Acetonitrile phase equilibria

Figure 14.20 Effect of pressure on the (solid -f liquid) and (liquid + liquid) phase equilibrium for (X1H2O + JC2CH3CN). L is liquid, (L1+L2) gives the phases in equilibrium in a two-phase liquid region, and A is solid acetonitrile (CH3CN). Reprinted by permission from G. Schneider, Z. Phys. Chem., 41, 327-338 (1964). Copyright by R. Oldenbourg Verlag.

Figure 17.13a shows H for the (cyclohexane + acetonitrile) system at (1), T = 348.15 K and (2), T = 323.15 K. At the higher temperature, a large positive is obtained, as expected for a (nonpolar + polar) mixture. At the lower temperature, (liquid + liquid) phase equilibrium with phase separation is present and a break occurs in the curve. The breaks at points (i) and (ii) are at compositions corresponding to the solubilities given by the (liquid + liquid) phase diagram shown in Figure 17.13b. Breaks in H curves such as at (i) and...

Five moles of benzene(l), five moles of acetonitrile(2), and five moles of water(3) are confined to a closed vessel at 1.0133 bar, 333 K. The mixture is observed to be in three-phase equilibrium a water-rich liquid (a), an organic-rich liquid (P), and a vapor. Analyses of samples drawn from each phase give the following mole fractions ... 

Pb(CH3)4 is soluble In absolute ethanol [1, 2], but not in 96% ethanol [3]. It is soluble in ether [1, 2], hydrocarbons, benzene, toluene, and other usual organic solvents [3], but Insoluble in liquid ammonia at —78 C [4]. Solutions of Pb(CH3)4 in C7Hi4-n (components are about equally volatile) are used as standards in atomic absorption spectrometry. Such solutions are stable for more than 6 months In contrast to Pb(C2H5)4 solutions [27]. For a phase equilibrium study of the systems Pb(CH3)4-toluene and Pb(CH3)4-benzene, see [38]. Estimated values of free energies of transfer of Pb(CH3)4 from methanol to water, alcohols, and several other solvents are given in [12, 21]. A solution of 17 to 90% methanol in 0.1 molar acetate buffer is used as a mobile phase for the separation of (CH3)4 nPb(C2H5)n by HPLC [39, 40]. Pb(CH3)4 and acetonitrile form an azeotrope [6]. Pb(CH3)4 is quantitatively extracted from dust samples into cold ammoniacal methanol [24]. The llpophllicity of Pb(CH3)4 is lower than that of Pb(C2H5)4 [41]. 

Park, S. J. Oh, J. H. Choi, H. J. Peak, S. K. Fischer, K. Gmehling, J. Phase equilibrium and excess molar volume for systems acrylonitrile + water, acetonitrile + water, and acetonitrile-acrylonitrile [Kor]. Hwahak Konghak 1997, 35, 678-683. 

Figure 17 shows results for the acetonitrile-n-heptane-benzene system. Here, however, the two-phase region is somewhat smaller ternary equilibrium calculations using binary data alone considerably overestimate the two-phase region. Upon including a single ternary tie line, satisfactory ternary representation is obtained. Unfortunately, there is some loss of accuracy in the representation of the binary VLB (particularly for the acetonitrile-benzene system where the shift of the aceotrope is evident) but the loss is not severe. 

The data in Table 17.3 are for vapor pressure and vapor and liquid composition of solutions of methyl tert-butyl ether (1) and acetonitrile (2), (9). The symbol Xi represents the mole fraction of (1) in the liquid phase, and yi represents the mole fraction of (1) in the vapor phase. P is the equilibrium vapor pressure of the solution. The temperature is 313.15 K. 

A mechanistic study of acetophenone keto-enol tautomerism has been reported, and intramolecular and external factors determining the enol-enol equilibria in the cw-enol forms of 1,3-dicarbonyl compounds have been analysed. The effects of substituents, solvents, concentration, and temperature on the tautomerization of ethyl 3-oxobutyrate and its 2-alkyl derivatives have been studied, and the keto-enol tautomerism of mono-substituted phenylpyruvic acids has been investigated. Equilibrium constants have been measured for the keto-enol tautomers of 2-, 3- and 4-phenylacetylpyridines in aqueous solution. A procedure has been developed for the acylation of phosphoryl- and thiophosphoryl-acetonitriles under phase-transfer catalysis conditions, and the keto-enol tautomerism of the resulting phosphoryl(thiophosphoryl)-substituted acylacetonitriles has been studied. The equilibrium (388) (389) has been catalysed by acid, base and by iron(III). Whereas... 

The anthocyanins exist in solution as various structural forms in equilibrium, depending on the pH and temperature. In order to obtain reproducible results in HPLC, it is essential to control the pH of the mobile phase and to work with thermostatically controlled columns. For the best resolution, anthocyanin equilibria have to be displaced toward their flavylium forms — peak tailing is thus minimized and peak sharpness improved. Flavylium cations are colored and can be selectively detected in the visible region at about 520 nm, avoiding the interference of other phenolics and flavonoids that may be present in the same extracts. Typically, the pH of elution should be lower than 2. A comparison of reversed-phase columns (Ci8, Ci2, and phenyl-bonded) for the separation of 20 wine anthocyanins, including mono-glucosides, diglucosides, and acylated derivatives was made by Berente et al. It was found that the best results were obtained with a C12 4 p,m column, with acetonitrile-phosphate buffer as mobile phase, at pH 1.6 and 50°C. 

An unsuccessful attempt has been made to determine the separate electronic and steric effects of alkyl groups on the acidities of hydrocarbons, acetophenone derivatives, and acetone derivatives CH3COCHR1R2 (at either site) by multivariational analyses of experimental and theoretical acidities for each set.15 A thermodynamic cycle has been used to estimate the aqueous phase p/C, = 22.7 1.0 for the methyl group of acetic acid and p/C, = 3.3 1.0 for the corresponding enol.16 Equilibrium acidities have been determined for several nitroaryl substituted nitroalkanes and cyanomethanes, 2,4,6-TNT, and 9-cyanofluorene17 in acetonitrile the influence of common cation BH+ on the electronic spectra of the anions obtained in the presence of strong guanidine bases (B) has been attributed to formation of two types of ion pair.18... 

A Quadruple Point Figure 14.20 shows phase diagrams for (water + acetonitrile) at five different pressures.16 The diagram in (a) at / = 0.1 MPa for this system is very similar to the (cyclohexane + methanol) diagram shown in Figure 14.19a that we described earlier, with a (liquid-I-liquid) equilibrium region present above the (solid + liquid) equilibrium curve for water. 

In Figure 14.20c, the pressure is 124 MPa. At this pressure the melting curves for water and for acetonitrile intersect the (liquid + liquid) curve at the same temperature. Thus, both ice and solid acetonitrile are in equilibrium with the two liquid phases. The temperature where this occurs is a quadruple point, with four phases in equilibrium. This quadruple point for a binary system, like the triple point for a pure substance, is invariant with zero degrees of freedom. That is, it occurs only at a specific pressure and temperature, and the compositions of all four phases in equilibrium are fixed. 

Theoretical calculations on the /Fdithietane 1,3-dioxide 3 =, sy -dithietane 1,3-dioxide 4 equilibrium in the gas phase at HF/6-31G level show that the anti-isomer 3 is slightly favored (by ca. 0.27 kcal moF1) over the, sy -isomer 4. The antijsyn-ratio is 1.6, with a ry -concentration of 36%. Due to different dipole moments of the anti 3 and syn 4, the solvents of low and medium-high polarity such as carbon tetrachloride, acetonitrile, and dimethyl sulfoxide (DMSO) exert a strong influence on the antisyn interconversion, producing an increase in the ry -concentration <2001BOC57>. 

For simple carbonyl compounds, the equilibrium between an aldehyde or a ketone and its corresponding enol is usually so shifted towards the keto form that the amount of enol at equilibrium can neither be measured nor detected by spectroscopy. Nevertheless, as recently emphasised by Hart (1979), this does not mean that the enol cannot exist free, not in equilibrium with ketones and aldehydes. Several examples of kinetically stable enols in the gas phase or in aprotic solvents have been reported. Broadly speaking, it appears that enols have relatively large life-times when they are prepared in proton-free media [e.g. the half-life of acetone enol was reported to be 14 s in acetonitrile (Laroff and Fischer, 1973 Blank et al., 1975) and 200 s in the gas phase (MacMillan et al., 1964)]. These life-times are related to an enhanced intramolecular rearrangement, indicated by the very high energies of activation (85 kcal mol-1 for acetaldehyde-vinyl alcohol tautomerization) which have been calculated (Bouma et al., 1977 Klopman and Andreozzi, 1979) It has therefore been possible to determine most of the spectroscopic properties of simple enols [ H nmr,l3C nmr (CIDNP technique), IR and microwave spectra of vinyl alcohol... 

In Table 7-1 we report the calculated and experimental vertical excitation energies in gas phase and in the two solvents. The calculated values refer to the ground state (GS) geometries (obtained at the B3LYP/6-311G(d,p)) and in the case of acetonitrile to non-equilibrium solvation. 

Reversed-phase high-performance liquid chromatography (RP-HPLC) is the usual method of choice for the separation of anthocyanins combined with an ultraviolet-visible (UV-Vis) or diode-array detector (DAD)(Hebrero et al., 1988 Hong et ah, 1990). With reversed-phase columns the elution pattern of anthocyanins is mainly dependent on the partition coefficients between the mobile phase and the Cjg stationary phase, and on the polarity of the analytes. The mobile phase consists normally of an aqueous solvent (water/carboxylic acid) and an organic solvent (methanol or acetonitrile/carboxylic acid). Typically the amount of carboxylic acid has been up to 10%, but with the addition of a mass spectrometer as a detector, the amount of acid has been decreased to as low as 1 % with a shift from trifluoroacetic acid to formic acid to prevent quenching of the ionization process that may occur with trifluoroacetic acid. The acidic media allows for the complete displacement of the equilibrium to the fiavylium cation, resulting in better resolution and a characteristic absorbance between 515 and 540 nm. HPLC separation methods, combined with electrochemical or DAD, are effective tools for anthocyanin analysis. The weakness of these detection methods is a lack of structural information and some nonspecificity leading to misattribution of peaks, particularly with electrochemical... 

An increase of the acetonitrile equilibrium concentration in this region leads to the linear decrease of the excessively adsorbed amount. The adsorbed layer has a finite volume (or finite thickness), and in this region of very high acetonitrile concentration it is possible to assume that the adsorbed phase is completely filled with acetonitrile, and therefore the following expression for only this region on the isotherm could be written ... 

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