Acidity measurements in THF. V. Heteroaromatic compounds containing 5-membered rings

The pKa's of 13 heterocyclic aromatic compounds have been measured in tetrahydrofuran (THF) using 13C nmr spectroscopy. The acidifying effect of a nitrogen heteroatom is seen to be quite large (4–6 pK units versus C—H) on an adjacent C—H bond but small on a more remote hydrogen. Most of these aromatic heterocycles are sufficiently acidic, pKa < 34, to be completely deprotonated by lithium tetramethylpiperidide in THF. Benzoxazole appears to be an extremely strong carbon acid, its ring-opened isomer having a pKa of less than 15.7.