pH-Dependent Tautomerism and pKa Values of Phloroglucinol (1,3,5-Trihydroxybenzene), Studied by 13C NMR and UV Spectroscopy
Open Access
- 1 April 1993
- journal article
- Published by Walter de Gruyter GmbH in Zeitschrift für Naturforschung B
- Vol. 48 (4) , 478-482
- https://doi.org/10.1515/znb-1993-0413
Abstract
13C NMR and UV spectroscopy of aqueous phloroglucinol over the pH range 2.6 to 15 confirms that the dianion exists predominantly as the keto form (3,5-dihydroxy-2,5-cyclohexadienone dianion 2c, δ(>CH2) = 46 ppm, δ(≥CH) = 101 ppm, δ(C3,C5) = 188 ppm, δ(C1) = 197 ppm). All other states of ionization, i.e. neutral la (δ(>CH) = 98 ppm, δ(5(C1,C3,C5) = 160.9 ppm), monoanion 1b (δ(>CH) = 98 ppm, δ(C1,C3,C5) = 164.4 ppm) and trianion 1d (δ(>CH) = 100 ppm, δ(C1,C3,C5) = 170 ppm) exist predominantly as the benzene forms. There is some evidence, consisting of a broadening of the signals from the 3,5-dihydroxy-2,5-cyclohexadienone dianion 2c in the pH range from ∾ 9 where they start to appear, to ∾10 where they approach their maximum values, for a small but noticeable equilibrium concentration of the cyclohexen-1-ol-3,5-dione (monoanion) 3b or the 3,5-dihydroxy-2,5-cyclohexadienone (monoanion) 2b forms. Other keto forms have not been detected. The third pKa turns out to be close to 14.Keywords
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