Substituent effects and isotope effects on the rate coefficients and equilibrium constant for opening an intramolecular hydrogen bond

Abstract

Proton removal from the intramolecularly hydrogen-bonded monoanions of substituted 4,6-bis(phenylazo)resorcinols in buffered 70%(v/v) Me₂SO–H₂O involves opening of the hydrogen bond followed by proton transfer between open form and buffer. At low buffer concentration the proton transfer is rate-limiting but opening of the hydrogen bond becomes rate-limiting at higher buffer concentration. Typical values of the rate coefficients for opening (k₁) and closing (k_–1) of the hydrogen bond are k₁ 3.8 ± 0.6 × 10⁴ s^–1 and k_–1ca. 8 × 10⁷ s^–1 found for 4,6-bis(phenylazo)resorcinol monoanion. The rate coefficients and the corresponding equilibrium constant are almost unaffected by substituents and by changing the solvent to 70%(v/v) Me₂SO–D₂O.