Acetonitrile equilibria

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 ... 

The time needed to reach equilibrium can be a few minutes to several hours. With water-miscible solvents such as acetonitrile, equilibrium is generally reached in less than 60 min Determine equilibrium time by preparing a 25-mL sample in a 50-mL test tube with acetonitrile-whole-broth ratio as determined in Note 1. Continuously shake test tube and take 2-mL samples at 10-min intervals. Immediately centrifuge a 2-mL sample to separate supernatant from solids and determine solute concentration in the supernatant. When the solute concentration of consecutive samples is the same, equilibrium has been reached. 

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 magnitude of the anomeric effect depends on the nature of the substituent and decreases with increasing dielectric constant of the medium. The effect of the substituent can be seen by comparing the related 2-chloro- and 2-methoxy-substituted tetrahydropy-rans in entries 2 apd 3. The 2-chloro compound exhibits a significantly greater preference for the axial orientation than the 2-methoxy compound. Entry 3 also provides data relative to the effect of solvent polarity it is observed that the equilibrium constant is larger in carbon tetrachloride (e = 2.2) than in acetonitrile (e = 37.5). 

A study of the solvent dependence of the equilibrium for this substituent gave the following re.3ults benzene (84% isoindole), acetonitrile (87%), ethanol (90%), and ether (99%). 

The method is very useful for the synthesis of physiologically interesting a-mcthylamino acids, e.g., methyl dopa from the 3,4-dimethoxybenzyl derivative. The excellent stereoselection achieved in the process, however, is caused by the preferential crystallization of one pure diastereomerfrom the equilibrium mixture formed in the reversible Strecker reaction. Thus, the pure diastcrcomers with benzyl substituents, dissolved in chloroform or acetonitrile, give equilibrium mixtures of both diastereomers in a ratio of about 7 347. This effect has also been found for other s-methylamino nitriles of quite different structure49. If the amino nitrile (R1 = Bn) is synthesized in acetonitrile solution, the diastereomers do not crystallize while immediate hydrolysis indicates a ratio of the diastereomeric amino nitriles (S)I(R) of 86 1447. 

I. Brown and F. Smith, "Liquid-Vapor Equilibriums. VI. The Systems Acetonitrile-Benzene at 45° and Acetonitrile-Nitromethane at 60c". Ansi. J. Chem., 8, 62-67 (1955). 

Methyl-4-phenyl-l,2,5-thiadiazole 1,1-dioxide 21 suffers proton abstraction in basic nonaqueous media to give a resonance stabilized anion 43, neutralization of which using anhydrous TFA gives the orange tautomer 4-methylene-3-phenyl-l,2,5-thiadiazoline 1,1-dioxide 44 (Scheme 3) <2001JP0217>. The tautomeric equilibrium is practically displaced toward 21 in acetonitrile and toward 44 in DMF. 

Solution EPR has been reported and consisted of a singlet of low intensity when recorded in acetonitrile. However, the signal intensity increased when the solvent used was trifluoroacetic acid, indicating a solvent-dependent dissociation equilibrium. No set of satellites due to coupling to Se were reported. 

The chemical properties of BFL are very similar to those of FL. The greatest difference is that under similar conditions there is more singlet reaction from BFL than from FL. This observation is reflected in the estimate of AGsr obtained, exactly as it was for FL, from the observed rate of reaction with methyl alcohol in acetonitrile. For BFL, use of (28) gives A(7st = 1.0 kcal mol 1. This value implies that there is a significant amount of BFL in the equilibrium mixture, and that the effect of the conjugating benzo-substituent is to stabilize the singlet carbene more than the triplet. 

The first indication that A-acyloxy-A-alkoxyamidcs reacted by an acid-catalysed process came from preliminary H NMR investigations in a homogeneous D20/ CD3CN mixture, which indicated that A-acetoxy-A-butoxybenzamide 25c reacted slowly in aqueous acetonitrile by an autocatalytic process according to Scheme 4 (.k is the unimolecular or pseudo unimolecular rate constant, K the dissociation constant of acetic acid and K the pre-equilibrium constant for protonation of 25c).38... 

Apart from the qualitative observations made previously about suitable solvents for study, the subject of solvates has two important bearings on the topics of thermochemistry which form the main body of this review. The first is that measured solubilities relate to the appropriate hydrate in equilibrium with the saturated solution, rather than to the anhydrous halide. Obviously, therefore, any estimate of enthalpy of solution from temperature dependence of solubility will refer to the appropriate solvate. The second area of relevance is to halide-solvent bonding strengths. These may be gauged to some extent from differential thermal analysis (DTA), thermogravimetric analysis (TGA), and differential scanning calorimetry (DSC) solvates of "aprotic solvents such as pyridine, tetrahydrofuran, and acetonitrile will give clearer pictures here than solvates of "protic solvents such as water or alcohols. 

As a measure of their thermodynamic stability, the pAfR+ values for the carbocation salts were determined spectrophotometrically in a buffer solution prepared in aqueous solution of acetonitrile. The KR+ scale is defined by the equilibrium constant for the reaction of a carbocation with water molecule (/CR+ = [R0H][H30+]/[R+]). Therefore, the larger p/CR+ index indicates higher stability for the carbocation. However, the neutralization of these cations was not completely reversible. This is attributable to instability of the neutralized products. The instability of the neutralized products should arise from production of unstable polyolefinic substructure by attack of the base at the aromatic core. 

The marked changes in the carbonyl IR bands accompanying the solvent variation from tetrahydrofuran to MeCN coincide with the pronounced differences in colour of the solutions. For example, the charge-transfer salt Q+ Co(CO)F is coloured intensely violet in tetrahydrofuran but imperceptibly orange in MeCN at the same concentration. The quantitative effects of such a solvatochromism are indicated by (a) the shifts in the absorption maxima and (b) the diminution in the absorbances at ACT. The concomitant bathochromic shift and hyperchromic increase in the charge-transfer bands follow the sizeable decrease in solvent polarity from acetonitrile to tetrahydrofuran as evaluated by the dielectric constants D = 37.5 and 7.6, respectively (Reichardt, 1988). The same but even more pronounced trend is apparent in passing from butyronitrile, dichloromethane to diethyl ether with D = 26, 9.1 and 4.3, respectively. The marked variation in ACT with solvent polarity parallels the behaviour of the carbonyl IR bands vide supra), and the solvatochromism is thus readily ascribed to the same displacement of the CIP equilibrium (13) and its associated charge-transfer band. As such, the reversible equilibrium between CIP and SSIP is described by (14), where the dissociation constant Kcip applies to a... 

The Aspen NRTL-SAC solvent database was identified from the list of solvents presented in the pharmaceutical based International Committee on Harmonization s guidelines for residual solvents in API [28], Hexane, Acetonitrile and Water were selected as the basis for the X, Y and Z segments respectively, the binary interaction parameters for the segments together with molecular descriptors in terms of X,Y and Z segments were then regressed from experimental vapour-liquid and liquid-liquid equilibrium data from the Dechema database. The list of solvent parameters that were used in the case study are given in Table 13. 

The second-order rate constant for the methylation of sodium 9-fluorenone oximate in 33.5% acetonitrile/66.5% t-butyl alcohol solution was found to decrease with increasing concentration of the salt, suggesting an equilibrium (13) between the reactive free anion [109] and the less reactive ion pair [110]... 

Any analytical method [312] suitable for determining equilibrium compositions of a reaction mixture at several temperatures can be used to obtain the enthalpy and the entropy of that reaction. The first example we describe involves a common analytical technique (infrared absorption spectroscopy) and addresses the energetics of the hydrogen bond between phenol and acetonitrile. This careful study on the equilibrium 14.6 was made by Sousa Lopes and Thompson more than 30 years ago [313]. 

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