Quantitative relationships between structure and physical properties of 4-substituted, 2-nitrophenols

Abstract

Chapter I discusses the correlation of the electronic spectra and acidity of 4-substituted, 2-nitrophenols with substituent constants. The apparent pK(subscript A)'s at 25° of a series of thirteen 4-substituted, 2-nitrophenols have been quantitatively related to Hammett's σ-values. A theoretical relation between the wave number shift [see equation in PDF] and pK(subscript A) has been derived from general thermodynamic principles, ΔV=-(2.303 RT/2.859) pK (subscript A) + (ΔE*-TΔS°)/2.859, where ΔE* is the difference in energy of the excited states, and ΔS° is the difference in entropy f the ground states. The deviation in the experimental plot of ΔV vs. pK (subscript A) are discussed. The wave-number shift, ΔV, does not correlate well with σ, but V (subscript B) and V (subscript A) have been separately correlated with σ �-values. An excellent correlation between V (subscript B and V (subscript A) has been found, and the quantitative relation between them has been used to assign structures to equivocal absorption maxima in the spectra of 4-amino-2-nitrophenol and 4-carboxy-2-nitrophenol. Some general considerations in the correlation of electronic spectra with substituent effects are discussed. The experimental evidence suggests that in the ground state there is no interaction between the 4-substituent and the 2-nitro group, but that such interaction does not exist in the excited state. Chapter II discusses some general consideration in the correlation of solubility phenomena with substituent constants. It is proposed that the intermolecular forces existing between solvent and solute in saturated solutions may sometimes be correlated with Hammett's σ-values. The ramifications of three experimental designs in which it is believed that both solvent-solvent and solute-solute forces are cancelled or largely eliminated, are discussed.