Ion-pair formation of hydroquinine by chromatography

The study of hydroquinine, a Cinchona alkaloid, by analytical TLC (silica gel) in a mixture of dichloromethane and methanol (9/1, v/v) has shown that a new compound, with a higher RF value, is formed during the migration. Its structure has been elucidated and was shown to be the hydrochloride salt of the alkaloid, which behaves like a tight ion-pair. X-ray analysis showed the unit cell to contain two independent monoprotonated hydroquinine hydrochloride ion-pairs, as well as two independent benzene molecules from the crystallisation solvent. The chloride ion appeared to be trapped in a positive cavity between the protonated nitrogen of the quinuclidine ring and the methoxy group of the quinoline ring. The geometry of the salt may explain its apparent lipophilicity.