A chain of intramolecular hydrogen bonds, an important factor for the acidity of oligo[(2‐hydroxy‐1,3‐phenylene)methylene

Abstract

AbstractThe first dissociation constant of oligonuclear phenolic compounds consisting of one nitrophenol unit besides further alkyl phenol units (10 dinuclear, 15 trinuclear, 6 tetranuclear compounds, and 4 nitrophenols as model) was determined spectroscopically in water/methanol (50/50 by vol.) at 25°C. For the ortho‐linked title compounds with the nitrophenol unit at the end of the molecule a decrease of pK1 is observed which becomes stronger with increasing chain length. The introduction of bulky groups in ortho‐position of the dissociating phenolic unit, as well as at the opposite end of the molecule causes a further decrease of pK1. The lowest value is found for a p‐nitrophenol unit in the middle of the molecule. The results do not only prove the stabilization of the monoanion by intramolecular hydrogen bonds, but they give also strong evidence for a chain of intramolecular hydrogen bonds in the undissociated molecule. The UV‐spectra of the monoanions directly show the existence of intramolecular hydrogen bonds. Especially the p‐nitrophenol system makes it possible, to differentiate according to their strength.