Structure-solubility relationships for some 2-substituted anthraquinones and estimation of heats of fusion from solubility data

Abstract

The mole fraction solubilities of eleven 2-substituted anthraquinones in chloroform (X₁) and in carbon tetrachloride (X₂) have been determined and are in the range of 10⁻² to 10⁻⁶, except that of 2-t-C₄H₉-anthraquinone which is considerably larger, 10⁻¹. This study indicates that 2-H₂N-, 2-HO- and even 2-CH₃COHN-anthraquinones, can undergo intermolecular hydrogen bonding. Multiple regression analyses for 2-H-, 2-H₃C-, 2-Br-, 2-Cl-, 2-O₂N-, 2-CH₃CO₂-, and 2-C₂H₅CO₂-anthraquinones of log(X₁/X₂) on σ[subscript m] and σ[subscript p], and on appropriate pairs of substituent constants of Dewar and Grisdale, show that: (1) the gross molecular structure is important in solubility studies, (2) the 9-carbonyl oxygen is the center of attack by the solvent molecules, and (3) σ[subscript m] and σ[subscript p] constants have field effects as well as π-mesomeric inductive effects. A method for evaluating heats of fusion, ΔH[subscript f], from solubility data and Hildebrand's equation is proposed. The heat of fusion for any member of a series of at least three solid solutes can be estimated from the regression of log(X/X[subscript i]) on substituent constants for the other members, where X and X[subscript i] are the mole fraction solubilities in a common solvent and the ideal mole fraction solubilities. The application of the method is discussed and shows that: (a) the average deviation of the calculated AHj from the experimental ΔH[subscript f] is 4%, (b) the maximum deviation is ±20% for ΔH[subscript f] values of about 5000 cal/mole, (c) the smaller the ΔH[subscript f], the larger the percentage deviation, with a maximum deviation increase of about 10% for each 1000 cal/mole decrease in ΔH[subscript f].