6533b7d4fe1ef96bd126312d

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The 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.