Protonation of 1,3,5-tripyrrolidinobenzene in basic aqueous solution. Thermodynamics and kinetics of the formation of a stable σ-complex

Abstract

In aqueous solution, 1,3,5-tripyrrolidinobenzene is protonated at the aromatic ring, thus forming a σ-complex. The dissociation constant of this acid is K_a=(2.50 ± 0.25)× 10^–10 mol dm^–3 in 9 : 1 v/v water–ethanol. The kinetics of protonation have been studied in basic solutions, where 1,3,5-tripyrrolidinobenzene reacts with water with the release of a hydroxide ion and free protons do not contribute to the reaction rate. The reaction is relatively slow and the rate constants are k₁=(0.62 ± 0.04) s^–1 for the protonation of TPB by water and k_–1=(24 300 ± 400) cm³ mol^–1 s^–1 for the reaction of the conjugate acid with hydroxide ions.