A Priori Prediction of the Vapor–Liquid Equilibria of Mixtures of Acetic Acid and Water or Alcohols by Explicit Consideration of Hydrogen-Bonded Dimers

Self-association of diphenylpnictoginic acids in solution and solid state: covalent vs. hydrogen bonding

Triphenylpnictogens were oxidized to access diphenylpnictioginic acids Ph₂XOOH (X = P, As, Sb, Bi). It was shown that oxidation with chloramine-T does not lead to the cleavage of a C-pnictogen bond. The preliminary reductive cleavage with sodium in liquid ammonia followed by the oxidation with hydrogen peroxide was successfully utilised for the synthesis of diphenylphosphinic and diphenylarsinic acids. It was shown that in solid state (by means of XRD), all diphenylpnictoginic acids form polymeric chains. Diphenylbismuthinic and diphenylantimonic acids form polymeric covalent adducts, while diphenylphosphinic and diphenylarsinic chains are associated through hydrogen bonding. Unlike diphenylphosphinic acid, diphenilarsinic acid forms two polymorphs of hydrogen-bonded infinite chains. In solution in a polar aprotic solvent diphenylarsinic acid, similarly to dimethylarsinic, forms hydrogen-bonded cyclic dimers together with a small amount of cyclic trimers.

Carboxylic acids in aqueous solutions: Hydrogen bonds, hydrophobic effects, concentration fluctuations, ionization, and catalysis

In the frequency range between 100 kHz and 2 GHz, ultrasonic absorption spectra have been measured for a series of carboxylic acids from formic to enanthic acid, including constitutional isomers. Also investigated have been the spectra for mixtures with water of short-chain formic, acetic, propionic, butyric, and isobutyric acid, in each case covering the complete composition range. The neat carboxylic acids feature two Debye-type relaxation terms with relaxation times between 5.6 and 260 ns as well as 0.14 and 1.4 ns, respectively, at room temperature. Depending on the composition, mixtures with water reveal an additional Debye relaxation term in the intermediate frequency range (acetic acid) or a term subject to a relaxation time distribution (propionic, butyric, and isobutyric acid). The relaxations of the neat acids are assigned to the equilibrium between monomers and single-hydrogen-bonded linear dimers and between linear and twofold-hydrogen-bonded cyclic dimers. The latter equilibrium is considerably catalyzed by hydronium and carboxylate ions. Several mixtures with water indicate one of the up to three Debye relaxations to reflect the protolysis of the organic acid. The term with underlying relaxation time distribution is due to noncritical fluctuations in the local concentrations. The Debye relaxations are evaluated to yield the parameters of the relevant elementary chemical reactions, such as the rate and equilibrium constants and the isentropic reaction volumes. A comparison of the correlation length of concentration fluctuations with data for other aqueous systems confirms the idea that the hydrophobic part of the organic constituent promotes the formation of a micro-heterogeneous liquid structure, whereas the hydrophilic moiety is of minor importance in this respect. The high-frequency limiting absorption suggests the equilibrium between conformers of linear dimers to contribute to the spectra well above the frequency range of measurements.