Visual distinction of dicarboxylic acids and their salts by 1-phenyl-3-(quinolin-5-yl)urea

The ability to recognize dicarboxylic acids by 1-phenyl-3-(quinolin-5-yl)urea (I) as a receptor is studied. The solution studies on receptor binding with various dicarboxylic acids show salt and co-crystal formation by I with different dicarboxylic acids. The salts and co-crystals can be distinguished by visible spectroscopy. The crystal structures of five such salts or co-crystals of I with maleic acid, fumaric acid, malonic acid, 1,4-phenylenediacetic acid and pyridine 2,3-dicarboxylic acid are determined. A similar isomeric compound 1-phenyl-3-(quinolin-8-yl)urea (II) is a receptor for mineral acids, but it is not suitable for binding to dicarboxylic acids. The advantage of higher binding ability of receptor I over II with maleic acid provides not only a means for separation of fumaric acid from maleic acid but also helps in separation of mixture of the receptor I and II.