Strongly basic systems
Creators: name: Albagli, Alain
Publisher: name: University of British Columbia
Year: 2011

description: Supervisor : Professor Ross Stewart In the last decade there has been considerable interest in strongly basic solutions and Hammett H_ acidity functions have been defined for various solvent systems. However some doubts still remain about the ionization modes of certain indicators which were used to determine these H_ functions, and the pKa values assigned to weak carbon acids. A careful anchoring of the H_ scale for the system dimethylsulfoxide-water-0.011 molar tetramethylammonium hydroxide with 2,4,4'-trinitrodiphenylamine was undertaken. The lower region of this scale correlates well with the recently published H_ function for the more basic regions. To study the dependence of H_ functions on indicator structure, the ionization of nitrogen and oxygen acids were compared in strongly basic systems. The solvent systems used in the study were dimethylsulfoxide-methanol-0.01 molar sodium methoxide and dimethylsulfoxide-ethanol-0.01 molar sodium ethoxide. The shallow rise of the function for phenols as compared to that of the HN for diphenylamine is believed to be caused by the smaller degree of charge delocalization in the phenolate anions than in the amide ions. The dissociation constants of 15 substituted nitrodiphenyl-amines were determined in the two previously mentioned alcoholic dimethylsulfoxide systems, and in the aqueous dimethylsulfoxide system. The "similar colour indicator" postulate was elaborated on the observation that the relative acidity of Hammett-type indicators remains constant through changes in solvent environment only if their spectral characteristics are similar. Linear free energy relationships between the pK values of 30 substituted diphenylamines and the substituent constant, σ°, were drawn. The ρ value in excess of 4 for monosubstituted diphenylamines compared with ρ values of the order of 2 for nitrosubstituted diphenylamines, results from the low degree of conjugation between the central N atom and the ring substituent whenever a nitro group is present in the other ring. The base-catalyzed rates of detritiation of various hydrocarbons were determined in dimethylsulfoxide mixtures. The logarithm of these rates were correlated with the thermodynamic basicity of the medium as measured by the H N function and gave excellent straight lines with slopes around 0.8. The theoretical basis of such correlations is discussed, and it was concluded that the logarithm of the rates should be correlated with H_ + log aROH. In the case of methanolic dimethylsulfoxide the activity of methanol has been determined, giving corrected correlations with slopes around 0.9. The qualitative limitation due to the use of HN instead of HC functions is also discussed. The activation parameters for these detritiations have been determined. The enthalpies of activation were found to increase with decreasing acidity of the hydrocarbon. These variations in enthalpy of activation, coupled with variation in the kinetic isotope effect and the observed Bronsted relationship have been interpreted in terms of a mechanism involving a rate-determining pre-equilibrium step with a transition state of increasing asymmetry. A critical examination of the HC scale for carbon acids in dimethylsulfoxide-ethanol-0.01 molar sodium ethoxide was made. The failure of the scale probably results from the fact that, for a number of indicators, proton abstraction is followed by a reaction between the carbanion and the neutral molecule to give a radical and radical anion. The carbanions of three nitrophenylmethanes were prepared in hexamethylphosphoramide and unequivocally identified by high resolution nuclear magnetic resonance. The spectral characteristics of the carbanions were recorded and compared with previously published values. The radical anions of these nitrophenylmethanes were generated electrochemically and their e.s.r. spectra measured. The variation in ¹⁴N hyperfine coupling constant (aN) of these radical anions indicated a large solvent shift in the absorption maxima.