PH-DEPENDENT TAUTOMERISM AND PK(A) VALUES OF PHLOROGLUCINOL (1,3,5-TRIHYDROXYBENZENE), STUDIED BY C-13 NMR AND UV SPECTROSCOPY

C-13 NMR and UV spectroscopy of aqueous phloroglucinol over the pH ran ge 2.6 to 15 confirms that the dianion exists predominantly as the ket o form (3,5-dihydroxy-2,5-cyclohexadienone dianion 2c, delta(>CH2) = 4 6 ppm, delta(greater-than-or-equal-to CH) = 101 ppm, delta(C3,C5) = 18 8 ppm, delta(C1) = 197 ppm). All other states of ionization, i. e. neu tral 1a (delta(>CH) = 98 ppm, delta(C1,C3,C5) = 160.9 ppm), monoanion 1b (delta(>CH) = 98 ppm, delta(C1,C3,C5) = 164.4 ppm) and trianion 1d (delta(>CH) = 100 ppm, delta(C1,C3,C5) = 170 ppm) exist predominantly as the benzene forms. There is some evidence, consisting of a broadeni ng of the signals from the 3,5-dihydroxy-2,5-cyclohexadienone dianion 2 c in the pH range from approximately 9 where they start to appear, t o approximately 10 where they approach their maximum values, for a sma ll but noticeable equilibrium concentration of the cyclohexen-1-ol-3,5 -dione (monoanion) 3b or the 3,5-dihydroxy-2,5-cyclohexadienone (monoa nion) 2b forms. Other keto forms have not been detected. The third pK( a) turns out to be close to 14.