Study of a novel reaction between p-benzoquinone and resorcinol in aqueous solution

Abstract

The kinetics of the simultaneous formation and decay of the charge-transfer (CT) complex between p-benzoquinone (A) as acceptor and resorcinol (B) as donor have been studied in aqueous solution. At 550 nm, the wavelength of absorption of the complex alone, the variation of the absorbance of the system with time suggests the following reaction path: A + B [graphic omitted] AB [graphic omitted] product(s). The rate constants k₁, k_–1 and k₂ have been found from an analysis of the absorbance data at different instants of time to be 0.25 min^–1, 0.05 × 10^–1 min^–1 and 2.48 × 10^–2 min^–1, respectively, and the molar absorption coefficient of AB was found to be 543 ± 9.5 at the experimental temperature, 25 °C. The stability constant of the intermediate CT complex, AB, was found to be k₁/k_–1= 50, a value somewhat greater than that found in methanol where AB does not decay into other product(s). It has been shown that, according to the above reaction scheme, t_max, the time required to reach the maximum absorbance, should be independent of the initial concentration [A]₀ of A with a fixed initial concentration [B]₀ of B and that A_max, the absorbance of the intermediate CT complex at t=t_max should be directly proportional to [A]₀ with a fixed [B]₀. These theoretical predictions of the kinetics of the above reaction scheme have been verified experimentally with [A]₀ and [B]₀ of ca. 10^–3 and 10^–1 mol dm^–3, respectively.

Using aqueous solutions of A and B at concentrations of 10^–2 and 10^–1 mol dm^–3 respectively, the product(s) could be isolated in solid form. The solid, however, could not be precipitated from mixtures of non-aqueous solutions of A and B at similar concentrations, where the coloured CT complex is formed instantaneously, and is stable, not showing a gradual disappearance of colour. The solid (P) obtained from the aqueous mixture was found by NMR studies in [²H₆] dimethyl sulfoxide ([²H₆] DMSO) to be a 1 : 1 resorcinol–p-benzoquinone molecular complex. This was further confirmed by some chemical tests in solutions of P in organic solvent and by conductometric titration of the solution of P in water–dioxan mixtures with Na₂S₂O₃. Elemental (C, H) analysis of P also confirmed the 1 : 1 stoichiometry. The IR spectrum of the solid in the form of a KBr pellet showed that it is either an H-bonded adduct of one molecule of A and one molecule of B or an H-bonded polymeric chain A⋯B⋯A⋯B⋯.