pH-Dependent Tautomerism and pKa Values of Phloroglucinol (1,3,5-Trihydroxybenzene), Studied by 13C NMR and UV Spectroscopy

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.