6533b82dfe1ef96bd12912dd

RESEARCH PRODUCT

The acidity of calix[5]arenes and their linear analogues

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Abstract Five new calix[5]arenes containing a single p-nitrophenol unit as the most acidic phenolic unit have been synthesised by (3+2) fragment condensation of a trimer with a p-nitrophenol in the middle with various bishydroxymethylated alkanediyl diphenols. Their first acid constant (pKal) has been determined in 2-methoxyethanol/water (9:1) at 22°C by optical titration. The pKal values are distinctly lower (ΔpKa > 2) than for the trimer, while no difference has been found in comparison to structurally analogous calix[4]arenes. Rigidification of the calix[5]arene skeleton by introducing a single alkanediyl bridge opposite to the p-nitrophenol unit has no effect on pKal. All values are in agreement with a stabilisation of the monoanion via two chains of consecutive intramolecular hydrogen bonds OH…OH…O(−), a system which is favoured, if the acidity of the first donating OH group is increased. This effect is observed also in linear pentamers which show pKal-values higher by only 0.8–1 units than those of the calix[5]arenes.