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 R-F 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. (C) 2001 Elsevier Science B.V. All rights reserved.