No abstract

No abstract

Equilibrium acidities and homohydrogen-bonding constants are reported for phenol and seven substituted phenols in dimethyl sulfoxide solution. A Hammett plot gave p = 5.3, which is 2.5 times larger them that in water but 2.7 times smaller than that in the gas phase. The p is similar in size to values obtained in other dipolar nonhydroxylic ("aprotic") solvents. Hydrogen bonding of phenol to phenoxide ion (eq 1) is stronger by 6.1 kcal/mol than hydrogen bonding between phenol and Me2SO. The homohydrogen-bonding constants for these phenols remain essentially constant (log ifhb = 3.2-3.4) over a pffa range of 4.5 units.Phenols and benzoic acids have historically been the most studied organic acids. The substituted acids ionize in a convenient range in water, and the effects of substituents on their acidities were responsible for the original concept of free energy relationships.1 2"3 45Many well-established ideas about charge dispersal through induction and resonance are based on these two acid classes. More recently their acidities in other solvents and in the gas phase have been instrumental in understanding medium effects on chemical properties and reactions.4,5 Because dipolar nonhydroxylic ("aprotic") solvents lack the ability to stabilize anions through hydrogen bonding, the properties of ions in these solvents are often intermediate between those in water and the gase phase. Thus they form an important bridge between these two extremes. The lack of anion solvation in dipolar nonhydroxylic solvents can lead to the formation of hydrogen-bonded species (eq 1).This "homohydrogen bonding" complicates the measurement of the acidities of oxygen acids in these solvents.PhO" + PhOH 7=1 PhO"-HOPh(1) Kolthoff has measured acidities in Me2SO of 22, mostly dior trisubstituted, phenols.6 Arnett has measured the enthalpies of ionization of 13 monosubstituted phenols in Me2S0.7 From Hammett plots of these data two different slopes (p) have been derived: 4.3 from the free energies and 5.3 from the enthalpies. This difference suggested that there might be an interesting entropy effect on the ionization of phenols. This would be in contrast with the ionization of a variety of other weak acids in Me2SO, for which the entropies of ionization are constant and near zero.7c However, it would be similar to the situation in water, where entropy changes dominate enthalpy changes in determining phenol acidities.8(1) Hammett, L. P. "Physical Organic Chemistry", 2nd ed.; McGraw-Hill: New York, 1970. (2) Taft, R. W. "Steric Effects in Organic Chemistry"; Newman, M.

The method of structure-oriented lumping (SOL) for describing the compositions, chemical reactions, and properties of complex hydrocarbon mixtures has been extended to molecules found in vacuum residua. The SOL approach was initially developed for gas oil and lighter fractions (i.e., those boiling at <1100 °F), where molecular compositions can be organized in terms of homologous series of single-core molecules (e.g., benzene and alkylated benzenes). In this work, residua molecules are represented as multicore molecules comprising linked assemblies of singlecore species. The original SOL molecular description, using vectors of structural increments, is retained, along with the addition of information about the core linkages, and the extension meshes seamlessly with the conventional SOL notation. Using this formalism, properties of residua may be readily calculated and kinetic models of residua conversion processes developed.

Field desorption (FD) ionization is studied for the measurement of the molecular weight distribution of heavy petroleum, vacuum residua, and asphaltene derived from them. Hydrocarbon polymers, heavy petroleum distillation cuts, and chemical and solubility fractions of heavy petroleums were examined to determine the ionization efficiency. FD showed effective ionization of molecules of a wide molecular weight range (300−5000 Da) with different chemical compositions (saturates versus aromatics) and solubility (maltene versus asphaltene) properties. Electrospray ionization (ESI) was explored and compared with FD. Previous studies had shown that ESI is selective toward the ionization of polar molecules (mostly basic and acidic molecules) while FD is a more universal ionization method. The comparison revealed general agreement between the two techniques. Small variation in the molecular weight distribution of polar and nonpolar molecules was observed. ESI yields lower average molecular weights. The difference increases with the boiling point.