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Acid-base properties of 4-substituted indoles

The protonation and deprotonation equilibria of a set of 4-substituted indoles have been studied spectrophotometrically at 25 o C. Ionization data have been analyzed by the Hammett acidity function and the excess acidity methods. The existence of close relationships between the Hammett's σ M constants of the substitutents and the ionization constants for both types of acid-base equilibria is reported