Reaction of 2-naphthol with substituted benzenediazonium salts in [bmim][BF 4

Abstract

Diazo-coupling reaction of 4-substituted benzenediazonium tetrafluoroborates with 2-naphthol in [bmim][BF 4] has been studied in the presence of triethylamine (B) and triethylammonium tetrafluoroborate (BH +) (1:1) under the conditions of pseudo-first-order reaction at 25 °C. It was found that the observed rate constant ( k obs) of the diazo-coupling reaction increases linearly with increasing concentration of triethylamine. On the basis of kinetic isotope effect measured with 1-deuterio-2-naphthol ( k H/ k D = 1), it was shown that the linear increase in k obs vs. [B] is not due to general base catalysis, and the rate-limiting step is, like in molecular solvents, the formation of Wheland tetrahedral intermediate. This fact was also confirmed by the value of the reaction constant ρ + = 1.2 ± 0.1. The linear increase in k obs vs. [B] was explained by the effect of increasing concentration of [B] upon the activity coefficient γ B and, hence, also upon the position of the acid–base equilibrium between the non-reactive 2-naphthol and reactive 2-naphthoxide or its proton-transfer complex (ion-pair). This acid–base equilibrium was studied using 4-nitrophenol as a model. It was proved that even at a constant ratio of [B]/[BH +] the concentration of 4-nitrophenoxide (or its proton-transfer complex with triethylamine) increases with increasing concentration [B]. Furthermore, it was found that triethylamine in [bmim][BF 4] exhibits a comparable basicity as 4-nitrophenoxide (Δp K a ≈ 0), while their basicities in water differ by more than 3.5 orders of magnitude.