Ion associates and hydrogen bonded complexes of bromocresol green and quinine in dichloromethane

Abstract

Quinine base (Q) reacts with bromocresol green (BCGH₂) in dichloromethane forming compounds of 2 : 1, 1 : 1 and 1 : 2 stoichiometric balance depending on the dye–quinine molar relation. In an excess of dye, 2 : 1 and 1 : 1 ion associates are formed clearly in a chemical equilibrium, and in an excess of quinine base 1 : 1 and 1 : 2 compounds in equilibrium are also formed. The 1 : 2 compound is a hydrogen bonded complex (HBC) formed through hydrogen bonding between –O–H in BCGH₂ and amino nitrogen of quinine. Canonical resonance structures of the hydrogen bond justify the large value of the molar absorptivity, the appearance of the band in the visible region, the large value of equilibrium constant and large negative values of the heat of reaction. These values indicate that the HBC is very stable. Quinine hydrochloride (Cl^–QH⁺) reacts with BCGH₂ forming 1 : 1 ion pairs in chemical equilibrium for all molar relations. The maximum absorption for 2 : 1, 1 : 1 and 1 : 2 compounds are at 415, 415 and 547 nm respectively. With a large excess of quinine base, a third absorption band with maximum at 620 nm appears in the solution which proves that the excess of quinine base also reacts with dichloromethane forming an ammonium quaternary compound. The presence in the solution of BCGH₂–ammonium associates justifies the third absorption band (620 nm). The ammonium compound generated is N-chloromethylquininium chloride whose molecular weight is the sum of the amine and that of dichloromethane.