Dissociation kinetics of picric acid and dipicrylamine in methanol. Steric effect on a proton transfer rate

Abstract

The acid–base dissociation kinetics of picric acid and 2,2′,4,4′,6,6′-hexanitrodiphenylamine(I) in methanol at 25°C have been investigated using the electric field jump method. The rate constants for the reaction [graphic omitted], where HA denotes picric acid, are k₁= 4.3 × 10¹⁰ dm³ mol^–1 s^–1 and k_–1= 9.2 × 10⁶ s^–1. For (I) the corresponding values are k₁= 2.28 × 10⁹ dm³ mol^–1 s^–1 and k_–1= 1.31 × 10⁵ s^–1. The dissociation constant of (I) in methanol was determined to be K= 5.4 × 10^–5 in agreement with the value derived from the kinetics. The protonation rate constant k₁ of the picrate ion indicates a diffusion controlled reaction. It is suggested that the surprisingly small value of k₁ for the protonation of the anion of (I) can be ascribed to steric shielding of the acidic nitrogen atom by the adjacent nitro groups in this compound.