Ronald A. Schulz

Single-ion gibbs energies, enthalpies and entropies of transfer from water to aqueous methanol based on the (Ph4P+, Ph4As+)= Ph4B– assumption

Enthalpies of solution of 298 K of the electrolytes NaCl, Ph4PCl and NaBPh4 in water and aqueous methanol mixtures are reported. These measurements lead to single-ion enthalpies of transfer based on the assumption that ΔH⊖t(Ph4P+)=ΔH⊖t(Ph4B–). Together with literature values for enthalpies of transfer of electrolytes and additional measurements made in the present work, a set of single-ion ΔH⊖t values for 14 ions is constructed. Literature values for Gibbs energies are used to construct a similar set of ΔG⊖t values based on ΔG⊖t(Ph4P+)=ΔG⊖t(Ph4B–); the corresponding entropies of transfer are also reported. It is shown that for transfer from water to pure methanol transfer values for Ph4P+ are, within experimental error, the same as those for Ph4As+.On the (Ph4P+, Ph4As+)= Ph4B– assumption it is found that ΔG⊖t(H+) is slightly negative for transfer to most aqueous methanol mixtures but is very positive (2.49 kcal mol–1) for transfer to pure methanol. Enthalpies and entropies of transfer of ions do not vary monotonically with solvent composition but reflect changes in solvent structure. There is quite good agreement between the single-ion entropies of transfer and those obtained by the correspondence-plot method for transfer from water to up to 80 wt% methanol mixtures, but not for transfers to 90 wt% methanol mixtures and to pure methanol.

Thermodynamic studies of cryptand 222 and cryptates in water and methanol

Enthalpies of solution in water and in methanol are reported for a number of cryptate electrolytes ([M+222]+X–) and in conjunction with data on X– yield values for the enthalpy of transfer, ΔH°t, of [M+222] ions from water to methanol. Enthalpies of complexing of cations with cryptand 222 have been determined in water and methanol; when combined with known values of ΔH°t(M+) and the presently determined value of ΔH°t(222), they yield, via a thermodynamic cycle, values of ΔH°t for the ions [M+222] where M+= Li+, Na+, K+, Rb+, Cs+ and Ag+. The two methods of obtaining the ΔH°t values are in good agreement with each other.Solubility measurements on the perchlorates of [Na+222] and [K+222] yield values of ΔG°t([M+222]) from water to methanol. These values are also obtained from a thermodynamic cycle involving known values of ΔG°t(M+), the free energies of complex formation in water and methanol and the presently determined value of ΔG°t(222). The direct values and the cycle values are again in good agreement.ΔH°t, ΔG°t and ΔS°t values for transfer of the complexed ions [M+222] vary considerably with the central cation M+(M+= Li+, Na+, K+, Rb+, Cs+ and Ag+) and it is clear that the surrounding cryptand does not isolate the central ion from the environment. Also, various single-ion assumptions that require the constancy of ΔG°t([M+222]) and ΔH°t([M+222]) with M+ are not valid for the water to methanol transfer.Partition coefficients for the hypothetical extraction process M+(aq)+222(aq)→[M+222](methanol) have been obtained and it is shown that by comparison with the simple partition M+(aq)→ M+(methanol), greatly enhanced cation selectivities are observed. The largest selectivity enhancement occurs with the ions Ag+ and Li+, where the complex extraction equilibrium favours the extraction of Ag+ by a factor of 4 × 109 over the simple partition.Ion-pair partition coefficients have also been obtained for the species [Na+222]ClO–4 and [K+222]ClO–4; extraction of the latter is favoured by a factor of 1.6 × 102. This may be compared to a factor of 2.7 × 102 in favour of the potassium salt when extracted as the pair of ions ([M+222]+ ClO–4) and to a factor of 0.61 for extraction as the uncomplexed K+ and Na+ cations.

Heats of solution of 1:1 electrolytes in 1,2- and 1,1-dichloroethane and derived enthalpies and entropies of transfer of electrolytes from water to these solvents

Heats of solution of nine electrolytes in 1,2-dichloroethane and of three electrolytes in 1,1-dichloroethane have been determined calorimetrically at various electrolyte concentrations and extrapolated to zero concentration to yield ΔHso values for these electrolytes. It is shown that values of ΔHto for transfer from water to the dichloroethanes of 1∶1 electrolytes are often negative, so that these electrolytes can be more stable enthalpically in the less polar solvents. Combinations of the ΔHto values with previously determined ΔGto values yield values of ΔSto for transfer of 1∶1 electrolytes from water to the dichloroethanes. These ΔSto values are mostly very negative; they can be correlated very well by the method of Abraham, and in this way ΔSto values for transfer of numerous other anions and cations have been predicted. The Ph4As+/Ph4B− convention yields single-ion entropies of transfer from water to the dichloroethanes in reasonable agreement with values calculated by the correspondence-plot method.

Heats of solution of 1∶1 electrolytes in 1-propanol, and derived enthalpies of transfer from water

Heats of solution of 13 1∶1 electrolytes in 1-propanol have been determined calorimetrically at various electrolyte concentrations, and extrapolated to zero concentration to give ΔHso values for these electrolytes. Together with literature data on three additional 1∶1 electrolytes, these measurements yield a self-consistent set of single-ion enthalpies of transfer from water to 1-propanol. Values are tabulated for 10 univalent cations and five univalent anions. It is shown that the ΔHto (Ph4As+)=ΔHto(Ph4B−) assumption yields chemically reasonable single-ion values. Using this assumption, it may be deduced that all the univalent ions studied have about the same enthalpy in 1-propanol as in methanol.

Hydrogen bonding. Part 1.—Equilibrium constants and enthalpies of complexation for monomeric carboxylic acids with N-methylpyrrolidinone in 1,1,1-trichloroethane

A novel calorimetric method has been derived for the simultaneous determination of equilibrium constants and enthalpies of complexation of monomeric carboxylic acids with bases in an inert solvent. The method requires a knowledge of the corresponding equilibrium constants and enthalpies for the monomer/dimer equilibrium in the same solvent. Both sets of K° and ΔH° values have been obtained for a number of carboxylic acids (and some other hydrogen-bond donors) in 1,1,1-trichloroethane at 298 K, using N-methylpyrrolidinone as a standard base. For the first time, it is possible to evaluate the relative hydrogen-bonding strength of monomeric carboxylic acids and other hydrogen-bonding species in an inert solvent. It is shown that unactivated carboxylic acids are no stronger than simple phenols: equilibrium constants for hydrogen bonding towards N-methylpyrrolidinone are acetic acid (109), benzoic acid (118) and phenol (137). It is further shown that the monomeric carboxylic acids are ca. 20 times as strong as the dimeric acids towards N-methylpyrrolidinone (NMP) in 1,1,1-trichloroethane, with respect to the formation of the species RCO2H·NMP in each case.

Solution and co-ordination enthalpies (solid state) of lithium and sodium macrocycle (ethyl p-tert-butylcalix(4)arene tetraethanoate, cryptand 222 and crown ethers) systems

A series of lithium and sodium ethyl p-tert-butylcalix(4)arene tetraethanoate and cryptand 222 salts has been isolated and thermochemically characterised in acetonitrile at 298.15 K. Also reported are the standard enthalpies of solution of common sodium salts (tetrafluoroborate, trifluoromethanesulfonate, iodide and bromide) in this solvent at the same temperature. Solution data for the free and the complexed salts and for the ligand are combined with complexation data for the appropriate cation and macrocycle in acetonitrile to derive the enthalpies of co-ordination of these systems for the process where the product and the reactants are in the solid state. Comparison of data involving lithium and the calix(4)arene ester and the cryptand 222 with those for crown ethers reflects considerable weakening of cation–anion interactions in the former ligands relative to crown ethers. It is concluded that for a given ligand and metal cation, the anion effect is reflected in the co-ordination enthalpies. On the other hand, for systems containing the same ligand and anion, there is a decrease in enthalpic stability in moving from lithium to sodium. The need to obtain experimental data on crystal lattice enthalpies of these ligands and their metal ion complex salts is emphasised.

Enthalpies of transfer from water to methanol of cations complexed with 18-crown-6

Abstract Enthalpies of transfer from water to methanol have been obtained via a thermochemical cycle for the [M + 18C6] complexes where M +  Na + , K + , Rb + , Cs + , and Ag + . Variation of the transfer enthalpy with M + is small.

Heats of solution of electrolytes in ethanol and derived enthalpies of transfer from water

Heats of solution of 12 1 : 1 electrolytes in ethanol have been determined calorimetrically, and have been extrapolated to zero electrolyte concentration to give ΔH°s values for these electrolytes. Together with literature data for 9 other 1 : 1 electrolytes, these measurements yield a set of single ion enthalpies of transfer from water to ethanol for 11 univalent cations and 6 univalent anions. Using the assumption that ΔH°t(Ph4As+)=ΔH°t(Ph4B–), it is shown that small univalent cations are enthalpically more stable in ethanol than in water, but that the larger tetra-alkylammonium ions are less stable in ethanol. With the same assumption, it is also shown that all the univalent cations and anions studied are of almost the same enthalpy (within about ± 0.5 kcal mol–1) in the three alcohols methanol, ethanol and 1-propanol.

Hydrogen bonding. Part 3.—Enthalpies of transfer from 1,1,1-trichloroethane to tetrachloromethane of phenols, N-methylpyrrolidinone (NMP) and phenol–NMP complexes

Enthalpies of solution of seven phenols and of NMP have been determined in 1,1,1-trichloroethane and in tetrachloromethane at 298 K. Combination with our previously determined enthalpies of complexation, ΔH°, of the phenols with NMP leads to enthalpies of transfer of the hydrogen-bond complexes ArOH ‥ NMP from 1,1,1-trichloroethane to tetrachloromethane. The substituent-dependent behaviour of values of ΔH° in the two solvents arises exclusively from the effect of the solvents on the reactant phenols and not on the complexes themselves. This finding confirms the suggestion that the ΔH° values in 1,1,1-trichloroethane contain a contribution from association of the phenols with the solvent itself, probably of the dipole–dipole type.

Activation parameters for the solvolysis of t-butyl chloride in water–ethanol mixtures. Glycine as a transition state model

Variations in ΔG‡, ΔH‡, and ΔS‡ for the solvolysis of t-butyl chloride in water–ethanol mixtures are accounted for when glycine is used as a transition state model, provided that differences in molar volumes are taken into account.