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The rate constants of the S(N)2 reaction of sodium 4-nitrophenoxide (1) and iodomethane were determined by UV-visible spectrophotometry in acetone-water mixtures at 25, 30, and 35 degrees C. The rate-Xwater (mole fraction of water) profile shows that the reaction depends strongly on the medium. The fastest rate constant was obtained in pure acetone, and a minimum occurred at Xwater= 0.4, whereas the observed second-order rate constants increases again in the water-rich region. In pure acetone, in the presence of dicyclohexano-[18]-crown-6, increases linearly with the concentration of the crown ether as a result of the complexation of the sodium ion (KS = 104.8 M) of the ion-pair and the increase in the effective concentration of free 4-nitrophenoxide ion, which was assumed to be the only reactive species. Ion-pairing was also detected at Xwater= 0.65 with a dissociation constant Kd = 7.82 x 10(-4) M(-1). The solvatochromic behaviors of 2,6-diphenyl-4-(2,4,6-triphenyl-1-pyridinio)-1-phenoxide (2), 4-[(1-methyl-4(1H)-pyridinylidene)ethylidene]-2,5-cyclohexadien-1-one (3), and 1-methyl-8-oxy-quinolinium betaine (4) were investigated in the entire range of acetone-water mixtures. The dyes presented an increasing order of hydrophilicity compatible with their chemical structure, i.e., 2 < 3 < 4. Kinetic parameters for the methylation of 1 and the ET values of the dyes show a linear correlation of the polarity in the region of Xwater = 1.0-0.40 for 3 and 4, and it was observed that the more hydrophilic the dye the better the correlation coefficient, because of the structural similarity with 1. The activation parameter-Xwater profile shows extrema at Xwater < 0.4, reflecting an important change in the structure of the solvent that is responsible for the changes in the solvation of the reactive species including ion-pairs. These results suggest that the addition of water to acetone reduces abruptly the rate of substitution due to the preferential solvation (PS) of the phenoxide ion by the hydrogen-bonding donor (HBD) solvent. Nevertheless, the real second-order rate constant is "masked" by the association involving Na+ and 4-nitrophenoxide that extends even to water-rich mixtures. A model, based on the assumption that the free-energy terms involved in the second-order rate constant and the dissociation constant of the ion-pair have two components, is invoked to explain the kinetic data. One of the components depends on electrostatic interactions for which the main variable is the dielectric constant of the solvent mixture, and the other depends on the specific solute-solvent interactions, expressed by the activity coefficients of transfer of the species involved. The model indicates that in the range of Xwater = 1.0-0.40 the interactions are exclusively electrostatic, while for the rest of the acetone-rich region they are specific with a large contribution of the 4-nitrophenoxide ion.
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PMID:Ion-dipole S(N)2 reaction in acetone-water mixtures. Electrostatic and specific solute-solvent interactions. 1131 43

The molar transition energy (E(T)) polarity values for the solvatochromic probes 2,6-diphenyl-4-(2,4,6-triphenylpyridinium)phenolate (1), 4[(1-methyl-4-(1H)-pyridinylidene)-ethylidene]-2,5-cyclohexadien-1-one (2), and 4-[4-(dimethylamino)styryl]-1-methylpyridinium iodide (3) were collected in binary mixtures comprising chloroform and a hydrogen-bond accepting (HBA) solvent [dimethyl sulfoxide (DMSO), N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMA), acetone or acetonitrile], aiming to investigate the ability of the chlorinated component to act as hydrogen-bond donating (HBD) solvent. Plots of E(T) as a function of X(2), the mole fraction of chloroform, were obtained and the data were analysed to investigate the preferential solvation (PS) of each probe in terms of both solute-solvent and solvent-solvent interactions. For dyes 1 and 2 a strong synergistic behavior was observed for all mixtures studied, indicating that the dyes are preferentially solvated by complexes formed through hydrogen bonding between chloroform and the HBA component in the mixtures. A study of 1 in deuterated chloroform with an HBA component (DMF and DMA) demonstrated that while almost no differences occur with the DMF mixtures, the presence of deuterated chloroform in its mixtures with DMA increases the synergistic effect, suggesting that it interacts more strongly with DMA, making its mixtures more polar. These data were successfully fitted to a model based on solvent-exchange equilibria. The features of the mixtures with dye 3 revealed a very different profile in comparison with the other two dyes, which suggests that in mixtures containing chloroform, the microenvironment of the dye seems to be important in determining the contribution of the structure resonances responsible for the stability of the dye.
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PMID:Merocyanine solvatochromic dyes in the study of synergistic effects in mixtures of chloroform with hydrogen-bond accepting solvents. 1867 25