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 condensatio n, Their first acid constant (pK(al)) has been determined in 2-methoxy ethanol-water (9:1) by optical titration, Relative to the correspondin g 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 wa y electron-withdrawing p-substituents in the opposite phenolic unit le ad to a further decrease in pK(al), while a distortion of the cone con formation by m-methyl groups causes a slight increase, The structure o f one calix[4]arene was further confirmed by single crystal X-ray anal ysis showing the molecule in the usual cone conformation.