water–cosolvent mixtures. Part 14.—Free energies of transfer of single ions from water into water–ethylene carbonate and water–propylene carbonate mixtures

The spectrophotometric solvent-sorting method for determining the free energies of transfer for individual ions, ΔG°t(i), from water into a mixture of water and cosolvent has now been applied to cosolvents which increase the dielectric constant and enhance the solvent structure when added in small concentrations to water. The spread of ΔG°t(i) values found, viz.ΔG°t(i) for i= a cation is negative and ΔG°t(i) for i= an anion is positive, is similar to those found in mixtures where the cosolvent decreases the dielectric constant and enhances the solvent structure or increases the dielectric constant and decreases the solvent structure. Values for ΔG°t(i)e, the free energy of transfer of the charge alone, are found for some large organic anions.

Kinetics of the solvolysis of [Co(Rpy)4Cl4]+ with R = 4-t-Bu, 3-Me or 3-Et in mixtures of urea with water

SummaryThe kinetics of the solvolysis of complex ions trans-[Co(Rpy)4Cl2]+, with R = 4-t-Bu, 3-Me and 3-Et, have been investigated in mixtures formed by adding urea to water, which enhances the dielectric constant and decreases solvent structure. Differential effects of the changes in solvent structure on the initial and transition states are found to be important factors controlling changes in the rate constant with solvent composition. The variation of the enthalpy and the entropy of activation with solvent composition are contrasted with their variations found for the solvolysis of [Co(Rpy)4Cl2]+ in mixtures where solvent structure is enhanced by additions of a co-solvent to water. The application of a free energy cycle to the process of the initial state going to the transition state suggests that the Co3+ cation in the transition state is more stable than the Co3+ cation in the initial state in the water + urea mixtures.

Kinetics of the solvolysis of a cationic and of an anionic chlorocobalt(III) complex in water–glucose mixtures: anomalous effects on the enthalpy and entropy of activation

The kinetics of the solvolysis of the ions [Co(4-Butpy)4Cl2]+ and [Co(CN)5Cl]3– have been investigated in a range of water-rich mixtures of water with hydrophilic glucose over a range of temperatures. The variations of the enthalpies and entropies of activation with solvent composition show extrema such as found in mixtures of water with hydrophobic co-solvents even though the extrema found in the physical properties of the latter mixtures are not present in those of the mixtures of water with glucose. The application of a Gibbs energy cycle to the process of the initial state going to the final state is compared for both complexes with the results from the same cycle applied using both hydrophilic and hydrophobic solvents mixed with water. Although there are differences between the behaviour of the two complexes with varying co-solvent, in all cases the emergent solvated cobalt ion in the transition state is more stable in the mixtures than the cobalt ion in the initial state.

Kinetics of the solvolysis of trans-dichlorotetra(4-t-butylpyridine)cobalt(III) ions in water and in water–propan-2-ol mixtures

[Co(4Butpy)4Cl2]Cl has been prepared to investigate the solvolysis kinetics of a complex which presents a largely hydrocarbon or hydrophobic surface to the solution in water and in water-rich mixtures of water with a co-solvent, propan-2-ol, which has a hydrophobic effect in water. Non-linear plots for log k(k= first-order rate constant) against the reciprocal of the dielectric constant and the Grunwald–Winstein Y function are found. Although the free energy of activation changes little with solvent composition, the enthalpy and entropy of activation each have a compensating broad extremum at a mole fraction of propan-;2-ol x2≈ 0.04–0.15. From the application of a free-energy cycle to the free energy of activation, it is found by comparison with data for other complexes that [ΔG°t(cation in the transition state)–ΔG°t(cation in the initial state)] is largely independent of the ‘hydrocarbon content’ of the coordination shell (ΔG°t= free energy of transfer water → mixture) for any particular co-solvent, but does vary with the identity of the co-solvent.

Kinetics of the solvolysis of chloropentacyanocobaltate(III) ions in water and in water–methanol mixtures

Rates of solvolysis of the anion [Co(CN)5Cl]3– have been investigated in water and in a range of water–methanol mixtures over a range of temperatures. The variations of the rate constants and the enthalpy and entropy of activation with solvent composition are compared with changes in the physical properties of water–methanol mixtures. The application of a free-energy cycle to the dissociative process of the initial state [Co(CN)Cl3–5] going to the transition state [Co(CN)2–5⋯Cl–] in water and in the mixtures suggests that changes in solvent structure in going from water into the mixture have a stabilising influence on the emergent Co(CN)2–5 ion in the transition state greater than that on the Co(CN)Cl3–5 ion in the initial state. These variations are compared with the variations of similar properties found earlier for the solvolyses of complex cations of Co3+ in mixtures of methanol and of other alcohols with water.

Ionic solvation in water + co-solvent mixtures Part 23. Free energies of transfer of single ions from water into water + diethylene glycol mixtures

Abstract The spectrophotometric solvent sorting method for determining the free energy of transfer of the proton Δ G t ⊖ (H + ) from water into mixtures of water with a co-solvent has been applied to mixtures of water with diethylene glycol. These values for Δ G t ⊖ (H + ) have been used to calculate Δ G t ⊖ (X − ) for X − = Cl − , Br − and I − from the data for Δ G t ⊖ (HX). The variation of these values of Δ G t ⊖ ( i ) with solvent composition obtained for single ions i in these mixtures is compared and contrasted with similar variations which have been found for Δ G t ⊖ (i) for mixtures of water with hydrophobic alcohols and for mixtures of water with other multi-hydroxy co-solvents and alkoxyethanols by correlating the different variations with the physical properties of the mixtures. To aid this, the densities of water + diethylene glycol mixtures have been measured and the relative partial molar volumes of water and glycol in the mixtures have been calculated.

Preparation and chlorination of cobalt(II) and nickel(II) complexes of 3-hydroxypyridine

Summary[MCl2L4] complexes (M = Co or Ni; L = 3-hydroxypyridine) were prepared by the reaction of stoichiometric amounts of 3-hydroxypyridine and MCl2 in EtOH solution. The complexes were chlorinated by passing Cl2 gas through EtOH solutions containing the metal(II) chloride and 3-hydroxypyridine to yield [CoCl2(LCl)2]·2H2O and [NiCl2(LCl)4]·4H2O, respectively. Structural assignments have been inferred from elemental and spectral analyses. Magnetic properties of the complexes are assigned and the electronic transitions are discussed.

Kinetics of the solvolysis of chloropentacyanocobaltate(III) ions in water + t-butanol mixtures

SummaryRates of the solvolysis of [Co(CN)5Cl]3− ions have been determined in mixtures of water with the hydrophobic alcohol, t-butanol over a range of temperatures. No linear correlation of log k with the reciprocal of the dielectric constant is found, suggesting that changes in solvent structure are an important factor influencing these rates. This result is confirmed by the good correlation found for the extrema in the enthalpy and entropy of activation with the extrema in the physical properties of the mixtures influenced by changes in solvent structure. The application of a free energy cycle to the loss of the chloride ion from the Co3+ in the transition state shows that [Co(CN)5]2− in the transition state is stabilised in the mixtures relative to [Co(CN)5Cl]3− in the initial state.