Topochemistry. Part XXV. The absorption spectra of some N-salicylideneanilines and related anils in solution

Abstract

The dependence of the absorption spectra of the title compounds in solution on solvent and on solute concentration has been investigated. These anils generally exist in solution with the aryl groups trans about the central CN bond, with hydrogen covalently bonded to oxygen, and with an intramolecular hydrogen-bond to nitrogen. The aniline ring is rotated out of the plane of the rest of the molecule. In anils with substituents which increase the basicity of the nitrogen there is indication of the presence of some tautomer with hydrogen covalently bonded to nitrogen. The concentration of this tautomer increases with the polarity of the solvent, and in particular with its efficiency as a hydrogen-bond donor and acceptor, and is independent of the solute concentration. Nitrosalicylideneanilines have spectra which vary with solvent, concentration of solute, and time after dissolution. In solvents such as ethanol the anil molecule probably ionises, and the ion is cleaved to give the anion of the aldehyde (plus aniline). The spectra of 2-(hydroxyphenyl)-benzoxazole and -benzothiazole are solvent- and concentration-sensitive; in polar solvents they change with time. However, the species finally formed in polar solvents is identical with that in non-polar solvents. The interpretation of these changes is obscure.