The thermodynamic ionization constants of aromatic arsonic acids

Ionization constants have been obtained for several aromatic arsonic acids and the values correlated with the structural effects of the substituents. The Hammett equation was obeyed in the case of para-substituted acids and ϱ values for the successive ionizations of phenylarsonic acids were determined. Hammett σ values for the substituents AsO₃H₂, AsO₃H⁻ and AsO₃²⁻ were also obtained.     

Spot test detection of phenylhydrazine and arsonic acids

If phenylhydrazine or its salts are heated with arsenic acid or with one of its organic derivatives (aliphatic or aromatic arsonic acids) phenol results. The latter can easily be detected in the gas phase by the indophenol reaction with 2:6-dichloroquinone-4-chloroimine. This has been made the basis of characteristic tests for phenylhydrazine (also phenylhydrazones and osazones) as well as for arsonic acids. When conducted within the bounds of spot test analysis, the limits of detection are microanalytical.     

Ionization and complexation equilibria involving arsonic acids containing azo linkages

Ionization constants have been obtained for a series of hydroxyazo compounds, some of which contain arsonic acid substituents. In addition, the complexing behaviour of these arsonic acids has been investigated. Potentiometric and spectrometric methods were used in these studies.     

Esterification of polymeric carboxylic acids

Polymers containing pendant carboxyl groups have been essentially completely esterified with little apparent change in molecular weight by the use of one of the following reagents: (1) trialkyl orthoesters, (2) dimethylformamide dialkyl acetals, (3) tertiary amines followed by alkyl halides, (4) quaternary ammonium hydroxide followed by alkyl halides. The use of some of these methods permits preparation of certain polymers that were previously not readily accessible.     

Some equilibrium constants of tellurous acid

The various literature values of the equilibrium constants for the system TeO₃²⁻H⁺ are tabulated, and the wide divergence in the values is noted. Solubility measurements and potentiometric titrations were used to re-evaluate some of the constants. The determinations were made in aqueous solutions, and at a range of ionic strengths (sodium perchlorate). The values found for aqueous solution, with no perchlorate, were as follows: pK_HL^H = 9·56 ± 0·02, pK_H2L^H = 6·08 ± 0·06, pK_H3L^H = 2·8 ± 0·2. There was no evidence of polynuclear species at concentrations below 0·05 M.     

Esterification of pyrazole-3- and 4-carboxylic acids

Russian Journal of General Chemistry -     

Spectrophotometric determination of equilibrium constants in solution

This paper reviews critically a number of published methods for using ultraviolet or visible spectrophotometric data for the determination of equilibrium constants in solution. Single-stage or overlapping equilibria are considered, and a number of ways in which the experimental data can be handled by electronic computers are suggested.     

Dissociation constants of polyfluorocarboxylic acids

The effect of the structure and molecular weight of fluorocarboxylic acids on their dissociation constants in acetonitrile and its mixtures with water and methanol was studied. Original Russian Text O.N. Chechina, S.V. Sokolov, V.V. Berenblit, V.A. Soshin, 2007, published in Zhurnal Prikladnoi Khimii, 2007, Vol. 80, No. 10, pp. 1741–1743.     

Esterification of 1,2,5-oxadiazolylacetic acids with polynitro alcohols

Reactions of 4-substituted (1,2,5-oxadiazol-3-yl)acetic acids with 2,2,2-trinitro- and 2-fluoro-2,2-dinitroethanols afford the corresponding esters that can be useful as components of gas generating compositions.     

Cyclization equilibrium constants and the conformations of polydimethylsiloxane chains

Experimental molar cyclization equilibrium constants for dimethylsiloxane cyclics having from 8 to ca 400 skeletal bonds are presented. The results for an undiluted equilibrate, an equilibrate in toluene solution, and an equilibrate in diglyme solution are compared. The increases in the equilibrium concentrations of cyclic oligomers, previously found to be large in good solvents (e.g., toluene) are found to be small in the poor solvent (diglyme). For macrocyclics, the cyclization equilibrium constants in the undiluted equilibrate and in diglyme (theta conditions) are found to be in good agreement with calculations which were carried out by assuming that chains are unperturbed and by using the rotational isomeric state model of Flory, Cresenzi, and Mark for polydimethylsiloxane. However, with chain polymer in dilute solution in a good solvent (toluene), the macrocyclization equilibrium constants are found to be lower than in the other two equilibrates. This result confirms the work of other authors who suggested that in this case the chains were expanded by excluded-volume effects.     

Determination of equilibrium constants from chromatographic and electrophoretic measurements

Chemical interactions, such as acid-base, complex-forming, ion association and other equilibria, are widely exploited to improve the separation efficiency in liquid chromatography as well as in electrophoresis. On the other hand, these techniques can be advantageously used to study the chemical equilibria affecting the separations. If the equilibium is sufficiently fast in comparison with the separation process, then the retention characteristics in chromatography (retention factors) or the migration characteristics in electrophoresis (effective mobilities) may be expressed as functions of the composition of mobile phase or background electrolyte (BGE), respectively. Using a proper experimental arrangement, the dependencies of retention (migration) characteristics on the mobile phase (background electrolyte) composition can be measured and utilized to calculate the equilibrium constants for equlibria taking place in the mobile phase (background electrolyte). Although principles of these measurements have been known for a long time, only more recent studies utilizing HPLC and capillary electrophoretic techniques are reviewed in this paper.     

Esterification of sterically hindered acids and alcohols in fluorous media

Sterically hindered esterification reactions are best performed in specific fluorous media in the presence of catalytic amounts of diphenylammonium triflate. Fluorous media, in addition to their positive effect on yields, have inherent favorable properties respecting environment and permitting simple work-up. Highly congested reagents, however, react only marginally.     

On the direct reduction of arsonic acids to arsenoso compounds: mechanisms and preparations†

By using a variety of reducing agents, either alone or in the presence of iodide as a catalyst, the reduction of arsonic acids to arsenoso compounds and/or arsonous acids has been studied. The solvent and temperature are important for a clean reduction. The mechanism of the reduction involves prior protonation of the  AsO₃H₂ group, and can be rationalized in the framework of the ‘hard and soft acids and bases’ principle, which is used to predict other reducing systems and to explain other literature data. For preparative purposes, triphenylphosphine/iodine, hexamethylphosphorous triamide/iodine and ascorbic acid/iodine give flexibility of choice depending on the substrate. The first two of these systems decompose arsonic acids with a weak C–As bond but the last system is sufficiently mild towards the same arsonic acids. Copyright © 2000 John Wiley & Sons, Ltd.