The first acid constant of calix[4]arenes

A series of calix[4]arenes containing a single-p-nitrophenol unit (or two p-nitrophenol units) have been synthesized by fragment condensation, Their first acid constant (pK(al)) has been determined in 2-methoxyethanol-water (9:1) by optical titration, Relative to the corresponding linear trimers with a p-nitrophenol in the middle, a decrease of pK(al) by 2.1 units or more is observed, which can be explained entirely by intramolecular hydrogen bonds stabilising the monoanion. In this way electron-withdrawing p-substituents in the opposite phenolic unit lead to a further decrease in pK(al), while a distortion of the cone conformation by m-methyl groups causes a slight increase, The structure of one calix[4]arene was further confirmed by single crystal X-ray analysis showing the molecule in the usual cone conformation.