Ultraviolet Absorption Spectra and Acid Dissociation Constants of N-(2-Hydroxybenzylidene)-2-hydroxy-5-(substituted) anilines

Abstract

The ultraviolet absorption spectra of N-(2-hydroxybenzylidene)-2-hydroxy-5-(substituted)anilines[1](CH₃[1a], H[1b], SO₃H [1c], Cl[1d], and NO₂[1e]) were investigated in methanol, methyl cellosolve, and N, N-dimethylformamide, the mixed solvents of these with water. It was found that[1]exsisted as a species with an intramolecular hydrogen bond of I in the neat solvents, and as a molecular species of H₂L which has no intramolecular hydrogen bond in the mixed solvents with water. The absorption bands of the molecular species were assigned to the first and the second bands (A-1 and A-2)for the benzene ring of benzylidene side (hereafter denoted A) and those (B-1 and B-2)for the benzene ring of aniline side (hereafter denoted B) (Tables 1 and 2). In the mixed solvents with water (7/3 v/v) of pH 8.1-13.0, the absorption bands observed at 258-269 nm (A-1) and 378-381 nm (A-2) are due to I-HL-([1a]-[1b]) and II-HL-([1e]) a nd those observed at 294-442 nm (B-2) are due to L^2-, respectively. By use of these absorption bands corresponding to molecular and ionic species, the acid dissociation constants (pKa) were determined in the mixed solvents with water (7/3 v/v) at 25±0.1°C. It was found that the pK_a1 (9.1-10.1) and pK_a2 (10.1-12.9) of[1a]-[1d]were acid dissociation constants corresponding to the hydroxyl group on A and B, and pK_a1 (8.O-8.6) and pK_a2 (8.4-9.2) of[1e] were those corresponding to the hydroxyl group on B and A, respectively. Due to the inductive effect of the substituents on B, the pK_a1 values of the hydroxyl group on B decreased in the order of[1a]>[1b]>[1c]>[1d]>[1e]. On the contrary, field effect plays an important factor decreasing the pK_a values of the hydroxyl group on A.