Ionization and dissociation equilibria in sulfur dioxide solution. Part 1: dissociation of ion pairs. Part II: equilibria of meta phenyl derivatives of trityl chloride. Part III: the apparent ionization of hexaphenylethane
Leftin, Harry Paul
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Part I Dissociation of Ion Pairs. The conductivity of potassium chloride, bromide and iodide and of tetramethylammonium bromide were measured over a dilution range of 10^2 to 10^5 liters per mole in liquid sulfur dioxide solution at 0.12°C. and at -8.93°C. employing the internal dilution technique described by Lichtin and Glazer. Similar measurements were carried out on tetramethylammonium sulfate at 0.12°C. Equilibrium constants were evaluated from these data by the method of Shedlovsky. The method of least mean squares was applied to establish the best straight line representing the Shedlovsky equation for each compound at each temperature. The experimental data and calculations presented in this dissertation clearly demonstrate that the theory of ionic association of Bjerrum is an accurate representation of the behavior of 1-1 electrolytes in liquid sulfur dioxide solution. The distance of closest approach of the ions were calculated from the experimental equilibrium constants by the Bjerrum equation. Values obtained from the data on pure ionic compounds in sulfur dioxide solution are in excellent agreement (± 0.1Å) with the sums of crystallographic radii of the corresponding ions. Conversely, ion pair dissoc:iation constants calculated from crystal radii agree closely with the experimental values obtained in this research. In the case of tetramethylammonium bromide, where a crystallographic radius is not available for the cation, a value estimated as the largest Van der Waals radius from the center of the molecule by direct measurement of a Fisher-Herschfelder-Taylor model gave excellent agreement with experiment. An equation for ΔH° derived solely from the Bjerrum theory gave values which were in good agreement within the uncertainties inherent in the experimental values of this property. On the basis of these observations it is possible to conclude that the Bjerrum treatment is quantitatively exact for 1-1 electrolytes in sulfur dioxide solution. This is the first demonstration of quantitative adherence to this theory. Part II Equilibria of m-Phenyl Derivatives of Trityl Chloride. All attempts to utilize conductivity data for ring substituted derivatives of triphenylchloromethane in sulfur dioxide solutions for the direct estimation of the electronic influence of the ring substituents have, in the past, met with little quantitative success due to the complications arising from short range ionic interactions which give rise to ion pairs and higher aggregates in solvents of low dielectric constant. In a qualitative manner Lichtin and Bartlett were able to demonstrate that ionic association equilibria introduce only minor errors in the relative equilibrium constants for trityl chloride and those ring substituted derivatives which are weaker electrolytes than trityl chloride. In this way these workers were able to estimate the qualitative electronic influences of those substituents which stabilize triphenylchloromethane more than they stabilize the triphenyl carbonium ion in sulfur dioxide solution. Since, however, many theoretically interesting substituents exert an effect resulting in an enhanced ionization of triphenylchloromethane it is both interesting and valuable to develop a method of evaluating an ion pair correction term to be used with the experimental data for these compounds. A method is proposed for the quantitative evaluation of an ion pair correction term to be applied to experimental conductivity data for ring substituted trityl chlorides in sulfur dioxide solution. With this method it is now possible to obtain a quantitative measure of the electronic influence of substituents from conductivity data in this solvent. The assumptions involved in this treatment are as follows: (1) The Bjerrum equation is an exact representation of ionic association behavior of 1-1 electrolytes in this solvent. This assumption is supported by the evidence presented in Part I. (2) The triarylmethyl carbonium ion in solution presents a spherical appearance to the anion by virtue of a tumbling motion about its center of gravity. The ion sweeps out an effective volume equal to a sphere whose radius is the largest Van der Waals radius from the center of gravity of the ion. (3) The Bjerrum radius of the triarylcarbonium ion is equal to the radius of the swept out volume and can be estimated directly from molecular models as being the largest Van der Waals distance from the center of gravity. Experimental equilibrium constants were determined by applying the Shedlovsky and least mean squares method to the conductivity data for mono-,di-, and tri-m-phenyl derivatives of trityl chloride in liquid sulfur dioxide at 0.12°C. and -8.93°C. obtained in this research. These values combined with calculated ion pair dissociation constants permitted the calculation of the experimentally inaccessible ionization constants for these compounds. It was demonstrated that the influence of stepwise introduction of m-phenyl substituents on the calculated free energy of ionization of the corresponding trityl chlorides could be described by equal free energy increments for each successive substitution. A sigma constant for the m-phenyl group was determined from acid strength measurements on benzoic and m-phenyl benzoic acids. With this value it was possible to calculate a Hammett rho parameter for the ionization of trityl chlorides in sulfur dioxide solution. Resonance sigma constants were calculated for p-phenyl, p-methyl and p-t-butyl groups. Hammett correlation plots were constructed for the ionization reaction in sulfur dioxide employing all available experimental data from this research and from the literature. It was found that poor correlations could be obtained with experimental dissociation constants while, on the other hand excellent agreement resulted when ionization constants calculated on the basis of the ion pair treatment were employed. An electron supplying resonance sigma for the para phenyl group of -0.148 is proposed. Values of -0.3 have been calculated for both the p-methyl and p-t-butyl group. This research has provided a useful tool for evaluation of substituent effects. Part III The Apparent Ionization of Hexaphenylethane. The conductivity which has been observed with solutions of hexaphenylethane in liquid sulfur dioxide has been subject to several chemical interpretations which differ in detail but which all assume an ionization mechanism involving only hexaphenylethane and sulfur dioxide. This conductivity is now found to be an artifact of at least two processes, namely, reaction with dissolved oxygen and a photochemical transformation. Experiments employing crystalline samples of ethane of purity established by quantitative oxygenation and a refinement of the conductivity technique of Lichtin and Glazer reveal a lack of reproducibility like that apparent in older work. Although irradiation with a Burton ultraviolet lamp produces slow but large increases in conductivity, variable exposure to light cannot be the sole source of the discrepancies since consistent data do not result from experiments performed in the dark. The fact that increasingly efficient degassing of the solvent prior to dissolution leads to progressive diminution of the conductivity suggests production of an electrolyte by reaction with a gaseous impurity. This reagent has been identified as oxygen. The conductivity of an oxygenated solution of hexaphenylethane is somewhat greater than the highest comparable values obtained without degassing the solvent. The conductivity of this solution does not change upon irradiation whereas that of the solutions in degassed sulfur dioxide increases.
Thesis (Ph.D.)--Boston University
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