Water-methanol mixtures entropies

As in the case of heats and heat capacities, ionic entropies have been examined more extensively in methanol and methanol-water mixtures than in other solvents. The earliest study was that of Latimer and Slansky who report entropies for NaCl, KCI, KBr, and HCl in water-methanol mixture from 0 to 100% methanol. Jakuszewski and Taniewska-Osinska " have determined the entropies of numerous halide salts and HCl at 25 C, and more recently Franks and Reid, using data from the literature, have calculated standard ionic entropies for several species in water-methanol mixtures covering the whole concentration range. It should be observed that these authors chose the mol fraction standard state for the solute in solution and ideal ionic gas as the standard state for the pure solute instead of the conventional hypothetical one molal solution and pure solid at OK. The former standard states are convenient when comparing entropies of solvation for the various species. Ionic entropies in methanol are considerably more negative than in water. 

We examined more closely the thermodynamics of the reaction between the diene 2p and but-2-enone by measuring the activation parameters of the reactions conducted in water and in a water-methanol mixture (50 50, v/v, the rate of the reaction in pure methanol is too small to be recorded with reliability). To check if there is a special effect due to the presence of the sugar moiety linked to die buta-1,3-diene unit, we studied the cycloaddition between methoxybutadiene and but-2-enone under the same conditions. TTie results are summarized in Table IV. As anticipated, the second-order rate constant increases in pure water and the rate enhancement comes only from an increase of the activation entropy (+32.3 J.mole . T" from water-methanol to pure water). It must also be noted that the increase of the activation entropy is even more important in the case of the diene 2P than for methoxybutadiene. This could be related with the interaction of the glucose moiety with the water structure and could have some importance in connection with biological problems. In fact, it seems that glucose acts as a structure-making compound and enhances the hydrophobic effect. We have thus demonstrated, as previously postulated, that the rate enhancements in aqueous cycloadditions have an entropic origin. 

Under certain conditions, one can observe a double reentrant transition. For example, a weakly charged gel based on NIPA-sodium acrylate copolymer swollen in a water-methanol mixture undergoes reentrant phase transition two times when the methanol concentration is gradually increased. This behavior can be connected with leveling off the solvent compositions inside and outside the gel at equal volume fractions of both the solvent components in the system, which is caused by a gain in the entropy of mixing. 

Extrema in the enthalpy and entropy of activation for the substitution of 2,2 -bipyridyl into Ni q in a series of water/acetonitrile mixtures correlate welf with the extrema in the physical properties of the mixture which are related to sharp changes in the solvent structure. Further reports have appeared on rate enhancements for the reaction of Ni " with pyridine-2-azo-p-dimethylaniline (pada) in micellar systems,and the kinetics of complex formation with lasalocid (X-537A) in methanol " and with poly(4-vinylpyridine) in aqueous methanol " have been reported. 

In addition, the activation enthalpy and entropy for the cyclopentadiene and methyl acrylate reaction is 10.2 and -40.9 cal/mol K, respectively, in a methanol/water mixture [81], Therefore, analogous to the cyclopentadiene and methyl acrylate reaction, the butadiene and acrolein activation enthalpy (adjusted by 0.7 kcal/mol)... 

A similar change was observed by Grenthe and Williams [Gr 67] in a study of the protonation of the acetate ion in water and in a 95% methanol-water mixture. The heat of protonation of the acetate ion changed from AH = —0.22 (in water) to AH= —2.12 in the methanol-water mixture, and the entropy change increased from dS = 6.0 to AS = 1A. The change in the medium had barely any effect on the protonation of the amino group. 

The excess activation free energy AG, enthalpy Aff, and entropy A5, and their partial molar quantities for alcohols AG, A//, and A5 were also calculated for the dominating processes with T, in methanol, ethanol, and 1-propanol water mixtures [67-69] and are depicted in Figure 6.6. These thermodynamic quantities were calculated according to the Eyring transition state theory [93]. Based on the curves in Figure 6.6 characteristic molar fraction values (0.30,0.18, and 0.14 for methanol, ethanol, and 1-propanol water mixtures, respectively) were also reported above and below which the behavior of the partial molar excess activation quantities... 

The activation enthalpies and entropies for the formation of the nickel(II) complexes of bipy, phen and terpy in water-r-butanol mixtures pass through distinctive minima (as they do with the complexes of 4-phenylpyridine and isoquinoline). Comparison of kinetic and thermodynamic results for the reaction of Ni(II) with bipy in aqueous methanol shows that the small variation of with solvent composition represents the difference between large destabilizations of both the nickel(II) (initial state) and the transition state as the proportion of methanol increases. 

Palladium(ii).—Rate constants for aquation of /ra/ij-[Pd(NH8)2Cl2], /ra/w-[Pd(NH8)a(OH2)Cl]+, and /m -[Pd(NH8)a(0Ha)(N02)]+ in methanol-, acetone-, dioxan-, and dimethylformamide-water mixtures correlate linearly, though with only three points to define each line, with the function D — 1)/(2D + 1) of the dielectric constants (D) of the respective solvent mixtures for each pair of solvents. The slopes of these linear plots are very different for each organic co-solvent. An associative mechanism is assumed to operate since in all cases the activation entropies are large and negative. Aquation of [Pd(N02)4] is subject to acid catalysis but not, as would be expected, to base catalysis. The mechanism of aquation of the cis- and /ronr-isomers of [Pd(gly)2] involves a fast preequilibrium ... 

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